Probing High-speed Signals with the Agilent

Probing High-speed Signals
with the Agilent 86100 Series
of Wide-bandwidth
Sampling Oscilloscopes
Product Note 86100-6
The high bandwidth and low noise of equivalent-time
sampling oscilloscopes provide precise displays of
high-speed signals. Historically, if the signal could
not be routed to the oscilloscope through a high-quality
cable, thus requiring the use of a probe, accurate
measurements became difficult to achieve. Three
important measurement accessories help make probe
based measurements both simple and accurate for
the Agilent 86100 Wide Bandwidth Oscilloscope:
• Agilent 113X, 115X, and 116X series of high
performance probes with up to 13 GHz of bandwidth
• Agilent N1022A probe adapter
• Agilent 83496A/B clock recovery module
Probes
Agilent offers a wide variety of probes with the bandwidth
and connectivity required for high-speed waveform
analysis. These include differential and single-ended
probes with solder in, socketed, SMA, and browsing
probe heads offering variable spacing probe tips for
today’s high-density IC’s and circuit boards. Typical
bandwidths are available from 1.5 to 13 GHz (9/05).
All probes have a flat frequency response over the
entire probe bandwidth to eliminate distortion and
frequency-dependent loading effects.
Connecting the probe to the oscilloscope
The precision 3.5 mm connector used for the electrical
channels of several 86100 plug-in modules is not directly
compatible with the standard probe interface of Agilent
real-time sampling oscilloscopes. This problem is solved
with the N1022A Probe Adapter. The N1022A Probe
Adapter provides power, calibration, and a high-integrity
signal path between the probe and the 861XX series
plug-in module. The interface of the probe amplifier
section attaches to one side of the adapter. The other
side of the probe adapter is an instrumentation grade
3.5 mm connector that attaches to the bulkhead male
3.5 mm connector of the electrical channel of the
oscilloscope plug-in module. For power and identification,
an interface cable from the probe adapter attaches to
the probe power port of the oscilloscope plug-in.
Figure 1: infiniimax probes
N1022A
Probe amplifier
Figure 2: Connecting a probe through the N1022A adapter
2
The N1022A is directly compatible with the following
modules:
86101A
86103A and B
86106A
83481A
83485A
83487A
54753A
86102A and U
86105A
86112A
83483A
83486A
54751A
54754A
The 86100 oscilloscope mainframe has built-in calibration
routines (except as noted above) to compensate for probe
attenuation and offset to allow a direct display of the
actual signal levels found at the probe tip. Very high
frequency and small pitch probes such as the N5381A
12 GHz solder-in probe head are difficult to connect to
the 86100 BNC calibration connector. The E2655B Deskew
and Performance Verification fixture is recommended.
The following plug-in modules have 2.4 mm compatible
connectors. The 85130-60010 2.4 mm (f) to 3.5 mm
bulkhead adapter is required in addition to the N1022A:
86106A
86109A
86116A
86117A
83484A
86106B
86109B
86116B
86118A
(For non-oscilloscope applications, a type ‘N’ (m) to
3.5 mm bulkhead adapter (N1022-60014) is available.
There is also a 3.5 mm (f) to 3.5 mm bulkhead adapter
(85052-60034)).
The following plug-in modules do not have built-in probe
power supplies. The Agilent 1143 power supply is used
to provide probe power through the N1022A. (In this
configuration an automatic gain and offset calibration is
not available. A manual gain calibration can be performed
to account for probe attenuation and signal splitting).
The 01143-61602 probe power extension cable may be
useful to allow the 1143 power supply to be located over
1 meter away from the oscilloscope or other instrument.
Figure 3: The E2655B Deskew and Performance Verification fixture
allows for easy probe calibration with the 86100 DCA
86105B
86105C
86116B
86117A
86118A
3
Triggering the oscilloscope with a clock
extracted from the probed signal
Sampling oscilloscopes are different from real-time
oscilloscopes in that they require a triggering signal other
than the test signal itself. The triggering signal is often
a clock that is synchronous to the signal under test. In
a probing scenario, a separate system clock for triggering
may not be present. When the necessary synchronous
‘trigger’ is not available, one solution is to derive a clock
from the data being measured. This process is performed
with the 83496A/B clock recovery module.
standard/data rate is being tested. Loop bandwidth will
control what spectrum of jitter is observed and what is
tracked out from eye-mask and jitter tests. For example,
low frequency jitter can be removed, which is usually of
low importance since system receivers easily tolerate it.
Testing with an optimal loop bandwidth assures that good
parts do not appear to be bad, and bad parts do not
appear to be good. For more information on the use of
clock recovery loop bandwidth, refer to product note
86100-5 “Triggering Wide-bandwidth Sampling
Oscilloscopes for Accurate Displays of High-speed Digital
Communications Waveforms”.
The following configurations for the 83496A/B are available:
The N1022A and appropriate probe are connected to
the 83496A/B input port where the signal is evenly split
within the module (see Figure 4). Half is used for clock
extraction; the other half is routed to the 83496A/B front
panel and then connected to the input channel of the
adjacent plug-in module. The 83496A/B will derive a clock
from the data. The recovered clock signal rate is divided
by eight and routed internally in the 86100 mainframe.
