A frequency discriminator using two Rayleigh-wave filters and two rectifying networks is described. For each filter, one of the transducers has a finger length varying according to a (sin x/x)² law, so that the frequency response is almost triangular. The central frequencies of the two filters are different. The direct voltages, obtained after rectifying diodes are connected at the output of each filter, are subtracted. The frequency-discriminator response is almost linear over a 1.4 MHz frequency interval and is zero at 29.75 MHz.