User`s Manual
microDXP
Digital X-Ray Processor
Technical Reference Manual
Version 1.1.2
August 6, 2009
MicroDXP Hardware Revision: E
Micromanager Software Revision: 2.3.x
XIA LLC
31057 Genstar Rd.
Hayward, CA 94544 USA
Tel: (510) 401-5760; Fax: (510) 401-5761
http://www.xia.com/
Information furnished by XIA LLC (XIA) is believed to be accurate and reliable. However, no
responsibility is assumed by XIA for its use, nor for any infringements of patents or other rights
of third parties which may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of XIA. XIA reserves the right to change specifications at any
time without notice. Patents have been applied for to cover various aspects of the design of the
DXP Digital X-ray Processor
Micro Digital X-ray Processor - Technical ReferenceManual
mdo-microDXP-MAN-1.1.2
Safety ............................................................................................................................. iv
Power Source ........................................................................................................ iv
Detector and Preamplifier Damage ....................................................................... iv
Servicing and Cleaning ......................................................................................... iv
End Users Agreement ................................................................................................... v
Contact Information:......................................................................................... v
Manual Conventions ..................................................................................................... vi
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1
Introduction.............................................................................................................. 1
1.1 MicroDXP Overview ......................................................................................... 1
1.1.1 Features .............................................................................................. 1
1.1.2 Options and Specifications ................................................................. 3
1.1.3 Application Examples ......................................................................... 6
1.2 Hardware Requirements ................................................................................ 10
1.2.1 Host Computer or PDA ..................................................................... 10
1.2.2 Detector/Preamplifier ........................................................................ 10
1.2.3 Power Requirements ........................................................................ 11
1.2.4 Operating Environment ..................................................................... 12
1.2.5 Regulatory Compliance .................................................................... 12
1.3 Software Overview ......................................................................................... 12
1.3.1 User Interface, e.g. microManager ................................................... 12
1.3.2 Device Drivers, e.g. Handel .............................................................. 13
1.3.3 Physical Interface.............................................................................. 13
1.3.4 Firmware ........................................................................................... 13
1.4 Support .......................................................................................................... 14
1.4.1 Software and Firmware Updates ...................................................... 14
1.4.2 Related Documentation .................................................................... 14
1.4.3 Email and Phone Support ................................................................. 15
1.4.4 Feedback .......................................................................................... 16
1.4.5 The Accelerated DevelOPmenT (ADOPT) Program ........................ 16
2
Using the microDXP .............................................................................................. 18
2.1 Board State and Configuration ...................................................................... 18
2.1.1 Board Information and Status ........................................................... 18
2.1.2 Serial Number ................................................................................... 19
2.1.3 Firmware Version .............................................................................. 19
2.1.4 Idle and Sleep Modes ....................................................................... 19
2.1.5 On-Board Temperature ..................................................................... 19
2.2 Global Settings and the GLOBSET ............................................................... 19
2.2.1 Preamplifier Settings ......................................................................... 20
2.2.2 Advanced Processor Settings ........................................................... 21
2.3 MCA Settings and GENSETs ........................................................................ 21
2.3.1 Selecting the GENSET ..................................................................... 22
2.3.2 MCA Size .......................................................................................... 22
2.3.3 MCA Granularity (Bin Width) ............................................................ 22
2.3.4 Base Gain ......................................................................................... 23
2.3.5 Reading the Current GENSET .......................................................... 23
2.3.6 Saving the Current GENSET to Non-Volatile Memory ..................... 23
2.4 Spectrometer Settings and PARSETs ........................................................... 23
2.4.1 Selecting a FiPPI Decimation ........................................................... 24
2.4.2 Selecting a PARSET ......................................................................... 24
2.4.3 Filter Parameters .............................................................................. 24
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2.4.4 Baseline Average Length .................................................................. 25
2.4.5 Thresholds ........................................................................................ 25
2.4.6 Fine Gain Trim .................................................................................. 26
2.4.7 Reading the Current PARSET .......................................................... 26
2.4.8 Saving the Current PARSET to Non-Volatile Memory...................... 26
2.5 Repetitive Configuration of Identical Systems ............................................... 26
2.5.1 Create Master Parameter Set… ....................................................... 26
2.5.2 Download a Master Parameter Set… ............................................... 26
2.6 Data Acquisition: ............................................................................................ 27
2.6.1 Starting a Run ................................................................................... 27
2.6.2 Stopping a Run ................................................................................. 27
2.6.3 Reading a Spectrum ......................................................................... 27
2.6.4 Reading (and Calculating) the Run Statistics ................................... 28
2.6.5 Specifying fixed run lengths .............................................................. 28
2.7 Diagnostic Tools ............................................................................................ 29
2.7.1 ADC Trace Readout.......................................................................... 29
2.7.2 Baseline Diagnostics ........................................................................ 29
2.7.3 DSP Parameters Readout ................................................................ 29
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MicroDXP Functional Description ....................................................................... 30
3.1 Organizational Overview ................................................................................ 30
3.2 The Analog Signal Conditioner (ASC) ........................................................... 30
3.2.1 Dynamic Range Reduction ............................................................... 31
3.2.2 Nyquist Criterion ............................................................................... 33
3.3 The Analog to Digital Converter (ADC) .......................................................... 33
3.4 The Filter, Pulse Detector, & Pile-up Inspector (FiPPI) ................................. 33
3.4.1 FiPPI Decimation .............................................................................. 34
3.4.2 FiPPI Code Variants ......................................................................... 34
3.5 The Digital Signal Processor (DSP) ............................................................... 34
3.5.1 FLASH Memory ................................................................................ 34
3.5.2 Serial Port (SPORT) ......................................................................... 35
3.5.3 DMA Port .......................................................................................... 35
3.5.4 DSP Code Variants ........................................................................... 35
3.6 PIC MicroController........................................................................................ 35
3.6.1 RS-232 Serial Port ............................................................................ 35
2
3.6.2 I C Serial Bus .................................................................................... 36
2
3.6.3 I C Memory ....................................................................................... 36
2
3.6.4 I C Temperature Sensor ................................................................... 36
3.6.5 PIC Code Variants ............................................................................ 36
3.7 Interface to Host Computer/PDA ................................................................... 37
3.7.1 Flex Cable Interface .......................................................................... 37
3.7.2 High Speed Interface ........................................................................ 37
4
Digital Filtering: Theory of Operation and Implementation Methods .............. 39
4.1 X-ray Detection and Preamplifier Operation .................................................. 39
4.1.1 Reset-Type Preamplifiers ................................................................. 39
4.1.2 RC-Type Preamplifiers ..................................................................... 40
4.2 X-ray Energy Measurement & Noise Filtering ............................................... 41
4.3 Trapezoidal Filtering in the DXP .................................................................... 44
4.4 Baseline Issues .............................................................................................. 45
4.4.1 The Need for Baseline Averaging ..................................................... 45
4.4.2 Raw Baseline Measurement ............................................................. 47
4.4.3 Baseline Averaging in the DXP......................................................... 47
4.5 X-ray Detection & Threshold Setting ............................................................. 48
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4.6 Peak Capture Methods .................................................................................. 49
4.6.1 The Slow Filter Gap Length .............................................................. 49
4.6.2 Peak Sampling vs. Peak Finding ...................................................... 50
4.7 Energy Measurement with Resistive Feedback Preamplifiers ...................... 52
4.8 Pile-up Inspection .......................................................................................... 54
4.9 Input Count Rate (ICR) and Output Count Rate (OCR) ................................ 56
4.10
Throughput .............................................................................................. 56
4.11
Dead Time Corrections ........................................................................... 58
5
microDXP DSP Code Description ........................................................................ 59
5.1 Introduction and Program Overview .............................................................. 59
5.2 Program Flow................................................................................................. 60
5.3 Initialization .................................................................................................... 62
5.4 Event Processing ........................................................................................... 62
5.4.1 Run Start ........................................................................................... 62
5.4.2 Event Interrupt .................................................................................. 63
5.4.3 Event Loop ........................................................................................ 63
5.4.4 Spectrum Binning.............................................................................. 63
5.4.5 SCA Mapping .................................................................................... 63
5.5 Baseline Measurement .................................................................................. 64
5.5.1 IIR (Infinite Impulse Response) Filter ............................................... 64
5.5.2 FIR (Finite Impulse Response) Filter ................................................ 65
5.5.3 Baseline Histogram ........................................................................... 65
5.5.4 Residual Baseline ............................................................................. 66
5.5.5 Baseline Cut...................................................................................... 66
5.6 Interrupt Routines .......................................................................................... 66
5.6.1 ASC Monitoring ................................................................................. 67
5.6.2 Timer Interrupt .................................................................................. 67
5.7 Error Handling ................................................................................................ 68
5.8 Specifying Data Acquisition Tasks (RUNTASKS): ........................................ 68
5.9 Special Tasks (WHICHTEST)........................................................................ 69
5.10
DSP Parameter Descriptions .................................................................. 69
5.10.1
Specifying fixed run lengths........................................................... 71
5.10.2
Setting the slow filter parameters .................................................. 71
5.10.3
Setting the fast filter parameters.................................................... 72
5.10.4
Setting Thresholds ......................................................................... 72
5.10.5
Setting the Pile-up inspection parameters..................................... 74
5.10.6
Setting the Analog Gain (GAINDAC) ............................................. 74
5.11
Standard Program Variants ..................................................................... 75
5.11.1
MCA acquisition with reset-type preamplifiers .............................. 75
5.11.2
MCA acquisition with RC-type preamplifiers ................................. 75
Appendices ................................................................................................................... 77
Appendix A. GLOBSET Specification .................................................................. 77
Appendix B. GENSET Version 1 Specification .................................................... 78
Appendix C. PARSET Version 1 Specification ..................................................... 79
Appendix D. MicroDXP Hardware Specification .................................................. 80
Board Dimensions and Mounting ................................................................... 80
Preamplifier Type Selector Switch ................................................................. 81
Connector Locations and Pinouts .................................................................. 81
Power Supplies .............................................................................................. 84
Appendix E. RS-232 Communications ................................................................. 86
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Safety
Please take a moment to review these safety precautions. They are
provided both for your protection and to prevent damage to the Micro Digital Xray Processor (µDXP) and connected equipment. This safety information
applies to all operators and service personnel.
Power Source
The microDXP requires several DC voltage supplies to operate. In
such cases where the user will provide their own power supplies, they must
conform to the specifications contained in section Appendix D of this manual to
avoid damaging the microDXP and connected equipment, and nullifying the
product warranty.
We recommend that new users purchase either the microDXP RS232
Rapid Development Kit or the microDXP USB Rapid Development Kit. The
RS232 kit includes a wall-mount power supply intended to operate from a mains
supply voltage of 120VAC at 60Hz. The included microCOM interface circuit
board provides linear regulation of the DC voltages required by the microDXP.
The USB kit, introduced August 2009, includes a wall-mount power
supply that can accommodate 100VAC to 240VAC at 47Hz to 63Hz. In
addition, it can adapt to plug styles for North America, Japan, Europe, UK, and
Australia. The included MicroComU board generates the various DC voltages
required by the microDXP.
Use of either evaluation kit with any other mains voltage or power
supply could damage the unit and nullify the product warranty. Refer to the
Rapid Development Kit Manual for instructions on installing the power
supply.
Detector and Preamplifier Damage
The microDXP input impedance is 1kΩ, and should be compatible with
most preamplifiers. Please consult the documentation provided by the
preamplifier manufacturer to confirm that such a load is acceptable.
Because the microDXP does not provide power for the detector or
preamplifier there is little risk of damage to either resulting from the microDXP
itself. Nonetheless, please review all instructions and safety precautions
provided with these components before powering a connected system.
Servicing and Cleaning
To avoid personal injury, and/or damage to the microDXP or
connected equipment, do not attempt to repair or clean the unit. The microDXP
hardware is warranted against all defects for 1 year. Please contact the factory
or your distributor before returning items for service.
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End Users Agreement
XIA LLC warrants that this product will be free from defects in
materials and workmanship for a period of one (1) year from the date of
shipment. If any such product proves defective during this warranty period,
XIA LLC, at its option, will either repair the defective products without charge
for parts and labor, or will provide a replacement in exchange for the defective
product.
In order to obtain service under this warranty, Customer must notify
XIA LLC of the defect before the expiration of the warranty period and make
suitable arrangements for the performance of the service.
This warranty shall not apply to any defect, failure or damage caused
by improper uses or inadequate care. XIA LLC shall not be obligated to furnish
service under this warranty a) to repair damage resulting from attempts by
personnel other than XIA LLC representatives to repair or service the product;
or b) to repair damage resulting from improper use or connection to
incompatible equipment.
THIS WARRANTY IS GIVEN BY XIA LLC WITH RESPECT TO
THIS PRODUCT IN LIEU OF ANY OTHER WARRANTIES, EXPRESSED
OR IMPLIED. XIA LLC AND ITS VENDORS DISCLAIM ANY IMPLIED
WARRANTIES OF MERCHANTABILITYOR FITNESS FOR A
PARTICULAR PURPOSE. XIA LLC’S RESPONSIBILITY TO REPAIR OR
REPLACE DEFECTIVE PRODUCTS IS THE SOLE AND EXCLUSIVE
REMEDY PROVIDED TO THE CUSTOMER FOR BREACH OF THIS
WARRANTY. XIA LLC AND ITS VENDORS WILL NOT BE LIABLE FOR
ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
DAMAGES IRRESPECTIVE OF WHETHER XIA LLC OR THE VENDOR
HAS ADVANCE NOTICE OF THE POSSIBILITY OF SUCH DAMAGES.
Contact Information:
XIA LLC
31057 Genstar Rd.
Hayward, CA 94544 USA
[email protected] (for microDXP, microCOM, or MicroComU
hardware support)
[email protected] (for microManager software support)
(510) 401-5760
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Manual Conventions
Through out this manual we will use the following conventions:
Convention
»
Description
The » symbol leads you
through nested menu
items, DAQExplorer
items, and dialog box
options.
Bold
Bold text denotes items
that you must select or
click on in the software,
such as menu items,
and dialog box options.
Bold text within [ ]
denotes a command
button.
Items in this font
denote sections of code,
file contents, and syntax
examples.
Text in quotation refers
to window titles, and
quotations from other
sources
Italic text denotes a new
term being introduced ,
or simply emphasis
[Bold]
monospace
“window”
Italics
<Key>
<Shift-Alt-Delete>
or <Ctrl+D>
Bold italic
CAPITALS
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Angle brackets denote a
key on the keybord (not
case sensitive).
A hyphen or plus
between two or more
key names denotes that
the keys should be
pressed simultaneously
(not case sensitive).
Warnings and
cautionary text.
CAPITALS denote
DSP parameter names
Example
The sequence
File»Page Setup»Options
directs you to pull down the
File menu, select the Page
Setup item, and choose
Options from the sub menu.
...expand the Run Control
section of the DAQExplorer
to access the run presets.
[Start Run] indicates the
command button labeled
Start Run.
Setup.exe refers to a file
called “setup.exe” on the host
computer.
“Options” indicates the
window accessed via
Tools»Options.
peaking time refers to the
length of the slow filter.
...it is important first to set
the energy filter Gap so that
SLOWGAP to at least one
unit greater than the
preamplifier risetime...
<W> indicates the W key
<Ctrl+W> represents holding
the control key while pressing
the W key on the keyboard
CAUTION: Improper
connections or settings can
result in damage to system
components.
SLOWLEN is the length of
the slow energy filter
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1 Introduction
The Micro Digital X-ray Processor (microDXP) is a high rate, digitallybased, multi-channel analysis spectrometer designed for energy dispersive x-ray
or γ-ray measurements in benchtop, networked, portable and embedded systems.
Its versatile analog front-end accommodates most solid-state and gas detectors
and a wide range of common preamplifiers, including pulsed optical reset,
transistor reset, and resistive feedback types. The microDXP offers complete
computer/PDA control over all available amplifier and spectrometer controls
including gain (if applicable), filter peaking time, and pileup inspection criteria.
As with all DXP instruments, the firmware (i.e. DSP, Microcontroller and
FPGA code) can be upgraded in the field. Unlike other instruments in the DXP
family, microDXP firmware and parameters are stored locally in non-volatile
memory. The architecture supports custom auxiliary digital access including the
industry-standard I2C serial bus and four configurable digital I/O lines. Finally,
the microDXP consumes less than 600mW of power. The microDXP is thus a
flexible, cost-effective OEM component that can form the core of a broad range
of systems, from basic XRF to the most demanding process and control
applications emerging in research and industry.
1.1 MicroDXP Overview
The microDXP offers a set of standard and customizable features and
options intended to address the design requirements of a wide range of complex
spectroscopy data acquisition systems. Application examples are given below in
§1.1.3.
1.1.1
Features
The microDXP is the latest addition to the Digital X-ray Processor
(DXP) line of instruments from XIA. The DXP is a digitally based spectrometer
architecture that offers general control over all analog and digital settings as
outlined in §1.1.1.1 below. A unique feature of the microDXP is non-volatile
parameter storage on a per-peaking-time basis. For each Peaking Time the
DSP stores a complete set of all related spectrometer parameters into nonvolatile memory, as outlined in §1.1.1.2. Five Peaking Times are available for
each FiPPI decimation (each decimation provides a range of peaking times)
included in the purchase, yielding a maximum of fifteen (15) independently
optimized Peaking Times. Re-selecting an optimized Peaking Time
conveniently retrieves all relevant digital filtering and peak inspection
parameters (e.g. gap time, thresholds, pileup inspection interval, etc.) from nonvolatile flash memory. This functionality is particularly beneficial in embedded
systems: The configuration and optimization procedures can be delegated to the
development kit platform (i.e. the MicroComU companion board and
MicroManager software), leaving a very tight data acquisition command set for
the embedded system itself. The configuration process need only be performed
once, though of course the parameter sets be modified at any time.
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Under NRE terms, XIA develops customized firmware to support
specialized data acquisition modes, and to support auxiliary hardware functions
(see §1.1.1.3 Custom OEM Features). The I2C bus can be used to control
various devices, e.g. x-ray tubes and DACs, and the four auxiliary lines can be
individually configured as inputs (e.g. to accept a synchronization signal in a
time-resolved spectroscopy experiment) or as outputs (e.g. multiple SCAs).
1.1.1.1 General Spectrometer Features
•
Extremely compact unit replaces spectroscopy amplifier, shaping
amplifier and multi-channel analyzer (MCA) at significantly reduced
cost and power consumption.
•
Operates with a wide variety of x-ray or γ-ray detectors using
preamplifiers of pulsed optical reset, transistor reset or resistor
feedback types.
•
Multi-channel analysis with up to 8K (8132) bins, allowing for optimal
use of data to separate fluorescence signal from backgrounds.
•
Instantaneous throughput up to 164,000 counts per second (cps) in the
spectrum for the standard 8MHz pipeline clock speed; up to 328,000
cps for the 16MHz pipeline clock option (see §1.1.2.3).
•
Digital trapezoidal filtering, with programmable peaking times between
750 ns and 48 µs for the standard 8MHz pipeline clock speed, and
between 375 ns and 24 µs for the 16MHz pipeline clock option (see
§1.1.2.3).
•
Digitally-controlled internal gain (requires variable gain option, see
§1.1.2.5): 32dB range, 16 bit precision.
•
Pileup inspection criteria computer settable, including fast channel
peaking time, threshold, and rejection criterion.
•
Accurate ICR and live-time reporting for precise dead-time corrections.
1.1.1.2 Embedded Systems Features
•
Up to fifteen (15) sets of optimized spectrometer parameters can be
stored, and later retrieved with a single command.
•
Five (5) sets of MCA parameters, or spectrum formats, can be stored,
and later retrieved with a single command. Note: The microManager
software currently supports only one MCA set.
•
XIA provides the microManager software application for microDXP
parameter set configuration—intended for use both in evaluation and
production phases.
•
A small and powerful RS-232 data acquisition command set.
1.1.1.3 Custom OEM Features
•
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Customized firmware development for special applications such as
time-resolved spectroscopy, multiple SCA’s and pulse-shape analysis.
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Micro Digital X-ray Processor - Technical Reference Manual
Note: The hardware
options included with your
microDXP are specified on
the information sheet
included with the shipment.
mdo-microDXP-MAN-1.1.2
•
Flexible auxiliary digital I/O: 4 general purpose lines, a Gate signal to
externally control data acquisition, the (emerging) industry-standard
I2C bus interface and an external interrupt line.
•
Several assembly options are offered that exclude various hardware and
software features in order to reduce the cost for dedicated applications
(further details in the following section).
1.1.2
Options and Specifications
The microDXP hardware is both powerful and flexible. In many cases,
particularly for dedicated embedded systems, much of the microDXP’s
resources will not be used. XIA is pleased to offer a number of assembly
options that will produce the best value for a given set of requirements. The
specification sheet provided with your microDXP lists the options that have
been implemented. Note: The firmware options and clock speed can be
upgraded in the field, whereas the power supply and gain options require
physical modifications to the hardware. Please contact XIA at
[email protected] for more information about hardware options and
upgrades.
1.1.2.1 Communications and Power Interface
The microDXP hardware offers three communications options via two
high-density connectors: a flat-flex cable for low and medium speed serial
communications and a board-to-board connector that offers both serial protocols
plus high-speed parallel access. In all cases the power, communications and
auxiliary digital I/O is carried on a single connector, and the analog input signal
enters via a separate connection to ensure immunity from electro-magnetic
interference.
The standard assembly offers RS -232 communications and auxiliary
I/O via the flex-cable interconnect. RS-232 runs at 115kbaud, with burst rates
up to 10 Kbytes/sec.
The flex interconnect also supports Analog Devices DSP serial port
(SPORT) communications for faster data transfers, up to 2 Mbytes/sec. This
option is targeted for multi-channel systems and will require some user
hardware and DSP code development.
A third communications option offers parallel IDMA access to DSP
memory for transfer rates up to 10 Mbytes/sec. This interface is used as the
basis for the new MicroComU companion board and microDXP USB Rapid
Development Kit.
1.1.2.2 Power Supplies
Two power supply variants are available, corresponding to whether onboard regulators for the analog supply voltages are used or are bypassed. The
analog circuitry requires +/-5.0V, either supplied directly or indirectly through
on-board LDO regulators. Excessive voltage spikes and/or ripple on the analog
supplies, >20mVpp, can seriously degrade system performance. If +/-5.0V is
supplied directly, either linear regulated or high-quality switching supplies
should be used. If the on-board LDO regulators are used a minimum of +/5.50V is required, and the ripple requirement can be relaxed a bit.
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If planning to use the MicroComU companion board, choose the
variant of the microDXP that uses its on-board voltage regulators. Aside from
this, the MicroComU board will take care of generating all required voltages for
the microDXP, at the specified currents and noise performance.
