You use a frequency discriminator to compare one signal frequency with another one. A functional feature, retriggering, of a monostable, one-shot 74xx123 multivibrator can yield frequency discrimination. Figure 1 shows a frequency discriminator that determines the relation of input-pulse frequency to a reference frequency. The external components, R₁ and C₁, set the reference frequency. These values determine the 74xx123's reference frequency as follows: f_R=1/t_W, and t_W=kR₁C₁. The multiplication factor k depends on C₁'s value and the power-supply voltage. The rising edge of the input pulse starts the one-shot, whose output switches high for the interval t_W. The same pulse edge sets the 74xx174 flip-flop to the same state as the output of the one-shot. If the interval between pulses is longer than t_W, the next pulse arrives after the one-shot returns to its initial state. The one-shot's output is low, and the rising edge of the input pulse sets the flip-flop low. The low flip-flop output indicates that the input-pulse frequency, f_IN, is lower than f_R.

If the interval between input pulses is shorter than t_W, the next pulse arrives before the one-shot completes its cycle and returns to its initial state. The one-shot's output is high, and the rising edge of the input pulse sets the flip-flop high. A high flip-flop output indicates that the input-pulse frequency, f_IN, is higher than f_R. Doubling the circuit in Figure 1 implements frequency discrimination with a "window" characteristic (Figure 2). Two pairs of R and C values determine the lower and upper reference frequencies. An exclusive-OR circuit takes the outputs of the upper and lower flip-flops. The exclusive OR's output is high when f_IN is between f_RL and f_RH. When f_IN is outside the frequency band f_RL to f_RH the exclusive OR's output is low. Figure 3 shows the frequency-discrimination characteristic. With R and C values as in Figure 2, and the use of a 74LS123 one-shot, f_RL=16 kHz, and f_RH=46 kHz. Other types of one-shots could produce different results.