Jitter analysis is a critical component in characterizing high-speed digital designs.
Does your oscilloscope provide the breadth of jitter measurements you need to perform
that characterization?
With the faster edge speeds and shrinking data valid windows in today's high-speed digital
designs, insight into the causes of signal jitter is critical for ensuring the reliability
of your design.
To gain fast and accurate insight into your signal, you need advanced decomposition, analysis,
and views of jitter.
When evaluating an oscilloscope, you should be looking for jitter capabilities like:
Histograms and trend plots Advanced level clock and data measurements
such as time-interval error and unit interval measurements
Expert-level analysis with complete jitter separation in timing and noise
And phase noise measurements on clock signals, which you'll only find on Keysight oscilloscopes.
In this video, you'll learn about some of the different solutions to meet these requirements,
such as various histograms,
bathtub plots, spectrum graphs,
and threshold measurements.
The jitter setup process and results can often seem daunting.
This breakdown will help you better understand what the various graphs and charts are telling
you about the jitter in your system.
If you're just starting to learn about Jitter, check out my video "How to Measure Jitter
With an Oscilloscope."
In that video I walk through the basics of jitter, like:
What it is
What time interval error (or TIE) is
The difference between random jitter and deterministic jitter
How to read histograms, measurements, and trend plots
How to use and analyze the spectrum view
And I talk about methods for clock recovery as well
You'll notice that video uses a 6000 X-Series oscilloscope, but today we will be working
with an Infiniium S-Series oscilloscope.
While the concepts learned on the 6000X still hold true for this oscilloscope, the set-up
is very different.
The Infiniium EZJIT application found on the S-Series provides you with significantly more
insight into the types of jitter affecting your signal.
With that, the setup is a bit more complex.
But the setup wizards help make that a lot easier.
Wizards seem to be underused, but they can be extremely helpful in setting up complex
analysis, especially if you don't know exactly what your settings should be.
You can use the setup wizards for all levels of jitter and phase noise measurements.
These provide detailed help guides to walk you through every setup option.
So, once you're done setting up with the wizard, you're ready to analyze!
The analysis charts provide detailed information needed to properly separate the jitter of
your design.
Let's quickly remind ourselves of the various types of jitter there are.
Afterall, the reason we're performing this analysis is to determine what different types
of jitter exist in our signal and how much each type is affecting it.
I highly recommend taking a photo or screenshot of this chart to reference throughout the
rest of the video.
So, we want to understand what is data-correlated versus data-uncorrelated.
What is caused by your design versus what is out of your control.
Total Jitter (or TJ) encompasses all jitter in the signal.
Deterministic jitter (DJ) is any data-dependent jitter (DDJ).
This is caused by your design and can be seen as inter-symbol interreference (or ISI) or
duty cycle distortion (DCD).
ISI is caused by things like imperfections in the transmission path or imperfections
in matching rise and falls times of transmitters.
DJ also consists of Periodic Jitter (or PJ), which calls for different troubleshooting
techniques than DDJ does.
PJ could be caused by things like coupling from a switching power supply, or coupling
from another clock source in the system.
The other component of TJ is random jitter (or RJ).
RJ is caused by random noise which can't be designed out, but just designed around.
You have to understand what RJ is present so that you know what to multiply by to achieve
the specified bit error rate.
So let's take a look at a few of the charts available and how they can help you.
And note the jitter measurements are actually made on all cycles of the waveform, including
those not shown on the display window.
The RJ,PJ histogram measures the TIE function with the DDJ removed.
It shows RJ cross-correlated with PJ.
A Gaussian-shaped RJ,PJ histogram would indicate that there is negligible PJ in the source
waveform.
To extract the RJ, there are two different possible methods.
Spectral and tailfit.
The spectral method computes RJ by integrating the noise floor of the RJ,PJ spectrum.
There are two different options for defining that noise floor - wide or narrow.
In the jitter spectrum, PJ components are seen as tall, narrow spikes.
We want to remove these PJ components so that we are left with only the RJ.
In many cases, it's easy to define these spikes.
However, there are measurements that have shorter, wider peaks, such as 1/f noise, or
pink noise.
Pink noise has a large value near DC, and slowly tapers off.
The wide/narrow setting allows you to decide whether a broad peak like that should be considered
periodic, or part of the noise floor (the random jitter).
