Abstract: An apparatus for comparing frequency of an object signal with a given reference frequency, with a reference signal generator for developing a reference signal having the reference frequency. If the object signal is frequency-modulated, then its center frequency is compared with the reference frequency. A switching signal generator, switching circuitry actuated by the switching signal, a frequency discriminator, low-pass filters and a voltage comparator are also provided. Actuated by the switching signal, the discriminator alternately discriminates the frequencies of the object and reference signals, and the averaged discrimination output voltages are compared by the comparator to give the indication of comparison of the frequencies of the object and reference signals.