A circuit for extending the frequency range of a digitally generated FM signal (f). A digital frequency generator (1) generates a precise FM waveform at a frequency up to about 50 MHz. Coupled to the FM signal (f) is a phase lock loop (53, 55, 57) comprising a voltage controlled oscillator (VCO) (57) whose output frequency can be much higher than the frequency of the FM signal (f). In a first embodiment, a crystal oscillator (61), mixer (59), and low pass filter (63) are used to bring the output (f') of the VCO (57) down into the range of the digital frequency generator (1). In a second embodiment, an analog frequency divider (73) is used to bring the output (f') of the VCO (57) into the range of the digital frequency generator (1). In a third embodiment, the single sideband (SSB) generator (37) from the digital frequency generator (1) and a counter (66) are used in a partly analog, partly digital, negative feedback loop.