Allard,
One thing puzzles me about this. I can see that the signal from the synth is buffered quite a bit by the on-board analog VFO circuitry before being applied to the mixer. I don't understand how changing the drive level in firmware would affect the mixer diodes after all the buffering, as I would expect the level to be pretty much constant. I will try different drive levels to see what difference it makes, but I'd also like to understand how it actually affects what the mixer circuitry does.
73,
Dave, N6AFV
