Wavecrest SIA-3000 User manual

WAVECREST Corporation
Examining Clock Signals And Measuring Jitter
with the
WAVECREST SIA-3000™
Application Note No. 142
200142-00 REV A

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WAVECREST Corporation continually engages in research related to
product improvement. New material, production methods, and design
refinements are introduced into existing products without notice as a
routine expression of that philosophy. For this reason, any current
WAVECREST product may differ in some respect from its published
description but will always equal or exceed the original design
specifications unless otherwise stated.
Copyright 2002
WAVECREST Corporation
A Technologies Company
7626 Golden Triangle Drive
Eden Prairie, Minnesota 55344
[phone]
[phone]
www.wavecrest.com
All Rights Reserved
U.S. Patent Nos. 4,908,784 and 6,185,509, 6,194,925, 6,298,315 B1,
6,356,850, 6,393,088 and R.O.C. Invention Patent No. 146548; other United
States and foreign patents pending.

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Contents
Introduction................................................................................................1
Amplitude Measurements..........................................................................2
Measuring Jitter with Histogram Tool.........................................................3
Use Tail-Fit to Separate RJ and DJ ...........................................................4
Find the Causes of DJ; Identify PJ, Crosstalk and Modulation ..................5
Identify the Spectral Content of the Jitter...................................................6
Identify Possible Low Frequency Jitter Content.........................................7
Analyze Long-term Drift/Wander................................................................8
Measure Duty Cycle and Frequency........................................................10
Measure Cycle-to-Cycle........................................................................... 11
Measure Skew or Propagation Delay.......................................................12
Conclusion............................................................................................... 13
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EXAMINING CLOCK SIGNALS AND MEASURING JITTER
WITH THE
WAVECREST SIA-3000™
Application note #142
IntroductionThe purpose of this application note is to highlight the various clock measurement tools
available in the VISITM software. The basic steps for specific tool setup and
interpretation of the resulting plots are covered. More details for each tool are provided
in the following
WAVECREST Quick Reference Guides: Understanding Jitter, SIA-
3000 Measurement Technique, Oscilloscope, Histogram, and High Frequency
Modulation. These guides provide details of tool settings, measurement setup and
theory of measurement.
This application note will discuss how to use the various VISITM tools to fully
characterize your signal. These tools allow many different parameters of a clock signal
to be measured including:
• Random Jitter (RJ)
• Deterministic Jitter (DJ)
• Total Jitter (TJ)
• Periodic Jitter (PJ)
• Cycle-to-Cycle Jitter
• Duty Cycle
• Frequency
• Low Frequency Periodic Jitter
• Spectral View of PJ and RJ
• Timing Characteristics:
Positive Pulse width
Negative Pulse width
Period (rising edge to rising edge)
Negative Period (falling edge to falling edge)
• Long term changes (varying Vcc or Temperature)
• Skew or Propagation delay between any combination of rising and falling
edges on 2 channels
Clock Tools Overview Page 1 of 13

Oscilloscope
To Measure:
Risetime
Falltime
Voltage Parameters
Eye Mask
For more information
on this tool, refer
to:
Oscilloscope Quick
Reference Guide
available at
www.wavecrest.com
Amplitude Measurements
Assuming the engineer has an idea of the device’s pins or test setup probe points, it is
helpful to view the signal in the Oscilloscope Tool. Eye masks can be created using
the integrated sampling oscilloscope on the channel cards. The information in this tool
is for qualitative analysis as well as measuring voltage parameters including rise or fall
times. A different measurement engine is used for jitter analysis (those tools follow this
page).
• Connect the clock signal under test to IN1 on the front panel.
• Press the Oscilloscope button. This will open the Oscilloscope tool. The channel and
trigger settings default to IN1.
• Press the Auto Scale button to configure the display and perform an initial
measurement. Signal should be <= 1Vpk-pk single ended or <=2Vpk-pk differential.
• Press the SINGLE/STOP button to acquire the signal. The signal will be displayed.
Figure 1 - Oscilloscope example
Clock Tools Overview Page 2 of 13

