PERFORMANCE INDEX OF QUARTZ PLATES 223 



not to introduce disturbing capacitance. Splitting the series capacitance 

 into two parts, C. and Ca-, the latter fixed and large compared to C,,, provides 

 the impedance element across which the voltmeter is connected. The input 

 capacitance of the voltmeter is incorporated in the magnitude of Ca-. A 

 capacitance attenuator, A , of known or calibrated values interposed on the 

 input of the voltmeter enables the voltmeter to be used to indicate voltage 

 ratios in terms of the attenuator calibration. 



The measuring voltmeter and a shunting capacitance, Cp , are connected in 

 the plate circuit of the amplifier tube, V-1. This circuit provides sufficient 

 gain to furnish an output voltage of measurable magnitude and also provides 

 an output voltage inversely proportional to frequency. The indication of 

 the output voltage is proportional to PI. 



The utilization of a vacuum tube in a circuit leading to a quantitative 

 measuring instrument such as the voltmeter across Cp involves determination 

 of tube constants or calibration. The determination of these constants is 

 best evaluated experimentally. A calibrating circuit for that purpose is 

 shown in Fig. 15.3B. A capacitance, Ca, of high impedance in series with 

 comparatively negligible resistance, Rj, is connected across the driving volt- 

 age terminals of d with a voltmeter measuring g; giving a reading Cic. The 

 second subscript "c" indicates calibration conditions. By connecting the 

 input circuit of V-1 across this resistance, the attenuation variation with 

 frequency of the Ra — Ca network cancels the attenuation variation with 

 frequency in the plate circuit of V-1. The ratio of dc to Cpc will then be in- 

 dependent of frequency. In the "calibrate" circuit (Fig. 15. 3B), the capac- 

 tior attenuator, Ac, interposed in the grid circuit is set at unity (minimum 

 insertion loss) for a given deflection of the meter indicating Cp. In the oper- 

 ate circuit (Fig. 15.3A), the attenuator is readjusted so that voltage eo 

 produces the reading of Cp as obtained in the calibrate position. The quan- 

 titative action of the amplifier then m.ay be expressed in terms of Cp, Ra, 

 Ca and a reading from the attenuator A, as will be shown later, and it is 

 constant and independent of frequency. By placing this resulting constant 

 in an equation, which will also be derived later, the value of PI may be de- 

 termined in terms of such constant, of the reading of attenuator A, and of a 

 reading on the scale of Cs that has been calibrated in terms of Ct. 



To facilitate still further the operation of the PI meter, the voltage Ci is 

 produced as shown in Fig. 15.4 by arranging for the oscillator to have its 

 frequency controlled by the crystal through feedback from capacitor, Ck . 

 Automatic volume control is provided such that the amplitude of d is 

 essentially constant at all times and at all frequencies. The circuit is con- 

 structed to oscillate at the desired frequency, and adjustment for insuring 

 this operation is provided in the form of a phase shifting circuit with variable 