Regulated Supply Option: (<20mV pk-pk noise)
Voltage Range
Current (min) Current (max)
+3.3V +/- 150mV
100mA
130mA
+5.0V +/- 100mV
25mA
30mA
-5.0V +/- 100mV
25mA
30mA
Description
Decent switching
supply
Linear or highquality switching
Linear or highquality switching
Unregulated Supply Option (<100mV pk-pk noise)
Voltage Range
Current (min) Current (max) Description
+3.3V +/- 150mV
100mA
130mA
Decent switching
supply
+5.5V to +6.0V
25mA
35mA
Decent switching
supply
-5.5V to –6.0V
25mA
35mA
Decent switching
supply
Table 1.1: Power supply options and specifications for the microDXP.
For both variants, the onboard digital circuitry draws from a 3.3V
supply input, with on-board regulators employed to produce 2.5V for the DSP
and 3.0V for the ADC. The ripple requirements for this supply are not
particularly stringent, though excessive radiated noise is to be avoided. If a
switching supply is used, it should be well shielded from, and properly grounded
with respect to, the microDXP.
1.1.2.3 Pipeline Clock Speed Choice
The DSP operates with a 32MHz clock, and a clock-divider produces
either 16MHz or 8MHz for the more power-hungry ADC and the FiPPI (FilterPulse-Pileup-Inspector). Simply put, the clock speed determines the scale of
available peaking times (see Table 1.3 below), with a faster clock producing
shorter peaking times and thus a higher output count rate, or OCR. Note
however that a higher clock speed will result in slightly increased power
consumption. The following table illustrates these points:
Clock Speed
Sampling Period
Maximum OCR
8 MHz
16 MHz
125 ns
62.5 ns
164,000 cps
328,000 cps
Power
Consumption
514 mW
558 mW
Table 1.2: Data pipeline (ADC and FiPPI) clock speed options.
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1.1.2.4 Firmware Selection
The term firmware refers both to code running on the DSP and the
FPGA configuration code that comprises the Filter-Pulse-Pileup-Inspector, or
FiPPI. Updates to both the DSP and FPGA codes will be posted to the XIA
website. Please check the microDXP page at:
http://www.xia.com/microDXP.html
The on-board digital signal processor (DSP) monitors the analog
circuitry, manages spectrum scaling and binning, and carries out various highlevel calculations. The standard DSP code included with all orders will be
sufficient to satisfy most users’ requirements. In some cases custom DSP code
may be provided for specialized applications.
The reconfigurable digital shaping, triggering, and pileup-rejection
algorithms are contained in a field-programmable-gate-array (FPGA). Each
complete configuration code is referred to as a FiPPI (Filter Pulse Pileup
Inspector). FiPPIs are denoted by their decimation. Each FiPPI uses a
decimator circuit to pre-average the ADC codes before trapezoidal shaping is
applied. The so-called decimation determines the peaking time range supported
by a particular FiPPI. The microDXP hardware can store between one and three
FiPPI decimations, allowing great flexibility in choosing the proper peaking
time range at the lowest cost. Table 1.3 lists the peaking times offered for each
FiPPI decimation; the peaking time range of course depends on the chosen
pipeline clock speed.
FiPPI
Decimation
#ADC codes
pre-averaged
0
0
1
2
Peaking times
available
with 8MHz clock
0.75, 1.125, 1.5, 2.25,
3
1.5, 2.25, 3, 4.5, 6
2
3
4
4
8
16
3, 4.5, 6, 9, 12
6, 9, 12, 18, 24
12, 18, 24, 36, 48
Peaking times
available
with 16MHz clock
0.375, 0.5625, 0.75,
1.125, 1.5
0.75, 1.125, 1.5, 2.25,
3
1.5, 2.25, 3, 4.5, 6
3, 4.5, 6, 9, 12
6, 9, 12, 18, 24
Table 1.3: Firmware peaking time range options.
For each FiPPI decimation five sets of DSP parameters, or PARSETs,
are stored in non-volatile memory corresponding to the five available peaking
times. This allows for the procedural separation of parameter optimization and
data acquisition: During the setup process the parameters for each peaking time
are optimized once and saved, and are then automatically retrieved whenever the
peaking time is selected for acquisition. Parameter set storage and retrieval is
described in further detail the Getting Started section below.
1.1.2.5 Gain and Calibration Options
Two gain configurations are available as hardware options. With the
fixed-gain option, a user-selected analog gain is implemented in the on-board
circuitry. The gain tolerance will typically be at the one percent level; typically
an energy calibration must be handled offline by the host software.
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The variable gain option provides 32dB of 16-bit precision, digitally
controlled gain. This allows the microDXP to be optimized for a wide range of
x-ray energies, and allows for a real hardware calibration.
1.1.3
Application Examples
The microDXP miniaturized circuit-board can easily be incorporated
into a variety of benchtop, portable, networked and embedded x-ray and γ-ray
spectroscopy data acquisition systems. In the first example below, the
microDXP / MicroComU board set runs on a laboratory benchtop under the
control of an x86 Personal Computer. No user hardware design is required, no
power supplies are needed, and no microDXP hardware or firmware
modifications are necessary.
In the second example below, the microDXP runs either on the
laboratory benchtop as a peripheral device under the control of an x86 Personal
Computer, or similarly in portable systems under the control of a PDA.
Minimal user hardware design, and no microDXP hardware or firmware
modifications are required.
In the third example a more complex dedicated system is considered.
The I2C serial bus is used to control a ‘smart’ x-ray tube and detector HV bias
supply, and the auxiliary digital I/O drives electromechanical or pneumatic
components in real time based upon user defined metrics of acquired data.
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1.1.3.1 Example 1. General-Purpose Spectrometer Using USB 2.0
and MicroComU Companion Board
In this example the microDXP / microComU board set acts as a
general-purpose spectrometer, connected as a peripheral device under the
control of a host computer. No specialized data acquisition modes are required,
thus no firmware development is necessary. No user hardware design is
required. All required power is drawn from the USB port.
XIA non-recurring engineering (NRE) required: NONE.
User development required: ALMOST NONE.
1.
It may be necessary to design an enclosure.
Figure 1.1: A general-purpose spectrometer incorporating the microDXP and MicroComU companion
board. The board set communicates with a host PC using USB 2.0. The board set derives all
necessary power supplies from the USB port.
In this example, the MicroComU board acts as both a carrier and
companion board for the microDXP. Power for the MicroComU/microDXP
board set may be taken from the USB port as shown in the example above, or
may be provided externally, as with the AC wall adapter provided with the USB
Rapid Development Kit. MicroComU board dimensions and mounting
information, the connector locations and specifications, and the power supply
specifications are all found in the separate MicroComU Technical Reference
Manual available at:
http://www.xia.com/microDXP.html
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1.1.3.2 Example 2. General-Purpose Spectrometer Using RS232
and Custom Breakout Board
In this example the microDXP acts as a general-purpose spectrometer,
connected as a peripheral device under the control of a host computer or PDA.
No specialized data acquisition modes are required, thus no firmware
development is necessary. Some user hardware design is, however, required.
XIA non-recurring engineering (NRE) required: NONE.
User development required:
1.
To connect to the external host, only a simple routing adapter interface
unit is required to break out the microDXP high-density internal
connection to standard RS-232 and power connections. At a minimum,
this interface is a wire harness but could entail a printed circuit board
with a small number of passive components.
2.
Power supplies for the microDXP must be provided. Optional voltage
regulators for the analog circuitry are included on the microDXP for
systems in which high-quality power supplies are not available.
3.
Some additional mechanical design, i.e. enclosure design, may be
necessary.
MicroDXP board dimensions and mounting information, the connector
locations and specifications, and the power supply specifications are all found in
Appendix D of this manual.
Figure 1.2: A general-purpose spectrometer incorporating the microDXP. A simple user-designed
routing-adapter interface connects the microDXP to the host computer/PDA and power
supplies.
The microDXP, power supplies and ‘routing-adapter’ together
constitute a spectrometer that can be connected to virtually any controller with
RS-232 communications. Note: The microCOM interface board included with
the RS232 Rapid Development Kit falls into this category, though its large
form-factor nearly prohibits commercial use. However, the recently released
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MicroComU board (included with the USB Rapid Development Kit) may also
be used for this purpose.
1.1.3.3 Example 3. Dedicated Spectrometer Using RS232
This example considers a materials sorting application where objects
with certain pre-defined alloy ratios X, Y and Z are to be separated from others.
An x-ray source irradiates incoming samples, and incident x-rays are collected
by a solid-state detector connected to the microDXP. The microDXP is
configured to assert a combination of its auxiliary digital I/O lines whenever the
peak ratio X,Y, or Z is detected. The digital I/O lines drive electromechanical or
pneumatic components in real-time to execute the appropriate mechanical
operation, e.g. put the recognized object in the desired bin. User controls are
limited to starting and stopping the system, and selecting one out of a small
number of operating modes. Power supplies for the microDXP are also
included. Finally, an external data port (e.g. RS-232) is also included so that
ratios corresponding to new alloys can be defined, and new firmware uploaded
without dismantling the hardware; or, alternatively, the microDXP could
periodically be run in full MCA mode under computer control for diagnostic
purposes.
This example demonstrates a system that uses a very small data
acquisition command set (i.e. ‘start run’ and ‘stop run’) but that, conversely,
requires customizations to the microDXP as well as significantly more userdesigned hardware.
XIA non-recurring engineering (NRE) required:
1.
Customized PIC microcontroller code is required to implement the I2C
peripheral device control.
2.
Customized PIC microcontroller code is required to implement highlevel data acquisition routines controlled through the user pushbutton
interface.
3.
Customized DSP code is required for peak ratio calculations, possibly
implemented in lookup tables.
4.
Minimal FiPPI (FPGA) code modification is required to implement the
auxiliary digital I/O functionality.
User development required:
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1.
A more advanced interface unit is required to break out the microDXP
high-density internal connection to standard RS-232, auxiliary and
power connections. Still, this interface does not involve many active
components, i.e. the I2C and auxiliary digital I/O are simply routed to
additional connectors. The pushbutton interface might include an
additional microcontroller, but could be implemented simply in logic.
2.
As drawn, the power supply is integrated on the interface board, with
the same requirements as in the previous example. As stated there,
optional voltage regulators for the analog circuitry are included on the
microDXP for systems in which high-quality power supplies are not
available.
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mdo-microDXP-MAN-1.1.2
Again, some additional mechanical design, i.e. enclosure design, may
be necessary.
Figure 1.3: A system with a fully embedded host and user interface, with real time controls via the
Auxiliary digital I/O.
Although the figure above shows a flex cable used to interface the
microDXP board to the Advanced Interface board, it is also possible to use the
board-to-board connector on the microDXP, as was done with the recently
introduced MicroComU companion board.
Designing the system described above using conventional spectroscopy
components would be a much more complex (and considerably more expensive)
task, when compared with the other solutions proposed here.
1.2 Hardware Requirements
1.2.1
Host Computer or PDA
The microDXP can communicate with a host computer or PDA in a
variety of ways. The hardware supports RS-232 serial communications, DSP
serial communications, and parallel IDMA access to the DSP. The IDMA port
allows a variety of interface options to be implemented, such as the USB 2.0
high speed interface now implemented on the MicroComU companion board.
At present, DSP serial communication is not yet supported. A full description of
the RS-232 command set can be found in the microDXP RS-232
Communications Specification, available online at:
http://www.xia.com/microDXP.html
1.2.2
Preamplifier signal and
power specifications must
be verified.
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Detector/Preamplifier
The microDXP accommodates nearly all detector preamplifier signals.
The two primary capacitor-discharge topologies, pulsed-reset and resistivefeedback, are supported. The voltage compliance range in the DXP analog
circuitry imposes in the following constraints:
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Parameter
X-ray pulse-height
Input voltage range
Table 1.4:
1.2.3
Minimum
250uV
-
Maximum
375mV
+/-5V
Typical
25mV
+/-5V
Analog input signal constraints for pulsed-reset preamplifiers.
Parameter
X-ray pulse-height
Input voltage range
Decay time τ
Table 1.5:
mdo-microDXP-MAN-1.1.2
Minimum
250uV
100ns
Maximum
625mV
+/-5V
infinity
Typical
100mV
+/-3V
50us
Analog input signal constraints for resistive-feedback
preamplifiers. Note that the maximum input range is less than for
pulsed-reset preamplifiers.
Power Requirements
The microDXP requires several DC voltage supplies to operate. In
cases where the user will provide their own power supply to generate these
voltages, such power supplies should conform to the specifications described in
section Appendix D.
The onboard digital circuitry draws from a 3.3V supply input, with onboard regulators employed to produce 2.5V for the DSP and 3.0V for the ADC.
The analog circuitry runs on +/-5.0V, either supplied directly or indirectly
through on-board LDO regulators.
Customers not wishing to worry about power supply requirements are
advised to purchase MicroComU companion boards. The MicroComU boards
will take care of generating all required voltages, at the required currents and
noise performance. The MicroComU / microDXP board set may be powered
from a USB port or from a single external DC supply, such as the AC wall
adapter included with the USB Rapid Development Kit.
Clock Speed
[MHz]
8
Voltage
Supply
VCC
Current
[mA]
89.7
Power
[mW]
296.0
8
V+
20.3
101.5
8
V-
23.3
116.5
514 mW
16
VCC
103.0
339.9
16
V+
20.3
101.5
16
V-
23.3
116.5
557.9 mW
Comment
+3.3V digital – includes
ADC
+5V analog – includes
VGA
-5V analog
Total power
consumption at 8MHz
+3.3V digital – includes
ADC
+5V analog – includes
VGA
-5V analog
Total power
consumption at 16MHz
Table 1.6: Power consumption depends upon the pipeline clock speed.
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mdo-microDXP-MAN-1.1.2
Operating Environment
•
Temperature Range:
0° C - 50° C
•
Maximum Relative Humidity:
75%, non-condensing.
•
Maximum Altitude:
3,000 meters
•
Pollution degree 2
•
Not rated for use in high electromagnetic fields.
•
Not rated for use in environments with measurable neutron flux.
Neutron flux will cause permanent damage to silicon crystals and
permanently degrade or impair the performance of this system.
•
The components on the microDXP board are not radiation hardened.
Although there should not be a problem operating them in
environments with modest gamma or X-ray flux, above a certain level
this radiation will start to cause bit errors in the digital components. If
necessary, please contact XIA LLC to discuss a proposed radiation
environment.
1.2.5
Regulatory Compliance
The microDXP board is RoHS compliant.
1.3 Software Overview
Up to three layers of ‘software’ are used in a typical microDXP
instrumentation system: a user interface for data acquisition and control, a driver
layer that communicates between the host software and the RS-232 or USB port,
and firmware code (consisting of PIC, DSP and FPGA code) that is stored and
runs on the microDXP itself. Firmware code is factory pre-loaded into
nonvolatile memory on the microDXP circuit board, and can be updated in the
field via the XUP utility in the microManager software.
1.3.1
User Interface, e.g. microManager
The host software communicates with and directs the microDXP (via a
driver layer), and displays and analyzes data as it is received. XIA offers
microManager as a general-purpose configuration and data acquisition
application. MicroManager features full configuration of and control over the
microDXP, with intuitive data visualization, unlimited ROI’s (regions of
interest) Gaussian fitting algorithms and the exporting of collected spectra for
additional analysis. Please refer to the Rapid Development Kit Manual for
instructions on using microManager with the microDXP. Some users may
decide instead to develop their own software to communicate with the
microDXP directly via the RS-232 port. Please read §1.3.3 below for further
details.
XIA offers the Accelerated DevelOPmenT (ADOPT) support option at
an additional fee to such users. ADOPT is described in § 1.4.5.
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mdo-microDXP-MAN-1.1.2
Device Drivers, e.g. Handel
XIA will in the near future provide source code for Handel, its highlevel spectrometer driver layer that operates with all DXP instruments. Handel
provides an interface that is versed in lay spectroscopic units (eV, microseconds,
etc...) while still allowing for safe, direct-access to the DSP. The trade-off for
this ease of use is size. The Handel library is on the order of 2 Mbytes. Though
microManager does currently use the Handel driver, we are in the process of
refining Handel to work more efficiently with the microDXP, and thus are not
yet offering Handel to microDXP users. In fact, the feature omissions in this
release are due primarily to Handel, and not to the microDXP hardware or
firmware. Please contact XIA for further information.
1.3.3
Physical Interface
The physical interface to the microDXP may be either an RS232
(“COM”) port or a USB 2.0 port. The MicroComU companion board is
required in order to use USB communications.
Regardless of physical interface, the microDXP is controlled directly
via the RS-232 command set. This is the lowest level of access currently
supported and is appropriate for OEM customers who wish to design embedded
systems incorporating the microDXP. Such users are encouraged to at first get
up and running with microManager, and may in fact find it useful in the long run
to use microManager for board configuration and their own software simply for
data acquisition “in the field”. Keep in mind that certain configuration
procedures, namely firmware updates via the XUP specification, can only be
executed using microManager. For the complete command set please refer to
the RS-232 Command Specification, available at:
http://www.xia.com/microDXP.html
XIA offers the Accelerated DevelOPmenT (ADOPT) support option at
an additional fee to advanced users and OEM customers. ADOPT is described
in §1.4.5.
1.3.4
Firmware
Firmware refers to the PIC, DSP and FiPPI (FPGA) configuration code
that is stored in non-volatile memory on the microDXP. One PIC file, one DSP
file, and up to three FiPPI files can be stored. For simplicity XIA provides
complete firmware updates in files of the form “update_name.xup”. The XUP
utility in microManager supports firmware backup and update functions. The
XUP utility also supports parameters settings import and export, to aid in the
production of multiple identical systems. The FiPPI and DSP are discussed in
chapter 3 and chapter 5. The XUP utility is described in §1.4.1.1 and in the
Rapid Development Kit Manual.
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1.4 Support
A unique benefit of dealing with a small company like XIA is that the
same people who designed them often provide the technical support for our
sophisticated instruments. Our customers are thus able to get an in-depth
technical advice on how to fully utilize our products within the context of their
particular applications. Please read through this brief chapter before contacting
us.
XIA LLC
31057 Genstar Rd.
Hayward, CA 94544 USA
(510) 401-5760
Hardware Support:
Software Support:
Check for firmware and
software updates at:
http://www.xia.com/micro
DXP.html
1.4.1
[email protected]
[email protected]
Software and Firmware Updates
It is important that the DXP unit is being driven by the most recent
software/firmware combination, since most problems are actually solved at the
software level. Please check for the most up to date standard versions of the
microDXP software and firmware at:
http://www.xia.com/microDXP.html
Please contact XIA at [email protected] if you are running semicustom or proprietary firmware code. (Note: it is not a bad idea to make backup
copies of your existing software and firmware before you update).
1.4.1.1 XUP Utility and Firmware Updates
Firmware updates will be provided in the XUP format, which is only
supported by microManager. MicroManager’s XUP utility also supports the
import and export of parameter settings to and from non-volatile memory, such
that multiple identical systems can easily be configured.
1.4.2
Related Documentation
As a first step in diagnosing a problem, it is sometimes helpful to
consult the most recent data sheets and user manuals for a given DXP product,
available in the Adobe Portable Document Format (PDF) from the XIA web
site. Since these documents may have been updated since the DXP unit was
purchased, they may contain information that could help solving a problem in
question. All manuals, datasheets, and application notes, as well as software and
firmware downloads can be found on at:
http://www.xia.com/microDXP.html
In order to request printed copies, please send an e-mail to
[email protected], or call the company directly. In particular, we
recommend that you download the following documents:
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
Rapid Development Kit User Manual (covers both the RS232 and
USB Rapid Development Kits) – All users

MicroComU Technical Reference Manual -- Users interested in
using the MicroComU companion board to generate the microDXP’s
power supplies and/or communicate with the microDXP using a USB
2.0 high speed interface.

MicroDXP RS-232 Communications Specification – Users who wish
to develop their own software and/or hardware
1.4.3
Email and Phone Support
The microDXP comes with one year of email and phone support.
Support can be renewed for a nominal fee. Please call XIA if your support
agreement has expired.
The XIA Digital Processors (DGF & DXP) are digitally controlled,
high performance products for X-ray and gamma-ray spectroscopy. All settings
can be changed under computer control, including gains, peaking times, pileup
inspection criteria, and ADC conversion gain. The hardware itself is very
reliable. Most problems are not related to hardware failures, but rather to setup
procedures and to parameter settings. XIA's DXP software includes several
consistency checks to help select the best parameter values. However, due to
large number of possible combinations the user may occasionally request
parameter values which conflict among themselves. This can cause the DXP unit
to report data that apparently make no sense (such as bad peak resolution or
even empty spectra). Each time a problem is reported to us, we diagnose it and
include necessary modifications in the new versions of our DXP control
programs, as well as add the problem description to the FAQ list.
Submitting a problem report:
XIA encourages customers to report any problems encountered using
any of our software. Unfortunately, due to limited resources XIA is unable to
handle bug reports over the phone. In most cases, the XIA engineering team will
need to review the bug information and run tests on their hardware before being
able to respond.
All software-related bug reports should be emailed to
[email protected] and should contain the following information, which
will be used by our technical support personnel to diagnose and solve the
problem:
 Your name and organization
8/6/2009

Brief description of the application (type of detector, relevant
experimental conditions...etc.)

XIA hardware name and serial number

Version of the library (if applicable)

OS

Description of the problem; steps taken to re-create the bug

Supporting data:
The most important are digital settings of the spectrometer unit, i.e., the
values of the DSP parameters such as the decimation, filter length, etc.
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The values of these parameters can be captured into an ASCII file in
microManager. Please attach a copy of this file if possible. Capturing
an oscilloscope image of the preamp output will be extremely helpful.
This can done with the diagnostic tool included in microManager.
For general questions and DXP hardware issues please email:
[email protected]
1.4.4
Feedback
XIA strives to keep up with the needs of our users. Please send us your
feedback regarding the functionality and usability of the microDXP and
microManager software. In particular, we are considering the following
development issues:
1.4.4.1 Export File Formats
We would like to directly support as many spectrum file formats as
possible. If we do not yet support it, please send your specification to
[email protected]
1.4.4.2 Fast Communications
Currently the hardware supports three communications interfaces: an
RS-232 serial port, a synchronous DSP serial port and IDMA parallel DSP
access. Only the RS-232 interface and recently the IDMA parallel DSP
interface have been implemented thus far in software and firmware. We are
interested in feedback about the support of the DSP serial port interface.
1.4.4.3 Hardware Interfaces for Production
Recently the MicroComU companion board has been introduced in
order to provide customers with a USB 2.0 high speed interface to the
microDXP and power supply generation for the microDXP, all in a form factor
close to the size of the microDXP itself. We are interested in how well the new
MicroComU board satisfies customer requirements and/or what improvements
are desired.
1.4.5
The Accelerated DevelOPmenT (ADOPT) Program
The ADOPT program is a support plan for users developing custom
software using any of our driver libraries. It is intended for those who wish to
get direct access to the XIA software team and obtain hands-on training in the
use of XIA software tools as a method of reducing overall software development
time.