HISTOGRAM
To Measure:
For more information
on this tool, refer
to:
Histogram Quick
Reference Guide
available at
www.wavecrest.com
Measuring Jitter with Histogram tool: 1-sigma, peak-to-peak
• Press “Tool” to open the main menu
• Press Clock then Histogram to open the histogram tool.
• Press the SINGLE/STOP button on the front panel or the top menu. The tool
defaults to make a period measurement (rising edge to the next rising edge). The tool
defaults to a sample size of 10,000 measurements.
Basic statistics displayed on the Histogram Summary View are: Mean, Min, Max, 1-
sigma, peak-to-peak, and number of hits (Figure 2).
Results from the most recent pass are displayed as “Latest” text. Results from
accumulated statistics are display as “ACCUM” text. The Maximum Histogram view
(below) shows all accumulated measurements.
Figure 2 - Histogram Period Measurement
A histogram can be made of PERIOD (+), PERIOD (-), PULSE WIDTH (+) and PULSE
WIDTH (-) measurements.
These Histogram statistics may be sufficient for jitter analysis when no deterministic
jitter is present, but with the addition of DJ, it is helpful to perform a more
comprehensive analysis. When the histogram is Gaussian, the 1-sigma equals the
Random Jitter (RJ). The addition of Deterministic Jitter (DJ) will increase the 1-sigma.
WAVECREST’s Tail-FitTM algorithm can be enabled to better estimate the true RJ.
Clock Tools Overview Page 3 of 13

HISTOGRAM with
TAIL-FITTM enabled
To Measure:
For more
information on this
tool, refer to:
Histogram Quick
Reference Guide
available at
www.wavecrest.com
Use Tail-FitTM to separate RJ and DJ. Predict Total Jitter (TJ) at a
specific Bit Error Rate (BER) or amount of time
• Press the Tail-Fit button. This will open the Tail-Fit menu
• Under the Tail-Fit selection, change “off” to “Enabled”.
• Press Clear to clear your measurements.
• Press RUN. The measurement will stop when the tail regions can be fit. Additional
information can now be displayed.
Figure 3 - Results of Histogram with Tail-Fit™ enabled. Measurement is on
a 2GHz clock
• Add a view by pressing ADD VIEW on the front panel or press on the top menu bar.
Change this view to “bathtub” to display the bathtub curve.
We can now see additional values in Figure 3 summary view at the top (which were not
available in Figure 2). The first value is Total Jitter (TJ), which includes contributions
from all deterministic and random components, and is a pk-pk value specified for a Bit
Error Rate (BER), or amount of time. The Bathtub curve shows the TJ as a function of
BER or time. To predict TJ, we must measure DJ and RJ accurately. Random jitter (RJ)
is characterized by a Gaussian distribution and assumed to be unbounded. As a result, it
generally affects long-term device stability. Because RJ is Gaussian in nature, the
distribution is quantified by the standard deviation (1σ) and mean (µ). Because RJ can
be modeled as a Gaussian distribution, it can be used to predict pk-pk jitter as a function
of BER or time. The Tail-FitTM algorithm curve fits a Gaussian model independently to the
right and left sides of the distribution. The Values of DJ, LtRMS (left-tail RJ), and RtRMS
(right-tail RJ) are displayed below the TJ. Chi-squared values are also displayed as an
indication of the goodness of fit from model to acquired measurements. Once DJ has
been found, we can determine the causes.
Clock Tools Overview Page 4 of 13

High Frequency
Modulation
1-sigma view
To Measure:
For more
information on this
tool, refer to:
High Frequency
Modulation—Quick
Reference Guide
available at
www.wavecrest.com
Find the causes of DJ; Identify Periodic Jitter (PJ), crosstalk and
modulation
The High Frequency modulation tool shows accumulation of jitter over many cycles and
the frequency and amplitude. First, look at the 1-sigma view to see jitter accumulation.
This is important because this is the record that will be used for an FFT calculation
showing the PJ components.
• From the main menu, press Clock, then High Frequency Modulation to open the tool.
• Press the SINGLE/STOP button on the front panel or the top menu.
This will make a measurement from the “HPF -3db freq” up to ½ the clock rate. The HPF
corner frequency is set in Acquire Options menu. This determines the low frequency end
of the acquisition. The High-end maximum frequency is always ½ the clock frequency or
less.
Figure 4 - 1-Sigma values vs. number of periods skipped. Analysis of 2GHz
clock from Figure 2. Note that this is a zoomed view to show
details every couple of edges.
Each point on this plot is the 1-sigma from a histogram. Each histogram encompasses
an increasing number of edges, up to 82 in Figure 4. A histogram is made from rising
edge to rising edge; the 1-sigma value is plotted on the y-axis at Period 1 on the x-axis.
Then a histogram is made from a rising edge to the second rising edge (skipping an
edge and thus measuring two periods). This 1-sigma value now corresponds to Period 2
on the x-axis. This process is repeated until enough edges are skipped to encompass
the frequency set in “HPF –3dB freq”.
When modulation is present, these 1-sigma values change periodically, as in Figure 4.
The example shows alternating high and low values every other edge. Other periodicities
are apparent. From these measurements we can determine the periodic jitter
frequencies and magnitudes.
Clock Tools Overview Page 5 of 13

