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I'd like to build an audio interface cable from my smartphone to my Yaesu FT-60 which incorporates VOX (since the Yaesu doesn't have VOX functionality) for APRS apps on the phone.

I'm having problems with RFI on my interface cable from the 5W 144.39 MHz signal the Yaesu is outputting. I'd like to measure just the RFI signal that's being impressed on my interface cable so I can attempt to model it in SPICE.

I have several questions:

  1. Both devices are battery powered and their audio grounds are tied together. However, their grounds are floating relative to my scope's earth ground. The RFI signal appears on both the audio signal lines and the audio ground line connecting both devices. It seems like earth-grounding my audio ground with my scope probe might effect my measurements. Should use two probes--one on the audio signal, the other on the audio ground and leave the probe's grounds disconnected?
  2. Even with my scope probe disconnected from my interface cable, I get a significant RFI signal on that channel. This makes me wonder if the RFI I'm measuring on my scope is being conducted by the interface cable or by the scope probe and wire itself. Is there some way I can make sure I'm measuring the RFI on the audio interface cable in isolation?

I can think of two approaches for #2:

  1. Use two channels. Connect one to the audio interface cable and leave the other in roughly the same position as the first. Then subtract the second channel from the first to yield just the RFI that's on the interface cable. However, if there's any phase shift on one channel relative to the other, it seems like this subtraction won't work.
  2. I have a faraday box with 50 Ohm SMA pass-throughs on it. Put the FT-60 and interface cable in the faraday box. Connect the scope channel to the SMA passthrough and then put some sort of probe inside the box. I could use my scope probe again, but it seems like I would run into the same problem with RFI on the probe. Should I just use a SMA->BNC->mini-grabber inside the faraday box and connect that to the audio interface cable?
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I doubt that SPICE modeling will be successful. There are required criterion that you do not have available such as the impedances at frequency of each device connection, the impedance characteristics of the connecting cable, the skin depth and effectiveness of the shield, etc. As you have observed, you will also struggle measuring the common mode current at the frequencies even with a proper current probe.

A more pragmatic approach is to take remedial measures:

  • Wrap a few turns of the cable on a type 31 torroid core. Do this on both ends of the cable. This will mitigate common mode currents on the cable.

  • Install a 0.1 $\mu$F capacitor to ground from each signal line in the cable. This may be necessary on both ends of the cable.

  • Keep the cable as short as possible but avoid lengths close (+/- 10%) to odd multiples of 1/4 $\lambda$ long.

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  • $\begingroup$ I've actually tried all those things. They help, but sometimes an RF transmission still triggers the VOX circuit and that ends up locking the transceiver in TX. I noticed that many TNCs use opto-isolators. I picked a specific part and I'm trying it in my LTSPICE design, but I thought maybe I should characterize my RFI first. $\endgroup$
    – watkipet
    Commented Apr 30, 2018 at 14:35

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