Abstract: A frequency control system with a phase-locked loop for automatically adjusting the frequency of an alternating signal and an external signal derived coherently therefrom to the variant zero phase-shift frequency of a phase shifter adapted to receive the external signal, the control system comprising: a phase shifter responsive to the external signal for introducing a frequency sensitive bipolar phase-shift to the external signal to produce a phase shifted external signal thereat, the phase shifter introducing zero phase-shift at a drift-prone frequency, f.sub.o ; an alternating signal source for generating the alternating signal of adjustable frequency covering a range of frequencies including frequency, f.sub.