The standard ADOPT package provides 12 months of support divided
as follows:
•
1 month: on-site support and priority phone/email support.
•
11 months: priority phone/email support.
The specific number of hours for on-site support and priority
phone/email support depend on the driver library being used. Typically, the
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person who will be doing the majority of the development will visit XIA for a
hands-on tutorial with the XIA software team. The visitor will be encouraged to
work at XIA for anywhere from a few days to two weeks, depending on the
specific situation and complexity of the project. By working on-site, visitors will
have access to live experimental setups on which they will be able to test their
software. Furthermore, the XIA software team will be available to provide
assistance and help immediately without the limitations of either email or phone.
For situations where more time is required, additional hours of support
may be purchased at XIA's standard consulting rate.
This program supports both our Handel and Xerxes driver libraries as
well as custom driver development. Please contact XIA to determine which
driver library is right for your application ([email protected]).
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2 Using the microDXP
Note: Please refer to the
‘Getting Started’ section of
the Rapid Development Kit
User Manual for detailed
setup instructions.
This chapter provides a general outline of microDXP operational
procedures. XIA recommends using the microManager software as a microDXP
configuration platform in all phases of production. For a step-by-step ‘Getting
Started’ guide using microManager, including hardware setup instructions,
please refer to the Rapid Development Kit User Manual. Though
microManager also supports microDXP data acquisition (DAQ) procedures,
many customers will necessarily use their own software when acquiring data.
The most common procedures are explained below at the RS-232 command
level. Please refer to Appendix E for a condensed summary of the RS-232
command and response protocol. Users who wish to develop the configuration
routines into their software should refer to the RS-232 Command Specification
(a separate document) for a detailed presentation of all RS-232 commands.
DXP-related documents are available online at:
http://www.xia.com/microDXP.html
2.1 Board State and Configuration
The microDXP boots itself upon power up, and is shortly thereafter
ready to acquire data with the same set of operating parameters used in the
previous run. The first time the microDXP is powered on, detector and
preamplifier related parameters should be modified (unless the default
parameters have been factory-set to match the detector and preamplifier). Once
these settings have been saved to non-volatile on-board memory, they will
automatically load during subsequent boot operations.
2.1.1
Board Information and Status
General information about the hardware and firmware, and current
board status can be retrieved.
In microManager, select View > Board Info… to display the Board
Information dialog.
2.1.1.1 Board Information
In microManager, press [Read Information] to display information about the
hardware and firmware configuration, variants and versions.
The RS-232 command to read board information is 0x49. Please refer to the
RS-232 Command Specification for details.
2.1.1.2 Board Status
In microManager, press [Read Status] to display information about the current
state of the PIC, DSP and data acquisition run.
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The RS-232 command to read board information is 0x4B. Please refer to the
RS-232 Command Specification for details.
2.1.2
Serial Number
In microManager, the serial number is automatically read at startup. It is
displayed along the bottom of the window.
The RS-232 command to read the serial number is 0x48. Please refer to the RS232 Command Specification for details.
2.1.3
Firmware Version
In microManager this command is omitted. All the version information is
embedded in the board information command described above.
The RS-232 command to read the firmware version information is 0x4D. Please
refer to the RS-232 Command Specification for details.
2.1.4
Idle and Sleep Modes
If the Idle Mode is enabled, the microDXP will enter a low power state
a specified time after the end of a run. Several different subsystems can be
manually powered down via the Sleep Mode command to conserve energy. The
sleep mode can not be changed in microManager.
The RS-232 command to change the Idle Mode is 0x46. The RS-232 command
to change the Sleep Mode is 0x47. Please refer to the RS-232 Command
Specification for details.
2.1.5
On-Board Temperature
The microDXP hardware includes an I2C thermometer. The
temperature reading is not accessible in microManager.
The RS-232 command to read the on-board temperature is 0x41. Please refer to
the RS-232 Command Specification for details.
2.2 Global Settings and the GLOBSET
The GLOBSET, specified in Appendix A, contains global settings
including detector/preamplifier settings and system settings. There is only one
GLOBSET—these settings are used for all peaking times and MCA formats.
The GLOBSET includes detector/preamplifier settings, advanced
processor settings, run control settings and diagnostic control settings.
In microManager, GLOBSET settings are accessed via the Detector and
Advanced tabs of the Settings panel. Because these settings are global, changes
are simultaneously applied and saved to nonvolatile memory via the [Apply
And Save] button.
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mdo-microDXP-MAN-1.1.2
Preamplifier Settings
The microDXP’s analog signal conditioner must be configured for the
specific detector / preamplifier that is used in order for the downstream digital
processing pipeline to operate correctly. The microDXP must have firmware
pre-loaded in non-volatile memory that is appropriate for the preamplifier type
used (i.e. reset-type or RC-feedback), and parameters must be properly set based
upon the following information, which is normally included in the detector or
preamplifier documentation (or alternatively measured using an oscilloscope).


Note: The microDXP must
have appropriate firmware
for the preamplifier type
used. In addition the
HARDWARE must also be
set to the appropriate
preamplifier type via a
miniature DIP switch S1.
Set S1 to “RAMP” for
reset-type preamplifiers.
Set S1 to “OFFSET” for
RC-type preamplifiers.
The decay constant for RC-type preamplifiers, or the settling time of
the reset transient for reset-type preamplifiers.
The preamplifier signal polarity: Positive polarity means that an x-ray
produces a positive step in voltage at the preamplifier output. The
preamplifier’s signal polarity unrelated to the detector’s bias voltage
polarity.
2.2.1.1 Preamplifier Type
Please refer to § 4.1 for more detailed description of charge-sensitive
preamplifier topologies. Briefly, RC-feedback preamplifiers use resistive
feedback to provide a continuous discharge path for the feedback capacitor,
resulting in a characteristic decay time, e.g. 50µs. Reset preamplifiers employ a
switch to periodically discharge the capacitor very quickly, resulting in a
periodic ‘staircase’ waveform with many x-ray steps between each reset.
The microDXP must have DSP and FPGA code pre-loaded in nonvolatile memory that is appropriate for the preamplifier type used. In addition
the microDXP hardware must be set type via a miniature DIP switch (refer to
Appendix D for the switch location):
Set to ‘RAMP’ for reset-type preamplifiers.
Set to ‘OFFSET’ for RC-feedback preamplifiers.
In MicroManager, the preamplifier type corresponding to the loaded firmware is
displayed in the Detector tab of the Settings panel.
2.2.1.2 Decay Time: TAURC
If using an RC-feedback type preamplifier you must set the DSP
parameter TAURC, where the decay time constant τRC, is expressed in
microseconds [µs]:
TAURC = τRC
In microManager, TAURC is set via the Decay Time field in the Detector tab
of the Settings panel. Simply enter the desired value in microsecond units and
press the [Apply And Save] button.
The RS-232 command to set and save TAURC to nonvolatile memory is 0x89.
Please refer to the RS-232 Command Specification for details.
2.2.1.3 Reset Interval: RESETINT
If using a reset-type preamplifier you must set the DSP parameter
RESETINT to the reset delay time or reset interval. This is the period after each
preamplifier reset that the microDXP waits before re-enabling data acquisition.
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RESETINT, expressed in microseconds, is set based on the settling time of the
preamplifier reset transient waveform, which typically ranges from hundreds of
nanoseconds to hundreds of microseconds. Setting the delay shorter than the
transient settling time typically degrades the energy resolution and may even
introduce ‘reset artifact’ events into the spectrum. Setting the delay longer than
necessary introduces additional processor dead time, which will reduce the data
throughput at high count rates.
In microManager, RESETINT is set via the Reset Interval field in the Detector
tab of the Settings panel. Simply enter the desired value in microsecond units
and press the [Apply And Save] button.
The RS-232 command to set and save RESETINT to nonvolatile memory is
0x8A. Please refer to the RS-232 Command Specification for details.
2.2.1.4 Pre-Amplifier Signal Polarity
Preamplifier signal polarity denotes the polarity of the preamplifier
output signal. Please read through § 4.1 for a description and figures relating to
the preamplifier signal polarity. Briefly, a positive polarity preamplifier
produces a voltage step with a rising edge. The DSP parameter POLARITY
must be set correctly:
POLARITY = 0, for negative polarity
POLARITY = 1, for positive polarity
In microManager, POLARITY is set via the Polarity field in the Detector tab of
the Settings panel. Select the desired polarity and press the [Apply And Save]
button.
The RS-232 command to set and save POLARITY to nonvolatile memory is
0x87. Please refer to the RS-232 Command Specification for details.
2.2.2
Advanced Processor Settings
The advanced processor settings enable and disable various Digital Xray Processor functions, and correspond to bits of the DSP parameter
RUNTASKS. Typically these settings should only be modified as directed by
XIA LLC engineers.
In microManager, the advanced processor settings are accessed in the Advanced
tab of the Settings panel. Please refer to Chapter 5 and specifically section 5.8
for further details.
2.3 MCA Settings and GENSETs
The GENSET, specified in Appendix B, is a table of MCA-related
parameters, e.g. the number of bins and bin granularity, preset length of run, etc.
Parameters within the GENSET can be modified and stored such that a
standardized MCA format can be implemented with a single command. Five (5)
GENSETs, and thus five MCA formats, can be stored and retrieved.
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Selecting the GENSET
In microManager, GENSET settings are accessed via the Acquisition tab of the
Settings panel. GENSETs 0-4 can be selected via the GENSET drop-down list.
Modifications to MCA settings can be tested by pressing the [Apply] button,
and saved to nonvolatile memory via the [Save] button.
The RS-232 command to select a GENSET is 0x83. Note that this command
simply retrieves one of five tables of MCA settings from nonvolatile memory.
Please refer to the RS-232 Command Specification for details.
2.3.2
MCA Size
This command is used to change the number of bins in the multichannel analyzer (MCA) via the DSP parameter MCALEN.
In microManager, MCALEN is set via the Number of MCA Bins field in the
Acquisition tab of the Settings panel. Select the desired number and press the
[Apply] button to test the setting. Press the [Save] button to store the setting to
the currently selected GLOBSET in nonvolatile memory.
The RS-232 command to set MCALEN is 0x85. Note that this command does
not save the setting to nonvolatile memory. Please refer to the RS-232
Command Specification for details.
2.3.3
MCA Granularity (Bin Width)
The granularity setting determines the hardware (DSP) scaling factor
BINGRANULAR, with four standard settings and a custom as outlined in table
Error! Reference source not found. below. The raw energy filter sample (i.e.
the ‘energy’) is divided in the DSP by BINMULTIPLE. The combination of
MCA size and granularity must satisfy dynamic range constraints imposed by
the digital filter pipeline. If they do not, a dead region will be included at the
highest energies in the spectrum, i.e. there is no possibility of getting counts in
this region.
Recommended #
of MCA Bins
4096
2048
1024
512
< 512
(e.g. 128)
Table 2.1:
Absolute MAX
# of MCA Bins
8192
8192
4096
2048
-
BINGRANULAR
0
1
2
3
4
BINMULTIPLE
1
2
4
8
(Bin Size)
(e.g. 32)
Suggested Bin Granularity settings based upon the Number of
MCA Bins. Using a larger-than-recommended number of bins for
a given granularity will result in extended-range events—if such xrays are present—being displayed. If greater than the absolute
maximum, the spectrum will include a high-energy dead region.
In microManager, BINGRANULAR is set via the Bin Granularity field in the
Acquisition tab of the Settings panel. Select the desired setting and press the
[Apply] button to test. Note that the Bin Size changes as a result. Press the
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[Save] button to store the setting to the currently selected GLOBSET in
nonvolatile memory.
The RS-232 command to set BINGRANULAR is 0x84. Note that this
command does not save the setting to nonvolatile memory. Please refer to the
RS-232 Command Specification for details.
2.3.4
Base Gain
To maintain perfect energy calibration a unique analog gain setting can
be stored for every combination of peaking time and MCA format. The Base
Gain is the approximate gain value appropriate for a given MCA size and
dynamic range. The scale is 0 to 100; the value 100 applies maximum gain to
the input signal.
In microManager, we recommend using the ROI calibration routine described in
the Rapid Development Kit User Manual to set the Base Gain. Base Gain can
also be edited directly in the Acquisition tab of the Settings panel. Select the
desired setting and press the [Apply] button to test. Press the [Save] button to
store the setting to the currently selected GLOBSET in nonvolatile memory.
The RS-232 command to set the base GAINDAC value is 0x88. Note that this
command does not save the setting to nonvolatile memory. Please refer to the
RS-232 Command Specification for details.
2.3.5
Reading the Current GENSET
The RS-232 command to read the current GENSET table is 0x8E. Note that this
command does not save the setting to nonvolatile memory. Please refer to the
RS-232 Command Specification for details.
2.3.6
Saving the Current GENSET to Non-Volatile Memory
The RS-232 command to save the current GENSET table is 0x8F. Please refer
to the RS-232 Command Specification for details.
2.4 Spectrometer Settings and PARSETs
For a given FiPPI decimation (i.e. peaking time range) and pipeline
clock speed, the slow filter length SLOWLEN sets the peaking time. In practice
other FiPPI control parameters, as well as other DSP parameters such as the
analog gain, may also be require adjustment for optimal performance at each
peaking time. To avoid repeating the optimization procedure five sets of
parameters, and thus five optimized peaking times are available for each FiPPI
decimation.
The PARSET, specified in Appendix C, is a table of peaking-timerelated spectrometer parameters, e.g. filter values, thresholds, pileup inspection
settings, etc. Parameters within the PARSET can be modified and stored such
that a calibrated spectrum is achieved whenever the peaking time (PARSET) is
subsequently selected. The PARSETs are stored in the nonvolatile flash
memory. Five (5) PARSETs are available for each FiPPI decimation purchased
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(see §1.1.2.4). Up to three FiPPI decimations, and thus a maximum of 15
optimized peaking times can be stored.
The PARSET further contains 5 sets of gain tweaking and threshold
settings, each corresponding to the 5 GENSETs or MCA formats. Storing
thresholds and gain settings for every combination of peaking time and MCA
format eliminates the need for calibrating a given combination more than once.
The factory-set default spectrometer settings should be adequate to
acquire a recognizable spectrum. To achieve optimal performance the
spectrometer settings must be adjusted, and stored to non-volatile memory such
that the optimized settings will be accessible in the future. To maximize
throughput, the slow filter peaking time τPS should be chosen to be as short as
possible to meet energy resolution requirements, since the maximum throughput
scales as 1/τ PS (see §4.10 for a detailed discussion of throughput):
OCRmax = 1/(e τd) = 0.37/τd
2.4.1
Selecting a FiPPI Decimation
The microDXP comes pre-programmed with at least one and up to
three FiPPI decimations or peaking time ranges.
In microManager, the FiPPI decimation is accessed via the Acquisition tab of
the Settings panel. FiPPIs 0-2 can be selected via the Peaking Time Range
drop-down list. Notice that the Peaking Time and other PARSET settings
update when the new FiPPI decimation is selected.
The RS-232 command to select a FiPPI decimation is 0x81. Please refer to the
RS-232 Command Specification for details.
2.4.2
Selecting a PARSET
For each peaking time range, five (5) tables of parameters are stored in
nonvolatile memory. Each PARSET corresponds to a peaking time.
In microManager, PARSET settings are accessed via the Acquisition tab of the
Settings panel. PARSETs 0-4 can be selected via the Peaking Time drop-down
list. Modifications to spectrometer settings for the selected PARSET can be
tested by pressing the [Apply] button, and saved to nonvolatile memory via the
[Save] button.
The RS-232 command to select a PARSET is 0x82. Note that this command
simply retrieves one of five tables of spectrometer settings from nonvolatile
memory. Please refer to the RS-232 Command Specification for details.
2.4.3
Filter Parameters
Although the peaking time itself cannot be edited, several digital filter
parameters are available for modification. Please refer to Chapter 4 for a
thorough discussion of digital filtering with the DXP.
In microManager, the filter parameters are accessed via the Acquisition tab of
the Settings panel. Press the [Edit Filter Parameters] button and modify
settings in the associated dialog. Modifications for the selected PARSET can be
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tested by pressing the [OK] button, and saved to nonvolatile memory via the
[Save] button.
The RS-232 command to modify filter parameters is 0x8B. Note that
this command does not save the change to nonvolatile memory. To do so it is
necessary to subsequently save the current PARSET as described below. Please
refer to the RS-232 Command Specification for details
2.4.4
Baseline Average Length
A running average of baseline measurements is computed, which is
then subtracted from sampled peak values to compute the energy of
corresponding incident x-rays. The number of baseline samples averaged is set
in microManager as Baseline Average Length. In the DSP this is converted
into the parameter BLFILTER according to the equation:
Baseline Average Length = 32768 / BLFILTER
Please refer to section 4.4 for a thorough discussion of baseline averaging.
In microManager, BLFILTER is accessed via the Acquisition tab of the
Settings panel. Select the desired Baseline Average Length from the dropdown list. Modifications for the selected PARSET can be tested by pressing the
[Apply] button, and saved to nonvolatile memory via the [Save] button.
The RS-232 command to modify BLFILTER is 0x92. Note that this command
does not save the change to nonvolatile memory. To do so it is necessary to
subsequently save the current PARSET as described below. Please refer to the
RS-232 Command Specification for details.
2.4.5
Thresholds
Proper triggering on input events depends on good threshold settings,
particularly for the so-called Trigger (fast filter) and Baseline (intermediate
filter) thresholds. The DSP parameters THRESHOLD, BASETHRESH and
SLOWTHRESH correspond to thresholds applied to the Trigger (fast),
Baseline (intermediate), and Energy (slow) filters, respectively. Please refer to
section 5.10.4 for a thorough discussion of thresholds.
Each PARSET includes 5 different settings for THRESHOLD,
BASETHRESH and SLOWTHRESH, corresponding to the 5 MCA formats, or
GENSETs.
In microManager, the threshold values for the current PARSET and GENSET
are accessed via the Acquisition tab of the Settings panel. Enter the desired
threshold settings. Settings for the current PARSET can be tested by pressing
the [Apply] button, and saved to nonvolatile memory via the [Save] button.
The RS-232 command to modify thresholds is 0x86. Note that this command
does not save the change to nonvolatile memory. To do so it is necessary to
subsequently save the current PARSET as described below. Please refer to the
RS-232 Command Specification for details.
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Fine Gain Trim
To maintain calibration across all combinations of peaking times and
MCA formats, slight modifications to the analog gain are necessary. The DSP
PARSET parameter GAINTWEAK is combined with GENSET parameter
GAINBASE to arrive at the final gain setting. Each PARSET includes 5
different settings for GAINTWEAK, corresponding to the 5 MCA formats, or
GENSETs.
In microManager, the threshold values for the current PARSET and GENSET
are accessed via the Acquisition tab of the Settings panel. Enter the desired
threshold settings. Settings for the current PARSET can be tested by pressing
the [Apply] button, and saved to nonvolatile memory via the [Save] button.
The RS-232 command to modify thresholds is 0x86. Note that this command
does not save the change to nonvolatile memory. To do so it is necessary to
subsequently save the current PARSET as described below. Please refer to the
RS-232 Command Specification for details.
2.4.7
Reading the Current PARSET
The RS-232 command to read the current PARSET table is 0x8C. Note that this
command does not save the setting to nonvolatile memory. Please refer to the
RS-232 Command Specification for details.
2.4.8
Saving the Current PARSET to Non-Volatile Memory
In microManager, all changes to the current PARSET can be saved to
nonvolatile memory via the [Save] button.
The RS-232 command to save the current PARSET table is 0x8D. Please refer
to the RS-232 Command Specification for details.
2.5 Repetitive Configuration of Identical Systems
In cases where many microDXPs are to be configured identically (or
nearly identical) it is very desirable to ‘carbon-copy’ user settings that have been
optimized. This procedure is supported in microManager. Note that gain
variations between modules make fine gain tuning necessary for each
microDXP.
2.5.1
Create Master Parameter Set…
In microManager select Firmware > Create Master Parameter Set…
to save all parameters to XUP file format. Select the desired filename and
location and press [Save].
2.5.2
Download a Master Parameter Set…
In microManager select Firmware > Download… to write the
parameters in the XUP file you created to a second microDXP. Browse to the
desired XUP file and location and press [Download].
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2.6 Data Acquisition:
This section describes the most common data acquisition procedures.
The most common commands (i.e. start/stop run, readout data) are covered in
§Error! Reference source not found.. Please refer to the RS-232 Command
Specification (a separate document) for a detailed presentation of all RS-232
commands. DXP-related documents are available online at:
http://www.xia.com/microDXP.html
In microManager select the MCA tab in the Data Acquisition panel.
2.6.1
Starting a Run
Data acquisition runs can be configured to automatically terminate the
run after a preset time or number of input or output counts has elapsed, as
described below in §2.6.5. By default (i.e. PRESET = 0) the run continues until
a ‘stop run’ command is issued (see §2.6.2).
In microManager, simply press the [Start Run] button.
The RS-232 command to start a data acquisition run is 0x00. Please refer to the
RS-232 Command Specification for details.
2.6.2
Stopping a Run
By default (i.e. PRESET = 0) a run in progress continues until a ‘stop
run’ command is issued. The microDXP can be configured to automatically
terminate until a preset time or number of input or output counts has elapsed, as
described below in §2.6.5. In such cases the ‘stop run’ command overrides the
run preset.
In microManager, simply press the [Stop Run] button.
The RS-232 command to stop a data acquisition run is 0x01. Please refer to the
RS-232 Command Specification for details.
2.6.3
Reading a Spectrum
The ‘read spectrum’ command supports readout of any contiguous
region of the MCA data, extending, of course to the entire spectrum. Each
MCA bin is represented in DSP program memory as a 24-bit word (i.e. 3 bytes),
supporting up to 16,777,215 counts per bin. In some cases (i.e. for short runs
and/or low count rates) the upper bits of each bin word will always be zero. The
readout speed can be increased by opting to read out a fewer number of bytes
per bin, or DEPTH. At DEPTH=2 up to 65,535 counts per bin are supported.
At DEPTH=1 up to 255 counts per bin are supported. Note: If the number of
counts in a bin exceeds the DEPTH, the resulting distribution will display sharp
discontinuities. The data is however always stored internally in the DSP at the
full 24 bits. It is thus not necessary to restart the run when the DEPTH is
exceeded: Simply change DEPTH and re-read the spectrum.
In microManager, the spectrum and statistics can be updated automatically or
manually. Check the Continuous? checkbox for automatic updates. If the
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checkbox is unchecked press the [Update] button to manually update the
spectrum and statistics.
The RS-232 command to read the MCA spectrum is 0x02. Please refer to the
RS-232 Command Specification for details.
