I'm seeing a 1.2GHz signal on the audio output

I’m seeing a 300mV, about 1.2GHz, signal on the audio output. At first I thought it was my TruSDX radio. It evens happens when the radio is unpowered. It happens on any of the USB ports on my Win 11 PC. It happens when WSJT-X is connected or not. It goes away when the DigiRig is unplugged from the radio Mic input.

I used my NanoVNA to characterize the Mic input to the TruSDX. It looks like about 0.01uF to ground. When I put a 0.01uF cap across the DigiRig audio output, it oscillates at 1.2MHz like before.

When I try to use WSJT-X with DigiRig it splatters noise across at least 20KHz.

On and O’scope, I can see the WSJT-X audio signal I expect riding on the 1.2MHz signal.

Any ideas?

Curt

I found a workaround. If I put a 600 ohm, 1:1, audio transformer in the audio path the the oscillation goes away.

Here is a video of what I did:

Curt, 73, KG7ETE

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Thank you for posting the info. If you look at the schematic of the interface, there is only a decupling cap between the codec’s output and the radio. It is possible that the oscillation is a side effect of the capacitive load on the IC and it normally expects low impedance resistive or inductive load (e.g. headphones).

TruSDX appears to be the only radio that causes this effect and fails to prevent the out-of-band signal from getting on the air.

Thanks for posting a transformer based solution to this rare case. As a potentially simpler solution try loading the output with a ~30 ohm resistor.

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