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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