Questionable Jitter Measurement Results

What Does it Mean if I Have a Question Mark in my RJ / DJ Measurement Results?

Jitter analysis separates PJ from RJ by inspecting the spectral content of jitter. The robustness of this separation methodology depends on having sufficient frequency resolution in the jitter spectrum. A warning is displayed, in the form of a question mark (?) to indicate when the frequency resolution drops below a threshold that could start to cause a reduction in RJ/PJ separation's accuracy. This question mark is displayed when the number of compete data patterns in the source waveform record falls below the somewhat arbitrary choice of 128 patterns.

Display of the question mark does not mean that the results are necessarily incorrect. It is only a caution that you should examine the complexity of the PJ's spectral content. If the RJ,PJ spectrum is comprised primarily of random noise with very few tall PJ spikes, then the questionable results are accurate. If there are a large number of PJ spikes in the RJ,PJ spectrum then another method of verifying the results is recommended. Repeat the jitter measurement several times while varying the acquisition record length. If the RJ and PJ results change significantly with record length, then the results cannot be trusted. Otherwise, they can be trusted.

The Jitter analysis uses another somewhat arbitrary threshold of at least 128 complete data patterns in the source data waveform to display the questionable results at all. Below this threshold the message "too few points" is displayed.

The number of complete data patterns in the source data waveform can be increased by increasing the oscilloscope's acquisition memory from within the Acquisition setup dialog box.

Note that some software clock recovery settings reduce the amount of the source waveform on which jitter can be measured. Software phase-locked loop (PLL) clock recovery with a low-frequency loop bandwidth, for example, will take many bit periods to settle, and the clock reference must settle before jitter measurements can begin.