Electronic Introspection

I have invented a metal detector which uses two channels in lockstep. This, I think, is new. Now, say it oscillates at 100,000 Hz in lockstep, and some "find" will increase this to 100,100 Hz. One needs to measure these frequencies, not parallel with another oscillator, but relative to both of them over time, so as not to disturb a very sensitive circuit. Most obviously, one could have a little computer which calculates 100,100 Hz at time T2 minus 100,000 Hz at time T1 = 100 Hz over 0 Hz. But try designing that. OBSERVATION: A super simple electronic "calculator" is required. My guess: one that would throw away most of 100,000 Hz and compare the remainder at T1 and T2 ...