Abstract: A frequency determining apparatus for receiving an input signal of unknown frequency and providing an output signal representing the frequency where the output signal is updated at substantially regular intervals regardless of the frequency of the input signal. A frequency divider or prescaler divides the input signal by a factor n to give an adjusted signal. A gated counter counts stable clock pulses for a time period related to the period of the adjusted signal to give a count inversely related to the frequency of the adjusted signal. A calculator corrects the count for the factor n and derives an output signal representing the frequency of the input signal. The calculator also determines if the count has increased above an upper limit or decreased below a lower limit, and if so it changes the factor n accordingly to maintain the count and correspondingly the adjusted signal within a predetermined range.