reference signal supplied by the generator 34 under these conditions the phase detector 23 produces an error signal which represents the difference between the carrier frequency of the cavity interrogation signal and the frequency of the stimulated emission this error signal is applied to the integrator 25 whose output signal adjusts the frequency of the oscillator 1 by way of the variable capacitor 26 so as to correct the frequency difference in the same way the detector 33 produces an error signal which represents the difference between the carrier frequency of the interrogation signal and the resonance frequency of the cavity this error signal is applied to the integrator 35 whose output signal adjusts the resonance frequency of the cavity by way of the variable capacitor 36 so as to correct the frequency difference the quality of the oscillator and cavity control action obviously depends on the precision with which the phase shifting circuits 42 and 44 produce a phase shift or difference in the components of the envelope signal produced by the amplitude detector 13 to bring them respectively into phase with the reference signal produced by the generator 34 in fact the above specified formulae express the theoretical values in respect of the phase shifts for the two components of the envelope signal but they do not take into account the phase shifts or differences due to the transmission lines and to the electronic equipment it is therefore necessary to effect adjustment in respect of the phase shifts and this adjustment can be performed as now described in order to provide for adjustment of the phase shift of the oscillator control circuit the modulation frequency can be shifted to a value of less than 1 hz and the.