Measuring RF Intrusion in Microphone Bodies

[a soBer Newt]

I'm currently grappling with RF interference issues affecting a condenser microphone, and I suspect it might be digital noise demodulation. In my troubleshooting arsenal, I have an HP3585A spectrum analyzer and a TekBox TBPS01 EMC Near-Field Probe Set. While the spectrum analyzer has a limited range of 40MHz, I believe it could offer valuable insights.

My initial strategy involves inserting the near-field probe into the microphone body through the XLR hole and comparing the obtained results with background noise. To expand my options, I'm contemplating crafting an XLR to SMA connector adapter or creating a custom small EMC probe that I can mount in place of the microphone capsule.

Additionally, I'm exploring the possibility of using the tracking generator output as a noise source to perform a frequency response analysis.

1. Has anyone done anything like this to address digital noise demodulation in condenser microphones, and do you have any recommendations for its effective use in this scenario?
2. Are there considerations or precautions I should be aware of when crafting XLR to SMA adapters or custom EMC probes for microphone interference troubleshooting?
3. Regarding the tracking generator output, has anyone used it as a noise source for conducting a frequency response analysis, and what insights can you share on this approach?

Any help would be highly valued, and any additional tips or alternative approaches to tackle noise demodulation in condenser microphones are welcome.

Thank you for your expertise!

 

[ccaudle]

My initial strategy involves inserting the near-field probe into the microphone body through the XLR hole

The problem is that a lot of the interference is coming in on the cable, so a near field probe won't necessarily pick up the interference in a way that gives you any useful information.
RFI is primarily a geometry problem in microphones. If you have an AES membership you can look up papers by Jim Brown and David Josephson, or you can read a somewhat abridged version at Jim's web page:
http://audiosystemsgroup.com/RF_in_Microphones.pdfJim has a lot of good information in general on RFI mitigation:
http://audiosystemsgroup.com/publish.htm
The starting point is look at your microphone layout, and eliminate pin 1 problems first.
http://audiosystemsgroup.com/Pin_1_Revisited.pdfhttp://audiosystemsgroup.com/Pin_1_Revisited_Part_2.pdf
Start from first principles, identify obvious mistakes in the layout and correct those first. That will be a faster path than trying to figure out where inside a microphone body the problem occurs, even if you can pick up the interference on your probe. There are three pins on an XLR, so you will still end up in the same place of inspecting the geometry of the layout and identifying whether there are any filter components, and if the shield pin and filter components all terminate directly to the chassis as they should.

Fixing pin 1 layout mistakes alone may solve the problem. The next level after that is RFI filtering of pins 2 and 3 with ferrite beads and small geometry capacitors (i.e. low inductance capacitor layout).

You can check performance with a Neutrik XLR-EMC connector, if that helps significantly then it points to the pin 1 problem bringing in RFI from the shield to the audio circuitry.
https://www.neutrik.us/en-us/product/nc3fxx-emc?c=audio

 

[Ike Zimbel]

Are you taking this approach because you want to learn about this, or are you trying to fix the RF noise in your mic?
If it's the latter, you are probably experiencing interference from a nearby cellular site, and there are mic cables that you can buy that will fix that issue. There's a thread that I participated in on here about that issue within the last six months or so that has all the info.
If it is cellular RF you're seeing, that's in the range of 618-652 MHz, so I don't know that a 40MHz SA is going to tell you much.

 

[Speedskater]

You might e-mail Jim Brown:
Jim Brown <[email protected]>