There was a different model, the PDC256 that was built with an internal offset feature. It had no antenna-coax sockets since it was built to tap directly into the radio's VCO the way you hooked up this one. There were four so-called "DIP" switches that served to select the frequency offset. This is what allowed it to display the channel frequency while receiving.
The "250" model is only set up to read the radio's AM carrier frequency while you are keyed, and the mike gain is turned down.
This Ebay widget will pretty much do the same thing. The offset frequency isn't set with hard switches, but with a setup sequence controlled by two pushbuttons on the board.
https://www.ebay.com/itm/3215178846...d=link&campid=5336136228&toolid=20001&mkevt=1
Just three drawbacks:
No enclosure.
And it needs a lower power-supply voltage than the 13.8 Volts that runs the radio. We use a TO-220 "7808" 3-terminal regulator for this. And this regulator serves to isolate annoying digital noises from the radio's receive audio.
It has only one offset number. A sideband radio will need three of them, one for AM, and one each for upper and lower sideband. You can set it up to read accurately for only one mode. It will be wrong on the other two modes.
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