Date: Sat Mar 24 2001 - 08:50:45 PST
You got the idea, Larry. But just to clarify for other readers, the -12 dB
(or 25%) of the allowable jitter gives adequate measurement headroom to
identify downstream contributors to the jitter (i.e., the buffers, mode
converters, etcetera). The -3 dB (subjective level) for the indicated test
condition indicates a high probability that both the delayed oscillator
(reference my original E-mail) measurement and the total system jitter
measurement will grossly exceed the allowable limits. This comment is based
on the presumption that reasonable care has been taken in the selection,
powering, and packaging/shielding of the reference oscillator so that the
downstream elements would be expected to contribute equal or lower levels of
added jitter. If jitter specification data are known for these additional
elements, they can all be RSSed (along with the delayed measurement on the
oscillator) for an approximation of the total system jitter.
If one unfortunately finds the "-3 dB" case, a check of the jitter of a
known, clean, precise frequency reference for the same minimum-delay test
conditions should identify whether the oscillator is at fault, or the test
system. Such a test is, of course, the first step to rectifying the problem.
Enjoy the weekend.
Michael L. Conn
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