High Accuracy Heterodyne Oscillators 



By T. SLONCZEWSKI 



The accuracy of a heterodyne oscillator after the low frequency 

 check is made is of the same order of magnitude as that of an 

 ordinary type of oscillator in which circuit elements of the same sta- 

 bility are used. It depends on the constants of the variable fre- 

 quency oscillator only. This accuracy can be improved by a ratio 

 of 10 to 1 by adding another and higher check frequency. The 

 temperature coefficient of the circuit elements can be kept down to 

 less than 6 parts per million. Scale errors can be reduced to a value 

 comparable with the oscillator accuracy by spreading the scale. 

 A precision oscillator having a frequency range up to 150 kc. and 

 an accuracy of zh 25 cycles including a scale mechanism whereby 

 a large scale spread is obtained on a direct reading scale is described. 



Introduction 



'"T^HE output 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 

 output 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 output is raised to the desired level. 



The earliest designs of heterodyne oscillator were confined to the 

 audio frequency range, but recently carrier-frequency applications 

 have become more numerous. As the frequency range of the oscilla- 

 tors has increased, their per cent accuracy requirement has increased 

 also. The required frequency accuracy of the oscillator is determined 

 by the maximum 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 re- 

 sult 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 transmis- 

 sion requirements independent of its location in the carrier band. 

 Therefore, the maximum slope of the characteristics is independent of 

 the frequency and an oscillator used for measuring purposes has to 

 meet a constant frequency error requirement. In addition the accur- 



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