The use of a bidirectional square wave for frequency modulation of a radio‐frequency spectrometer offers certain advantages, the principal one being that the background signal due to incidental amplitude modulation can be effectively eliminated by appropriate adjustment of the relative amplitudes of the positive‐ and negative‐going excursions. A quadrupole resonance spectrometer employing this principle has given excellent sensitivity and faithful line shape presentation. The oscillator, of the regenerative type, is based on a circuit recently published by Knight. The frequency modulation method should be widely applicable to other types of spectrometers.