I think I can make NEC-2 work for this. It is a wire antenna modeling
program but I can make it do much more.
Dave
From: Robert A. Macy [mailto:m...@california.com]
Sent: Monday, October 08, 2007 3:34 PM
To: David Cuthbert
Cc: emc-p...@ieee.org
Subject: Re: RFI/EMI and Susceptibility on PCB for Analog Signal
Distribution
David,
Thank you for your response.
It seemed counter intuitive that the second approach would
have less crosstalk than the first.
That expectation is based upon picturing the fields between
the two signal traces as better contained when the traces
were side by side on the same layer, than if the traces
were on top each other. As in the second structure it
seemed that the field of one pair is polarized to better
inject into an adjacent pair. Another way to say this, is
to think in general of a field from a current loop on axis
and off axis. Off axis, like in the first structure there
is inherently 6 dB less than when on axis like in the
second structure.
I agree about modelling to get a better representation of
what is going on.
Will the modelling show the effect of 3V/m DC to 1GHz
fields?
Robert
On Mon, 8 Oct 2007 14:27:28 -0600
"David Cuthbert" <dcuthb...@linear.com> wrote:
> Robert,
>
> The second PCB stack-up will give less channel-to-channel
> crosstalk. Imagine
> the stack-up with the ground traces on layers 2 and 3
> removed. The coupling
> of the layer 2 and layer 3 traces will be almost equal to
> the adjacent
> channel. The ground stitching will help a lot. But you
> need to know if 120
> dB is possible. A 3-D field solver is the tool to use
> unless you like doing
> tedious calculations (from your EM class). I can run this
> in NEC-2 this
> evening and see what we get.
>
> Dave Cuthbert
> Linear Technology
>
> -----Original Message-----
> From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On
> Behalf Of Robert A.
> Macy
> Sent: Monday, October 08, 2007 1:10 PM
> To: emc-p...@ieee.org
> Subject: RFI/EMI and Susceptibility on PCB for Analog
> Signal Distribution
>
> Background
>
> Trying to 'Engineer' the design of transmitting more than
> eight balanced analog signals of very high quality over a
> length of 30-40 cm using PCB structures that simulate
> twinax cabling.
>
> The signals themselves are energetic with bandwidth
> exceeding 50MHz at levels more than 200 Volts. Impedance
> of the twinax is 150 ohm to a capacitive load, so only
> 150mA spikes occur.
>
> These high quality analog signals cannot stand more than
> 1/4 mV injected back into them. We're talking 120dB
> analog
> distribution system here.
>
>
> Need Help with E3
>
> Anybody have experience, or data, regarding RFI/EMI
> generation *and* susceptibility from PCB structures that
> are made to simulate twinax? Interested especially in
> susceptibility.
>
>
> Here are two possible PCB constructions:
> First, use 3 copper layers:
> GND layer,
> then side by side traces
> GND : S+ : S- : GND
> GND layer
> Second, use 4 copper layers:
> GND layer
> then single trace per layer
> GND : S+ : GND
> GND : S- : GND
> GND layer
>
> which is better? It seems that four layers is worse,
> because with four layers the adjacent traces are more
> likely to 'talk' through to adjacent structures.
>
> Does anyone know of a better structure?
>
>
> Also, given thickness of the insulation, what should be
> spacing? and spacing of 'stitching' vias from the top GND
> through guard GND traces through to bottom GND layer?
>
> Then, after selecting a structure, what is susceptibility
> of that structure? How does one predict the amount of
> signal corruption that will appear on a pair of lines?
>
> Robert
>
> -
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