Automatic Frequency Control for a Marginal-Oscillator Magnetic Absorption Spectrometer

A closed-loop frequency-control circuit is added to an rf spectrometer of the Pound-Knight-Watkins type. Frequency variations are detected by a discriminator circuit operating at a heterodyne frequency of 175 kc/sec. Correction signals affect a saturable reactor which is added to the spectrometer. Frequency variations are reduced by factors of 5 to 11. Larger factors can be obtained without instability. The objects are (a) to reduce frequency drift due to physical changes of the coil surrounding the sample; (b) to reduce distortion of absorption line shape, due to frequency modulation arising from the dispersion component in magnetic resonance.