[SI-LIST] Re: RMS jitter
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxxxxxx>
- To: <khalida@xxxxxxxxxxx>
- Date: Thu, 13 Dec 2001 18:29:02 -0500
If you are eyeballing jitter from a scope, I think it is next to
impossible to determine RMS jitter with any reasonable accuracy. Even
eyeballing peak-to-peak jitter is only approximate, because it may vary
depending on how long you look.
However, I recall seeing an application note, a long time ago, about a
way to estimate the RMS jitter (or maybe it was RMS noise voltage?) by
eye. I think there was something about superimposing two displays and
moving them together until the density just becomes "continuous" ...
that is, just to the point where it ceases to look bi-modal and looks
like one smooth area of "fuzz". But I don't recall the details. Maybe
someone else does. With smarter instruments today, there is less need
to use this technique.
The relation between RMS and peak-to-peak jitter is undefined. If and
only if you are talking about noise with a gaussian distribution, then
there is a relationship between RMS jitter and the peak (or pk-pk)
value, as a function of a probability. But gaussian noise is
theoretically unbounded, so there is a small but non-zero probability
that you may witness a pk-pk value that is 1000 times the RMS.
If your objective is to achieve a certain BER, then that BER sets the
probability, or the frequency that pk-pk jitter may exceed whatever
jitter tolerance your circuit has; and from that, you can derive the
ratio to RMS jitter. *IF* you know that it's gaussian.
If it's gaussian, the longer you store a histogram, the greater the
pk-pk becomes. But the RMS stays the same (1 sigma).
Whenever jitter doesn't follow a gaussian distribution, all of the above
no longer holds. It is possible for RMS to exceed peak.
Regards,
Andy
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