so ive been working on this little project for a few weeks now, basically trying to get an arduino uno to key my IC-7300 via the CI-V interface and also trigger the PTT line with a transistor. the idea is to automate some beacon transmissions on 10m while im at work, nothing fancy just ID every 10 minutes or so with a CW message.

anyway everything was going fine, i had the timing worked out and the morse code library was doing its thing, but now whenever the arduino tries to key up i get this nasty voltage spike on the audio line and it ends up with a weird chirp at the start of every transmission. i measured the PTT line and its clean but somewhere between the arduino firing and the final RF going out something is causing that audio artifact. i put a 100 ohm resistor inline thinking it was a grounding issue but no dice.

has anyone dealt with this before? im using a 2N2222 as the switching transistor and the arduino is powered off a separate USB supply from the radio. maybe thats the problem actually, ground loops are the bane of my existence with this stuff.

yeah that sounds like a ground loop to me honestly. if your arduino and the radio are on separate supplies and you dont have a common ground between them youll get exactly that kind of garbage on the audio. i had the same thing happen with a raspberry pi zero i was using to control my old 857. what fixed it for me was running a single wire from the arduino ground to the radio chassis ground, not through the USB supply, just directly. the 2N2222 should be fine for PTT switching as long as you have a pulldown on the base, which im assuming you do.

the other thing worth checking is whether the CI-V and PTT are fighting each other timing-wise. if PTT fires like 5ms before the CI-V transaction finishes you can get some weird startup artifacts. i added a 50ms delay in my sketch after the CI-V command before actually pulling PTT low and that cleaned things up a lot.

might also want to throw a small cap across the transistor collector to ground, like 0.1uf ceramic, ive seen that tame some of the switching transients on the PTT line. also are you sure youre not accidentally feeding audio from the arduino into the mic input somehow? some of those boards can leak a tiny bit of signal on the GPIO when they switch states and if thats anywhere near the mic circuit on the 7300 itll pick it up for sure.