A device for sensing the frequency of an alternating electrical input signal. An autocorrelator at an input to the device generates a signal having a periodic component of frequency equal to four times the frequency of any periodic component in the input signal. The amplitude of non-periodic components is substantially reduced. The autocorrelator is followed by a series of sixteen frequency dividers whose outputs are applied to respective comparators. Within each comparator the signal from a divider is compared with one of sixteen window signals, each representing a predetermined range of periods. Any output from the comparator is converted to a binary decimal form and is applied to a qualifier circuit. The qualifier circuit only gives an output if at least eleven of the last eighteen signals applied thereto have represented a signal whose period lies within the same one of the predetermined ranges of periods.