High Frequency
Modulation
FFT N-clk view
To Measure:
For more
information on this
tool, refer to:
High Frequency
Modulation—Quick
Reference Guide
available at
www.wavecrest.com
Continue analysis of Periodic Jitter (PJ). Identify the Spectral
content of the jitter
To provide frequency and amplitude of jitter, we use Accumulated Time Analysis, a
patented
WAVECREST algorithm that creates a frequency vs. jitter diagram.
• While still in the High Frequency Modulation tool, you can now change views from the
1-sigma view to the FFT N-clk view.
• Add a view by pressing ADD VIEW on the front panel or press on the top menu
bar. Change this view to Summary.
Figure 5 - FFT shows jitter frequencies and amplitudes. Analysis of 2GHz
clock from Figure 3. Shows jitter at 866MHz
The cause of the bimodal histogram in Figure 3 is now apparent in this FFT. For the
purposes of this example, Periodic Jitter was added to the 2GHz clock. The summary
page shows the amplitude of this highest peak in the N-Clk (Pk-Pk) column as 7.6ps at
866MHz. Having identified the major contributor to jitter, the engineer can now work to
eliminate it. Usually, it is at a frequency that is familiar, such as some multiple of a
reference clock or crosstalk from another clock.
Clock Tools Overview Page 6 of 13

Low Frequency
Modulation
FFT 1-clk view
To Measure:
For more
information on this
tool, refer to:
VISI Online Help
Continue analysis of Periodic Jitter (PJ); Identify possible Low
Frequency Jitter content
Often it is not necessary to identify jitter at very low frequencies (in this tool <100kHz).
Low frequency modulation can be analyzed in the High Frequency Modulation tool, but
the acquisition time becomes very long for lower “HPF –3dB freq” settings. So, the Low
Frequency modulation tool acquires measurements in a different way allowing faster
acquisition when looking for low frequency jitter or wander on a high speed clock.
• From the main menu, press Clock, then Low Frequency Modulation to open the tool.
• Choose the “Maximum Frequency”, which must be 100kHz or less.
• “Minimum Data Points” determines the number of time measurements in the record.
Increasing this value will reduce the possibility of aliasing, increase FFT resolution
and allow lower frequency components to be acquired.
• Press the SINGLE/STOP button on the front panel or the top menu.
Figure 6 - FFT shows jitter frequencies and amplitudes. Analysis of 2GHz
clock from Figure 3. Shows jitter at ~62kHz.
This 1-clk FFT view shows jitter components lower than 100kHz. This tool is useful when
very low frequency jitter or wander may be suspected. The example in Figure 6 shows
that we can see a Periodic jitter component at ~62kHz on the 2GHz clock signal; this
jitter was added to the clock signal for the purposes of displaying the capabilities of the
tool.
We have now identified low frequency jitter, but there may be other factors that affect the
timing stability such as drift due to temperature or voltage changes.
Clock Tools Overview Page 7 of 13

Strip Chart
Min/mean/max view
To Measure:
On:
For more
information on this
tool, refer to:
VISI Online Help
Analyze long-term drift/wander or effects of temperature change or
supply voltage change on mean measurement values
• From the main menu, press Clock, then Strip Chart to open the tool.
• The default interval between histogram measurements is 0.1seconds.
• Press the RUN button on the front panel or the top menu to run continuously.
Figure 7 - Shows drift of period by logging histogram statistics at user
defined intervals
Figure 7 shows the results of “logged” histogram values. The Strip Chart tool allows
histograms to be performed at regular intervals chosen by the user. In other words, when
a histogram is performed, the tool waits for a specified time interval and then performs
another histogram measurement (see Figure 8). The Max, Average, Min values are
displayed relative to when the histogram
was created. This tool is useful to plot the
effects of environmental changes such as
temperature, Vcc, or ground changes. By
enabling Markers in Figure 7, we can see
the total deviation of the mean period
measurement—4.16ps over the total
acquisition time—51.6 seconds on the x-
axis.
Long term studies of environmental
effects on timing parameters can be
performed using this tool.
Figure 8 - Shows Histogram values
plotted relative to time of
acquisition
Clock Tools Overview Page 8 of 13

