230 BELL SYSTEM TECHNICAL JOURNAL 



circuit, a condition which permits damping of transients set up by changes 

 in gain occurring from AVC action. The input of the Unear amphfier 

 is held constant by the hmiter. Gain changes in the Hnear amplifier pro- 

 duced by the variation of Cr (Fig. 15.4) are absorbed by AVC, while the 

 variation of activity in the crystal is absorbed by the limiting amplifier. 



15.50 Evaluating Performance Index 



From (15.28) it can be seen that the attenuator, A, the effective variable 

 capacitance, C(, together with the vacuum tube voltmeters, d and Cp, 

 offer a number of possible variations in the method of evaluating the con- 

 stants used to determine the PI of quartz crystals. There are, however, 

 two principal methods — the first provides direct reading, while the second is 

 more accurate but requires an indirect evaluation. 



The first method utihzes a means of calibration of the meter scales directly 

 in terms of PI. The attenuator is adjusted such that its indicator reading 

 times the multiplying factor associated with the dial attached to Cs is some 

 multiple of 10. If in the calibrate position, Ac is set at unity, and Cpc and 

 Cic are adjusted by varying the capacitive load to some reference deflection, 

 then the expression for PI becomes 



where 



PI = -^ K^ (15.34) 



^ic . Ck Ra Ca 



_6pc {^t ^x) 



The Performance Index then is indicated by the two readings of ep and d . 

 The absolute magnitudes of ei and ep need not be known since it is possible 

 to use as a reference, the arbitrary calibrating deflections of e^ and epc . 

 The magnitude of Cp indicates the significant figures while d is a multiplying 

 factor. 



The second method of evaluating Performance Index eliminates any 

 calibration errors in the two vacuum tube voltmeters, Ci and Cp . This 





method utilizes the attenuator to adjust == 1- In the "cahbrate" 



\_ei epc A 



operation, e.c is set to equal e. and then epc is adjusted for full scale or a 



convenient deflection. In the "operate" position, the attenuator is varied 



until ep equals Cpc . In this manner, the two readings of the attenuator 



Cp 



are used to determine the ratio of — and the measurement is independent 



enc 



