Rigol MSO8000 Series Manual de usuario
RIGOL –Innovation or nothing Page | 1
Introduction
Debugging embedded communications is one of
the most common tasks for electronic design
engineers. Efficient Analysis of serial
communications requires more than simple
triggering and decoding but historically there
has been a significant cost difference between
the oscilloscopes with mixed signal, serial
triggering, and serial decode capabilities and the
high performance instruments with advanced
analysis. Engineers need the ability to test long
term signal quality characteristics including
jitter and eye patterns without investing in high
performance, high cost solutions.
The MSO8000 (Figure 1) provides the most
complete analysis capabilities, deepest memory,
and highest sample rate in its class. Built for
embedded design and debug, the MSO8000 is
designed to enable engineers to speed
verification and debug of serial communications
on a budget. Let’s look at how class leading
sampling, memory, and analysis can be used
debug complex signal quickly and easily with the
help of Jitter and Eye Analysis.
Characterizing Jitter
Clock precision is critical to high
performance digital data transmission. Subtle
changes in clock frequency affect error rates and
data throughput, but these timing errors can’t be
visualized easily in a traditional oscilloscope
view. Jitter analysis can be done on these types
of signals. Utilizing the high sample rate and the
deep memory, the oscilloscope compares the
changes in time between thousands of clock
transitions. This makes it possible to visualize
timing fluctuations below 100 picoseconds while
also tracking changes in clock timing over long
time periods.
Advanced
Embedded Debug
with Jitter and
Real-Time Eye
Analysis
Figure 1: The MSO8000 High Performance Oscilloscope
RIGOL –Innovation or nothing Page | 2
One of the keys to visualizing jitter is the TIE or
Time Interval Error. The Time Interval Error is
the difference in time between the occurrences
of the expected and the actual clock edge. There
are two main visualization tools we use with TIE
for debugging. First, is the TIE trend graph. This
shows the accumulated error in time of the TIE
values. This trend is a valuable debug tool since
it highlights periodic types of jitter. Figure 2
shows the high speed clock signal on channel 1
(yellow) and the TIE Jitter trend in purple. The
vertical axis units for the TIE trend shown here
is 10 nanoseconds per division. The trend shows
that the jitter TIE accumulates periodically. That
implies a periodic signal or event is affecting the
clock frequency. Next, investigate the TIE trend
with measurements or cursors as shown in
Figure 3. The cursors make it easy to view the
period of the signal and calculate the frequency
as well (1/DX). Direct measurements of the TIE
Trend can also be made. The period of these
changes are an important clue as to the root
cause of any jitter issues.
In addition to the TIE trend you can also
calculate the distribution of the TIE values. The
shape and standard deviation of the TIE values is
an important component of determining root
cause. For the signal above the histogram is
shown in Figure 4.
When using the TIE trend and distribution to
debug and solve jitter related issues it is
important to understand the nature of the TIE
values and trend. Remember, that TIE is
calculated as the accumulated changes in the
period of the underlying signal. This means that
the TIE graph looks like the integral of changes
in the period. Therefore, the triangle wave
shown in the figures so far represent a square
wave change in the period. This is critical to
understanding how to debug signal jitter. The
TIE trend shows the period changes lengthening
linearly (triangle rising) and then the period
Figure 2: TIE trend graph showing periodic jitter
Figure 3: TIE trend graph cursor measurements
Figure 4: TIE trend and TIE Histogram
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changes shortening linearly (triangle falling).
When TIE is increasing linearly, the period must
be longer than the expected period, but at a fixed
value since the sum is linear. When TIE is
decreasing linearly, the period is at a fixed value
shorter than expected. Therefore, the period is
changing between 2 fixed values at this
frequency. One is just above and one is just
below the expected period. We are therefore
looking for a 10 kHz square wave that is
somehow affecting our clock timing. We can
learn from the histogram that this fluctuation
appears to be constant as the TIE distribution is
evenly and symmetrically spread across those
values.
After testing different nearby signals on our
device under test, we find a time correlated
square wave (Figure 5) shown in blue at that
frequency that is affecting our serial clock
timing.
Jitter can often be caused by issues with PLLs,
power fluctuations, or emissions, among others.
Let’s look at some use cases where the histogram
data is important to correct jitter analysis.
Figure 6 shows a bimodal distribution of the TIE
values in the histogram with a sinusoidal TIE
trend. Here we can see the standard deviation
(sigma in the histogram statistics) is about 3.2
ns. Since these trends have a mean value of close
to zero, the standard deviation can also be
approximated as the RMS value of the signal
(shown in the RMS measurement in the bottom
left). Since both sinusoidal waves and triangle
waves have an integral that appear visually
sinusoidal it can be difficult to discern whether
the underlying changes to the clock period are
more triangular or sinusoidal. The standard
deviation and the histogram are additional tools
that can help to determine what signals might be
interfering. Often, signal timing shows a sharper
correction or snapback to the nominal timing as
Figure 5: Finding root cause with Jitter analysis
Figure 6: Standard Deviation of the histogram
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the clock is realigned. Visually this can look like
more of a ramp or saw wave. The histogram can
help to visualize the asymmetry if the signal
drifts slowly and then is corrected quickly.
Figure 7 is a good example of an asymmetrical
jitter distribution that still appears nearly
sinusoidal in the jitter TIE trend. A distribution
like this makes it easier to pinpoint the process
that might be causing the fluctuations.
