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BELL SYSTEM TECHNICAL JOURNAL 



Figures 15.7 (A) and (B) show the ampUtude correlation between the 

 Performance Index meter and the grid current of the "Tuned-Plate" and 

 'Tierce" oscillators, respectively. The P.I. vs. grid current characteristic 

 was arbitrarily taken at three frequencies — 4.5 mcs, 5.66 mcs and 7.81 mcs. 

 The change in grid current of the oscillator shown in Fig. 15.7 (A) with 

 frequency is caused by the varying L-C ratio in the plate circuit. (See 

 curv-es A, B and C for constant crystal activity.) The curves A and D 

 represent the effect of changing bands by switching coils, varying the L-C 

 ratio 2 to 1 in the plate circuit for the same crystal frequency. Fig. 15.7 



A AND D 7.810 MC. 

 B 5.660 MC 



C 4.50 MC. 



(A) 



A 7.810 MC. 

 B 5.660 MC. 

 C 4.50 MC. 



.^^' 



(B) 



PI PI 



Fig. 15.7 — Typical oscillator characteristics. 



(B) represents the correlation between P.I. and grid current of the "Pierce" 

 type oscillator. The results of this correlation indicate that the grid cur- 

 rent is essentially independent of the operating frequency. 



The measurement of P.I. is independent of the level of crystal vibration, 

 provided that the electrical equivalent circuit parameters of Fig. 15.1 

 become constants; however, in actual practice these are not constants, 

 particularly Ri . Variations of this type, as previously discussed, make 

 it necessary to duplicate the amplitude of oscillation of the P.I. meter with 

 the oscillator. Fig. 15.8 represents the variation of P.I. as a function of 

 voltage across the cr}'-stal terminals for five crystals arbitrarily selected. 

 It is readily observed that P.I. may be a random function of amplitude. 



