An adjustable frequency signal generator is described which utilizes a stable fixed frequency oscillator in combination with an adjustable modulus frequency divider, to obtain a frequency divided output signal whose frequency may vary within a wide range, while maintaining spectral purity of a signal generated by the stable oscillator. In the preferred embodiment the modulus of the frequency divider is controlled by an up/down counter. The frequency of the frequency divided output signal is detected with respect to a reference signal frequency. When the detected output signal frequency is higher, the counter is incremented, thereby incrementing the modulus of the divider and decreasing the frequency of the output signal. Similarly, when the detected output signal frequency is lower than the reference signal frequency, the counter is decremented, whereby the modulus of the divider decreases and the output signal frequency increases. When the detected frequency is within predetermined limits of the reference signal frequency, the modulus is maintained constant. The rate of change of the output signal frequency is independent of any signal frequencies in the system and may be selected by selecting a desired clock rate for the up/down counter.