Abstract: An improved controller for vibratory seismic energy sources insures that the energy imparted to the ground by the source is in phase with the reference signal. Phase lock is achieved in a manner which minimizes the effects of the attendant frequency distortion. On upsweeps when the phase error is greater than some threshold level, phase lock is achieved by slowing down the drive signal, thereby introducing frequency distortion which falls below the bandpass of the sweep signal. On downsweeps when the phase error is above some selected threshold, phase lock is achieved by speeding up the drive signal, thereby resulting in frequency distortion which falls above the bandpass of the sweep signal.