Re: The Talking House newer upgrade?

I don't know, but I think if they used the same design as the old transmitters, then it would still have unintentional FM. My opinion is the FMing is caused by modulated RF feedback into the oscillator section. The transmitter is PLL controlled to stop frequency variations, but the PLL isn't fast enough to remove FM caused at an audio rate; audio is too fast to correct for.

This has happens with many transmitters, and it's a design issue. The oscillator should be fully RF-shielded and its power supply highly regulated and filtered, and have a good buffer amp between the oscillator and final.

I would think the designers of the Talking House knew this, but didn't consider the FM to be a problem in real estate voice announcement use, so they took the compromise to keep the unit cheap.

Another circuit that has FM in this way is the Ramsey AM-25 kit. The PLL's tank circuit inductor is probably picking up RF from the output.