Hi Scoupe,

A few things to consider;
Since it looks like the AMPLITUDE of the ac signal is Frequency dependent, ie Lower Frequency means Lower voltage peak on the sine(?) wave, it may be the easiest way to go is with a EXTERNAL comparator or use a op-amp to convert to a square wave. Any good book on intro to op-amps or The ARRL Handbook for Radio Amateurs chapters on op-amps or comparators. You also could try Googling op-amps or comparators.

Hope this makes things less muddy.
Tryin to help
Terry K9HA