Abstract: A continuous wave laser and modulator drives a fibre optic delay line whose output is detected by a photo detector and gain control amplifier. The signal from the amplifier is phase advanced in a modulator by a phase advance signal whose period is equal to the loop delay including the delay line. The resulting signal is added to an input signal in an adder whose output is the modulating signal for the modulator. Cycles of the phase advanced signal are counted to establish a processing time of N circulations round the loop where N might be 160 or significantly larger. At the end of the processing time, the output signal from the adder, over a segment equal to the delay time, represents the frequency spectrum of the input signal. N can be varied to vary the resolution of the measurement. The fibre optic delay line provides a highly stable determinate delay and permits very wide band operation.