In wide mode, the pink noise will be seen as PJ, whereas in narrow mode, it will be
seen as RJ.
The other method of RJ extraction is called tailfit.
Tailfit is used for aperiodic bounded uncorrelated jitter, such as crosstalk.
Often this can be seen as RJ, when its in fact very different and its affecting your
signal.
This algorithm works by fitting a gaussian function to the tails of the RJ, PJ histogram.
In general, the tailfit method requires more data to converge than the spectrial method,
so we recommend using the spectral method unless you expect crosstalk or other aperiodic
bounded uncorrelated jitter.
In the case that you are not sure which method to use, you can look at the tailfit/spectral
bathtub plot to determine which method works best.
Next, you can find the DDJ histogram.
This is calculated directly from the DDJ versus bit function, which you'll learn about shortly.
This histogram provides a graphical representation of the ISI and DCD within the total measured
jitter.
And now the TJ histogram is similar to the histogram of the TIE function you saw in my
last video.
The Infiniium EZJIT complete software calculates the TJ histogram by cross-correlating the
DDJ histogram with the RJ/PJ histogram.
This technique reduces the variance in the histogram function compared to a histogram
produced directly from the TIE function.
So, it gives you a more accurate overall view of the TJ in your signal.
I mentioned the DDJ versus bit function.
This graph displays the average time error associated with each bit of the source waveform's
repeating bit pattern.
A positive value of DDJ indicates that the transitions preceding those bits arrive later
than they should.
This graph shows which patterns and which bits within a pattern are most likely to cause
transmission errors.
Since the DDJ versus bit function is used to analyze the waveform's repeating bit pattern,
it naturally is only used on pattern waveforms.
Alternatively, if you're analyzing an arbitrary waveform you can utilize the ISI filter graph.
This indicates which transitions relative to specific bit positions add or subtract
delay to the total DDJ.
The bit error rate bathtub plot, or BER bathub, plots the width of the data valid window of
a signal on the horizontal axis versus the bit error rate on the vertical axis.
The two sides of the bathtub plot each consist of a measured section and an extrapolated
section.
The top section, which is the high bit error rate, is calculated directly from the measured
TJ time interval error.
The lower section, which consists of low bit error rates, is calculated by extrapolation,
ether with the spectral method or the tailfit method.
The bathtub plot is useful in visualizing how the desired BER affects the data valid
widow, or alternatively, what BER can be expected from a desired data valid window.
And note that the EZJIT complete application provides both timing jitter separation and
vertical noise separation.
You'll find similar charts for the vertical noise separation.
Lastly, if you're analyzing clock jitter, another critical measurement to make is phase
noise.
Dedicated phase noise measurements are only available on Keysight Infiniium oscilloscopes.
Phase noise is related to clock TIE, and is generally used to measure change in an oscillator's
frequency, either in the long term or short term.
When you look at the spectrum of an imperfect clock or oscillator, there will be energy
radiated slightly off the nominal clock frequency (or the carrier), called sidebands.
Phase noise is generally measured as a ratio of the spectral power in the carrier vs. the
phase noise in the sidebands, normalized to 1 Hz of bandwidth.
The results are presented in a log frequency plot, where the amplitude units are dBc/Hz.
That is, decibels relative to the carrier power, normalized to a 1 Hz bandwidth.
The X-axis is the frequency offset from the carrier frequency.
Resulting spurs can be normalized, omitted, or be represented separately to better show
their energy levels.
These various graphs and measurements that you'll find in the Infiniium EZJIT complete
application help to fully characterize the jitter in your signal.
In order to eliminate as much jitter as possible, you have to indicate the specific types of
jitter that are present in your signal and what's causing them.
Be sure that your oscilloscope has the variety of jitter analysis capabilities needed to
design the best high-speed digital device.
Look for functions like: Histograms for each type of jitter
Trend plots Clock and data measurements like time-interval
error and unit interval measurements Complete jitter separation in timing and noise
And phase noise measurements In order to test the true jitter in your signal,
you have to be sure that your oscilloscope has both very low noise and high signal integrity.
There are a few key specifications you should consider to ensure your oscilloscope gives
you the highest signal integrity and best measurement accuracy.
Learn about those specifications and some of the common misconceptions around signal
integrity in the eBook, How to Determine Oscilloscope Signal Integrity.
Thanks for joining today.
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