Isolated BNC connector in metal chassis, EMC considerations

Question

I consider using an isolated BNC connector for a precision application, in which the BNC Shell will be either grounded via an internal switch or used as a Guard electrode. The motivation behind this is to avoid the cost and "nicheness" of Triax connectors. The circuit looks like this:

U1 is a buffer amplifier with high impedance. Signal frequency is DC to kHz, source impedance can be from low to >GOhm. Therefore, the two different operation modes of the Shell. If Switch W1 is closed, the BNC would work as a regular shielded, single-ended connector, which is the main purpose. For very high source impedance inputs to be measured, a guarded input is needed. Opening switch W1 allows this.

Problems:

When using the guard function, of course the ungrounded cable is a fasttrack for common-mode noise into and out-of the chassis bringing all sorts of EMI problems. The answer of Dan Mills to this post suggests that isolated BNC connectors are a bad idea for EMI. I suppose, the same would apply to other measurement devices such as table-top multimeters using unshielded banana connectors for example. So how do such devices tackle EMI with the unshielded cables?

a) One option I thought of was tackling this with a common-mode-choke. But: Won't that commom-mode-choke hinder the guard drive function. And when using the grounded shell, won't that same common-mode-choke couple the chassis noise into the sensitive circuit?

b) Another idea would be to RF-bond the BNC Shell to the chassis anyway, leaving the guard function only functional for low frequencies (sub-kHz). But I wonder how to achieve this? A capacitor is baaad at RF-bonding from what I have read.

I would be happy to hear your thoughts on my two ideas (a) and (b), or about ideas of your own on how to make isolated BNC ports work well EMC-wise.

Answer 1

Hard ground is preferred for coax because the impedance are low (50,75) and good coax is a must where semi-ridged coax is preferred for critical circuits in the GHz range. ( used by all and HP spectrum Analyzers and sub-ns scopes).

But you implied avoidance of Tri-ax connectors implies high impedance audio sources. Therefore balanced source impedances are best to reject common mode noise. Depending on the spectrum of interference also demands a range of solutions. High L CM chokes at source for line noise and low L CM chokes for RF with an RF shunt cap to make a Pi filter.

In the old days, signals often went thru capacitive shunting feedthru’s for cable signals into a box, and RF AM can get demodulated if sufficiently coupled into high impedance preamps from long cables which need to be shunted by an RF cap near entry.

Details require the source, cable and load impedance over the entire spectrum of signal and noise.

If all the signals are very long wavelength compared to cable length then cable impedance and reflections won’t occur on the signal but maybe on RF noise.