One important key to jitter measurements is to
remember that this is about data integrity and
ultimately about errors that cost the system time
or bandwidth. In other words, it isn’t just about
how much the timing might fluctuate but how
your receiver views the data. For this reason it is
important to test the signal for jitter in the way
that the receiver is also determining the clock
settings. In serial communications the clock can
be explicit, meaning that there is a clock line
transmitted for this purpose. There may also be a
constant clock speed defined by the
communication standard. It is also common for
the receiver to ‘recover’the clock from the signal
itself using a PLL circuit.
The design of the receiver has an outsized effect
on jitter and timing. If the receiver uses a
constant clock rate at a 70 Mb/sec rate the jitter
appears as shown in the figures above. If the
receiver uses a 1st order PLL with 200 kHz of
bandwidth it can eliminate much of the low
frequency jitter we saw at 10 kHz. This is shown
in Figure 8. The MSO8000 can emulate explicit,
constant, 1st order PLL, and 2nd order PLL clock
recovery systems to precisely measure the jitter
or eye diagram as it will be seen by the receiver.
These are important capabilities to accurately
debug critical timing issues and ignoring
insignificant issues. Once we are correctly
emulating the clock recovery and have removed
key causes of jitter, we can zoom in on the TIE
Trend to 500 picoseconds per division (Figure
9). We still see some periodic fluctuations but
Figure 7: Asymmetry in the histogram results
Figure 8: Dynamic Clock Recovery
Figure 9: Remaining Jitter
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they have been reduced significantly and may no
longer have any impact on the bit error rate.
Once, all the jitter has been adequately
addressed in the system you can view noise
sources below 500 picoseconds per division as
shown in Figure 10. The MSO8000 Oscilloscope
also enables a direct statistical table view of the
TIE values as well as Cycle to Cycle values and
values calculated from both the positive and
negative widths. Together, these jitter tools
make it possible to carefully visualize and
analyze timing issues in vital serial
communication links.
Signal Quality & the Eye Diagram
Timing is only one of the characteristics
that contribute to overall signal quality. The goal
of all signal quality analysis is to reduce data
error in the transceiver link. Errors are often
caused by timing and clock issues, but problems
stemming from bandwidth, grounding, noise,
and impedance matching all can impact how a bit
is interpreted by the receiver. The best method
for visualizing the holistic data signal quality is
the eye pattern or eye diagram test. Real-time
eye diagrams are a great way to validate and
debug serial data links where throughput and bit
error rate are important to system performance.
The eye diagram analyzes the data line aligning
the bit timing with the recovered clock. The
same clock recovery options are available here
as in the jitter toolkit. They eye diagram is then
created by lining up and overlaying each bit. A
density plot is then created from what can be
thousands of bit sequences. This is called an eye
pattern or eye diagram because the shape in the
center resembles an open eye that closes to a
point on each side. The goal is to have an open
eye where the bit level (0 or 1) is correctly
interpreted at the center of the eye.
Figure 10: Jitter Measurements
RIGOL –Innovation or nothing Page | 6
The eye pattern in Figure 11 shows some
potential issues. Depending on the user settable
thresholds, the instrument calculates the width
and height of the eye. This signal has a
bandwidth limitation. We can interpret that
because the slope of the rising and falling edges
on each side of the eye is not as steep as our
design planned for. Analytically, we can
determine this by comparing the eye height, eye
width, and signal risetime on the screen to our
design documents. There also appears to be
some frequency uncertainty with regard to the
recovered clock. We can see from the histogram
that the distribution of the period is not Gaussian
implying some non-random causality to the
frequency shifts. Lastly, there is some noise
causing the amplitude to fluctuate. This closes
the eye vertically.
Using the eye diagram as a visual debug tool,
evaluate the cables and connections. Also, look
for layout, cross-talk, or other emissions that
might be impacting signal quality. Once we
remove the signal causing the frequency
fluctuations we see the eye start to open in
Figure 12. The purple histogram now shows
that the remaining timing errors are at least
symmetrical. This also improves the eye width in
the eye measurements window.
Figure 13 is the result once we identified and
removed the nearby noise source. This improves
the eye height and width and the entire signal is
more precise. Now it is clear that the rising and
falling bit transitions don’t reach the same high
or low level as the non-transitioning bits. We
also see that the rising and falling edges
themselves have about a 45° slope with these
time and voltage settings. The design document
indicates that this should be higher. This is likely
a bandwidth issue that is both limiting the
risetime of the transitions and causing the eye to
close vertically when the signal does not return
Figure 11: Eye Diagram of signal causing errors
Figure 12: Eye Diagram with improved timing
Figure 13: Eye Diagram with improved noise
RIGOL –Innovation or nothing Page | 7
all the way to its peak or base by the middle of
the bit.
Finally, Figure 14 demonstrates improved
bandwidth after changing our transmitter circuit.
The histogram distribution shows that this has
also removed some of the outliers in the signal
timing. The improved bandwidth shows clearly
in the improved Risetime as well as a completely
open vertical eye.
Conclusions
Embedded design and debugging of
digital data is a critical requirement in modern
electronic products. Modern high performance
digital oscilloscopes expand the analysis
capabilities available to the everyday engineer’s
bench. RIGOL’s UltraVision II technology and our
MSO8000 Series oscilloscopes (Figure 15)
complete these tools with jitter and eye diagram
analysis options making complete signal quality
analysis affordable and easy to use. Jitter and eye
diagram analysis on the MSO8000 simplifies
viewing, analyzing, and resolving issues
involving timing, noise, bandwidth, and overall
signal quality in serial data links. With new
analysis capabilities built on the deep memory
and high sample rate of the UltraVision II
platform, RIGOL’s MSO8000 Series oscilloscopes
are the high performance debugging tool of
choice for today’s embedded engineer.
Figure 14: Eye Diagram after debugging and improvements
Figure 15: The MSO8000 High Performance Oscilloscope
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