HIGH ACCURACY HETERODYNE OSCILLATORS 



411 



Oscillators With a Double Frequency Check 

 For carrier frequency applications the tolerable error takes a con- 

 stant value over the entire frequency range and it is found that if a 

 single frequency check is used it is not possible to obtain sufficiently 

 stable elements to maintain the required accuracy at points on the 

 scale removed from the check frequency. 



An increase in the accuracy of heterodyne oscillators has been ob- 

 tained, however, by adding an adjustable condenser to Co and checking 

 the oscillator at two frequencies, the low frequency i^o.and at another, 

 higher, frequency Fs. Adjustment of this condenser by ACo introduces 

 a frequency change — ycofo^Col2Co adjustable in sign and magnitude 

 and this can be made to cancel the error AFca + ^Pl for at least one 

 frequency, the check frequency F^. Obviously if the adjustment is 

 made to correct for variations in Co no residual error remains. The 

 residual errors which remain after correcting for Ai^ca and AF/, are 

 shown on Fig. 2. The residuals of AFco and AFl differ from each other 



0.008 



0.004 







-0.004 



-0.008 



-0.012 



-0.016 



-0.020 



-0.024 



-0.028 



0.04 



0.12 



0.16 

 X 



0.24 



0.28 



Fig. 2 — The frequency errors In a heterod^'ne oscillator at a frequency F = x/o 

 + ^0 after the low and high frequency checks had been made can be obtained from 



the plot as follows:For a variation in thcair condenser capacitance C,,, AFco = y -p—fo', 

 for a variation in the inductance L, AFl = J -f Jo- 



SO little that only one set of curves was drawn. The values of 3' were 

 obtained by forming the sum y = Kyc„ + yca ^^^ choosing K so that 

 y = oior x, = (Fs — Fo)lfo. 



For Xs = .1 better compensation is obtained at the lower end than 

 at the higher. For a very wide frequency range up to x = .25 the best 



