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A new arrangement is described in which a single resonance in a centimetre-wave cavity is used to give a discriminator action with high sensitivity and stability. The construction is considerably simpler than that of discriminators described previously, whilst the performance is free from all the errors that have been eliminated from the best of such discriminators. Only a single loop is used to couple with the cavity, and the discriminator action is obtained by arranging that this loop is self-inductive, so that the total impedance across it varies linearly with frequency near the cavity resonance. The output of this discriminator arrangement, however, contains a steady direct voltage, independent of frequency, but changing with the input power. This unwanted effect can be avoided by a switching arrangement used to vary the coupling between the loop and the cavity at some suitable frequency; 400 kc/s has been used. A tuned amplifier can then extract the output component at this frequency, and with a phase-sensitive-rectifier it is possible to obtain a discriminator characteristic output which is both amplified and free of serious errors due to changes of input power and drift effects in the crystal detector. An application of this discriminator to stabilize the frequency of a reflex-klystron oscillator is then described in which errors in the oscillation frequency are detected by the discriminator and are made to alter the reflector voltage on the klystron in such a direction as to reduce the error. The apparatus described has been in use for over a year and has made possible the generation of oscillations constant in frequency to one part in a million for periods of up to half an hour without readjustment.