412 



BELL SYSTEM TECHNICAL JOURNAL 



check frequency would be :!Cs = .2. A good practical limit to x is at 

 1.9 and here a value of Xs around .15 is best. A further improvement 

 of about 50 per cent could be obtained by choosing a higher value of Fo. 

 When comparing Fig. 2 with Fig. 1 it should be borne in mind that 

 the scale spread for y on Fig. 2 is ten times that of the Fig. 1 which 

 shows that an improvement in accuracy of at least ten to one is obtained 

 by the adjustment. This means, that given two frequency standards 

 Fo and Fs of sufficient accuracy a heterodyne oscillator can be built 

 having a much higher accuracy than an ordinary oscillator having the 

 same frequency range and same quality of circuit elements. This is 

 somewhat contrary to what we are accustomed to think. 



10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 3 — Frequency-output characteristic. 



One detail involved in the procedure of checking the oscillator which 

 permits this high degree of accuracy to be obtained needs elaboration. 

 As Co is varied during the adjustment by the amount ACo the value of/o 

 is changed and this destroys the low frequency adjustment at Fo. It is 

 possible to obtain the adjustment by a process of successive approxima- 

 tions but the procedure is tedious. The difficulty can be overcome by 

 the use of a mechanical device, however, as follows. The condenser 

 ACo is ganged to another condenser in the resonant circuit of the fixed 

 oscillator, and the two condensers are so proportioned that the change 

 in the fixed oscillator frequency is equal to the change in jo as the con- 

 denser is adjusted. This makes the low frequency adjustment inde- 

 pendent of the high frequency one. The oscillator is just set to the 

 required reading at F^ and ACo is adjusted until the frequency value is 

 correct. Theoretically instead of two condensers two coupled inducto- 

 meters could have been used to adjust the inductances in the resonant 

 circuits. The net result obtained would have been the same and the 

 mathematical treatment would be like the one given above. Conden- 

 sers lend themselves better to such construction, however. 



