RE: [SI-LIST] : Signal Dispersion

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From: Ron Miller (rmiller@Brocade.COM)
Date: Wed Sep 13 2000 - 09:37:17 PDT


Hi George

While I agree with you about the causes of distortion that you mention, there
are others and other effects aw well.

The parasitics L and C you mention below are certainly present, and their effect
is to cause "group delay" which causes edge dispersion.

However, in addition to scin effect losses there are dielectric losses and ratiation
losses, all of which contribute to edge dispersion. Cable/PCB equalizers in fact
are designed for the frequency dependent loss compensation and for group delay
compensation. The literature is sparse and much is old but it is around.

Above 1 Ghz Dielectric losses dominate.

Ron Miller

> -----Original Message-----
> From: George_Tang@Dell.com
> Sent: Tuesday, September 12, 2000 4:56 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Signal Dispersion
>
>
> But in a pcb, there are more
> dominating causes of waveform distortion than dispersion due to dielectric
> constant variation. The IC packaging bondwires are usually seen as lumped
> inductors by the signal, and the vias in a signal path can be seen as a
> lumped capacitor. The solder pads for ICs and termination resistors are
> also lumped capacitors. All these inductors and capacitors can distort the
> signal by changing the amplitude and phase of the frequencies in the signal
> spectrum. People have also brought up distortions caused by skin effects.
> These distortions are more dominating than dispersion for signals in the
> pcbs at this time.
>
> George Tang
>
>
> -----Original Message-----
> From: Bob Perlman [mailto:bobperl@best.com]
> Sent: Monday, September 04, 2000 1:18 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Signal Dispersion
>
>
> Hi -
>
> A happy Labor Day to all.
>
> I'm looking into effects that can distort very high frequency
> waveforms. There's already been a lengthy discussion of skin
> effect here, and in the past folks have discussed dielectric
> absorption.
>
> One thing I haven't heard much about is dispersion. My
> understanding is that, for long signal traces or cables, slight
> frequency-dependent differences in propagation delay can cause
> the various frequency components in a signal transition to de-align,
> causing the edge to disperse. Some of the causes to which this is
> attributed seem to be:
>
> - different frequencies propagating in different transmission modes
> - changes in dielectric constant as a function of frequency.
>
> Of course, I could be totally wrong; this is just what I've been able
> to piece together from things I've read. Can anyone explain
> dispersion, or point to a good text that does? Under what
> conditions does dispersion become significant?
>
> Thanks,
> Bob Perlman
>
>
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