Envelope detector circuits
simulate this circuit &ndash; Schematic created using CircuitLab

This is a negative envelope peak detector.  

If f is too high (100 kHz) then each cycle does not have enough time to charge up the negative peak , so it takes several cycles. Fig 1 looks like ~3 cycles to 64% peak of attack time or 30us must be around T1=30 us.

If f is too low ( 100 Hz) then the peak decays too much within each cycle of 10ms so the decay time T2 is approaching 10ms. 

Both R values may be scaled down with C scaled up to get similar results.

Values are estimated quickly due to less than ideal photos.