Simplified Automatic Stabilization of a Frequency-Modulated Oscillator

A frequency-modulated exciter which incorporates a quartz-crystal discriminator for center-frequency stabilization of the modulated oscillator is described. The unit was designed as the exciter for a frequency-modulated broadcast transmitter. The frequency-stabilizing circuits are the unique portion and are described in detail. Circuit simplicity is attained by operating the discriminator at the modulated-oscillator frequency, thus eliminating heterodyning and mixing circuits. A special bridge circuit, operating on the modulator bias, maintains the average frequency at the midpoint of the discriminator. Performance and stability of a completed unit are briefly discussed.