RE: [SI-LIST] : Signal Dispersion

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From: George_Tang@Dell.com
Date: Tue Sep 12 2000 - 16:55:45 PDT


Dispersion is most talked about in topics concerning waveguides (optical or
metal) and not as much in that of pcb signals. To understand why, we need
to know how dispersion occurs. Let's take a single mode fiber for example.
We can use z-axis for the circular fiber's center axis. When a light pulse
is injected into the 'core' of the fiber, the light radiates outward and
strikes the cylindrical core boundaries at multiple angles. The light
bounces inside the core as it propagates in the z direction. For this light
pulse to be retrieved at the other end of the fiber, the 'rays' of the light
pulse need to propagate in-phase in the z direction, so the pulse does not
spread. It turns out, for a single mode fiber, there is only one
propagation constant (or one bouncing angle) for a specific carrier
frequency, fo. But different fo has different beta. The light pulse
usually takes the shape of a gaussian pulse, which is narrow band (fo +/-
delta f) in the frequency spectrum. Since 'fo + delta f' has a different
beta compared to 'fo - delta f', they will be out of phase when they get to
the other end of the fiber. This is one of the cause of dispersion with
non-TEM wave. But on a stripline in PCB, the dominating mode is TEM, so the
wave is propagating down the z-axis and not bouncing off the guiding
structure. The dispersion due to different bouncing angle does not come
into play. If the board dielectric constant changes with respect to
frequency, dispersion can still occur. 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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