Creating a filter/demodulator

[kc6ete]

Is there a template or example for creating a filter or demodulator?

This is very much jumping in at the deep end, but I'm wanting to try something like a boxcar integrator, where the data is averaged over many carrier cycles worth of time, and this process is repeated for various frequencies. It's used in radio astronomy, and there is a similar thing going on when they are looking at stellar Doppler shifts to find exoplanets, looking for periodic wobble.

I might be averaging several milliseconds of carrier, maybe as many as 100mS.