2.6.4
Reading (and Calculating) the Run Statistics
The ‘read run statistics’ command retrieves the fast-filter livetime
(LIVETIME), the realtime (REALTIME), the number of input counts
(FASTPKS) and the number of output counts (EVENTSINRUN). These
parameters can be used to directly calculate the input count rate (ICR), output
count rate (OCR) and the deadtime percentage (%DEADTIME):
ICR = FASTPKS/LIVETIME
OCR = EVENTSINRUN/REALTIME
%DEADTIME = OCR/ICR
Realtime and livetime are expressed in units of 500 ns. Note: The LIVETIME
corresponds to the fast filter—NOT the energy filter—and thus does not alone
determine the relationship between input and output count rates, i.e. the
deadtime percentage.
In microManager, the spectrum and statistics can be updated automatically or
manually. Check the Continuous? checkbox for automatic updates. If the
checkbox is unchecked press the [Update] button to manually update the
spectrum and statistics.
The RS-232 command to read the run statistics is 0x06. Please refer to the RS232 Command Specification for details.
2.6.5
Specifying fixed run lengths
By default, the microDXP acquires data until a stop command is
received from the host. Alternatively the microDXP can automatically
terminate data acquisition runs based upon the realtime, livetime or the number
of input or output counts exceeding a preset value. Note: Although the realtime
and livetime are expressed in (measured accurately to) units of 500
nanoseconds, the process that monitors the realtime and livetime is only updated
every 500 microseconds. Similarly, the input and output counts are tallied every
500 microseconds. The result is that a preset of 100,000 input counts may
terminate with a slightly larger number of input events than 100,000.
Nonetheless, the run statistics (see §2.6.4 above) are all mutually consistent and
accurate.
In microManager, select the desired Preset Run Type from the drop-down list,
and enter the desired value in the Preset Value field.
The RS-232 command to control preset run settings is 0x07. Please refer to the
RS-232 Command Specification for details.
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2.7 Diagnostic Tools
The microDXP provides for diagnostic features including ADC trace
readout, baseline trace and histogram readout and DSP parameters readout.
2.7.1
ADC Trace Readout
An 8000 point digital trace of the signal at the ADC is available for
readout as a diagnostic aid. Remember that this signal is NOT the raw
preamplifier signal, but has been conditioned as described in section 3.2.
In microManager, select the ADC tab in the Data Acquisition panel. Enter the
desired value in the Sampling Interval field. The minimum value is 0.125
microseconds. Press the [Read ADC] button to refresh the display. Press the
[Save ADC] to save the data to file in ASCII format.
The RS-232 command to read the ADC is 0x11. Please refer to the RS-232
Command Specification for details.
2.7.2
Baseline Diagnostics
An 8000 point digital trace of the running baseline average is available
for readout as a diagnostic aid. A histogram of instantaneous baseline samples
is also available. Refer to sections 4.4 and 5.5 for discussion of baseline
acquisition and averaging.
In microManager, select the Baseline tab in the Data Acquisition panel. Press
the [Get Baseline] button to display the baseline histogram. Press the [Get
Baseline History] button to display the baseline average vs. time. Press the
[Save Baseline] to save the data to file in ASCII format.
The RS-232 command to read the baseline histogram is 0x10. The RS-232
command to read the baseline history is 0x12. Please refer to the RS-232
Command Specification for details.
2.7.3
DSP Parameters Readout
In microManager, the spectrum and statistics can be updated automatically or
manually. Check the Continuous? checkbox for automatic updates. If the
checkbox is unchecked press the [Update] button to manually update the
spectrum and statistics.
The RS-232 command to read the run statistics is 0x06. Please refer to the RS232 Command Specification for details.
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3 MicroDXP Functional Description
3.1 Organizational Overview
The DXP channel architecture is shown in Figure 3.1. The four major
operating blocks are the Analog Signal Conditioner (ASC), Digital Filter, Peak
Detector, and Pileup Inspector (FiPPI), Digital Signal Processor (DSP), and PIC
microcontroller. Also depicted are the ADC, the two host interface connections,
a digital temperature sensor and the nonvolatile memory. The functions of each
block are summarized below. This chapter does assume the reader has some
familiarity with x-ray pulse processing theory and electronic devices. Please see
Chapter 4 of this manual for a brief review.
Figure 3.1: Block diagram of the DXP channel architecture, showing the major functional sections and
interface port options.
3.2 The Analog Signal Conditioner (ASC)
The ASC has two major functions: to reduce the dynamic range of the
input signal so that it can be adequately digitized by a 12 bit converter and to
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reduce the bandwidth of the resultant signal to meet the Nyquist criterion based
upon the ADC sample rate.
3.2.1
Dynamic Range Reduction
In many cases, and particularly for reset-type preamplifiers, the fullscale output voltage range is many times greater than the voltage step produced
by a single x-ray event (see Figure 4.2). A high sampling rate is necessary to
provide good pulse pileup detection, as described in §4.8, and sufficient ADC
resolution is required to accurately sample the noise prior to the digital filters.
For high count rates, pulse pair resolution less than 200 ns is desirable, which
implies a sampling rate of 10 MSA or more. In order to reduce the noise σ in
measuring Vx (see Figure 4.1 and Figure 4.3), experience shows that σ must be
at least 4 times the ADC’s single bit resolution ∆V1. This effectively sets the
gain of the amplifier stages preceding the ADC. Then, if the preamplifier’s full
scale voltage range is Vmax, it must digitize to N bits, where N is given by:
N = log 10 (Vmax/∆V1)/log 10 (2)
Equation 3-1
For a typical high-resolution spectrometer, N must at least 14. An
ADC with 14 effective bits resolution that supports 10 MSA is quite expensive.
The alternative approach is to first reduce the dynamic range of the preamplifier
output signal such that a moderately priced ADC can be used.
3.2.1.1 Reset-Type Preamplifiers
For reset-type preamplifiers the dynamic range reduction is
accomplished using a novel dynamic range technology, for which XIA has
received a patent, and which is indicated in Figure 3.2. Here a preamplifier
output is shown which cycles between about -3.0 and -0.5 volts. We observe
that it is not the overall function which is of interest, but rather the individual
steps, such as shown in Figure 4.2 of the next chapter, that carry the x-ray
amplitude information. Thus, if we know the average slope of the preamp
output, we can generate a saw tooth function that has this average slope and
restarts each time the preamplifier is reset, as shown in Figure 3.2. If we then
subtract this saw tooth from the preamplifier signal, we can amplify the
difference signal to match the ADC’s input range, also as indicated in the
Figure.
The generator required to produce this saw tooth function is quite
simple, comprising a current integrator with an adjustable slope and a reset
switch. A DAC (SLOPEDAC) controls the current, which sets the slope. The
DAC input value is set by the DSP, which thereby gains the power to adjust the
saw tooth generator in order to maintain the ASC output (i.e. the “Amplified
Saw tooth Subtracted Data” of Figure 3.2) within the ASC input range.
In practice, the large-signal dynamic range can be reduced by a factor
of 8 to 16, thus reducing the required number of bits necessary to achieve the
same resolution from 14 to 10.
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ADC Max Input
3.0
2.0
Amplified Sawtooth Subtracted Data
Preamp Output (V)
1.0
ADC Min Input
Preamp
Output
0.0
-1.0
-2.0
Reset
Level
-3.0
0
1
Sawtooth
Function
Preamp-Sawtooth kfig 960923
2
3
4
5
Time (ms)
Figure 3.2: A saw tooth function having the same average slope as the preamp
output is subtracted from it and the difference amplified and offset
to match the input range of the ADC.
Occasionally, as also shown in Figure 3.2, fluctuations in data arrival
rate will cause the conditioned signal to pass outside the ADC input range. This
condition is detected by the FiPPI, which has digital discrimination levels set to
ADC zero and full scale, which then interrupts the DSP, demanding ASC
attention. The DSP remedies the situation by quickly closing the reset switch.
During this time, data passed to the FiPPI are invalid. Preamplifier resets are
detected similarly. When detected the DSP responded by resetting the current
integrator until the signal comes back into range. Note: Data acquisition is
halted until the time period defined by RESETINT has expired (see also
§2.2.1.3).
3.2.1.2 RC-Feedback Preamplifiers
RC-feedback preamplifiers inherently produce a smaller dynamic
range: At low rates the preamplifier output decays to baseline after each event,
producing a voltage range on the order of a single event; at higher rates
successive events add up, however, the larger the sum, the steeper the decay
slope back to baseline. The result, as of course intended, is to yield a full-scale
signal that increases only logarithmically with count rate.
Another consideration with RC-feedback preamplifiers is the DC
offset. Although many such preamplifiers produce a ground-referenced output,
many do not. Both problems are solved in the microDXP by adding a digitally
controlled DC offset voltage to the input signal. The offset DAC is used both to
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subtract the DC shift that results when running at high rates, and to account for
any DC offset voltage in the preamplifier output signal.
3.2.2
Nyquist Criterion
The Nyquist criterion states that there should be no frequency
component in the signal that exceeds half of the sampling frequency.
Frequencies above this value are aliased into the digitized signal at where they
are indistinguishable from original components at those frequencies. In
particular, high frequency noise would appear as excess low frequency noise,
spoiling the spectrometer’s energy resolution.
3.3 The Analog to Digital Converter (ADC)
Signal digitization occurs in the Analog-to-Digital converter (ADC),
which lies between the ASC and the FiPPI. The ADC is a 12 bit device, which
is currently run at either 8 or 16MHz. Although the chosen ADC supports both
sampling rates, because the Nyquist criterion must be satisfied in both cases (i.e.
4MHz at 8MSA and 8MHz at 16MSA) the passive components are different in
each case. Changing the pipeline (i.e. ADC and digital filtering) clock speed
thus requires modification to the microDXP hardware.
Pipeline Clock
Speed
8 MHz
16 MHz
ADC Sampling
Rate
8 MSA
16 MSA
Nyquist
Frequency
4 MHz
8 MHz
Sample Period
125 ns
62.5 ns
Figure 3.3: Nyquist frequencies and sample periods for the two pipeline clock
speed offerings.
3.4 The Filter, Pulse Detector, & Pile-up Inspector (FiPPI)
The FiPPI is implemented in a field programmable gate array (FPGA)
to accomplish the various filtering, pulse detection and pileup inspection tasks
discussed in chapter 4. As described there, it utilizes up to three digital filters
running simultaneously for the purposes of pulse detection, pileup inspection
and rejection, noise reduction and peak selection.
The FiPPI also includes a livetime counter that is activated any time the
DSP is enabled to collect x-ray pulse values from the FiPPI and therefore
provides an extremely accurate measure of the system livetime. In particular, as
described in §3, the DSP is not live, while an x-ray pulse is being processed,
during preamplifier resets or during ASC out-of-ranges; in the latter case both
because it is adjusting the ASC and because the ADC inputs to the FiPPI are
invalid. Thus the DXP measures livetime more accurately than an external
clock, which is insensitive to resets and includes them as part of the total
livetime. While the average number of resets/sec scales linearly with the count
rate, in any given measurement period there will be fluctuations in the number
of resets which may affect counting statistics in the most precise measurements.
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3.4.1
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FiPPI Decimation
FiPPI’s are distinguished also by ‘decimation’. Decimation refers to
pre-averaging of the ADC signal prior to the FPGA processing pipeline. Each
decimation accommodates a specific range of peaking times, i.e. shaping or
integration times. Up to three (3) FiPPI configuration files can be stored in the
microDXP’s nonvolatile memory. When the peaking time is changed such that
a range boundary is crossed, the host software downloads the appropriate FiPPI
configuration to the MicroDXP.
FiPPI
Decimation
#ADC Samples
in Average
0
1
2
3
4
5
6
1
2
4
8
16
32
64
Table 3.1:
Peaking Time
Range: 16MHz
Pipeline Clock
125 ns – 750 ns
250 ns – 1.5 µs
500 ns – 3.0 µs
1.0 µs – 6.0 µs
2.0 µs – 12.0 µs
4.0 µs – 24.0 µs
8.0 µs – 48.0 µs
Peaking Time
Range: 8MHz
Pipeline Clock
250 ns – 1.5 µs
500 ns – 3.0 µs
1.0 µs – 6.0 µs
2.0 µs – 12.0 µs
4.0 µs – 24.0 µs
8.0 µs – 48.0 µs
16.0 µs – 96.0 µs
Available peaking time ranges by FiPPI decimation and pipeline
clock speed.
3.4.2
FiPPI Code Variants
The FiPPI pipeline topology for RC-type preamplifiers is different than
for reset-type preamplifiers, thus two standard code variants are offered for each
decimation. Additionally, any use of the auxiliary digital I/O will require perinstance FiPPI configuration variant. Please contact XIA to discuss this
development.
3.5 The Digital Signal Processor (DSP)
The Digital Signal Processor acquires and processes event data from
the FiPPI, and controls the ASC through DACs. The processor is an Analog
Devices ADSP-2183 16 bit Fixed-Point DSP optimized for fixed-point
arithmetic and high I/O rates. Different DSP program variants are used for
different types of data acquisition and different preamplifier types. Chapter 5
describes in detail the DSP operation, its tasks, and parameters which control
them.
The ADSP-2183 has 16K words of 16-bit wide data memory and 16K
words of 24-bit wide program memory, part of which is used as data memory to
hold the MCA spectrum. (If more memory is required for special purposes, up
to 4 Mbytes of extended memory can be added by specifying option M).
Transferring data to/from these memory spaces is done through the DSP’s builtin DMA port, which does not interfere with the DSP program operation.
3.5.1
FLASH Memory
A new feature implemented on the microDXP is the inclusion of on
board non-volatile memory, which allows for firmware storage and retrieval.
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The flash memory, accessed by the DSP, used to store FiPPI configuration codes
and parameter sets, called GENSETS and PARSETS. The FiPPI is thus
configured and optimized independently, with only a short command issued
from the host.
Parameter sets simplify data acquisition procedures. The DXP works
well only if the internal parameters that govern digital filtering, peak detection
and pileup inspection are properly set. For the lay user the optimization process
can become overwhelming. The new approach is to optimize the relevant
parameters for each peaking time only one time, and store the entire parameter
in a unique location in the flash memory. The exact state can be subsequently
reproduced simply by selecting the saved parameter set. The flash memory can
be updated with new FiPPI code via the RS-232, SPORT or IDMA ports.
3.5.2
Serial Port (SPORT)
The Analog Devices DSP synchronous serial port, or SPORT, supports
a variety of serial data communications protocols, and offers a maximum
transfer rate of approximately 1Mbyte/sec. The SPORT interface is a interface
candidate in multi-microDXP systems, and is available with either interface
connector option (see §3.7 below),
3.5.3
DMA Port
Parallel Direct Memory Access (DMA) provides the highest bandwidth
communications path to the DSP data memory. Transfer rates up to 16
Mbytes/sec are possible. The DMA bus is available to the host computer/PDA
only if the high-speed board-to-board interface option (see §3.7.2 below) is
chosen.
3.5.4
DSP Code Variants
The FiPPI pipeline topology for RC-type preamplifiers is different than
for reset-type preamplifiers, thus two standard code variants are offered for each
FiPPI decimation. Additionally, special data acquisition modes (e.g. timeresolved spectroscopy, multi-SCA’s, etc.) require variation in the DSP code.
Please contact XIA to discuss this development.
3.6 PIC MicroController
The PIC microprocessor serves as the system controller, carrying out
procedures to boot the board, loading appropriate DSP code from memory, and
running acquisition routines. In addition the PIC handles I/O including RS-232
standard communications and an I2C bus for controlling dedicated peripheral
devices.
3.6.1
RS-232 Serial Port
The RS-232 serial port is the default communications interface for the
microDXP, and is wired to both the flex-cable connector and the high-speed
DMA port connector. Though relatively slow (115 kbaud) the RS-232 port is in
fact adequate for most applications. As one of the oldest communications
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standards, RS-232 enjoys wide compatibility with existing devices and is
supported by all x86 Personal Computers.
3.6.2
I2C Serial Bus
The I2C serial protocol allows for several serial devices to share the
same two-wire bus through a device ID handshaking procedure. I2C devices are
pre-programmed with a four-bit device ID (e.g. 1001 is used for some digital
temperature sensors) appended with a 3-bit suffix that is typically set by
hardwiring the appropriate pins to either the supply voltage or to ground. Two
I2C devices are included on the microDXP itself (described below), and the bus
is wired to both interface connectors to provide for microDXP control over, or
monitoring of, other devices.
3.6.3
I2C Memory
A new feature implemented on the microDXP is the inclusion of on
board non-volatile memory, which allows for firmware storage and retrieval.
The I2C memory, accessed by the PIC, is used to store the DSP code and general
system information. The DSP is booted automatically upon power-up. The I2C
memory can be updated with new DSP code via the RS-232 serial port.
3.6.4
I2C Temperature Sensor
An I2C temperature sensor is included on the microDXP. The
temperature measurement range is –55°C to +125°C, with ± 1°C accuracy.
3.6.5
PIC Code Variants
Semi-custom and custom PIC code will be necessary for applications
utilizing the I2C bus. Please contact XIA to discuss this development.
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3.7 Interface to Host Computer/PDA
NOTE: As of August 2009,
the IDMA parallel
communications interface
is now implemented, in
order to communicate with
the new MicroComU
companion board. The RS232 communications port
has always been supported.
The SPORT interface is
still not implemented at this
time.
The microDXP interfaces to a computer or PDA via one of two
connectors: the standard flex-cable port or the high-speed DMA port. Currently
only the flex-cable port is supported.
3.7.1
Flex Cable Interface
A 0.5mm-pitch flex-cable provides the connection to power, serial
communications and auxiliary digital lines. The flex cable provides for two
dimensions of freedom, but does require alignment along the axis that bisects all
of the contacts. Please refer to Appendix D for connector locations and pinouts.
Flex Cable Interface Resources
Resource
Function
RS-232
low-rate serial
communications
I2C
low-rate serial
communications
SPORT
mid-rate serial
communications
AUX0-3
GATE*
EXTINT*
3.7.2
Reserved
DAQ control
DSP interrupt
Description
Default communications interface.
Peripheral device interface, e.g.
indicators, DACs, etc.
Alternate serial communications
interface, e.g. for multi-channel
systems that require moderate
readout bandwidth.
Auxiliary digital I/O lines.
Connect to FiPPI.
Inhibits data acquisition when low.
Extra interrupt line, active low.
High Speed Interface
A high-density board-to-board connection is also included on the
microDXP. The so-called high-speed interface includes all resources carried on
the flex-cable interface, plus Direct Memory Access (DMA) to the DSP. It was
included for applications requiring very fast data transfer rates.
High Speed (board-to-board) Interface Resources
Resource
Function
Description
DMA
High-rate parallel
Direct Memory Access to DSP
communications
memory, for the highest bandwidth
data transfers.
RS-232
Low-rate serial
Default communications interface.
communications
2
IC
Low-rate serial
Peripheral device interface, e.g.
communications
indicators, DACs, etc.
SPORT
Mid-rate serial
Alternate serial communications
communications
interface, e.g. for multi-channel
systems that require moderate
readout bandwidth.
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AUX0-3
GATE*
EXTINT*
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Reserved
DAQ control
DSP interrupt
mdo-microDXP-MAN-1.1.2
Auxiliary digital I/O lines.
Connect to FiPPI.
Inhibits data acquisition when low.
Extra interrupt line, active low.
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4 Digital Filtering: Theory of Operation and
Implementation Methods
This chapter provides an in-depth discussion of x-ray pulse-processing
theory both generally and as implemented in the microDXP. The topics include
how digital filters work, x-ray detection, thresholds, baselines, pileup inspection,
and input and output count rates. Topics are covered to illustrate the theoretical
issues, practical implementation, and how to adjust parameters to obtain best
performance.
The acronym DXP stands for “Digital X-ray Processor” and refers to
XIA’s standard digital processing technology, which is included in many XIA
products, including the microDXP.
4.1 X-ray Detection and Preamplifier Operation
Energy dispersive detectors, which include such solid state detectors as
Si(Li), HPGe, HgI2, CdTe and CZT detectors, are generally operated with
charge sensitive preamplifiers. When an x-ray is absorbed in the detector
material it releases an electric charge Qx = Ex/ε, where the material constant ε is
the amount of energy needed to form an electron-hole pair. Qx is integrated onto
the preamplifier’s feedback capacitor Cf, to produce the voltage Vx = Qx/Cf =
Ex/(εCf). Measuring the energy Ex of the x-ray therefore requires a
measurement of the voltage step Vx in the presence of the amplifier’s noise σ.
Figure 4.1 and Figure 4.3 depict reset-type and RC-type charge sensitive
amplifiers, respectively. In both figures the detector D is biased by voltage
source HV (either positive or negative) and connected to the input of amplifier
A. Note that the signal polarity must be distinguished from the bias voltage
polarity. The signal polarity is positive if the voltage step Vx is a rising edge, as
displayed in Figure 4.1. Whether signal polarity is positive or negative depends
upon the preamplifier’s design and does not depend upon bias voltage polarity,
which is specified on the detector and is determined by its design.
4.1.1
Reset-Type Preamplifiers
Figure 4.3a is a simplified schematic of a reset-type preamplifier,
wherein Cf is discharged through the switch S from time to time when the
amplifier’s output voltage gets so large that it behaves nonlinearly. Switch S
may be an actual transistor switch, or may operate equivalently by another
mechanism. In pulsed optical reset preamps light is directed at the amplifier A’s
input FET to cause it to discharge Cf. In transistor reset preamps, the input FET
may have an additional electrode which can be pulsed to discharge Cf. The
output of a reset-type preamplifier following the absorption of an x-ray of
energy Ex in detector D is a voltage step of amplitude Vx. Two x-ray steps are
shown in Figure 4.3b as a step.
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Figure 4.1: a) Reset-type charge sensitive preamplifier with a negatively biased detector; b) Output on
absorption of x-ray rays. Note that the steps have a rising edge, so that the signal polarity is
positive.
Figure 4.2 depicts the large-signal saw tooth waveform that results
from successive x-ray steps followed by the reset. Note that the units here are
Volts and milliseconds vs. millivolts and microseconds in the previous figure.
Figure 4.2: The large-signal reset waveform for a reset-type preamplifier with positive signal polarity, as
displayed on a real oscilloscope. Note that the large signal character is not displayed in the
microDXP diagnostic ADC readout, e.g. used in microManager’s “ADC Trace” diagnostic
tool, looks quite different because of the dynamic range reduction carried out in the ASC, as
described in §3.2.1.
4.1.2
RC-Type Preamplifiers
Figure 4.3a is a simplified schematic of an RC-type preamplifier,
wherein Cf is discharged continuously through feedback resistor Rf. The output
of an RC-type preamplifier following the absorption of an x-ray of energy Ex in
detector D is, again, a voltage step of amplitude Vx. The continuous discharge
of Cf through Rf results in an exponential voltage decay after the x-ray step, with
decay constant τ, where:
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τ = Rf Cf
Equation 4-1
In practice the decay time may depend on subsequent circuitry, i.e. if a
pole-zero cancellation circuit is used, thus τ may not be directly related to the
feedback elements of the front-end. The point of this simplified model is that
the resulting waveform is a single-pole RC decay. The discussion in § 4.2
through § 0 assumes a reset-type preamplifier, but is mostly applicable to RCtype preamplifiers. § 4.7 describes the few key differences in the processing of
RC-type preamplifier signals.
