Texas Instruments | AN-1810 LMX9830 Design Checklist (Rev. A) | Application notes | Texas Instruments AN-1810 LMX9830 Design Checklist (Rev. A) Application notes

Texas Instruments AN-1810 LMX9830 Design Checklist (Rev. A) Application notes
Application Report
SNOA518A – March 2008 – Revised March 2008
AN-1810 LMX9830 Design Checklist
.....................................................................................................................................................
ABSTRACT
This application note provides guidelines for designing with the LMX9830 Bluetooth® serial port module,
particularly for critical aspects of the PCB layout. The guidelines have been organized in a simple flow that
walks you through the first steps on what documentation to review, key design considerations and then
final steps of certification and production. Figure 1 summarizes the product design flow.
1
2
3
4
5
6
Contents
Scope ......................................................................................................................... 2
Introduction .................................................................................................................. 2
Product Design Flow ....................................................................................................... 3
3.1
DOCUMENTATION ................................................................................................ 3
3.2
KEY DESIGN CONSIDERATIONS .............................................................................. 3
3.3
CERTIFICATION AND REGISTRATION ....................................................................... 3
Design Flow Details ......................................................................................................... 3
4.1
DOCUMENTATION STRUCTURE .............................................................................. 3
4.2
PCB DESIGN ....................................................................................................... 4
Certification and Production ............................................................................................... 6
5.1
EXPECTED RESULTS ............................................................................................ 6
5.2
FIRST PROTOTYPE CHECKLIST .............................................................................. 9
5.3
MOST COMMON DESIGN ERRORS ........................................................................... 9
5.4
PRODUCT CERTIFICATION ..................................................................................... 9
References ................................................................................................................. 10
List of Figures
1
2
3
4
5
6
7
8
.......................................................................................................
Example Schematic with Critical Areas Indicated ......................................................................
Example PCB Layout with Critical Components Indicated ............................................................
Example Ground Layer Design with Critical Features Indicated......................................................
Deviation Due to Modulation ..............................................................................................
Eye Diagram .................................................................................................................
BER vs. Input Power Level (dBm) ........................................................................................
Transmit Spectrum ..........................................................................................................
Product Design Flow
2
4
5
5
7
7
8
8
List of Tables
1
Critical Areas
................................................................................................................
6
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Scope
1
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Scope
This application note provides guidelines for designing with the LMX9830 Bluetooth® serial port module,
particularly for critical aspects of the PCB layout. The guidelines have been organized in a simple flow that
walks you through the first steps on what documentation to review, key design considerations and then
final steps of certification and production. Figure 1 summarizes the product design flow.
Figure 1. Product Design Flow
2
Introduction
The National Semiconductor LMX9830 Bluetooth Serial Port module is a highly integrated Bluetooth 2.0
baseband controller and 2.4 GHz radio, combined to form a complete small form factor (6.1 mm × 9.1 mm
× 1.2 mm) Bluetooth node. All hardware and firmware is included to provide a complete solution from
antenna through the complete lower and upper layers of the Bluetooth stack, up to the application
including the Generic Access Profile (GAP), the Service Discovery Application Profile (SDAP), and the
Serial Port Profile (SPP). The module includes a configurable service database to fulfill service requests
for additional profiles on the host. The LMX9830 is pre-qualified as a Bluetooth Integrated Component.
Conformance testing through the Bluetooth qualification program enables a short time to market after
system integration by insuring a high probability of compliance and interoperability. Based on National’s
CompactRISC® 16-bit processor architecture and Digital Smart Radio technology, the LMX9830 is
2
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Product Design Flow
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optimized to handle the data and link management processing requirements of a Bluetooth node. The
firmware supplied in the on-chip ROM offers a complete Bluetooth (v2.0) stack including profiles and
command interface. This firmware features point-to-point and point-to-multipoint link management and
supports data rates up to the theoretical maximum over RFCOMM of 704 kbps (Best in Class in the
industry). The internal memory supports up to 7 active Bluetooth data links and one active SCO link.
