This is a followup clarification (or at least a mind tickling comment) on the original question I raised as to the ADDED shielding effectiveness of an outer cable shield over a group of (inner) twisted, shielded pairs carrying high frequency digital signals (balanced ECL drivers and receivers).
The ability to reliably model circuit and cable performance to 22 GHz is more difficult to prove than to verify by test; however, knowing "ballpark" numbers and gaining the insight that models offer can lead to innovative designs and potentially save many man-hours of cost. In this particular case, the models are most useful in predicting the spectral content of the potentially offensive radiated emissions. As a qualitative comparison, I see that a small shield current (for a given ECL-driven, balanced wire pair) will flow on the INTERIOR of its associated shield. By consideration of skin depth, some (but less) current will flow on the OUTER surface of that same shield. This latter surface current can be likened to the surface current on a wire inside another shield (i.e., the outer, added cable assembly shield). Hence, I see the added cable shield like the outer shield of a coaxial cable, except the "center" wire is randomly displaced from the center, and mixed with other wires. From practical experience or otherwise, do you have an estimate for the ADDED attenuation (or SE)? I am anticipating 30 to 40 dB added reduction of radiated emissions from addition of this outer cable shield, assuming 360 degree termination at both ends of the cable (two to ten feet in length). Mike Conn Owner/Principal Consultant Mikon Consulting