Figure 4.3: a) RC-type charge sensitive preamplifier with a positively biased detector; b) Output on
absorption of an x-ray. Note that the step has a falling edge, thus the signal polarity is
negative.
4.2 X-ray Energy Measurement & Noise Filtering
Reducing noise in an electrical measurement is accomplished by
filtering. Traditional analog filters use combinations of a differentiation stage
and multiple integration stages to convert the preamp output steps, such as
shown in Figure 4.1b, into either triangular or semi-Gaussian pulses whose
amplitudes (with respect to their baselines) are then proportional to Vx and thus
to the x-ray’s energy.
Digital filtering proceeds from a slightly different perspective. Here
the signal has been digitized and is no longer continuous, but is instead a string
of discrete values, such as shown in Figure 4.4. The data displayed are actually
just a subset of Figure 4.3b, which was digitized by a Tektronix 544 TDS digital
oscilloscope at 10 MSA (million samples per second). Given this data set, and
some kind of arithmetic processor, the obvious approach to determining Vx is to
take some sort of average over the points before the step and subtract it from the
value of the average over the points after the step. That is, as shown in Figure
4.4, averages are computed over the two regions marked “Length” (the “Gap”
region is omitted because the signal is changing rapidly here), and their
difference taken as a measure of Vx. Thus the value Vx may be found from the
equation:
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Vx,k = – Σ wivi +
i (before)
Σ
i (after)
wi vi
Equation 4-2
Where the values of the weighting constants wi determine the type of average
being computed. The sums of the values of the two sets of weights must be
individually normalized.
Preamp Output (mV)
4
2
Length
Gap
0
Length
-2
Digitized Step 960919
-4
20
22
24
26
28
30
Time ( µs)
Figure 4.4: Digitized version of one of the x-ray steps of Figure 4.3b.
The primary differences between different digital signal processors lie
in two areas: what set of weights {wi} is used and how the regions are selected
for the computation of Equation 4-2. Thus, for example, when the weighting
values decrease with separation from the step, then the equation produces “cusplike” filters. When the weighting values are constant, one obtains triangular (if
the gap is zero) or trapezoidal filters. The concept behind cusp-like filters is
that, since the points nearest the step carry more information about its height,
they should be more strongly weighted in the averaging process. How one
chooses the filter lengths results in time variant (the lengths vary from pulse to
pulse) or time invariant (the lengths are the same for all pulses) filters.
Traditional analog filters are time invariant. The concept behind time variant
filters is that, since the x-rays arrive randomly and the lengths between them
vary accordingly, one can make maximum use of the available information by
adjusting Length on a pulse-by-pulse basis.
In principal, the very best filtering is accomplished by using cusp-like
weights and time variant filter length selection. There are serious costs
associated with this approach however, both in terms of computational power
required to evaluate the sums in real time and in the complexity of the
electronics required to generate (usually from stored coefficients) normalized
{wi} sets on a pulse-by-pulse basis. A few such systems have been produced
but typically cost about $13K per channel and are count rate limited to about 30
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Kcps. Even time invariant systems with cusp-like filters are still expensive due
to the computational power required to rapidly execute strings of multiply and
adds. One commercial system exists which can process over 100 Kcps, but it
too costs over $12K per channel.
The DXP processing system developed by XIA takes a different
approach because it was optimized for very high-speed operation and low cost
per channel. It implements a fixed length filter with all wi values equal to unity
and in fact computes this sum afresh for each new signal value k. Thus the
equation implemented is:
L Vx , k =
–Σ
k –L – G
i = k – 2L – G + 1
vi +
Σ
k
i = k – L+ 1
vi
Equation 4-3
where the filter length is L and the gap is G. The factor L multiplying Vx,k
arises because the sum of the weights here is not normalized. Accommodating
this factor is trivial for the DXP’s host software. The operations are carried out
using hardwired logic in a field programmable gate array (FPGA) that is called
the FiPPI because is implements Filtering, Peak capture, and Pileup Inspection.
In the FiPPI, Equation 4-3 is actually implemented by noting the recursion
relationship between Vx,k and Vx,k-1, which is:
L Vx,k = L Vx,k-1+ vk - vk-L - vk-L-G + vk-2L-G
Equation 4-4
Decimation by N means to
break up the data into
sequential sums of length
D = 2 N.
Peaking Time ranges vs
Decimation N
(8MHz clock assumed)
N = 0 0.75 – 3.00 µs
N = 1 1.50 – 6.00 µs
N = 2 3.00 – 12.0 µs
N = 3 6.00 – 24.0 µs
N = 4 12.0 – 48.0 µs
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While this relationship is very simple, it is still very effective. In the
first place, this is the digital equivalent of triangular (or trapezoidal if G = 0)
filtering which is the analog industry’s standard for high rate processing. In the
second place, one can show theoretically that if the noise in the signal is white
(i.e. Gaussian distributed) above and below the step, which is typically the case
for the short shaping times used for high signal rate processing, then the average
in Equation 4-4 actually gives the best estimate of Vx in the least squares sense.
This, of course, is why triangular filtering has been preferred at high rates.
Triangular filtering with time variant filter lengths can, in principle, achieve
both somewhat superior resolution and higher throughputs but comes at the cost
of a significantly more complex circuit and a rate dependent resolution, which is
unacceptable for many types of precise analysis. In practice, XIA’s design has
been found to duplicate the energy resolution of the best analog shapers while
approximately doubling their throughput, providing experimental confirmation
of the validity of the approach.
A practical limitation on the implementation of Equation 4-4 is that two
FIFO memories are required, one of length L and one of Length L+G. Since
memory space is limited in FPGAs, we have restricted our designs to values of
L+G less than 32. Since the microDXP samples at 20 MSA, this corresponds to
a peaking time of about 1.5 µs, a significant limitation. XIA overcomes this
limitation by first “decimating” the data stream from the ADC by performing
sequential sums of D data points, where D = 2N. We refer to this filtering
procedure as “Decimating by N”. By feeding the decimated data in an Equation
4-4 filter, we now obtain peaking times that are extended to L*D. It is important
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to understand that no data are lost in this procedure, we have merely rearranged
the order of the summations represented in Equation 4-3. By extension, a
“Decimation N FiPPI” is one that decimates the data by N before applying the
energy filter. The common decimation values in the microDXP are 0, 2, and 4,
corresponding to averaging times of 125 ns, 500 ns, and 2.0 µs. The associated
peaking time ranges are 0.75 – 3.0 µs, 3.0 – 12.0 µs, and 12.0 – 48.0 µs. Note:
This assumes the standard 8MHz pipeline clock. For a 16MHz pipeline clock
(optional), the averaging times and peaking time ranges are exactly one-half of
above values.
4.3 Trapezoidal Filtering in the DXP
From this point onward, we will only consider trapezoidal filtering as it
is implemented in the DXP according to Equation 4-3 and Equation 4-4. The
result of applying such a filter with Length L = 20 and Gap G = 4 to the same
data set of Figure 4.4 is shown in Figure 4.5. The filter output Vx is clearly
trapezoidal in shape and has a risetime equal to L, a flattop equal to G, and a
symmetrical falltime equal to L. The basewidth, which is a first-order measure
of the filter’s noise reduction properties, is thus 2L+G. This raises several
important points in comparing the noise performance of the DXP to analog
filtering amplifiers. First, semi-Gaussian filters are usually specified by a
shaping time. Their peaking time is typically twice this and their pulses are not
symmetric so that the basewidth is about 5.6 times the shaping time or 2.8 times
their peaking time. Thus a semi-Gaussian filter typically has a slightly better
energy resolution than a triangular filter of the same peaking time because it has
a longer filtering time. This is typically accommodated in amplifiers offering
both triangular and semi-Gaussian filtering by stretching the triangular peaking
time a bit, so that the true triangular peaking time is typically 1.2 times the
selected semi-Gaussian peaking time. This also leads to an apparent advantage
for the analog system when its energy resolution is compared to a digital system
with the same nominal peaking time.
One extremely important characteristic of a digitally shaped trapezoidal
pulse is its extremely sharp termination on completion of the basewidth 2L+G.
This may be compared to analog filtered pulses which have tails which may
persist up to 40% of the peaking time, a phenomenon due to the finite bandwidth
of the analog filter. As we shall see below, this sharp termination gives the
digital filter a definite rate advantage in pileup free throughput.
In practice it is also important to realize that implementing an energy
filter in a Decimation N FiPPI sets certain limitations on the practical flat-top
lengths that can be obtained in trapezoidal filters. Because the decimation
process is uncorrelated with the arrival of x-rays, the gap G must be 3 or greater
to assure that the filter’s peak truly represents the x-ray’s energy. Therefore, the
minimum Decimation N gap time is G*2N*∆t, where ∆t is the ADC’s sampling
interval. With the microDXP’s ∆t = 125 ns sampling interval, for instance, the
smallest useful flat-top in Decimation 4 is 6.0 µs. Note: Again, this assumes the
standard 8MHz pipeline clock. For a 16MHz pipeline clock (optional), the
sampling interval and gap time would be exactly one-half of above values.
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6
Filtered Step S.kfig 960920
Output (mV)
4
2
0
L
L+G/2
2L+G
-2
Preamp Output (mV)
Filter Output (mV)
-4
24
26
28
Time ( µs)
30
32
Figure 4.5: Trapezoidal filtering the Preamp Output data of Figure 4.4 with
L = 20 and G = 4.
4.4 Baseline Issues
4.4.1
The Need for Baseline Averaging
Figure 4.6 shows the same event as is Figure 4.5 but over a longer time
interval to show how the filter treats the preamplifier noise in regions when no
x-ray pulses are present. As may be seen, the effect of the filter is both to
reduce the amplitude of the fluctuations and reduce their high frequency content.
This signal is termed the baseline because it establishes the reference level or
offset from which the x-ray peak amplitude Vx is to be measured. The
fluctuations in the baseline have a standard deviation σe which is referred to as
the electronic noise of the system, a number which depends on the peaking time
of the filter used. Riding on top of this noise, the x-ray peaks contribute an
additional noise term, the Fano noise, which arises from statistical fluctuations
in the amount of charge Qx produced when the x-ray is absorbed in the detector.
This Fano noise σf adds in quadrature with the electronic noise, so that the total
noise σt in measuring Vx is found from
σt = sqrt( σf2 + σe2)
Equation 4-5
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Filtered Step L.kfig 960920
σt
Output (mV)
4
Vx
2
σe
0
-2
Filter Output (mV)
-4
5
10
15
20
25
30
Time ( µs)
35
40
45
Figure 4.6: The event of Figure 4.5 displayed over a longer time period to
show baseline noise.
The Fano noise is only a property of the detector material. The
electronic noise, on the other hand, may have contributions from both the
preamplifier and the amplifier. When the preamplifier and amplifier are both
well designed and well matched, however, the amplifier’s noise contribution
should be essentially negligible. Achieving this in the mixed analog-digital
environment of a digital pulse processor is a non-trivial task, however.
In the general case, the mean baseline value is not zero. This situation
arises whenever the slope of the preamplifier signal is not zero between x-ray
pulses. This can be seen from Equation 4-3. When the slope is not zero, the
mean values of the two sums will differ because they are taken over regions
separated in time by L+G, on average. Such non-zero slopes can arise from
various causes, of which the most common is detector leakage current.
When the mean baseline value is not zero, it must be determined and
subtracted from measured peak values in order to determine Vx values
accurately. If the error introduced by this subtraction is not to significantly
increase σt, then the error in the baseline estimate σb must be small compared to
σe. Because the error in a single baseline measurement is σe, by definition, this
means that multiple baseline measurements will have to be averaged. This
number, NB is the Baseline Average. For example, if NB = 128 measurements
are averagedthen the total noise will be as shown in Equation 4-6.
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σt = sqrt( σf2 + (1+1/128)σe2)
Equation 4-6
This results in less than 0.5 eV degradation in resolution, even for very
long peaking times, when resolutions of order 130 eV are obtained.
4.4.2
Applying a Baseline Cut
can improve performance
when the Baseline
Histogram is nonGaussian. Outlying data
points are‘cut’ from the
running Baseline Average
(though still included in the
histogram)
Raw Baseline Measurement
The output of the baseline filter (either the energy filter itself, or a
derivative of it) is sampled periodically in the explicit absence of an x-ray step,
defined by a baseline threshold. In practice, the DXP initially makes a series of
NB baseline measurements to compute a starting baseline mean. It then makes
additional baseline measurements at quasi-periodic intervals to keep the estimate
up to date. These values are stored internally and can be read out to construct a
spectrum of baseline noise, referred to as the Baseline Histogram. This is
recommended because of its excellent diagnostic properties. When all
components in the spectrometer system are working properly, the baseline
spectrum should be Gaussian in shape with a standard deviation reflecting σn.
Deviations from this shape indicate various pathological conditions which may
also cause the x-ray spectrum to be distorted and therefore have to be fixed.
The situation is remedied by removing (“cutting”) outlying samples
from the baseline average described below. If the maximum in the baseline
distribution lies at E0, then captured baseline values that deviate from E0 by
more than ∆E+ and ∆E-, respectively, are not included in the running baseline
average. Note that all captured baseline values are included in the Baseline
Histogram, however, so that it is always a valid representation of the system’s
behavior.
4.4.3
Baseline Averaging in the DXP
A running average of baseline measurements is computed, which is
then subtracted from sampled peak values to compute the energy of
corresponding incident x-rays. The number of baseline samples averaged is set
in microManager as “Number of Baselines to Average”. In the DSP this is
converted into the parameter BLFILTER according to the equation:
# baseline samples averaged = 32768 / BLFILTER
Decimation
# Baseline Samples to
Average
BLFILTER
(DSP Parameter)
0
2
4
64
128
256
512
256
128
Table 4.1:
Typical values used for baseline averaging. The best value for
each decimation should be determined empirically, though the
general trend illustrated in the table, i.e. larger number to average
for higher decimations, should be followed.
Physically, the baseline is a measure of the instantaneous slope
(volts/sec) for a pulsed-reset detector, and a measure of the DC offset for an RCfeedback preamplifier. For a perfect detector and preamplifier the baseline
value is independent of time. In fact, the variation in leakage current of the
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detector and offset drift and 1/f noise of the preamplifier contribute to a baseline
value that wanders at low frequencies. The goal is to achieve a baseline average
that has a sufficient number of samples to average out the high frequency noise,
but which still reflects the ‘local’ instantaneous baseline. Generally speaking,
Number of Baselines to Average is set to achieve the best energy resolution
performance over the desired range of input count rate. There are two
considerations worth emphasizing:
1.
Excess detector/preamplifier noise and pickup (all decimations): The values
in the table above implicitly assume a flat noise spectrum from the
preamplifier. A high-frequency noise peak can result in poor relative
performance at the corresponding ‘resonant’ peaking time. Often this
problem can be mediated, though not eliminated, by increasing the number
of baseline samples in the average for the affected peaking times. On the
other hand, excess low-frequency noise, i.e. wandering, can be remedied by
reducing the number baseline samples in the average.
2.
High rate performance (decimation 0): At higher rates, i.e. > 50%
deadtime, the slow filter returns less and less often to baseline, thus the time
between baseline samples grows longer. This is the primary cause of
degraded energy resolution at high rates. Firmware of decimation 2 and
above now employs a proprietary circuit that virtually eliminates this
problem, resulting in industry-leading count rate stability. This
improvement cannot however be implemented in the decimation 0
firmware. The resolution can nonetheless be improved in most cases by
reducing the number of baseline samples in the average.
4.5 X-ray Detection & Threshold Setting
Before capturing a value of Vx we must first detect the x-ray. X-ray
steps (in the preamp output) are detected by digitally comparing the output of a
trapezoidal filter to a threshold.
In the DXP up to three trapezoidal filters are implemented: fast,
intermediate and slow; each with a threshold that can be individually enabled or
disabled. A fast filter very quickly detects larger x-ray steps. A slow (energy)
filter averages out the most noise and can thus detect smaller x-ray steps, but has
a response that is much slower. An intermediate filter (used in decimations 2
and 4 only) is a derivative of the slow filter that provides a balance between the
speed of the fast filter and the noise reduction of the slow filter.
The fast filter is used solely for x-ray detection, i.e. a threshold crossing
initiates event processing. Its short basewidth (2L+G) means that successive
pulses that would ‘pile-up’ in slower filters can be resolved in the fast filter and
rejected from the spectrum (see Figure 4.11 below). Conversely, little noise
reduction is achieved in the fast filter, thus the fast threshold cannot be set to
detect particularly low x-ray energies.
The intermediate filter is used for all decimations other than 0. Its
threshold is applied as part of the baseline acquisition circuitry, i.e. baseline
measurements are taken when the signal is below this threshold. Intermediate
threshold crossings by default also trigger event processing, extending the
detectable energy range significantly below the fast filter threshold. Note that
this threshold is initialized to the maximum, i.e. most conservative, value, and
should be adjusted downward by the user for best performance.
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After an x-ray has been detected, the step height is measured at the
slow filter output. Although its excellent noise reduction also allows detection
of the very lowest energy x-rays, its slow response precludes an accurate
determination of pulse pileup. For this reason the slow threshold should be
disabled in almost all cases.
4.6 Peak Capture Methods
As noted above, we wish to capture a value of Vx for each x-ray
detected and use these values to construct a spectrum. This process is also
significantly different between digital and analog systems. In the analog system
the peak value must be “captured” into an analog storage device, usually a
capacitor, and “held” until it is digitized. Then the digital value is used to
update a memory location to build the desired spectrum. During this analog to
digital conversion process the system is dead to other events, which can severely
reduce system throughput. Even single channel analyzer systems introduce
significant deadtime at this stage since they must wait some period (typically a
few microseconds) to determine whether or not the window condition is
satisfied.
Digital systems are much more efficient in this regard, since the values
output by the filter are already digital values. All that is required is to capture
the peak value – it is immediately ready to be added to the spectrum. If the
addition process can be done in less than one peaking time, which is usually
trivial digitally, then no system deadtime is produced by the capture and store
operation. This is a significant source of the enhanced throughput found in
digital systems.
Once an active threshold is exceeded, the microDXP employs one of
two methods to capture the slow energy filter output such that the best measure
of Vx results. For decimations other than 0 the slow filter output is monitored
over a finite interval of time in the region of its maximum, and the maximum
value within that interval is captured. This method is referred to as “peak
finding” or “max capture”. For decimation 0, the slow filter is sampled at a
fixed time interval after the pulse is detected by the fast filter. This method is
referred to as “peak sampling”.
After describing in §4.6.1 below how to set the Gap parameter so that
there will be a quality value of the energy filter to capture, we describe the two
methods in detail in §4.6.2.
4.6.1
The Slow Filter Gap Length
When starting with a new detector, it is important first to set
SLOWGAP to a minimum of 3, and at least one unit greater than the smallest
value, in decimated clock cycles (see Table 4.2), that encloses the entire
preamplifier risetime, per §5.10.2.
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Decimation
# ADC
Samples
averaged
Decimated
Clock
frequency
Decimated
Clock cycle
interval
Peaking Time
Range
(in µs)
0
1
2
3
4
1
2
4
8
16
8 MHz
4 MHz
2 MHz
1 MHz
500 kHz
125 ns
250
500 ns
1.0 µs
2.0 µs
0.75 – 3
1.5 – 6
3 - 12
6 - 24
12 - 48
Table 4.2:
For decimation 0 the slow filter output is sampled a fixed time
after the x-ray is detected. PEAKSAM must be set properly to
achieve optimum performance.
For example, consider a preamplifier with a pulse risetime of 460ns.
For decimations 1, 2, 3, and 4 SLOWGAP would be set to 3 or greater. For
decimation 0 SLOWGAP would be set to 5 or greater.
4.6.2
Peak Sampling vs. Peak Finding
The figures below demonstrate the two approaches. For decimations 2
and 4 the slow filter output is monitored over a finite interval of time, and the
maximum value within that interval is selected. This method is referred to as
‘peak finding’. The interval is set automatically, solely based on the values of
the DXP parameters SLOWLEN and PEAKINT. SLOWLEN and PEAKINT are
both automatically derived from the peaking time value selected in
microManager and should normally not be adjusted by the user. PEAKINT is
also a pileup inspection parameter, as will be discussed in further detail in §4.8.
Figure 4.7 For decimations 2 and 4 the slow filter output is monitored and the peak value is selected.
For decimation 0, the slow filter output is instead sampled a fixed time
after the x-ray is detected. This method is referred to as ‘peak sampling’. An
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additional ‘Peak Sampling’ timer is started when an x-ray step is detected which
expires after PEAKSAM decimated clock cycles. PEAKSAM must be less than
PEAKINT, and should typically be set such that the sample point lies in the
‘flat-top’ region of the slow filter output:
SLOWLEN ≤ PEAKSAM ≤ SLOWLEN + SLOWGAP
Equation 4-7
The precise setting has a strong effect on energy resolution and should
be determined empirically for each new detector. More on this below...
Figure 4.8: For decimation 0 the slow filter output is sampled a fixed time after the x-ray is detected.
PEAKSAM must be set properly to achieve optimum performance.
In our experience values at the low end (i.e. PEAKSAM ~ SLOWLEN)
tend to work better. We recommend that you record the initial value of
PEAKSAM and then change it in steps of 1, working out from the initial value.
Certain PEAKSAM values may cause the Saturn to crash. Do not be alarmed,
just restart and be sure to enter a valid PEAKSAM value before proceeding.
Making a plot of energy resolution versus PEAKSAM will indicate the best
value to select.
This determination need only be done for one peaking time per
decimation. The result can then be applied to any value of SLOWLEN and
SLOWGAP using the following recipe:
PEAKSAM = (SLOWLEN + SLOWGAP) – X
Equation 4-8
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4.7 Energy Measurement with Resistive Feedback Preamplifiers
In previous sections, the pulse height measurement was shown for the
case of reset-type preamplifiers. The reset-type scheme is most often used for
optimum energy resolution x-ray detectors. Other detectors use an RC-type
preamplifier, as described in § 4.1.2. Resistive feedback is most often used for
gamma-ray detectors that cover a larger dynamic range and where the electronic
noise is not as significant a contribution to energy resolution.
Where analog shaping amplifiers typically have a “pole-zero”
adjustment to cancel out the exponential decay, the DXP uses a patented
exponential decay correction to achieve good energy resolution without a polezero correction. Figure 4.9 and Figure 4.10 illustrate the method used. The first
shows the output voltage of a RC feedback preamplifier with a x-ray or γ-ray
step of amplitude A appearing at t=0. Ve is the voltage just before the step pulse
arrives and V0 is the asymptotic value that the signal would decay to in the
absence of steps. t1 is the earliest time used in the slow filter, L and G are the
length and gap of the trapezoidal filter in clock units, and ∆t is the clock period,
In addition to the normal slow filter measurement of the step height, the ADC
amplitude, VD is made at time tD. In the following discussion, it is assumed that
the signal rise-time is negligible.
