[SI-LIST] Re: Differential Signalling through Tri-axial Cable ?....

Peter:
Although "the customer is always right" generally applies to retail sales, it 
sounds like he/she is creating major problems with the stated cable 
configuration. The most common use for triaxial (otherwise known as 
double-shielded) 
cable is for increased isolation/shielding for both radiated emissions and 
susceptibility. You are right to be concerned about the common-mode voltage 
conversion that will result from the triaxial cable being driven from a 
differential 
source. The result will undoubtedly be an increase in EMI (radiated emissions).

Depending on the frequency content of the signal (which you failed to 
mention), the use of a twisted, shielded pair (TSP) would appear to be a 
superior 
choice from both the technical and cost points of view. Many TSP designs are 
closer to 100 Ohms, but there are probably multiple sources for lower impedance 
TSPs. The stated differential impedance of 75 Ohms for your PCB might also be 
changed to 100 Ohms which would more closely match widely available, low cost 
TSP cable. If the 75 Ohms is related to TV or other communication circuits, I 
would expect it would be single-ended, not differential, in nature; therefore, 
a 
different approach altogether may be in order.

I can't comment further with the limited info you provided.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

*** Serving Your Needs with Technical Excellence ***


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