Interesting indeed.
Some more pictures.??
First with x69 = 60, agc enabled
And with the gain fixed:
Perhaps some explanation - the response of? my SAW filters as measured with a NanoVNA:
I tried with and without matching for the filters.? With matching (pink trace) the response is much flatter but has the feature around 438MHz.? I figured, perhaps wrongly, that the narrow response at the IF without the matching (brown trace) would be an advantage, and the other peaks far enough away to not be an issue, and no worse than having a flat passband.? My filter is on a double sided home etched PCB.? With the matching components fitted having enough vias in place proved important.
This filter response explains I think the odd agc action, but not the spurious signals present when the agc changes the gain.
Dave