3
Product Design Flow
3.1
DOCUMENTATION
•
•
•
•
•
•
3.2
KEY DESIGN CONSIDERATIONS
•
•
•
•
•
•
•
3.3
LMX9830 Data Sheet [1]
LMX9830 Dongle Reference Design and Layout [2] [7]
Software User’s Guide [3]
Bluetooth Antenna Design Application Note [4]
IEEE Bluetooth Device Address Documentation [5]
Bluetooth Certification Guide [6]
Power Supply Decoupling—refer to the data sheet [1] and reference design schematics and layout [2].
Antenna—refer to the antenna design application note [4].
Crystal—refer to the data sheet [1].
Power-On Reset (POR)—refer to the data sheet [1].
Loop Filter Tuning - refer to the datasheet [1] and LMX9830 Dongle reference design documents [7].
EEPROM—refer to the data sheet for the NVS contents [1].
Low-Power Mode—refer to the data sheet [1].
CERTIFICATION AND REGISTRATION
•
•
Bluetooth Certification Guide [6]
Bluetooth Device Address Management—refer to the IEEE Bluetooth device address application note
[5]
4
Design Flow Details
4.1
DOCUMENTATION STRUCTURE
4.1.1
•
•
•
•
Design Documents
LMX9830 Data Sheet—contains key performance details regarding the device, pin description,
functional details, programming details, package information, and application information.
LMX9830 Dongle Reference Design and Layout—a design example with circuit schematics and PCB
layout. It is strongly recommended to use the reference design documentation to complete the layout
for the LMX9830. Reference design files (Gerber) and Bill of Materials are all provided on the
reference design CD. These files can easily be imported into schematic capture/layout design
packages such as Orcad and PADS.
Software User’s Guide—reference for implementing the LMX9830 module into a system. A getting
started session gives a very detailed entry point for starting software development. The Advanced
Usage section describes all features and configuration parameters in detail and gives examples for
using the LMX9830 as an active Bluetooth node. Finally all commands and events are listed and
explained in the command section.
Bluetooth Antenna Design Application Note—contains detailed information about the design of the
Bluetooth antenna including matching to the module and a list of antenna component suppliers.
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Design Flow Details
4.1.2
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Certification and Production Documents
Bluetooth Certification Guide—best practices and details for obtaining Bluetooth qualification and also
regulatory certification such as FCC or CE that is required for all designs before entering the
production stage.
• IEEE Bluetooth Device Address Documentation—information on obtaining a unique Bluetooth device
address from the IEEE, including the procedure and costs.
•
4.2
PCB DESIGN
The following section discusses the layout requirements for the PCB. Figure 2, Figure 3 and Figure 4 are
taken from the LMX9830 UART buffer Dongle reference design. Capital letters mark critical areas which
are discussed further in Section 4.2.4.
4.2.1
Schematic
Figure 2. Example Schematic with Critical Areas Indicated
4
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4.2.2
Layout
Figure 3. Example PCB Layout with Critical Components Indicated
4.2.3
Grounding
Figure 4. Example Ground Layer Design with Critical Features Indicated
4.2.4
List of Critical Components and Features
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Table 1. Critical Areas
Critical Area
4.2.5
•
•
•
•
•
•
Description
A
Loop filter between PLL_OUT and V_tune. This is the external 3rd order loop filter for the LMX9830 PLL. Its
design is key to the overall performance of the device. Refer to the LMX9830 data sheet [1]. This is the most
critical part of the design. It is important to match layout and components for this circuit to the reference design
documents. Further tuning can be done to ensure that performance is optimized.
B
Crystal oscillator. This forms part of the clock for the radio and baseband. Because this is a large source of
noise, a crystal with jitter less than the specified datasheet limit and <20 ppm tolerance is required. Refer to
LMX9830 data sheet [1].
C
Load capacitors for the crystal. These fine-tune the frequency of the oscillator to make the transmitted frequency
error <75 kHz, in order to comply with the Bluetooth specification. High tolerance capacitors(<5%) are
recommended to achieve good part-to-part stability.
