external microphone line levels

[DrMag]

I've been working on a project that would send a specified audio signal to my radio (could be hand-held, could be mobile, etc.) microphone line. For the life of me, I can't find anywhere in any radio's manual information on what signal amplitude the radio expects. The only thing that's listed is the microphone impedance (600 ohms seems common). Not wanting to destroy something in the radio by applying too high of a voltage to the microphone line, I'm trying to figure out where the line level should be. Is there a way to find that out or determine it from the microphone impedance, or am I stuck doing it manually by attenuating the signal to zero and gradually turning it up until the audio level sounds right, then measuring the output voltage that gives that level?

My current radios are an old IC-V8 (mic impedance not listed in the manual) and a TM-V71A (listed as 600 ohms) if that's of any use.

[DrGeoff]

It sounds like a dynamic mic. The output from the mic is likely to be around 20mV or so, depending on how close you have it and how loud you shout into it.
The radio probably will have an ALC circuit as well, to adjust the gain level to compensate for variations in input level. This also helps maintain better signal modulation.

[DaJMasta]

+1 it's very common to see dynamic mics in radios, so I would expect that as well.  You can check to be sure, and see if there's any DC bias coming out of the mic jack, if there is, then it's using a different kind of mic that involves active circuitry (like your typical electret or various condensers).

Regardless, if you know what mic would have come with it or some that are known compatible - you could see if they're specified.  If you've got a working mic, you can always just tap into the line with a high impedance probe and take a look for yourself too!

[4CX35000]

You can measure the voltage output using a oscilloscope with a pair of 1k2 resistors in parallel and speak into the mic. Use the second oscilloscope channel as a marker to show where the peak of the audio output is.

If the mic is 600 ohm then it will probably be similar to the mic in the transceiver, so you can probably just add the mic to the transceiver input and adjust the FM deviation settings on the transceiver to adjust the output. Listen on your second transceiver for maximum performance with no clipping.