Strip Chart
1-sigma/pk-pk
view
To Measure:
On:
For more
information on this
tool, refer to:
VISI Online Help
Analyze long-term drift/wander or effects of temperature change or
supply voltage change on the 1-sigma and Peak-to-peak values of
histograms
• While still in the Strip Chart tool, change the view from “Avg/Min/Max” to “1-Sigma/Pk-
Pk”. The default interval between histogram measurements is 0.1seconds.
• Press the RUN button on the front panel or the top menu .
Figure 9 - Shows the change of 1-sigma and Pk-Pk values by logging
histogram statistics at user defined intervals.
Figure 9 shows the results from the same tool, but a different view of the measurements
from Figure 7. In this case, the 1-sigma and Peak-to-Peak values are plotted relative to
when the histogram was created. This specific example shows that as the mean (from
Figure 7) changes, the 1-sigma and Peak-to-Peak values of the histograms remain level.
In some cases, as temperature or supply voltage to the part are varied, the period mean
will remain constant, but the jitter will increase. This plot would show that effect. If the
histogram grew in width with the addition of DJ or RJ, the Pk-Pk and 1-sigma lines would
trend upwards across the plot.
Clock Tools Overview Page 9 of 13

Statistics:
Duty Cycle/
Frequency count
To Measure:
For more
information on this
tool, refer to:
VISI Online Help
Measure Duty Cycle and Frequency
• From the main menu, press Clock, then Statistics to open the tool.
• Press SINGLE/STOP button on the front panel or the top menu or Press the RUN
button on the front panel or the top menu .
Figure 10 - Shows statistics from histogram measurements of Period +, Period-,
Pulsewidth +, Pulsewidth -, Duty Cycle, and Frequency Counter.
The Statistics tool is valuable in that it is the only tool to show Duty Cycle and Frequency
Counter information. Additionally, it shows the basic statistics of histograms of all
measurements of Period +, Period -, PW +, PW-. It is important to remember, though,
that while the raw statistics may convey some information, the plot views convey so
much more. Before accepting any values from this tool “as is” you should also perform at
least a histogram analysis of the measurement and understand the graphical results.
Clock Tools Overview Page 10 of 13

Cycle-to-cycle
Accumulated view
To Measure:
For more
information on this
tool, refer to:
VISI Online Help
Measure Cycle to Cycle
• From the main menu, press Advanced Clock, then Adjacent Cycle to open the tool.
• Press SINGLE/STOP button on the front panel or the top menu or Press the RUN
button on the front panel or the top menu .
Figure 11 - Shows statistics from histogram measurements of Cycle-to-Cycle (or
the difference of two adjacent cycles), Period, and Duty Cycle.
This tool can measure cycle-to-cycle, which is the difference between two adjacent
cycles. Rising edges or falling edges can be measured and Tail-FitTM can be performed
on the resulting histogram.
Clock Tools Overview Page 11 of 13

Skew
Propagation
delay
To Measure:
For more
information on this
tool, refer to:
VISI Online Help
Measure Skew or Propagation Delay
• From the main menu, press Chan to Chan then Propagation Delay and Skew to open
the tool.
• Press SINGLE/STOP button on the front panel or the top menu or Press the RUN
button on the front panel or the top menu .
Figure 12 - Shows statistics from histogram measurements of Skew or
Propagation delay between two or more channels.
This tool is useful for measuring the skew between two or more signals. One example is
the propagation delay through a part from input to multiple outputs such as a clock fan
out. The example above shows the skew referenced from Channel 1 to Channels 2 and
3. A window or guide can be enabled to quickly identify if all channels occur within a
specified amount of time from one another.
Clock Tools Overview Page 12 of 13

Conclusion
The SIA-3000 provides the ability to measure and graphically display many different critical clock
parameters. Using the tools described in this application note, a characterization or test engineer can
understand the output of the part under test in just a few minutes. Each tool has additional capabilities
not described in this application note. Most notably are DSP post-processing filters that can be
enabled. For more details on these capabilities please refer to specific Quick Reference Guides or the
online help.
WAVECREST also provides software tools for advanced analysis of PLLs. The PLL
analysis module can provide Transfer function, Poles and Zero, Bode Plot, Carrier Frequency,
Damping Factor, Natural Frequency, PSD of Noise, Lock Range, Lock-in Time, Pull-in Time, Pull-out
Range and Noise Bandwidth. This tool allows comparison of actual measurements to the design
simulations.
These different analysis tools provide the flexibility to look at many different components of Signal
Integrity. The SIA-3000 combines different hardware techniques with industry-accepted algorithms
for the measurement of Deterministic, Random, and Total Jitter. These capabilities make the SIA-
3000 the most complete and easy to use Signal Integrity Analysis system available today.
Clock Tools Overview Page 13 of 13

200142-00 REV A
WAVECREST Corporation
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