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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:

enter image description here

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.

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  • \$\begingroup\$ Show each scenario schematically and indicate the driving circuit, the connector, the grounding points, the cable to the remote device, the remote device's connector and input circuit and, the remote device's ground bonding mechanisms. It's not a trivial problem to be handled by any manner of words attempting to describe a scenario. The devil lies in the detail; any bit of detail missed can result in an incorrect analysis. \$\endgroup\$
    – Andy aka
    Commented Apr 13, 2021 at 16:04
  • \$\begingroup\$ You are right, the topic is too broad..I will close this one and make another post with more details. \$\endgroup\$
    – tobalt
    Commented Apr 13, 2021 at 16:47
  • \$\begingroup\$ It's an interesting question but scenarios are needed else any answer becomes a book and isn't worth the effort. \$\endgroup\$
    – Andy aka
    Commented Apr 13, 2021 at 16:48
  • \$\begingroup\$ How about converting to balanced signal, isolate with transformer on sending side, and connect one RX to chassis ground. Should reduce common-mode. \$\endgroup\$
    – rdtsc
    Commented Apr 13, 2021 at 17:02
  • \$\begingroup\$ @Andy aka Ok I edited the post with more details, but wouldnt mind it closed. I guess my main question is: "How to achieve RF bonding of the connector shell to the chassis, without making it also DC-bonded?" \$\endgroup\$
    – tobalt
    Commented Apr 13, 2021 at 18:02

1 Answer 1

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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.

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    \$\begingroup\$ Who says I don't upvote you! \$\endgroup\$
    – Andy aka
    Commented Apr 13, 2021 at 16:49
  • \$\begingroup\$ I have added more details. I have no control over source balancing and sources range from low impedance to >GOhm. I am looking to bond the Shell the chassis at >10 MHz permanently while controlling its bonding at lower frequencies using the switch. I am aware of the feedthrough cap entries. It sounds exactly like what I need, but as I want to stick to BNC, I am wondering if there is a way to achieve the effect of a feedthrough cap entry with the Shell of an isolated BNC connector. \$\endgroup\$
    – tobalt
    Commented Apr 13, 2021 at 18:41
  • \$\begingroup\$ Physically it would be soldered to the pin and a solder lug on the rear. With a ferrite bead on the signal wire or trace. But CM noise is usually worse and gets converted to differential by unbalanced impedances which is why one uses a BALUN or CM choke. It depends on length of cable and SNR and Z mismatch and noise ingress from Transfer impedance. You might be better off with RJ-45 and STP with magnetics, but application is to vague to say for sure.. for long umbilicals , I always used a diff amp that is also balanced unlike yours. \$\endgroup\$ Commented Apr 13, 2021 at 20:42

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