D
Antenna matching network and blocking capacitor. The antenna can be a source of ESD and therefore the
module should be protected. A blocking capacitor between the module and the antenna prevents any DC voltages
on the antenna being applied directly to the module, thus preventing ESD damage. The blocking capacitor can be
placed by itself or, when necessary, be part of a matching network to improve power transfer to the antenna. If
used by itself then typically 4pF to 5 pF should be used for lossless transmission at 2.4 GHz. Refer to Bluetooth
Antenna Design Application Note [4].
E
Antenna. Several designs and types are available. Figure 3 shows the placement of a chip ceramic antenna. A
cheaper but larger solution is to use a Printed Inverted-F Antenna (PIFA) which is formed using micro-strip traces,
making it very inexpensive to manufacture. Refer to the Bluetooth Antenna Design Application Note [4].
F
Decoupling capacitors on power supply pins. Capacitor values should be calculated based on the magnitude
of the voltage ripple and the frequencies present. Single, double, or triple arrangements may be used. These
capacitors must be placed as close to the module power supply pins as possible for maximum effect.
G
Component to surface ground layer. A component which is to be grounded should not be soldered directly to a
ground plane because the large heat dissipation will give an unsatisfactory solder joint. Therefore a component
pad should be used and two short tracks used to minimize inductance to ground.
H
Track length from a pin to grounded via. A pin which is connected to a ground via should use a short track to
minimize inductance.
Other Considerations
Ideally, a decoupling capacitor pair should be used per VCC pin of the device, though single capacitors
may be used in some cases without degrading performance significantly.
Shielding around the Bluetooth area may be needed when in very close proximity to an interference
source along with a blocking filter on the antenna.
One solid ground plane should be used for digital and RF.
A 50-ohm trace is needed for RF input/output. Programs such as AppCad or LineCalc can assist in the
determination of the required width of the trace.
Do not route digital signals under the radio, because this causes cross-talk and degrades performance,
especially for out-of-band blocking.
A 32.768-kHz oscillator may be used to support lower power mode. The modular structure of the
LMX9830 allows firmware to shut-down unused modules such as the radio LLC and UART interface.
Refer to the data sheet [1] for further information.
5
Certification and Production
5.1
EXPECTED RESULTS
Once the design is completed and boards have been assembled, testing can start. Bluetooth test sets are
typically used to evaluate performance, with several suitable models being available. The following plots
were collected using an Anritsu 8852A. Additional plots below are collected with standard spectrum
analyzers.
Figure 5 shows the frequency deviation due to modulation, which is measured during Bluetooth
qualification. The payload deviation is used to assess whether the device meets the specification.
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Figure 5. Deviation Due to Modulation
Figure 6 is an eye diagram, which shows three features of the transmitter:
• The resolution or clarity of the diagram itself is proportional to the amount of noise present within the
transmit spectrum. A noisy spectrum will have a much smaller eye opening.
• The center crossing point shows the frequency error.
• The opening of the eye is the payload deviation.
Figure 6. Eye Diagram
The Frame Error Rate (FER) and the Bit Error Rate (BER) are a measure of receiver quality. Figure 7
shows typical BER performance. The Sensitivity Specification states the at -70dBm the BER should be
better than 0.1%. The BER and FER should both be close to 0% when signal level is >-70 dBm. The BER
should only exceed 0.1% at <-80 dBm.
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Figure 7. BER vs. Input Power Level (dBm)
The modulation spectrum in Figure 8 shows the 20 dBc Bandwidth is less than 1000 kHz, as required by
the Bluetooth specification. The limit prevents interference with neighboring channels.
Figure 8. Transmit Spectrum
8
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5.2
FIRST PROTOTYPE CHECKLIST
If the expected results cannot be obtained from the first prototypes, the following points should be
checked:
• Test at least 3 to 5 boards to make sure the failure is consistent with the design and not boarddependent.
• Inspect the boards under a PCB microscope, checking for dry or short-circuited joints.
• Check power supply, digital I/O, and radio are within the data sheet specified range.
• X-ray can be used to inspect the quality of soldering under the module. Too little solder will form dry
gaps, excess will lead to overspill and shorts between pins.
• Check that the crystal is turning on during power-up using a high-impedance probe. Check that the
frequency is ±240 Hz of the desired frequency. The crystal can be tuned using internal registers or the
load capacitors.
