High Accuracy Heterodyne Oscillators

T HE o u t p u t frequency of a heterodyne oscillator is obtained by modulating the outputs of two oscillators of appreciably higher frequency, one of the oscillators having a fixed frequency, the other being continuously variable over a band width equal to the required o u t p u t frequency range. The circuit consists essentially of the two so-called local oscillators, the modulator, where the difference frequency is generated, and an amplifier where the modulator o u t p u t is raised to the desired level. The earliest designs of heterodyne oscillator were confined to the audio frequency range, b u t recently carrier-frequency applications have become more numerous. As the frequency range of the oscillators has increased, their per cent accuracy requirement has increased also. The required frequency accuracy of the oscillator is determined by the m a x i m u m slope of the frequency characteristic of the apparatus being measured. If this slope is great, as in the case of a sharply tuned circuit a relatively small displacement of the frequency will result in a large error in the value to be measured. In carrier-frequency systems where the signal is displaced upwards in the frequency scale by modulation, each channel has to meet same crosstalk and transmission requirements independent of its location in the carrier band. Therefore, the m a x i m u m slope of the characteristics is independent of the frequency and an oscillator used for measuring purposes has to meet a constant frequency error requirement.