This circuit converts frequency to voltage by taking the average dc value of the pulses from the 74121 monostable multivibrator. The one shot is triggered by the positive-going ac signal at the input of the 529 comparator. The amplifier acts as a dc filter, and also provides zeroing. The accuracy is 2% over a 5 decade range
The input signal to the comparator should Jbe greater than 0 volt peak-to-peak, and less than 12 volts peak-to-peak for proper operation.