[SI-LIST] Re: Peaks and Nulls in spectral density
- From: Aaditya Kandibanda <aaditya.kandibanda@xxxxxxxxx>
- To: a.ingraham@xxxxxxxx
- Date: Tue, 17 Jul 2012 22:54:07 -0700
Thank you very much Andy! I find it very useful
By the way, your guess is correct. I am new student! I am learning concepts
of signal integrity!
On Sat, Jul 14, 2012 at 11:01 AM, A. Ingraham <a.ingraham@xxxxxxxx> wrote:
> > 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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