The automatic frequency control of a nuclear magnetic resonance field discriminator oscillator, using an external crystal reference and a germanium diode as a high Q voltage-dependent capacitor, is described. A frequency stability of 1 part in 10⁷ is achieved, while allowing the frequency to be scanned through a range of 30 kc/s about the centre frequency of 30 Mc/s. The discriminator unit provides fine control of the 7000 G field of a large electromagnet and produces a corresponding 0.1% field scan with a stability better than 1 part in 10⁶.