A
VD
Ve
L∆ t
V0
L∆ t
t1
t2
G∆ t
tD
t
0
Figure 4.9: RC preamplifier output voltage. An x-ray step of amplitude A
occurs at time t=0.
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10000
ICR = 41 kcps
8000
Zr Kα
Step
6000
Zr Kβ
4000
2000
Ge escape
peaks
0
Noise
-2000
0
4000
8000
Amplitude
12000
Figure 4.10: Correlation between step size and amplitude for Zr Kα x-ray
events measured with the DXP-4C.
As Figure 4.10 makes clear, there is a linear correlation between the
step height from the trapezoidal filter and the ADC amplitude, for pulses of a
given energy. This is due to the fact that the exponential decay causes a deficit
in the measured step height, which grows linearly with the distance from the
asymptotic ADC offset at zero count rate.
The DSP reads these two values for each event that passes the FiPPI’s
trigger criteria, and makes a correction of the form:
E = k1 ( SX + k2 VX - < SB + k2 VB > )
Equation 4-9
Here the quantities SX and VX are the step height and ADC amplitude
measured for the step, and the corresponding values with the B subscript are
“baseline” values, which are measured frequently at times when there is no
trigger. The brackets <> indicate that the baseline values are averaged over a
large enough number of events to not introduce additional noise in the
measurement. The constant k2 (the DSP parameter called RCFCOR) is inversely
proportional to the exponential decay time; this correction factor is a constant
for a detector channel at a fixed gain and shaping time. The constant k1 is
effectively a gain factor, and is taken into account with a detector gain
calibration.
The parameter RCFCOR is a function of the digital filter parameters
(SLOWLEN, SLOWGAP and DECIMATION) and the preamplifier decay time
(the DSP parameter TAURC). The user-entered decay time TAURC is in units
of 50 ns clock ticks. At the start of an acquisition run, the DSP calculates
RCFCOR using the following approximate expression:
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RCFCOR = 2DEC * (LEN + GAP) / (TAURC – (LEN + GAP/2 + 3)*2DEC)
Equation 4-10
The above expression is valid for peaking times less than about
TAURC/2. Alternatively, RCFCOR can be determined empirically in a special
test run from a linear fit of data, as in Figure 4.10.
4.8 Pile-up Inspection
The captured value Vx (see Figure 4.6) will only be a valid measure of
t\its associated x-ray’s energy provided that its filtered pulse is sufficiently well
separated in time from its preceding and succeeding neighbor pulses so that its
peak amplitude is not distorted by the action of the trapezoidal filter on those
neighbor pulses. That is, if the pulse is not piled up. The relevant issues may be
understood by reference to Figure 4.11, which shows 5 x-rays arriving separated
by various intervals.
Because the triangular filter is a linear filter, its output for a series of
pulses is the linear sum of its outputs for the individual members in the series.
In Figure 4.11 the pulses are separated by intervals of 3.2, 1.8, 5.7, and 0.7 µs,
respectively. The fast filter has a peaking time of 0.4 µs with no gap. The slow
filter has a peaking time of 2.0 µs with a gap of 0.4 µs.
The first kind of pileup is slow pileup, which refers to pileup in the
slow channel. This occurs when the rising (or falling) edge of one pulse lies
under the peak (specifically the sampling point) of its neighbor. Thus peaks 1
and 2 are sufficiently well separated so that the leading edge (point 2a) of peak 2
falls after the peak of pulse 1. Because the trapezoidal filter function is
symmetrical, this also means that pulse 1’s trailing edge (point 1c) also does not
fall under the peak of pulse 2. For this to be true, the two pulses must be
separated by at least an interval of L + G/2. Peaks 2 and 3, which are separated
by only 1.8 µs, are thus seen to pileup in the present example with a 2.0 µs
peaking time.
This leads to an important first point: whether pulses suffer slow pileup
depends critically on the peaking time of the filter being used. The amount of
pileup which occurs at a given average signal rate will increase with longer
peaking times. We will quantify this in §4.10, where we discuss throughput.
Because the fast filter peaking time is only 0.4 µs, these x-ray pulses do
not pileup in the fast filter channel. The DXP can therefore test for slow channel
pileup by measuring for the interval PEAKINT after a pulse arrival time. If no
second pulse occurs in this interval, then there is no trailing edge pileup.
PEAKINT is usually set to a value close to L + G/2 + 1. Pulse 1 passes this test,
as shown in the figure. Pulse 2, however, fails the PEAKINT test because pulse
3 follows in 1.8 µs, which is less than PEAKINT = 2.3 µs. Notice, by the
symmetry of the trapezoidal filter, if pulse 2 is rejected because of pulse 3, then
pulse 3 is similarly rejected because of pulse 2.
Pulses 4 and 5 are so close together that the output of the fast filter does
not fall below the threshold between them and so they are detected by the pulse
detector as only being a single x-ray pulse. Indeed, only a single (though
somewhat distorted) pulse emerges from the slow filter, but its peak amplitude
corresponds to the energy of neither x-ray 4 nor x-ray 5. In order to reject as
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many of these fast channel pileup cases as possible, the DXP implements a fast
channel pileup inspection test as well.
The fast channel pileup test is based on the observation that, to the
extent that the risetime of the preamplifier pulses is independent of the x-rays’
energies (which is generally the case in x-ray work except for some room
temperature, compound semiconductor detectors) the basewidth of the fast
digital filter (i.e. 2Lf + Gf) will also be energy independent and will never
exceed some maximum width MAXWIDTH. Thus, if the width of the fast filter
output pulses is measured at threshold and found to exceed MAXWIDTH, then
fast channel pileup must have occurred. This is shown graphically in the figure
where pulse 3 passes the MAXWIDTH test, while the piled up pair of pulses 4
and 5 fail the MAXWIDTH test. Thus, in Figure 4.11, only pulse 1 passes both
pileup inspection tests and, indeed, it is the only pulse to have a well defined
flattop region at time PEAKSAMP in the slow filter output.
25
Digitized MultiPile kfig 960921
Preamp
4 5
20
Passes
PEAKINT
Test
2
1
3
Fails
PEAKINT
Test
Passes
MAXWIDTH
Test
15
Fails
MAXWIDTH
Test
Fast Filter
10
2
1
4
3
5
PEAKSAMP
1b
5
1a
2a 1c
Slow Filter
0
5
10
15
20
25
30
Time ( µs)
Figure 4.11: A sequence of 5 x-ray pulses separated by various intervals to show the origin of both slow
channel and fast channel pileup and demonstrate how the two cases are detected by the DXP.
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Note that PEAKINT and MAXWIDTH are both DSP parameters and
are normally set automatically. In particular, there is almost never any benefit to
a longer value of PEAKINT than the standard value as it does not improve
energy resolution and only decreases throughput for a given input rate. Please
see §5.10.5 for details on how to adjust MAXWIDTH.
4.9 Input Count Rate (ICR) and Output Count Rate (OCR)
During data acquisition, x-rays will be absorbed in the detector at some
rate. This is the true input count rate , which we will refer to as ICRt. Because
of fast channel pileup, not all of these will be detected by the DXP’s x-ray pulse
detection circuitry, which will thus report a measured input count rate ICRm,
which will be less than ICRt. This phenomenon, it should be noted, is a
characteristic of all x-ray detection circuits, whether analog or digital, and is not
specific to the DXP.
Of the detected x-rays, some fraction will also satisfy both fast and
slow channel pileup tests and have their values of Vx captured and placed into
the spectrum. This number is the output count rate , which we refer to as the
OCR. The DXP normally returns, in addition to the collected spectrum, the
REALTIME for which data was collected, the fast channel LIVETIME for
which the fast channel was below threshold (and thus ready to detect a
subsequent x-ray) together with the number FASTPEAKS of fast peaks detected
and the number of Vx captured events EVTSINRUN. From these values, both
the OCR and ICRm can be computed according to Equation 4-11. These values
can then be used to make deadtime corrections as discussed in the next section.
ICRm = FASTPEAKS/LIVETIME; OCR = EVTSINRUN/REALTIME
Equation 4-11
Note: The fast channel LIVETIME should only be used to determine
the input count rate according to Equation 4-11. Specifically, it is NOT related
to the energy filter livetime and should not be interpreted as the inverse of the
processor deadtime. The DSP does calculate the energy filter livetime
ELIVETIME, however, it is only an approximation. The most accurate
deadtime measurement is obtained from ICRm and OCR in Equation 4-11, as
discussed in §4.11
4.10 Throughput
Figure 4.12 shows how the values of ICRm and OCR vary with true
input count rate for the DXP and compare these results to those from a common
analog shaping amplifier plus SCA system. The data were taken at a
synchrotron source using a detector looking at a CuO target illuminated by xrays slightly above the Cu K absorption edge. Intensity was varied by adjusting
two pairs of crossed slits in front of the input x-ray beam so that the harmonic
content of the x-ray beam striking the detector remained constant with varying
intensity.
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200
DXP OCR
DXP ICRm
Analog OCR
Analog ICRm
True ICRt
150
Output Count Rate (kcps)
NOTE: The DXP’s peaking
time is twice as long as the
analog system peaking time
in this comparison, and yet
the throughput is nearly the
same.
100
50
ICR/OCR Plot kfig 960922
0
0
50
100
150
200
Input Count Rate (kcps)
Figure 4.12: Curves of ICRm and OCR for the DXP using 2 µs peaking time,
compared to a common analog SCA system using 1 µs peaking
time.
System
OCR Deadtime (µs)
ICR Deadtime (µs)
DXP (2 µs τp, 0.6 µs τg)
4.73
0.83
Analog Triangular Filter Amp (τp = 1 µs)
4.47
0.40
Table 4.3:
Comparing the deadtime per event for the DXP and an analog shaping amplifier. Notice that
that the DXP produces a comparable output count rate even though its peaking time is nearly
twice as long.
Functionally, the OCR in both cases is seen to initially rise with
increasing ICR and then saturate at higher ICR levels. The theoretical form,
from Poisson statistics, for a channel that suffers from paralyzable (extending)
dead time is given by:
OCR = ICRt * exp( - ICRt * τd ),
Equation 4-12
where τd is the dead time. Both the DXP and analog systems’ OCRs are so
describable, with the slow channel dead times - τd - shown in Table 4.3. The
measured ICRm values for both the DXP and analog systems are similarly
describable, with the fast channel dead times - τdf - as shown. The maximum
value of OCR can be found by differentiating Equation 4-12 and setting the
result to zero. This occurs when the value of the exponent is -1, i.e. when ICRt
equals 1/τd. At this point, the maximum OCRmax is 1/e the ICR, or
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OCRmax = 1/(e τd) = 0.37/τd
Equation 4-13
These are general results and are very useful for estimating experimental data
rates.
Table 4.3 illustrates a very important result for using the DXP: the slow
channel deadtime is nearly the minimum value that is theoretically possible,
namely the pulse basewidth. For the shown example, the basewidth is 4.6 µs
(2Ls + Gs) while the deadtime is 4.73 µs. The slight increase is because, as
noted above, PEAKINT is always set slightly longer than Ls - Gs/2 to assure
that pileup does not distort collected values of Vx.
The deadtime for the analog system, on the other hand is much larger.
In fact, as shown, the throughput for the digital system is almost twice as high,
since it attains the same throughput for a 2 µs peaking time as the analog system
achieves for a 1 µs peaking time. The slower analog rate arises, as noted earlier
both from the longer tails on the pulses from the analog triangular filter and on
additional deadtime introduced by the operation of the SCA. In spectroscopy
applications where the system can be profitably run at close to maximum
throughput, then, a single DXP channel will then effectively count as rapidly as
two analog channels.
4.11 Dead Time Corrections
The fact that both OCR and ICRm are describable by Equation 4-12
makes it possible to correct DXP spectra quite accurately for deadtime effects.
Because deadtime losses are energy independent, the measured counts Nmi in
any spectral channel i are related to the true number Nti which would have been
collected in the same channel i in the absence of deadtime effects by:
Nti = Nmi ICRt/OCR
Equation 4-14
Looking at Figure 4.12, it is clear that a first order correction can be
made by using ICRm in Equation 4-11 instead of ICRt, particularly for OCR
values less than about 50% of the maximum OCR value. For a more accurate
correction, the fast channel deadtime τdf should be measured from a fit to the
equation:
ICRm = ICTt * exp( - ICRt τdf )
Equation 4-15
Then, for each recorded spectrum, the associated value of ICRm is
noted and Equation 4-15 inverted (there are simple numerical routines to do this
for transcendental equations) to obtain ICRt. Then the spectrum can be
corrected on a channel-by-channel basis using Equation 4-12. In experiments
with a DXP prototype, we found that, for a 4 µs peaking time (for which the
maximum ICR is 125 kcps), we could correct the area of a reference peak to
better than 0.5% between 1 and 120 kcps.
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5 microDXP DSP Code Description
NOTICE: if you are curious about how the DSP operates in controlling the DXP
and processing data from the FiPPI, then please read on. You will also find this
information useful if you wish to develop your own control code for the
microDXP. However, in the latter case, we strongly advise you to use XIA’s
support libraries (Handel) to interface between your program and the microDXP
module.
5.1 Introduction and Program Overview
The following sections are intended to provide the DXP user with a
good understanding of the various tasks performed by the DSP in the
microDXP. The DSP performs several functions:
1.
Respond to input and output calls from the host computer to start and
stop data collection runs, download control parameters, and download
collected data.
2.
Perform system calibration measurements by varying the various DAC
settings under its control and noting the output change at the ADC.
3.
Make initial measurements of the slow filter baseline and preamplifier
slope value at the start of data taking runs to assure optimum starting
parameter values.
4.
Collect data:
a.
Read energy values Ex from the FiPPI, under interrupt control,
and store them in DSP buffer memory in less than 0.25 µs.
b.
Adjust the ASC control parameters, under interrupt control, to
maintain its output within the ADC’s input range.
c.
Process captured Ex values to build the x-ray spectrum in DSP
memory.
d.
Sample the FiPPI slow filter baseline and build a spectrum of
its values in order to compute the baseline offset for Ex
values.
Several DSP program variants are available to cover a range of
applications. The standard program provided with the microDXP is for typical
x-ray fluorescence spectroscopy using a reset-type preamplifier. Additional
program variants are available for other applications, including hardware
diagnostics and other specialized measurements, e.g.:
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•
X-ray mapping
•
Quick XAFS scanning
•
Switching between multiple spectra synchronously with an
experimentally derived signal (e.g. “Phased locked EXAFS”)
•
Time resolved spectroscopy (e.g. “multi-channel scaling”)
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Standard variants available to all users via our website are described in
§5.11. Several other variants have been developed for particular customers and
may be made available upon request.
By convention, the DSP programs are named “NAMEmmnn.HEX”,
where NAME is the variant name listed in the table, mm and nn are major and
minor version numbers, respectively. The hex file format is in ASCII, with the
parameter table at the top followed by the code generated by the Analog Devices
218x development system.
The internal data memory area is subdivided into three sections. The
first section, starting at location 0x4000, contains DSP parameters and constants,
both those used for controlling the DSP's actions and those produced by the DSP
during normal running. These parameters and their addresses are listed and
described in the following sections. When these parameters are referred to they
will be denoted by all capital letters (e.g. RUNTASKS ). The locations of
parameters can (and, for forward compatibility should) be determined from the
symbol table.
The second section of data memory contains acquired monitoring data
such as the baseline event histogram. The third section of internal data memory
is used as a circular buffer for storing events from the FiPPI. Note that future
hardware revisions may eliminate the need for this buffer area, in which case it
could be switched to more histogramming area.
5.2 Program Flow
The flow of the DSP program is illustrated in Figure 5.1. It is
essentially identical for all program variants. The structure is very simple; after
initialization, the DSP enters an idle phase, waiting for a signal from the host to
start a run. During this idle phase, the DSP is continuously collecting baseline
events from the FiPPI as well as monitoring the Analog Signal Conditioner
(ASC) to keep the ADC input signal in the proper range and to adjust the slope
generator to match the current input rate. When the Begin Run signal is
received (from the host through the CSR register), the DSP first determines
whether the run is a normal data acquisition run or a special run.
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Figure 5.1 DSP code flow diagram.
In a normal run, ASC monitoring and baseline collection continue as in
the idle phase. Event interrupts are enabled; when the FiPPI detects an event, it
interrupts the DSP, which quickly responds and reads the energy value from the
FiPPI into an internal buffer in data memory. The events in the buffer are then
used to build the x-ray spectrum (or fill regions of interest).
In a special run, the action is determined by the value of the parameter
WHICHTEST. The special runs include calibration tasks such as collecting an
ADC trace, as well as ways of putting the DSP code into a special state (such as
putting it into a dormant state to allow reprogramming the FiPPI on the fly).
Special runs normally end automatically and the DSP returns to the idle state.
After the initialization phase, the Timer interrupt is enabled. This
interrupt is used to handle the housekeeping type chores, such as updating the
statistics during a run, controlling the rate LED, and handling fixed length runs.
The Timer interrupt occurs with a period of 500 µsec.
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If the DSP encounters an error condition, the DSP turns on the red
status LED and waits for the host to set the parameter RUNERROR to 0 (after
finding and fixing the problem that resulted in the error condition).
Each phase of the DSP program is discussed in more detail below.
5.3 Initialization
The DSP code starts running immediately after the power is turned on.
During the initialization phase, several tasks are performed:
1.
Setup internal DSP control registers
2.
Zero spectrum and data memory, then initialize parameters to default
values.
3.
Set ASC DACs to initial default values
4.
Initialize FiPPI and download default filter parameters
5.
Perform initial calibrations for controlling the ASC:
6.
a.
Find the SlopeDAC setting corresponding to zero slope
b.
Measure conversion factor used to calculate the contribution
of the slope generator to the FiPPI baseline.
Enable the ASC and timer interrupts.
After the interrupts are enabled, the DSP is alive and ready to take data.
After completing the initialization phase, the DSP enters the idle phase. In the
idle phase, the DSP continuously samples the FiPPI baseline and updates the
baseline subtraction register in the FiPPI so that the FiPPI is always ready to
take data as soon as a run is started. There are two primary tasks performed
during a normal data acquisition run: event processing and baseline
measurement. These tasks are described in detail below.
5.4 Event Processing
5.4.1
Run Start
Prior to the start of a normal the run, the DSP performs several tasks:
1.
Sets the stored gain (by setting the GAINDAC).
7.
Sets the stored polarity.
8.
Downloads FiPPI parameters (SLOWLEN, SLOWGAP, etc) stored in
the current PARSET to obtain the desired peaking time.
9.
Updates the internal calibrations with the new gain and FiPPI values.
10. If desired, the run statistics and the MCA are cleared. Otherwise, the
run is treated as a continuation of the previous run. Note that for a run
continuation, no gain or FiPPI changes are performed. In either case,
the run number (parameter RUNIDENT) is incremented.
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Event Interrupt
When the FiPPI detects a good event, it triggers a high priority interrupt
in the DSP. Upon receiving the interrupt, the DSP immediately reads the event
energy from the FiPPI into an internal circular buffer and increments the write
pointer into that buffer. The normal event loop compares the write pointer to the
read pointer to determine that there is a new event to process.
5.4.3
Event Loop
The processing that takes place during a normal collection run is very
simple, in order to allow high event rates. The structure of the event loop is
illustrated below in pseudocode:
while (RunInProgress)
{
if (EventToProcess)
ProcessEvent
else
CollectBaseline
endif
}
RunFinish
goto IdleLoop
The run can be stopped by the host, or can be stopped internally for
fixed length runs; see §5.10.1 below.
The event processing involves either binning the energy into an MCA
or determining whether the event falls into a defined SCA window, depending
upon the DSP code variant. If there is no event to process, the DSP reads a
baseline value from the FiPPI; see below for a detailed description of the
baseline processing. Once the run is over, the statistics are finalized and the
DSP returns to the idle state where it continuously samples baseline and waits
for a command to start a new run.
5.4.4
Spectrum Binning
The primary event-processing task is to use the energies measured in
the FiPPI to build up a full energy spectrum (MCA). The MCA bin width is
determined by the analog gain, the FiPPI filter length and the binning parameter
BINFACT1. The DSP determines the spectrum bin by multiplying the FiPPI
energy output by (1/BINFACT1). If the bin is outside the range determined by
the parameters MCALIMLO and MCALIMHI, the event is classified as an
underflow or overflow. Otherwise, the appropriate bin is incremented. A 24-bit
word is used to store the contents of each bin, allowing nearly 16.8 million
events per MCA channel.
5.4.5
SCA Mapping
An alternate variant of the DSP code allows the user to define up to 24
SCA regions and count the number of events that fall into each region. The
regions are defined in terms of MCA bin number, and can overlap. A useful
method for defining the SCA windows is to take a run with the full MCA
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spectrum, and use the spectrum as an aid in choosing the limits for each SCA.
The reduced amount of data storage in SCA mapping mode is very useful in
time resolved spectroscopy or scanning applications, where separate spectral
data are desired for many different time or spatial points.
5.5 Baseline Measurement
The DSP collects baseline data from the FiPPI whenever there are no
events to process, both during a run and between runs (when there are never
events to process). The DSP keeps a running average of the most recent
baseline samples; this average is written back into the FiPPI where it is
subtracted from the raw energy filter value to get the true energy. The baseline
data read from the FiPPI is just the raw output of the energy filter. One bit of
the baseline register is used to indicate whether the sample occurred while an
event was in progress, in which case it is not used.
Two methods are available to determine the average baseline value. By
default, an infinite impulse response (IIR) filter is used, where the baseline
average is calculated by combining a new baseline sample with the old average,
using weights x and (1-x) respectively, where x is typically 1/128. By setting
the appropriate bit in the parameter RUNTASKS (see below), a finite impulse
response (FIR) filter is used, where the baseline mean is just the straight average
of the N most recent baseline samples. Both averaging methods are described in
more detail in the following sections. The baseline mean is stored with 32 bit
precision in the parameters BASEMEAN0 (high order word) and
BASEMEAN1.
5.5.1
IIR (Infinite Impulse Response) Filter
By default, the baseline mean is calculated using an infinite impulse
response filter, characterized in the following way:
< Bi >=
N −1
1
< Bi − 1 > + Bi
N
N
Equation 5-1
Where <Bi> is the baseline mean after the ith baseline sample, Bi is the ith
baseline sample, and <Bi-1> is the baseline mean before the ith sample. With
this filter, the most recent baseline samples are weighted the most, but (up to the
precision of the stored mean value) all baseline values have a small effect on the
mean (hence the infinite in the name).
