588 



BELL SYSTEM TECHNICAL JOURNAL 



harmonics in the output current were respectively 67 db and 80 db 

 below that of the fundamental. This purity of wave form is of course 

 largely dependent upon the absence of overloading. 



To correct any small initial frequency error of the crystal and to 

 allow for subsequent aging, a small reactance connected directly in 

 series with the crystal provides a convenient means of adjusting the 

 frequency as precisely as it is known. This added reactance may be 

 considered as modifying either of the reactances in the equivalent 

 series resonant circuit of the crystal. Figure 10 shows that for a small 



> 51 



OSCILLATOR OUTPUT LEVEL 

 INTO 150-OHM LOAD 



VOLTAGE AMPLIFICATION OF 

 VACUUM TUBE CIRCUIT 

 = 20LOG,o |}JL| 



8 9 10 



FILAMENT BATTERY POTENTIAL IN VOLTS 



Fig. 9 — Amplifier gain and oscillator output level vs. filament battery potential. 



range of frequencies the change introduced in this manner is accurately 

 proportional to the added reactance. Series inductance, of course, 

 lowers the frequency, while series capacitance raises it. The stability 

 requirement imposed on the adjusting reactance is only moderate, for 

 its total effect upon the frequency should not be more than a few parts 

 in a million. 



The frequency measurements here presented were obtained using 

 apparatus similar in principle to the frequency comparison equip- 

 ment of the National Bureau of Standards.^ Frequency differences 



* "Harmonic Methbd of Intercomparing the Oscillators of the National Standard 

 of Radio Frequency," E. G. Lapham, Jotirnal of Research of the National Bureau of 

 Standards, October 1936, p. 491, 



