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Old 4th July 2020
Lives for gear
jaddie's Avatar

A few things to think about:

1. The core to core (conductor to conductor) capacitance is not the same as the core to shield C, which may be lower. It may make more sense to float the unused conductor of a shielded pair for unbalanced wiring in a console.

2. Shielding, even with 100% coverage, is only part of RFI proofing. The other part is good output circuit design and RF immune input circuit design.

3. David noted, a thorough understanding of the interference you are primarily dealing with is essential. If your desk is to be used in a high RF field produced by a broadcast transmitter, you need to remember at what frequencies those operate. Same for mobile devices, etc. And along with frequency, field intensity. anecdote: many years ago I built a complex array of studio in an office building that faced a major broadcast transmitter center. Before starting construction I measured RF field intensity in the space at all known high power transmitter frequencies. The intensity was greatest in the 50 mHz band, US TV channel 2. In retrospect it was most likely due to some sort of resonant cavity effect, but we anticipated having RFI issues, so the solution was to enclose the entire technical core in RF shielding, and filter every pair of wires passing into and out of that shielded area. It was expensive, and effective. The offending TV station was not even receivable in the completed space. Overboard? Absolutely! Necessary? Not entirely, but you know, you get your one shot, you can't screw up, the company had money, and we, the engineering staff, didn't have enough experience to take a chance on something less intensive.

So we didn't have RFI issues at all, but we did have ground loop issues, resulting in furthering my personal understanding of balanced line receivers, and common mode rejection in the real world. We had two consoles that were completely unbalanced. That worked fine within their own control rooms, but not interconnecting to the rest of the system. We had purchased a flock of power amps that had a poorly designed balanced input with only 40dB of CMRR, which we had to modify to make work with long cable runs.

The lesson learned was that you have to consider the entire picture and not obsess about one small portion of it. The process of choosing the cable to use should include a study of ease of termination, diameter, and flexibility as the total cost of the cable should include installation time. Including of course the electrical properties.