Abstract: A narrow band level detector for use in the frequency generating equipment of an FDM-system is described. The detector includes a product modulator one input of which receives a sine wave signal, the other input receiving an incoming signal containing the periodic signal which should be detected together with disturbing periodic signals. The sine wave signal has a determined amplitude and frequency and can be generated by an oscillator or by a phase locked loop. The output of the modulator is connected to a bandpass filter whose pass band substantially coincides with the sideband defined by the frequencies of the disturbing periodic signals. A first peak rectifier is via an amplifier connected to the output of the bandpass filter and a second peak rectifier is connected directly to the input of the detector to form a signal whose level corresponds to the maximum value of the signal from the amplifier and the detector input signal, respectively.