The length of the filter is controlled by the parameter BLFILTER,
which holds the value 1/N in 16 bit fixed point notation, which has 1 sign bit
and 15 binary bits to the right of the decimal point. Expressed as a positive
integer, BLFILTER = (1/N)*2**15. The default value for BLFILTER
corresponds to N=64. Interpreting BLFILTER as an integer gives
(1/128)*2**15 = 2**9 = 256.
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mdo-microDXP-MAN-1.1.2
FIR (Finite Impulse Response) Filter
By setting the appropriate RUNTASKS bit, it is possible to choose a
finite impulse filter to calculate the baseline mean. With this filter, a straight
average of the N most recent valid baseline samples is used to calculate the
mean. To implement this filter, a buffer large enough to hold all N samples is
necessary. For this reason, the length of the finite response filter is limited to
1024. The filter length is stored in the parameter BLFILTERF.
5.5.3
Baseline Histogram
As part of the baseline processing, all valid baseline samples are
entered into the baseline histogram, which occupies 1024 words of data
memory. The baseline histogram can be very useful in monitoring or evaluating
the performance of the microDXP, as discussed in the Rapid Development Kit
Manual. The parameter BASESTART contains the pointer to the location of
the histogram in data memory, and the length (nominally 1K) is contained in the
parameter BASELEN.
The baseline histogram is centered about a zero baseline. The
parameter BASEBINNING determines the granularity of the histogram;
2**BASEBINNING baseline values are combined into one bin of the baseline
histogram. The default value of BASEBINNING is 2 (i.e., the baseline value is
divided by 4 to determine the bin). All valid baseline values are included in the
histogram, even if there is a baseline cut (see §5.5.5 below) in use.
The baseline histogram is only filled during a normal data acquisition
run; when the DSP is idle, the baseline average is calculated but the histogram is
not filled. Since the baseline histogram is stored in data memory, 16-bit words
are used to record the bin contents. As a result, the histogram overflows quite
often; the time to overflow depends on the baseline sample rate (typically
several 100 kHz) and the width of the baseline distribution. When the DSP
detects an overflow, all bins are scaled down by a factor of 2 and histogramming
continues.
The baseline distribution should be very Gaussian; the width of the
distribution reflects the electronic noise in the system (including the effects of
the energy filter). A tail on the positive side of the distribution indicates the
presence of energy in the baseline, resulting from undetected pileup or energy
depositions that did not satisfy the trigger threshold. The tail should be very
small compared to the peak of the histogram; it will grow with rate. If this tail is
too large, it can have a noticeable effect on the baseline mean, leading to
negative peak shifts. Under these circumstances, enabling the baseline cut is
useful in eliminating the bias.
A tail on the low energy side of the baseline distribution is usually
caused by baseline samples just after a preamplifier reset; the effects of the reset
can last quite a while (tens of microseconds), especially for optical reset
preamplifiers. It is usually best not to take data while the reset is in effect; the
dead time associated with a reset can be adjusted using the parameter
RESETWAIT, which sets the dead time in units of 250.
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mdo-microDXP-MAN-1.1.2
Residual Baseline
When operating with a reset type preamplifier, the raw baseline
measured in the FiPPI (which is just the output of the energy filter) comes from
two sources: the detector preamplifier and the slope generator in the microDXP
itself. At high rates, the slope gets rather large in order to balance the highenergy deposition rate in the detector; under these conditions, the baseline due to
the slope is by far the dominant factor in the baseline.
By default, the DSP continually adjusts the slope to match the current
rate; these slope adjustments result in an instantaneous change in the baseline. If
the baseline due to the slope generator is included in the baseline mean, the
change in the calculated mean would be delayed relative to the change in the
slope, due to the effect of all the baseline samples prior to the slope change. For
this reason, the baseline due to the slope is subtracted out of the overall baseline
prior to calculating the mean value (and added back in prior to loading the FiPPI
baseline subtraction register). The residual baseline included in the mean
reflects the detector leakage current, and should be fairly constant with rate (to
the extent that the leakage current does not depend on rate). The calibration
procedure used to determine the baseline due to the slope generator is performed
during the initial startup procedure.
By default, the baseline due to the slope generator is taken out of the
baseline average. You can choose to include the slope baseline in the mean (e.g.
for diagnostic purposes) you can do so by clearing the residual baseline bit (6) in
RUNTASKS.
5.5.5
Baseline Cut
As specified above, a baseline cut is available to exclude baseline
samples that include real event energy, which can lead to peak shifting at high
event rates. The cut is expressed as a fraction of the peak value of the baseline
distribution; by default, the baseline cut is set to 5%. The cut values are based
on the baseline histogram, and are recalculated every time the histogram
overflows (every few seconds). The DSP searches on either side of the peak of
the baseline distribution for the first bin whose contents are less than the cut (.05
by default) times the peak value; these bin numbers are used to calculate the
actual baseline cut.
The cut fraction is stored in the parameter BLCUT, expressed in 16-bit
fixed-point notation. Interpreted as an integer, BLCUT = (cut fraction)*2**15;
the default 5% cut corresponds to BLCUT=1638 decimal (or 666 hex). The
actual cut values determined by the DSP code are stored in BLMIN and
BLMAX. The baseline cut is enabled or disabled by setting or clearing a bit
(10) in the RUNTASKS parameter.
5.6 Interrupt Routines
There are several tasks performed under interrupt control within the
DSP on the microDXP. The event interrupt routine (which just transfers event
data from the FiPPI to an internal buffer) is described above in Section 5.4
above. There are two other interrupt routines: the ASC interrupt is used to keep
the analog signal within the input range of the ADC, and the timer interrupt is
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used to handle such housekeeping chores as updating statistics. These routines
are described in more detail below.
5.6.1
ASC Monitoring
There are four main tasks performed by the ASC interrupt routine:
1.
Detects Resets (reset-type detectors only)
2.
Adjusts the slope generator to match the event rate (reset-type
detectors only)
3.
Adjusts the offset value to keep the signal in range (RC feedback
detectors only)
4.
Moves the signal back to the center of the ADC range whenever it
drifts out of range (high or low).
The ASC interrupt routine is triggered whenever the FiPPI detects the
ADC going out of range. The DSP first triggers a short reset. If the signal
merely drifted out of range to begin with, the short reset will bring the signal
back to the center of the ADC range, and data taking resumes. If instead the
signal was driven out of range by a preamplifier reset, the short reset will not
bring the signal back into range. In this case the DSP triggers a long reset, and
then holds the signal at the center of the ADC range for a time determined by the
parameter RESETINT, which specifies the dead time after a reset in
microsecond units. After the reset interval, the signal is released and data taking
resumes.
The DSP keeps track of how many times the signal drifts out of range
in both directions, and adjusts the slope such that the number of drifts high
(NUMDRUPS) roughly matches the number of drifts low (NUMDRDOS). If
the DSP determines that the slope must be changed to match the rate, the
SlopeDAC value is modified by a constant fraction of the parameter
SLOPEVAL determined by the value of the parameter SGRANULAR. By
default, the slope adjustment granularity is 5%, which is a good compromise
between adjusting the slope quickly to match quickly changing input rates and
being able to set the SlopeDAC just right.
For an RC feedback detector, the offset added to the input signal is
adjusted such that the signal stays in range as much as possible.
5.6.2
Timer Interrupt
Every 500 µsec, the DSP is interrupted to take care of the regular
‘maintenance’ type tasks. These tasks include:
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1.
Update the run statistics EVTSINRUN, LIVETIME, REALTIME and
FASTPEAKS (only during a run).
2.
Control the Rate LED. This LED flashes whenever a reset is detected
(reset detector only), and during a run the color indicates the current
output/input ratio. By default, the LED flashes green for
OCR/ICR>0.5, flashes yellow (green plus red) for 0.5>OCR/ICR>1/e,
and flashes red for OCR/ICR<1/e. The thresholds are determined by
the parameters YELTHR and REDTHR.
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Handle fixed length runs. During a fixed length run, the current value
of EVTSINRUN (output events), FASTPEAKS (input events),
LIVETIME or REALTIME is compared to the desired run length.
Once the value exceeds the desired value, the run is ended.
5.7 Error Handling
When the DSP detects an error in the operation of the microDXP, the
red Status LED is turned on, and the source of the error is stored in the
parameter RUNERROR. The possible values for RUNERROR are listed below:
RUNERROR Value
0
1
2
3-5
6
Table 5.1:
Meaning
No Error
FiPPI communication error
ASC setup failure
Reserved
TrackDAC calibration error
Identification of DXP errors according to the DSP parameter
RUNERROR.
A FiPPI communication error could mean that the FiPPI configuration
was not successful. An ASC calibration error can indicate a hardware problem,
or possibly that a jumper is not set properly (for example, the DSP code for reset
preamplifiers will generate an error if the jumper is set to run in OFFSET
mode).
Once the source of the error has been located and cleared, the host can
set RUNERROR to 0 to force the DSP to exit the error loop and reinitialize the
system. Note that all system settings are saved when initialization is performed
coming out of the error loop. Of course, another valid method for clearing the
error is to re-download the DSP code after fixing the problem.
5.8 Specifying Data Acquisition Tasks (RUNTASKS):
Many aspects of the operation of the microDXP are controlled by
individual flag bits of the parameter RUNTASKS. The meaning of each
RUNTASKS bit is described below:
Bit
0
1
2
3
4
5
6
7
8
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Meaning if set (1)
Reserved (set to 0)
Update SlopeDAC or OffsetDAC value to match
current rate (DEFAULT)
Use Finite Impulse Response (FIR) filter to
calculate baseline average
Acquire baseline values for histogramming and
averaging (DEFAULT)
Adjust fast filter threshold to compensate for
rate shifts
Correct for baseline shift, either in FiPPI (pulse
reset) or DSP (RC feedback) (DEFAULT)
Apply residual baseline correction (DEFAULT)
Disable writing baseline values to baseline
history circular buffer
Indicates special task or calibration run specified
by WHICHTEST
Meaning if cleared (0)
Reserved (set to 0)
SlopeDAC or OffsetDAC adjustments disabled
Use Infinite Impulse Response (IIR) filter to
calculate baseline average (DEFAULT)
Disable baseline acquisition
Disable fast filter threshold adjustment
Disable baseline correction
No residual baseline correction
Continuously write baseline values to baseline
history circular buffer (DEFAULT)
Indicates normal acquisition run
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10
11-15
Histogram DeltaBaseline
(baseline - <baseline>)
Enable baseline cut
Reserved (set to 0)
mdo-microDXP-MAN-1.1.2
Histogram raw baseline (DEFAULT)
Disable baseline cut (DEFAULT)
Reserved (set to 0)
Table 5.2: Data acquisition tasks controlled by the DSP parameter RUNTASKS.
5.9 Special Tasks (WHICHTEST)
Special tasks are selected by starting a run with bit 8 of the
RUNTASKS parameter set. The following tasks are currently supported:
Number
0
1
2
3
4
5
6
7
8
9-10
11
12
13
14
15
16
Test Segment
Set ASC DAC values to current value of GAINDAC, SLOPEDAC or OFFSETDAC
Acquire ADC trace in history buffer
Gain calibration
Slope calibration (measure SLOPEMULT)
Measure ADC non-linearity
Not currently used
Put DSP to sleep while FPGA logic is downloaded
Not currently used
OffsetDAC calibration (measure OFFDACVAL)
Not currently used
Program FiPPI
Set internal polarity to current value of POLARITY parameter
Not currently used
Not currently used
RC feedback calibration trace of baseline filter and decimator values
RC feedback calibration trace of event filter and decimator values
Table 5.3: Special tasks and test segments that can be selected with the DSP parameter WHICHTEST.
5.10 DSP Parameter Descriptions
As noted above, DSP operation is based on a number of parameters.
Some are control parameters required to operate the DXP, some are calibration
values determined by the DSP, and others are run statistics.
NOTE: in general you will not want to modify these parameters
directly, but only through a host control program like microManager or, if you
are a programmer, through a software library like XIA’s Handel library.
Variable
PROGNUM
CODEREV
HDWRVAR
FIPPIREV
FIPPIVAR
DECIMATION
RUNIDENT
RUNERROR
BUSY
Acquisition Statistics:
LIVETIME0,1,2
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Type
Constant
Constant
Constant
Constant
Constant
Constant
Returned
Returned
Returned
Description
Program variant number.
Current DSP program revision.
Hardware variant. DSP reads this from interface FPGA.
FiPPI design revision. DSP reads this from FiPPI FPGA.
FiPPI design variant. DSP reads this from FiPPI FPGA.
Slow filter decimation factor. DSP reads this from FiPPI FPGA.
Run identifier
Error code if run is aborted, 0 for success
DSPs current acquisition status. Values listed below.
Statistic
Intermediate filter live time in 500 nsec units
Reference
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ELIVETIME0,1,2
REALTIME0,1,2
EVTSINRUN0,1
UNDRFLOWS0,1
OVERFLOWS0,1
FASTPEAKS0,1
NUMASCINT0,1
NUMRESETS0,1
NUMUPSETS0,1
NUMDRUPS0,1
NUMDRDOS0,1
NUMZIGZAG0,1
BASEEVTS0,1
BASEMEAN0,1
Control parameters:
WHICHTEST
RUNTASKS
BINFACT1
MCALIMLO
MCALIMHI
TRACEWAIT
ASCTIMOUT
YELLOWTHR
REDTHR
PRESET
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
Statistic
mdo-microDXP-MAN-1.1.2
Energy filter live time in 500 nsec units
Elapsed acquisition time in 500 nsec units
Number of events in MCA spectrum
Number of MCA underflow events
Number of MCA overflow events
Number of input events detected by FiPPI
Number of ASC interrupts
Number of "reset" events seen
Number of "upset" events seen
Number of "drift up" events seen
Number of "drift down" events seen.
Number of "zigzag" events seen
Number of baseline events acquired
Updating mean baseline value
Which test segment to execute.
Which tasks will be executed in run sequence
MCA binning factor
Lower limit of MCA spectrum
Upper limit of MCA spectrum
ADC trace time factor
Timeout for ASCSetup in tenths of seconds
Medium rate throughput threshold for front panel LED
High rate throughput threshold for front panel LED
Preset type (0:none; 1:real time; 2:live time; 3: output cts; 4: input
cts)
PRESETLEN0,1
Parameter Preset run length
FiPPI Digital Filter/Event selection parameters:
SLOWLEN
Parameter Slow filter length
SLOWGAP
Parameter Slow filter gap
PEAKINT
Parameter Peak interval
FASTLEN
Parameter Fast filter length
FASTGAP
Parameter Fast filter gap
THRESHOLD
Parameter Threshold value for fast filter trigger (range: 1-255, 0 disables)
MINWIDTH
Parameter Minimum peak width
MAXWIDTH
Parameter Maximum peak width
BASETHRESH
Parameter Automatically set threshold for intermediate filter trigger (range 1255, 0 disables both baseline and event discrimination—use
FIPCONTROL to disable event discrimination only)
BASETHRADJ
Parameter Coefficient for BASETHRESH auto-set algorithm (range 1-255,
smaller values result in tighter thresholds)
FIPCONTROL
Parameter FiPPI advanced control, bitwise flag register:
bit 0 – fast threshold (0: enabled)
bit 1 – intermediate threshold (0: enabled)
bit 2 – slow threshold (0: enabled)
SLOWTHRESH
Parameter Threshold for slow filter trigger (range 1-255, 0 disables)
PEAKSAM
Parameter Peak sampling time
Baseline Related Parameters:
BLFILTER
Parameter Filtering parameter for baseline (IIR filtering)
BLFILTERF
Parameter Filtering parameter for baseline (FIR filtering)
BASEBINNING
Parameter Baseline binning for histogram (0:finest to 6:coarsest)
BLCUT
Parameter DSP baseline cut (cut at BLCUT*FWHM, units defined below)
BLMIN
Calibration Min baseline value accepted in average (calculated from BLCUT)
BLMAX
Calibration Max baseline value accepted in average (calculated from BLCUT)
ASC Control Parameters and Calibrations (all variants)
POLARITY
Parameter
Preamplifier signal polarity (0:negative step; 1:positive step)
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Parameter
Parameter
Parameter
Parameter
Parameter
Parameter
Parameter
Parameter
Parameter
Parameter
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GAINDAC
Parameter
Current Gain DAC value (16 bit serial DAC, range 0-65535).
ASC Control Parameters and Calibrations (reset-type variants)
RESETWAIT
Parameter Quick Reset time, 25ns units
RESETINT
Parameter Reset time, 0.25 µsec units
SLOPEDAC
Calibration Current Slope DAC value (16 bit serial DAC, range 0-65535)
SLOPEZERO
Calibration Slope DAC zero value (approximately center of range)
SLOPEVAL
Calibration Absolute setting (SLOPEDAC-SLOPEZERO)
SGRANULAR
Parameter Slope DAC step size
TRKDACVAL
Parameter Tracking DAC value: 12-bit parallel
TDACWIDTH
Parameter Track DAC pulse width 50 ns units
TDQPERADC
Calibration
TDQPERADCE
Calibration
ASC Control Parameters and Calibrations (RC feedback variants)
OFFSETDAC
Parameter
Current offset DAC value (16 bit serial DAC, range 0-65535).
OFFSETSTEP
Parameter
Offset DAC step size
TAURC
Parameter
Preamplifier decay constant, in 25 ns units (RCF variant only)
RCFCOR
Calibration Preamplifier decay correction (RCF variant only)
Miscellaneous Constants:
SPECTSTART
Constant
Address of MCA spectrum in program memory
SPECTLEN
Constant
Length of MCA spectrum buffer
BASESTART
Constant
Address of baseline histogram in data memory (offset by 0x4000)
BASELEN
Constant
Length of baseline histogram
EVTBSTART
Constant
Address of event buffer in data memory (offset by 0x4000)
EVTBLEN
Constant
Length of baseline histogram
HSTSTART
Constant
Address of history buffer in data memory (offset by 0x4000)
HSTLEN
Constant
Length of history buffer
NUMSCA
Parameter Number of SCA regions defined (mapping variants only)
SCAxLO, x=0-23
Parameter Lower MCA channel for SCA region x (mapping variants only)
SCAxHI, x=0-23
Parameter Upper MCA channel for SCA region x (mapping variants only)
USER1-USER8
User
User variables. Host software can use these for any purposes
Table 5.4: Summary of DSP parameter definitions
5.10.1 Specifying fixed run lengths
(PRESET, PRESETLEN0,1)
By default, the microDXP acquires data until a stop command is
received from the host.
A fixed run length can be specified using the parameters PRESET and
PRESETLEN0,1, as follows:
PRESET specifies the type of run: 0 = indefinite (default)
1 = fixed realtime
2 = fixed (energy filter) livetime
3 = fixed output events
4 = fixed input counts
PRESETLEN0,PRESETLEN1 specifies the length of preset fixed
length run, as a 32 bit quantity. For fixed real time or live time, the units are 500
nanosecond intervals.
5.10.2 Setting the slow filter parameters
(SLOWLEN, SLOWGAP)
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The DXP uses a trapezoidal filter, characterized by the peaking time,
Tp, and gap time, Tg. The peaking time is determined by the SLOWLEN and
DECIMATION values. SLOWLEN is the interval of time, in units of decimated
clock cycles, during which the decimated ADC signal is integrated, referred to
as the peaking time. DECIMATION is automatically sensed by the DSP and
should not be modified. For Tp and Tg. in µsec, and the pipeline clock FCLK in
MHz, the following gives the value of SLOWLEN and SLOWGAP:
SLOWLEN = FCLK *Tp *2-DECIMATION
e.g. At DECIMATION = 4, Tp = 16 µsec yields SLOWLEN=8 for an 8MHz
clock, or SLOWLEN=16 for a 16MHz clock. The user will want to be able to
choose the peaking time based on resolution and throughput requirements as
described earlier in this document.
SLOWGAP is the gap time, visible as the ‘flat-top’ region of the
trapezoid.
SLOWGAP = FCLK*Tg*2-DECIMATION
Subject to the restriction that it must exceed 3, SLOWGAP is set such
that the flat-top interval is longer than the 0%-100% risetime of the preamplifier
output pulses by at least 1 decimation period:
Gap Time = (2 D * SLOWGAP * 125 ns) > pulse risetime+ 2D * 125 ns
or: (2 D * (SLOWGAP-1) * 125 ns) > pulse risetime
5.10.3 Setting the fast filter parameters
(FASTLEN, FASTGAP)
The fast filter is also trapezoidal but has a decimation of 0 for all FiPPI
designs. The values of FASTLEN and FASTGAP are given, for Tp’ fast
peaking time and Tg’ = fast gap time in µsec:
FASTLEN = 8*Tp’
FASTGAP = 8*Tg’
Typical values of these parameters are FASTLEN=4 and FASTGAP=0.
5.10.4 Setting Thresholds
(THRESHOLD, MINWIDTH, BASETHRESH, BASETHRADJ,
SLOWTHRESH, FIPCONTROL)
Note: BASETHRESH is NOT used in the decimation 0 FiPPI (i.e.
peaking time range 0.75 – 3 µsec for the standard 8MHz pipeline clock; or
peaking time range 0.375 – 1.5 µsec for a 16MHz pipeline clock). Instead
THRESHOLD is used to discriminate both for and against events.
X-rays are identified when a filter output goes above an active
threshold. Thresholds can be applied to the fast (THRESHOLD), intermediate
(BASETHRESH, BASETHRADJ) and energy filters (SLOWTHRESH), though
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in practice the energy threshold is rarely used. The user will typically only
adjust the fast filter threshold, THRESHOLD.
All thresholds are scaled by their respective filter lengths. Further, they
cannot be expressed in energy units until the DXP conversion gain (see §5.10.6
below), GDXP = number of ADC counts per keV at the DXP input, is known.
For an energy threshold Eth in keV,
THRESHOLD=GDXP*Eth*FASTLEN
BASETHRESH=GDXP*Eth*SLOWLEN
SLOWTHRES=GDXP*Eth*SLOWLEN
THRESHOLD is set by the user. A good procedure is to initially set
the value too high. Once a spectrum is observed, reduce THRESHOLD until the
zero energy noise peak starts to become significant, and then raise it again until
only a trace of the noise peak remains. The MINWIDTH parameter is used for
noise rejection: It is the minimum number of time bins the fast filter is above
threshold. A typical value that works with FASTLEN=4 is MINWIDTH=4.
Currently BASETHRESH must also be set by the user
(BASETHRESH will be automatically set by the DSP in the next release) and
applied to the intermediate filter as part of the baseline acquisition circuitry, i.e.
baseline measurements are taken when the signal is below this threshold.
BASETHRESH threshold crossings by default also trigger event processing,
effectively extending the detectable energy range significantly below the fast
filter THRESHOLD. The parameter BASETHRADJ controls the algorithm that
calculates BASETHRESH, with larger BASETHRADJ values resulting in more
conservative BASETHRESH values. In the rare case the baseline threshold is
set incorrectly by the DSP algorithm, we recommend adjusting BASETHRADJ
rather than BASETHRESH itself.
