[SI-LIST] Re: Peaks and Nulls in spectral density

> Why are there peaks and nulls in spectral density and in insertion/return
> loss measurement graphs?

I suspect this question is asked by someone new to this, perhaps a new
student.  Otherwise you probably would have known the answer.  (Sorry
if I am wrong.)

Signal spectra tend to have peaks and nulls because most signals do
not have random timing, and they have regular patterns, such as a bit
rate, where something about the signal repeats every X ns.  Also many
signals have a clock that is encoded within the data; or a way to
extract and replicate the clock.  That gives most signals some
frequency components that are either there and stronger than the
energy between those frequencies, or that may be missing ... depending
on how the data is encoded ... and there are many ways to encode data
in an electrical or optical signal.  An analysis of any of these ways
of encoding data will show frequencies that are stronger or weaker.  A
good text book that covers data encoding will show you that.

Perhaps the only way for a signal spectrum to be uniform without peaks
and dips, is if the data AND its timing is totally random.  We don't
work well with signals that have random timing.  We tend to like
things that have a known clock rate.

The peaks and nulls in insertion/return loss measurements are for a
different reason.  Any real wire, trace, transmission line, or
whatever, will have discontinuities where they aren't perfectly
uniform.  The discontinuity on the end of the signal path (the
mismatch between the wire and its load) is a big one.  These
discontinuities give you small resonances and the variations you see
in the measurements.  Those peaks and dips wouldn't be there IF the
line is perfectly uniform and is terminated exactly in its
characteristic impedance; which is an ideal that can never be
achieved, especially when you go looking towards higher frequencies.

Regards,
Andy
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