Impedance Mismatches
Another potential source for deterministic jitter is impedance mismatches. One way to recognize impedance mismatches is to look at the eye diagram of the signal. If the signal has an impedance mismatch somewhere, it often manifests itself as something strange looking in the eye diagram — perhaps over or undershoot, perhaps a hitch or shelf in the edge of the signal, or some other spurious wiggle in the waveform.
If you suspect that you might have an impedance mismatch then there are a couple of ways to isolate where the problem might be. You can use a time domain reflectometer (TDR) to examine your data channel and see where along the channel the mismatch might be — common culprits are at the locations of large components or connectors.
Once you have the plot on the TDR, if you see a bump in the response and want to relate that to a physical location on your board, take a pencil and touch the transmission trace while looking at the TDR trace. You will see a bump appear and disappear as you make and break contact with the trace. This will allow you to figure out where the various impedance mismatches are — move the pencil around until you are pointing at your problem.
Cable Equalization
Many deterministic jitter issues have the bandwidth limitation of the transmission medium as their cause — be this a PCB trace, or a cable. In many cases the easiest way to reduce this jitter is through the use of a cable equalizer, which understands the low pass filtering effect of the cable that was used, and reverses this effect. If you use an equalizer, it is important that it be placed in the signal path before any digital processing of the degraded signal has begun — once the signal has gone through a comparator, it becomes much harder to reverse the effect of the cable.
Figure 6: The effect of a cable equalizer
In Figure 6, the upper (yellow) trace shows a 3 Gbps SDI video signal after it has passed through 100m of coax cable. The deterministic jitter has completely closed the eye of the eye diagram, and as-is, this signal will have difficulty recovering error free data. The lower (blue) trace shows the output of an LMH0344 cable equalizer, which has the signal from the upper trace as its input. As you can see, the eye is now open enough to receive essentially error-free data. As such, the LMH0344 is a quick, effective way to get your jitter budget back on track.
NEXT: Sources of Random Jitter and Reclocking
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