🚀 go-pugleaf

use a super-hetrodyne principle


                                 
source ----X-----[fixed notch filter]---A/D---C64
           |  
           | 
           ------[Voltage Controlled Osc]---|
                                            |
                                            |
	[ramp voltager generator]-----------|

the idea is that the fixed notch filter can be much more
accurate than a sweeping filter.

multiply the source out to a higher freq, filter it out,
then shift the source by a small ammount more (ie up the freq of VCO output.

You can use an oscope with the ramp voltage as x and
notch filter output as Y -- this will give you a description of your input waveform.

I learned this in a Signals course which covered fourier and laplace transforms.  A good book with some practical circuits might have this circuit and would also give the math background as well.

Mike (dusting off the cobwebs) Seningen