HIGH ACCURACY HETERODYNE OSCILLATORS 413 



Stability of the Constants 



Having determined the oscillator errors from the variations in its 

 constants it will be of interest to inquire how large these may be. 



When the zero beat method is used the error AFpo will depend on 

 the value of the lowest beat frequency at which the local oscillators 

 can operate. With a reasonable amount of shielding and some pre- 

 cautions in order to avoid mutual inductance in wiring loops it is quite 

 practicable to keep this error below one cycle with local oscillators as 

 high as 200 kc. The beat frequency may be observed- on an ammeter 

 placed in the plate circuit of the modulator. When alternating current 

 from the power mains is used as a standard the accuracy is better than 

 one cycle. 



There are now available external frequency standards against which 

 the high frequency check could be made which have such high accuracy 

 that the resulting error in the heterodyne oscillator can be entirely 

 neglected. It is desirable, however, to make the oscillator independent 

 of external sources for its adjustment. A convenient checking circuit 

 consists of a quartz crystal which is thrown in with a key across the 

 grids of the output amplifier. At the series resonance frequency of the 

 crystal the loss introduced reduces the output so sharply that the 

 minimum output can be observed within 3 cycles at 100 kc. At any 

 other frequency the error is therefore 30 ppm (parts per million). By 

 using properly cut crystals the temperature variation error is made 

 negligible. 



The variations in L are chiefly due to temperature variations. 

 Ordinary potted coils having a large number of layers have temperature 

 coefficients up to 20 parts per million per degree Fahrenheit. The 

 variation is chiefly due to the expansion of the wire. 



This error is tolerable in audio frequency oscillators for most pur- 

 poses. For carrier frequency oscillators unpotted coils having a single 

 layer bank winding wound on a phenol plastic form may be used. Here 

 the lengthwise expansion of the form, which tends to decrease the 

 inductance partly compensates for the expansion of the winding which 

 tends to increase the inductance. Coefficients from to + 6 ppm per 

 °F. are obtained. 



The capacitance Ca in commercial air condensers has temperature 

 coefficients of up to 25 ppm per °F. This, again, gives sufficient ac- 

 curacy for audio frequency oscillators but is not satisfactory for carrier 

 applications. The variations in capacitance with temperature are 

 produced by increase in area of the plates with their expansion which 

 increases by an amount equal to twice the linear coefficient of expansion 

 of the material used. This change is partly compensated by the length- 



