Abstract: A high frequency SSB radio transmitter has an envelope amplitude modulator for varying the envelope of an r.f. signal source based on an error signal from envelope detectors detecting the envelope of the input and output waveform. It also has a first phase modulator in a main feedback loop for varying the phase of the input waveform based on differences detected in a phase detector between the instantaneous phase of the input and output r.f. signal. To overcome the problem of spurious outputs from the phase detector resulting from the cross-over points of the SSB waveform when there are carrier breaks and other problems, a subsidiary, phase lock loop feeds a signal derived from the error signal to a second phase modulator to tend to hold the inputs to the phase detector in such a phase relationship that the output is zero. To cope with large phase shift errors between the input waveform and the output which result when the power amplifier changes frequency or temperature variations e.g.