Abstract: There is disclosed a digital frequency synthesizer which converts an ultrnic burst from an echo-ranging sonar system into a continuous signal having a frequency exactly equal to the frequency of the cycles making up the burst. This burst is detected and a number representing the period of this burst generated and transmitted to a first counter. A clock pulse train is generated and transmitted to a second counter. When the numbers in the first and second counters coincide, a single output pulse is generated and the second counter reset. When this cycle is repeated, the frequency of the resultant pulse train reflects any change in frequency of the cycles making up the bursts.