A discriminator that selects input signals of constant amplitude and produces an output voltage proportional to the amount that the input frequency differs from a fixed frequency. Frequency discriminators are commonly used in automatic frequency control systems (when the output is used to correct the frequency) and in frequency-modulation systems to convert the frequency-modulated signals to amplitude-modulated signals. The design of frequency discriminators is such that noise due to amplitude variations in the received signal is almost completely eliminated.