Ken Kundert
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Generally you choose the noise post-processing options to match the way the noise will be observed. Presumably in the circuit the output of the buffer will be observed by some thresholding circuit such as a mixer or digital gate. In this case, jitter is the most interesting thing to observe because that is what the observing circuit is sensitive. Noise that occurs away from the threshold will not effect the observing circuit.
If you are trying to match a measurement with a spectrum analyzer, then it becomes the observing circuit and you need to match it. I took a quick look at the E5052, which appears to be an Agilent signal source analyzer. It claims to be able to measure phase noise and jitter, but it is not clear to me how it is doing it. Jitter assumes a threshold, but I did not see a thresholding circuit in their block diagram. Measuring phase noise generally requires a phase detector, but I did not see that either.
So, to summarize, you choose the style of noise post processing to match the characteristics of the observing circuit. You can think of the block being driven by the block being measured as the observing circuit, or if you are trying to match a measurement, you think of the measurement equipment as the observing circuit. Jitter is probably more appropriate for the former, and it is unclear what is best for the latter without a better understanding of the test equipment.
-Ken
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