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
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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