The full rate clock or user selected rate divided clock is
available at the 83496A/B front panel and can be useful as
a trigger for eye diagram analysis when data pattern
lengths are multiples of two. (A divided trigger with
an even divisor yields an incomplete eye, see Product
Note 86100-5). Above 7.1 Gb/s, the front panel recovered
clock has a minimum divide ratio of two. The 83496A/B
option 100 requires at least 150 mVpp at its input port
to accurately perform clock extraction. If a 10:1 probe is
used (such as the 1134A), the signal level at the probe
tip must be greater than 1.5 Vpp. If a 3.45:1 probe is used
(such as the 1169A), the signal level at the probe tip must
be greater than 500 mVpp. Note however, that a proper
probe calibration will compensate for both the probe
attenuation and signal splitting within the 83496A/B
and provide an accurate display of signal levels on the
oscilloscope screen. (83496A/B option 101, without
internal splitters, requires half the input signal of
83496A/B option 100 for clock extraction. Any signal
splitting outside the module must be considered when
determining system limits.)
The 83496A/B clock recovery module can derive a clock
from NRZ signals with rates as low as 50 MB/s, as high as
13.5 Gb/s, and any rate between, providing the ultimate
in flexibility and value. (As of 12/06 the data rate must
be entered into the 83496A/B to allow it to properly
synchronize to the signal being probed). As low as
300 femtoseconds rms, the residual jitter of the output
clock is virtually negligible, allowing accurate measurements
of very low levels of signal jitter. The 83496A/B can be
configured with a tunable loop bandwidth. This critical
feature allows the module to be operated as a “golden
PLL” with the optimal loop bandwidth for whatever
4
Option 100: Electrical differential or single-ended
clock recovery 50 Mb/s to 7.1 Gb/s. The input signal
is internally split and ~50% routed back out to the
measurement channel of the adjacent plug-in module.
When measuring differential signals, the most
convenient technique is to use a differential probe
tip, rather than provide two input signals to the
83496A/B. The probe provides a single-ended signal
to the 83496/861XX channel representative of the
difference between the signals at the two probe tips.
The 83496A/B Option 101 electrical/optical clock
recovery module is not recommended for probing
as it does not have an internal signal divider.
It can be used, but requires an external power
divider that precludes the installation of the
N1022A on the front of the clock recovery
module. A complicated adapter scheme is
required. See below.
Special Option 101-H05: This special option
version of the electrical/optical clock recovery
module integrates the signal splitting within the
module. Use for electrical signals is similar to
Option 100. No special adapters other than the
N1022A are required.
Option 200: Increase operating range to 50 Mb/s
to 13.5 Gb/s. Available for either Option 100
or Option 101 configurations. When measuring
clocks, rates from 25 MHz to 7.75 GHz are allowed.
Option 300: Add golden PLL (tunable loop bandwidth
capability). Loop bandwidth is tunable from 30 KHz
to over 10 MHz. (Without Option 300, the loop
bandwidth can be configured at two discrete settings,
dependent upon data rate).
Option 200 and 300 can also be added at a later date.
The module must be returned to an Agilent service center
for the upgrade.
Probe calibration procedures
The 86100 DCA has a built-in calibration process to
compensate for probe attenuation and offsets. The probe
calibration is achieved by attaching the probe tip to a
known DC signal that is available from the oscilloscope
mainframe. The process is Calibration/All calibrations/CH
‘X’ Probe compensation (for the channel the 83496A/B
output is routed to.).
For probe compensation with plug-in modules that do not
include a probe supply outlet (86105B, 86105C, 86117A,
86118A) the calibration procedure is similar. However,
if there is a voltage offset, it must be compensated for
manually (Setup/Channels/Channel X/Advanced with
the attenuation factor adjusted to account for signal loss
in splitting and/or probe voltage division).
861XX
83496A
Data in
Data in
Data out
Data out
N1022A
Probe
Figure 4: Connector scheme using the 83496A/B and N1022A
5
Probing configuration when using the
optical/electrical configuration of the
83496A/B (Option 101)
The primary usage of the 83496A/B Option 101 is for
extracting clocks from optical signals. However, electrical
inputs are provided to derive a clock from an electrical
signal. Unlike 83496A/B Option 100 or special Option
101-H05, there are no internal dividers to tap the
electrical signal and pass it to a measurement channel.
If an electrical signal is to provide a trigger and be
simultaneously observed, it must be divided before
entering the 83496A/B.
In a probing scenario, the chain of adapters required to
connect a probe and probe adapter becomes complex as
shown in the following sketch. Parts needed are:
• N1022A, probe adapter
• N1022-60014, adapter Type-N male to 3.5 NMD male
• 11636A, DC-18 GHz power divider, Type-N, 50 ohm
• 85054-60030, adapter: 3.5 mm female, Type-N male
• 1250-1745, adapter: 3.5 mm female, Type-N female
• 01143-61603, probe power extension cable
• 1143A probe power supply (optional)
Probe power supply
InfiniMax
probe amplifier
Probe power
extension cable
Differential browser
probe head
Probe adapter
N1022A
Male-to-male
adapter (N to 3.5 NMD)
N1022A-60014
11636A
1250-1745
Adapting and splitting signals for probing and clock extraction with the 83496A/B option 101
6
Wrench
spanner
85054-60030
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