The circuit can be used to tell whether or not an input signal is within a certain frequency range. The device consists of three !Cs, a dual monostable multivibrator, and two dual D-type flip-flops. The signal whose frequency is in question is fed to the clock input of one of the flip-flops. The Q output of that flipflop (IC1a) is cross coupled to its data input so that it acts like a divide-by-two counter. The trailing edge of the Q output is used to trigger the one shots formed by IC2.
The upper-and lower-frequency boundaries are determined by the two sections of IC2; the dual precision monostable multivibrator and their external rc networks. The upper-frequency boundary, fl, is set by the output of IC2a, and the lower-frequency boundary, f2, is set by the output of IC2b. The frequency of the input to the circuit can be anywhere from de to 100kHz. The states of the outputs of IC2, which determine the upper-and lower-frequency boundaries, are latched by IC3a and IC3b respectively. The output of IC3a will be high only when the input frequency is less than that of the output of IC2a, j 1. The output of IC3b will be high only when the frequency of the input is greater than that of the output of IC2b, f2.