Sources of Random Jitter
Random jitter has its source in a plethora of small sources such as electrical noise, minor fluctuations in the width of a PCB trace, shot noise in your driver, minor variations in local electrical fields, all of which perturb the waveforms in your system and many other sources.
Because random jitter is so complicated in its sources, it is often harder to control. However, as long as the random jitter component has not accumulated too much, you can regenerate the signal (thereby eliminating random jitter) and wipe out the accumulated debt in your jitter budget.
Reclocking
The most common way to regenerate the signal in a broadcast video system is through the use of a reclocker. A reclocker is a PLL-based circuit that uses the input signal to extract the embedded clock from the system, and then uses this clock to ‘reclock’ the data.
The PLL will eliminate any jitter that’s at a higher frequency than the loop filter bandwidth of the PLL, and since random jitter tends to be high frequency jitter, the reclocker works very well as a way to eliminate random jitter.
In some integrated circuits, components of equalization as well as reclocking are incorporated. For example, the LMH0356 is an SDI reclocker that also includes the function of an equalizer (optimized for PCB traces) on its input.
Figure 7: 3Gbps Signal that has passed through 1m of PCB trace
Figure 7 shows the eye diagram of a signal which has come through 1 meter of PCB trace, as well as some backplane connectors. As you can see, there is a significant amount of deterministic and random jitter on this signal.
NEXT: EQ and Reclocking Combined
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