Speed-dependent re-Hautzel

Signal generator for use in testing FM communication receivers including a triggered free-running multivibrator which may be synchronized to operate at a frequency equal to the spacing between adjacent channels of the FM receiver. An audio oscillator modulates the output of the multivibrator which output is applied to a step recovery diode. The output of the step recovery diode is a plurality of harmonics of the frequency of the multivibrator each frequency modulated at the frequency of the audio oscillator.