[SI-LIST] Re: Coaxial Cable crosstalk
- From: C Deibele <deibele@xxxxxxx>
- To: cgrassosprint1@xxxxxxxxxxxxx
- Date: Tue, 05 Feb 2002 10:46:54 -0500
Charles Grasso wrote:
>
> The limiting factor in a coax design will be
> its terminations not the cable itself in the
> first order. SO I would not expect much
> xtalk between two coax cables
>
> What you are describing Craig is a little different.
> If you jump onto Doug Smiths website (www.dsmith.org) or go to
> one of his seminars, you'll find a nice proof that
> the coax actually acts as a transformer and simply
> transfers noise on the shield right onto the signal.
>
> This is the effect you're looking for I think
>
I did look at Doug Smith's website - what a great resource. I wish I
could get away and go to a seminar -- it seems like Doug does an
impressive job!
While I don't quite see directly what you mean by transformer, I do
believe we are on the same page....perhaps just using different
language.
And you are right, one shouldn't expect must crosstalk. The question is
how does one evaluate it. The math gets *nasty* very quickly to do a
good prediction.
To get away from even the problems associated with cylindrical coords, I
thought that it would be fun to solve the problem of:
Region I Region II Region III
| |
eps_o mu_o | eps_1, mu_1 | eps_o mu_o
| sigma |
| |
(so, the problem is infinite in x and y, and the only spatial changes
are in the z axis)
have a TEM wave strike region II, and measure how much gets transmitted
to region III.
This problem was rather straightforward, and I can see how the thickness
of the conductor of region II plays on the isolation. Quite ironically,
I would have expected that the permeability was a strong function,
especially at low frequencies.
The flaw in this model, however, is that coax *guides* waves.
so, I tried to solve:
Region III eps_o, mu_o
--------------------------------------
Region II eps_1, mu_1, sigma
--------------------------------------
Region I eps_o, mu_o
-------------------------------------
perfect conductor
allow propagation into your computer screen, with the x and y axis
infinite again. The problem is the field matching at the boundaries.
The propagation is dispersive...and that made for some nastier math.
So, with some hand waving, I can say that the worst case scenario is the
first problem, and rely on these numbers.
Comments anyone?
Craig
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