The use of a slow threshold introduces significant errors in the counting
statistics. Specifically, the dead-time-per-event is markedly different for x-rays
above and below the threshold. SLOWTHRESH should only be used if:
 Your detector has a very thin window and operates in a vacuum.

You understand that it is not possible to compute input count rates for
x-ray peaks below the threshold relative to x-ray peaks above the
threshold.
Setting a threshold to zero disables that threshold. Individual
thresholds can also be enabled and disabled via the lowest 3 bits of the
parameter FIPCONTROL:
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FIPCONTROL
BIT
0
1
2
mdo-microDXP-MAN-1.1.2
Meaning, if = 0
Meaning, if = 1
THRESHOLD event
discrimination enabled
BASETHRESH event
discrimination enabled;
baseline discrimination
enabled
SLOWTHRESH event
discrimination enabled
THRESHOLD event
discrimination disabled
BASETHRESH event
discrimination disabled;
baseline discrimination
enabled
SLOWTHRESH event
discrimination disabled
Table 5.5: Threshold control via FIPCONTROL.
The two methods of disabling thresholds are equivalent, except in the case of the
intermediate filter threshold BASETHRESH, which is used for both baseline
and energy discrimination,
e.g. setting FIPCONTROL = XXXX XXXX XXXX X110 enables event
discrimination based on the fast filter THRESHOLD, disables event
discrimination based on the intermediate filter BASETHRESH and slow filter
SLOWTHRESH, however, BASETHRESH still discriminates baseline
measurements in the intermediate filter; alternatively, setting BASETHRESH to
zero would disable both event and baseline discrimination, regardless of the
value of FIPCONTROL.
5.10.5 Setting the Pile-up inspection parameters
(MAXWIDTH, PEAKINT)
MAXWIDTH is used to reject pulse pile-up on a time scale that is
comparable to FASTLEN as discussed in § 4.8. A typical value is
MAXWIDTH = 2*FASTLEN + FASTGAP +N
where N is in the range 4-8. If the signal rise-time depends on the x-ray energy
(e.g. bandwidth limited preamplifier or low field regions of the detector that are
preferentially sampled at some energy) this cut can bias the spectrum if it is too
small.
PEAKINT is used to reject energy channel pulse pile-up when the
pulses are well resolved by the fast channel. This value should be set as:
PEAKINT = SLOWLEN + SLOWGAP + N
where N = 1 typically.
5.10.6 Setting the Analog Gain (GAINDAC)
The DXP internal gain is chosen to set the ADC dynamic range
appropriately for the signals of interest. If it is set too low, the energy resolution
may be compromised, while if is set too high there may be excessive deadtime
and attenuation of higher energy x-rays. The ADC range is one (1) Volt full
scale. Two guidelines are suggested for the internal gain setting:
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This is appropriate when there is a single peak of interest: Set the gain
such that the typical pulse height is between 2 and 10% of the ADC
range.
2.
This is appropriate when looking at a fixed energy range, with no
particular peak of interest: Set the gain such that the maximum energy
pulses are around 200 displayed vertical units in the ADC Trace
readout.
The parameter GAINDAC sets the internal amplifier gain. The overall
gain can be expressed as follows:
Gtot = Gin’ * Gvar * Gbase
where
Gin’: the nominal input stage gain Gin ~ 1, is modified by the resistive divider
created by the output impedance of the preamplifier and the input
impedance of the microDXP to the value Gin’.
Gvar: variable gain setting = 0.36 to 36 depending on GAINDAC setting
Gbase: reference gain ~ 1.87
The total internal gain ranges from 0.67 VADC/VINPUT to 67.3 VADC/VINPUT.
The digital gain control is a 16-bit DAC that sets the gain of a “linear in
dB” variable gain amplifier. The gain setting accuracy is approximately one bit
(or 0.00061 dB = 0.007%). The relationship between Gvar and GAINDAC is:
Gain (in dB) = (GAINDAC/65536) * 40 dB
Gvar = 10**(Gain (in dB)/20)
The output impedance ROUT of the preamplifier creates a resistive
divider with the microDXP input impedance RIN =1.0kΩ and thus affects the
input gain term Gin:
Gin (ROUT)’ = 1.0kΩ /(1.0kΩ + ROUT))
5.11 Standard Program Variants
5.11.1 MCA acquisition with reset-type preamplifiers
Variant 0 is the standard firmware variant supplied with the microDXP,
as described in this manual. It is intended for use with reset-type preamplifiers
(described in §4.1.1). Note: To use this variant, the “Ramp/Offset” switch S1
(see Appendix D) should be in the “Ramp” position.
5.11.2 MCA acquisition with RC-type preamplifiers
This firmware variant is intended for use with resistive feedback
preamplifiers (described in §4.1.2). Additional parameters (described in §4.7):
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TAURC: Exponential decay time in 50 ns units.
RCFCOR: Correction factor (calculated automatically at start of run if
TAURC not 0)
Note: To use this variant, the “Ramp/Offset” switch S1 (see Appendix
D) should be in the “Offset” position.
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Appendices
Appendix A. GLOBSET Specification
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ID
0
Parameter
NUMGLOBSET
Format
16:0
1
GLOBVERSION
16:0
2
POLARITY
1:0
3
RUNTASKS
Bitwise flag
register
4
FIPCONTROL
5
6
7
PRESETLENLO
PRESETLENHI
PRESET
8
9
RESETINT
TAURC
10
IDLEMODE
11
IDLEDELAY
12
13
SLEEPMODE
TRACEWAIT
14
STATSMODE
Description
Number of GLOBSET parameters
NOT including NUMGLOBSET
and GLOBVERSION (used
internally)
Version of the GENSET (used
internally)
Preamplifier signal polarity (0:
negative, 1: positive)
Each bit controls a separate task.
Will be in manual.
Controls various FiPPI operations.
Used for debugging primarily.
Low word of preset run length
High word of preset run length
Preset type (0: no preset, 1: realtime,
2: livetime, 3: output events, 4:
input events)
Reset time in microseconds
Preamplifier RC decay time in
microseconds (unused for reset
preamplifier)
Idle mode, as defined in command
0x46
Delay before entering idle after end
run (in seconds)
Sleep mode, see command 0x47
Number of clock ticks between
successive samples when taking
ADC trace (see command 0x11)
Sets the response to the 0x06 “Read
Run Statistics” command.
STATSMODE=0 (default) yields
the short response (21 bytes).
STATSMODE=1 yields the long
response is (29 bytes, including
UNDERFLOWS and
OVERFLOWS).
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Appendix B. GENSET Version 1 Specification
Five MCA related GENSETs (i.e. five MCA formats) are available.
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ID
0
Parameter
NUMGENSET
Format
16:0
1
GENVERSION
16:0
2
3
MCALEN
MCALIMLO
4
5
MCALIMHI
BASEBINNING
6
BLCUT
7
BINMULTIPLE
8
BINGRANULAR
9
GAINBASE
Description
Number of GENSET parameters
NOT including NUMGENSET and
GENVERSION (used internally)
Version of the GENSET (used
internally)
Number of spectrum bins
Lowest spectrum bin (allows offset
spectrum)
Highest spectrum bin
Number of bins baseline values
combined into one bin in baseline
histogram (power of 2)
Baseline cut value, in percentage of
peak value in baseline histogram.
Standard value is 5%. Expressed as
x * 32768.
MCA bin size in terms of the
minimum.
Bin granularity (as defined in
command 0x83: 0 == very fine, etc)
16-bit GAINDAC base setting.
(Higher value == higher gain). This
value is modified per-peaking time
by the GAINTWEAK table in the
PARSET.
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Appendix C. PARSET Version 1 Specification
These are all the parameters that are stored and retrieved for each
peaking time.
ID
Parameter
Format
Description
0
NUMPARSET
Number of PARSET parameters
NOT including NUMPARSET and
PARVERSION (used internally)
1
PARVERSION
PARSET version number (used
internally)
2
BINSCALEXP
Energy scaling factor; internally
calculated
3
BINSCALE
Energy scaling factor, proportional
to SLOWLEN
4
FIPSCALE
Controls FiPPI energy scaling
(depends on SLOWLEN)
5
BLFILTER
Baseline filter average length =
32768/BLFILTER
6
SLOWLEN
Slow Filter Ramp length
7
SLOWGAP
Slow Filter gap
8
PEAKINT
Minimum spacing between pulses
9
FASTLEN
Fast filter ramp length
10
FASTGAP
Fast filter gap
11
THRESHOLD
Fast filter threshold corresponding
to the current GENSET.
12
MINWIDTH
Minimum time above threshold for
fast filter
13
MAXWIDTH
Maximum time above threshold for
fast filter (detects fast filter pileup)
14
SLOWTHRESH
Energy filter threshold
corresponding to the current
GENSET (only needed for light
elements – normally set to 0 to
disable)
15
BASETHRESH
Intermediate filter threshold
corresponding to the current
GENSET. Needs to be set properly
to get good baselines.
16
BASTHRADJ
Parameter used for future auto
adjustment of BASETHRESH
17
PEAKSAM
Energy Sampling point
18- GAINTWEAK0If non-zero, this per-GENSET
22
GAINTWEAK4
parameter modifies the GAINBASE
setting in the GENSET to arrive at
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2327
2327
2327
THRESHOLD0THRESHOLD4
BASETHRESH0BASETHRESH4
SLOWTHRESH0SLOWTHRESH4
mdo-microDXP-MAN-1.1.2
the GAINDAC setting. E.g. If
GENSET 3 is in use, GAINDAC =
GAINBASE + GAINTWEAK 3.
Per-GENSET fast filter threshold
values.
Per-GENSET baseline filter
threshold values.
Per-GENSET slow filter threshold
values.
Appendix D. MicroDXP Hardware Specification
This section describes the first steps that should be taken to design
hardware for a system incorporating the microDXP. XIA engineers will provide
limited assistance with the actual design, depending on the support agreement.
Board Dimensions and Mounting
The microDXP measures 3.375” x 2.125”, as shown in Figure. C.1,
with 0.120” non-plated mounting holes inset by 0.175” symmetrically with
respect to each of the four corners. These mounting holes are intended for use
with 4-40 or equivalent screws. An alternate board form factor includes 0.1875”
blank PCB rails on the two long sides. The rails were included for systems
where the microDXP board is to be mounted in a slot. The overall dimensions
for the slot-mounted board are thus 3.375” x 2.500”.
Figure C.1: Dimensions and mounting hole locations.
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Preamplifier Type Selector Switch
The only hardware setting on the microDXP board is the preamplifier
type selector switch. The location of the miniature two-position slide switch S1
is displayed in Figure C.2. The two positions are silkscreen-labeled RAMP and
OFFSET, for reset-type and RC-feedback preamplifiers, respectively
Connector Locations and Pinouts
Two connectors carry all electronic signals to and from the microDXP
standard assembly, as depicted in Figure C.2. A 2-conductor, 1.25mm pitch
connector carries the analog signal from the preamp. The mating connector is a
crimp-type socket that accommodates 26-30AWG stranded wire.
A single 30-conductor, 0.5mm pitch flat-flex interconnect carries all
communications, power and auxiliary I/O to and from the microDXP. The flex
cable provides for two dimensions of freedom, but does require alignment along
the axis that bisects all of the contacts. The most likely error would be
misalignment of this interconnect, or a reversal of the pinout. Table C.1 details
the pin assignments of the flex interconnect.
Figure C.2: Connector locations and part numbers (top side).
Figure C.3: Connector locations and part numbers (bottom side).
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J1 - Analog Input: 2-pin compact right-angle header; or thru-hole LEMO (Note 1).
Hirose P/N: DF13-2P-1.25H (mating P/N: DF13-2S-1.25C; crimp contact P/N: DF13-2630SCFR)
Pin Name
Description
#
1
SIGNAL
Preamplifier output signal
2
GND
Internal ground connection
Note 1: Customers wishing to use an SMA input cable are advised that this capability is provided by means of the
MicroComU companion board. See the separate MicroComU Technical Reference Manual. Also contact XIA in
order to get quotes for the appropriate variant of the microDXP board.
J12 – Flex Cable Port: 30-conductor, 0.5mm locking flex-cable connector; carries power,
communications and auxiliary digital I/O
Hirose P/N: FH12-30S-0.5SH (e.g. flat-flex cable, Parlex P/N: 0.5MM-30-x-B)
Pin Name
Description
#
1
+AVDD
Positive DC supply voltage for analog signal conditioner:
Regulated +5.0V; or unregulated +5.5V if on-board regulator present.
2
-AVSS
Negative DC supply voltage for analog signal conditioner:
Regulated -5.0V; or unregulated -5.5V if on-board regulator present.
3
GND
Internal ground connection
4
+3.3VCC
+3.3V DC supply for on-board digital components.
5
+3.3VCC
+3.3V DC supply for on-board digital components.
6
GND
Internal ground connection
7
SDA
I2C data line
8
SCL
I2C clock
9
ExtInt*
External interrupt line, active low.
10 Gate*
Inhibits data acquisition, active low.
11 GND
Internal ground connection
12 RX
RS-232 microDXP receive (host→microDXP)
13 TX
RS-232 microDXP transmit (microDXP→host)
14 GND
Internal ground connection
15 Vprog
PIC programming voltage
16 ProgData
PIC programming data line
17 ProgClk
PIC programming clock
18 Aux0
Auxiliary configurable digital I/O line: connects to FiPPI
19 Aux1
Auxiliary configurable digital I/O line: connects to FiPPI
20 GND
Internal ground connection
21 Aux2
Auxiliary configurable digital I/O line: connects to FiPPI
22 Aux3
Auxiliary configurable digital I/O line: connects to FiPPI
23 +3.3VCC
+3.3V DC supply for on-board digital components.
24 SPORT_CLK
DSP serial port clock line (ADSP218x SPORT)
25 GND
Internal ground connection
26 SPORT_TDATA
DSP serial port transmit data line (ADSP218x SPORT)
27 SPORT_TFS
DSP serial port transmit frame sync line (ADSP218x SPORT)
28 GND
Internal ground connection
29 SPORT_RDATA DSP serial port receive data line (ADSP218x SPORT)
30 SPORT_RFS
DSP serial port receive frame sync line (ADSP218x SPORT)
Table D.1: Pin assignments for the 30-conductor flat-flex interconnect.
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J11 – Board-to-Board Port: 50-conductor, 0.5mm mezzanine board-to-board receptacle; carries power,
communications and auxiliary digital I/O
Hirose P/N: DF12-50DS-0.5V (microCOM / MicroComU mating header P/N: DF12(5.0)-50DP-0.5V)
Pin # Name
Description
Odd-numbered pins (top to bottom along the right-side of the connector as shown in Figure C.3)
+3.3VCC
+3.3V DC supply for on-board digital components.
1
3
+3.3VCC
+3.3V DC supply for on-board digital components.
5
+3.3VCC
+3.3V DC supply for on-board digital components.
7
GND
Internal ground connection
9
EAD15
IDMA data/address I/O line (MSB)
11
EAD14
IDMA data/address I/O line
13
EAD13
IDMA data/address I/O line
15
EAD12
IDMA data/address I/O line
17
EAD11
IDMA data/address I/O line
19
EAD10
IDMA data/address I/O line
21
EAD9
IDMA data/address I/O line
23
EAD8
IDMA data/address I/O line
25
EAD7
IDMA data/address I/O line
27
EAD6
IDMA data/address I/O line
29
EAD5
IDMA data/address I/O line
31
EAD4
IDMA data/address I/O line
33
EAD3
IDMA data/address I/O line
35
EAD2
IDMA data/address I/O line
37
EAD1
IDMA data/address I/O line
39
EAD0
IDMA data/address I/O line (LSB)
41
GND
Internal ground connection
43
EWR*
IDMA write strobe (Active LO)
45
ESel*
IDMA device select INPUT (must be asserted LO for IDMA use)
47
ERdy*
IDMA data ready (Active LO) OUTPUT
ERD*
IDMA read strobe (Active LO)
49
Even-numbered pins (top to bottom along the left-side of the connector as shown in Figure C.3)
+AVDD
Positive DC supply voltage for analog signal conditioner:
2
Regulated +5.0V; or unregulated +5.5V if on-board regulator present.
4
-AVSS
Negative DC supply voltage for analog signal conditioner:
Regulated -5.0V; or unregulated -5.5V if on-board regulator present.
6
+3.3VCC
+3.3V DC supply for on-board digital components.
8
GND
Internal ground connection
10
SPORT_RFS
DSP serial port receive frame sync line (ADSP218x SPORT)
12
SPORT_RDATA
DSP serial port receive data line (ADSP218x SPORT)
14
GND
Internal ground connection
16
SPORT_TFS
DSP serial port transmit frame sync line (ADSP218x SPORT)
18
SPORT_TDATA
DSP serial port transmit data line (ADSP218x SPORT)
20
GND
Internal ground connection
22
SPORT_CLK
DSP serial port clock line (ADSP218x SPORT)
24
GND
Internal ground connection
26
Aux3
Auxiliary configurable digital I/O line: connects to FiPPI
28
Aux2
Auxiliary configurable digital I/O line: connects to FiPPI
30
Aux1
Auxiliary configurable digital I/O line: connects to FiPPI
32
Aux0
Auxiliary configurable digital I/O line: connects to FiPPI
34
Gate*
Inhibits data acquisition, active low.
36
SCL
I2C clock
38
SDA
I2C data line
40
GND
Internal ground connection
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42
44
46
48
50
RX (B)
TX (B)
GND
EA/D*
ExtInt*
mdo-microDXP-MAN-1.1.2
RS-232 microDXP receive (host→microDXP)
RS-232 microDXP transmit (microDXP→host)
Internal ground connection
IDMA address (HI) / data (LO) selector INPUT
External interrupt line, active low.
Table D.2: Pin assignments for the 50-conductor board-to-board interconnect.
Power Supplies
Note: Excessive ripple on
the analog supplies
(>20mVpp) can seriously
degrade system
performance. If +/-5.0V is
supplied directly, either
linear regulated or highquality switching supplies
should be used.
8/6/2009
The microDXP requires three supply voltages to operate. A supply
voltage of +3.3V is used to directly power most on-board digital circuitry, with
minimal LC filtering at the board entry point. On-board voltage regulators also
generate from this supply +3.0V for the ADC and +2.5V for the DSP. The total
current requirement depends on the selected clock speed, ranging from 80mA to
130mA. The ripple requirements for this supply are not particularly stringent,
though excessive radiated noise is to be avoided. If a switching supply is used,
it should be well shielded from the microDXP.
Supply voltages of +/-5.5V are regulated on-board by default to
generate +/-5.0V to power the analog components. The microDXP is thus
intended to tolerate some conducted EMI (<100mV pk-pk) from switching
supplies. The +/-5V analog regulators can be bypassed, and thus a slight
reduction in power achieved, if low-noise (ie. linear or carefully designed
switching) supplies are used. If the regulators are bypassed, only minimal LC
filtering will be applied at the board entry point.
Current draw on the analog supplies is dominated by the optional
variable-gain amplifier, which alone draws 12.5mA. The remaining analog
circuitry draws roughly 10mA. The total required current is, conservatively,
15mA without variable-gain; 30mA with variable gain.
XIA has recently introduced the MicroComU companion board to
complement the microDXP. The MicroComU will generate all voltages
required by the microDXP, at the required currents and noise performance. The
MicroComU/microDXP board set may be powered off the USB bus or off a
single external AC power adapter. Customers wishing not to have to worry
about power supply design are advised to purchase MicroComU companion
boards.
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Regulated Supply Option: (<20mV pk-pk noise)
Voltage Range
Current (min) Current (max)
+3.3V +/- 150mV
100mA
130mA
+5.0V +/- 100mV
25mA
30mA
-5.0V +/- 100mV
25mA
30mA
Description
Decent switching
supply
Linear or highquality switching
Linear or highquality switching
Unregulated Supply Option (<100mV pk-pk noise)
Voltage Range
Current (min) Current (max) Description
+3.3V +/- 150mV
100mA
130mA
Decent switching
supply
+5.5V to +6.0V
25mA
35mA
Decent switching
supply
-5.5V to –6.0V
25mA
35mA
Decent switching
supply
Table D.3: Power supply specifications for the microDXP.
Clock Speed
[MHz]
8
Voltage
Supply
VCC
Current
[mA]
89.7
Power
[mW]
296.0
8
V+
20.3
101.5
8
V-
23.3
116.5
514 mW
16
VCC
103.0
339.9
16
V+
20.3
101.5
16
V-
23.3
116.5
557.9 mW
Comment
+3.3V digital – includes
ADC
+5V analog – includes
VGA*
-5V analog– includes
VGA*
Total power
consumption at 8MHz
+3.3V digital – includes
ADC
+5V analog – includes
VGA*
-5V analog– includes
VGA*
Total power
consumption at 16MHz
Table D.4: Power consumption by pipeline clock speed.
*The (optional) variable gain stage draws approximated 12.5mA.
8/6/2009
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Appendix E. RS-232 Communications
This appendix describes the basic microDXP RS-232 command and
response protocol. Please refer to the RS-232 Command Specification (a
separate document) for a detailed presentation of all RS-232 commands. DXPrelated documents are available online at:
www.xia.com/DXP_Resources.html.
The general structure for commands and responses is as follows:
[Esc][Command][Ndata (2 bytes)][data1]…[dataN][xor CS]
where:
[Esc]
[Command]
[Ndata]
[data1]...[dataN]
[xor CS]
Escape (ASCII 0x1B) as a command start byte
Single byte for command number, allowing up to
255 commands. See the tables below for command
definitions.
Number of data bytes to follow. Two bytes, low
byte first.
The data bytes.
Exclusive-or checksum (bitwise xor of all bytes
except for the initial [Esc]). If the checksum is
not correct an error response is returned.
The format of responses echo the format of commands; that is, they
start with the [Esc]character and pass back the command # to which it is
responding, followed by appropriate data and checksum. The first data byte of
all responses is the return status, which is zero for a successful command. In
case of an error, only the error byte is returned – no other data bytes are sent.
The command for starting a run is given below as an example:
Command:
0x1B
0x00
0x01
0x00
0x01
0x00
(the escape character)
(the command)
(the low byte that sets the number of data bytes )
(the high byte that sets the number of data bytes)
(the data byte: new run, clear the data)
(bitwise XOR—excludes escape character)
Response (if successful):
0x1B
0x00
0x03
0x00
0x00
0x0B
0x10
0x18
8/6/2009
(the escape character)
(the command is always returned)
(the low byte that sets the number of data bytes)
(the high byte that sets the number of data bytes)
(status is ok)
(the low byte of the new RUNID=0x1B=27)
(the high byte of the new RUNID=0x1B=27)
(bitwise XOR—excludes escape character)
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Response (if unsuccessful):
0x1B
0x00
0x01
0x00
0x01
0x18
8/6/2009
(the escape character)
(the command is always returned)
(the low byte that sets the number of data bytes)
(the high byte that sets the number of data bytes)
(status indicates an error)
(bitwise XOR—excludes escape character)
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