Abstract: A device for detecting input signals whose frequency lies within a predetermined range of frequencies. Included in the device is an oscillator and two sampling circuits which are operated by switching signals derived from the oscillator. An input signal of unknown frequency is applied to a first sampling circuit, an "oscillator sampling circuit" which generates a control voltage if the frequency of the input signal is within the range. The control voltage is applied to the oscillator and causes the frequency of the switching signals to become equal to a multiple of the frequency of the input signal. Further switching signals, also having a frequency equal to a multiple of the frequency of the input signal, are applied to the second sampling circuit, the "word sampling circuit", which also receives the input signal. The word sampling circuit then provides an output if the amplitude of the input signal is above a predetermined threshold value.