• Check the power-supply noise ripple on the VCC pins is within 15 mVp-p. A large capacitor on the
output of the LDO will smooth low-frequency ripple.
• Loop filter tweaks may be required if performance measurements are generally poor. Refer to the data
sheet for details.
• Antenna matching may be required to improve radiated sensitivity and power, therefore range. Refer to
the Bluetooth antenna design application note.
• A front-end filter (either LC or ceramic) may be required for additional blocking margin. Refer to the
Bluetooth antenna design application note.
5.3
MOST COMMON DESIGN ERRORS
These are some of the more common errors when designing with the LMX9830.
5.3.1
Non-Compact Design
When there is more than enough space on the PCB which must be of a certain shape and size to conform
to the application, some designers tend to spread the external components over a larger area than is
required. This is a mistake. Though there may be space available, the external components such as loop
filter, decoupling capacitors, crystal, etc. must be placed as close to the LMX9830 as possible, otherwise
degraded performance due to unnecessary line parasitics will result.
5.3.2
Poor Grounding
Not using enough ground vias or ground lines that are too thin and long is a common error. Long tracks
are inductive, and high-frequency currents do not get down to the ground plane fast enough, resulting in
cross-coupling and spurious emissions.
5.3.3
No Antenna-Matching Network or External Filter
Though a purchased antenna may work well on its test PCB, it may not have the same performance on
the final design PCB. A common mistake is to forget the matching network or room for filtering that will
add versatility to the design and allow for antenna tuning and better blocking performance.
5.3.4
Badly Tuned Crystal
Like the antenna, the crystal oscillator’s frequency is subject to pull when placed on the final application
PCB due to parasitic stray capacitance. Not tuning the crystal on the final design is therefore a common
cause of initial poor performance.
5.4
PRODUCT CERTIFICATION
Refer to the Bluetooth certification guide which describes the steps and preparation needed to get full
regulatory and Bluetooth certification for a new product, including lists of required tests and detailed
procedures. The key steps required to achieve certification, as described in the certification guide, are:
1. Become an Adopter or Member of the Bluetooth SIG.
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References
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2. Review the design guide, data sheet and complete documentation package for a selected National
Semiconductor product.
3. Design the schematic and component layout. The critical grounding and decoupling required for the
radio must be studied in detail.
4. Generate the schematic and layout Gerber files for the final application.
5. Compare schematics and layout files to details in datasheet, reference designs and application notes.
6. Implement the design changes as agreed in the review.
7. Production of first prototypes (between 10 and 20 units).
8. At least five units must be tested fully over the temperature range.
9. Any failure or marginal pass of the specification must be corrected by component change or layout
modification.
10. Test the DUTs again to verify that all parameters are within specification.
11. Prepare three “golden units” for the BQTF, two with antenna connectors and one with an original
antenna.
12. Select the BQTF and prepare documentation for DUTs.
13. Submit the documentation and DUTs to the BQTF for regulatory and Bluetooth qualification tests.
14. Product will be listed on the official Bluetooth web page as Bluetooth compliant when all test cases
have passed.
15. Release product to manufacturing.
6
References
Ref.
Title
Type
File Name
1
LMX9830 Bluetooth Serial Port Module
Data Sheet
LMX9830DS.pdf
2
LMX9830 Dongle Hardware User Guide
User Guide
LMX9830DONGLE Hardware User Guide.pdf
3
LMX9830 Software Users Guide
Users Guide
LMX9830_SW_UG.pdf
4
Bluetooth Antenna Design
Application Note
Bluetooth Antenna Design.pdf
5
Bluetooth Device Address for the LMX9830
Application Note
BD_ADDR AN.pdf
6
Bluetooth Certification Guide
Application Note
BT Cert Guide Complete.pdf
LMX9830Dongle without UART Buffer.DSN
7
LMX9830 Dongle Reference Design
Design Files
LMX9830DONGLE PADS file.pcb
LMX9830DONGLE_BOM_without UART
Buffer.xls
NOTE: Dongle without UART buffer is a more accurate form factor design. UART buffer is used for
5V interface to PC for testing only. UART buffers is not needed for 3V designs.
10
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