MANUFACTURING DEVIATIONS IN CRYSTAL UNITS 



261 



values of the crystal unit in eitlier branch depart from their nominal values 

 by about one per cent. A negative departure in one branch results in about 

 the same efifect on performance as a positive departure in the other branch. 

 The difference between the two curves shown on Fig. 14.1 is that one as- 

 sumes a positive departure and the other a negative departure for the in- 

 ductance of a branch. 



Due to the close impedance balance which is required for these filters, 

 the effect of small departures in resonant frequency will produce rather large 

 variations in the transmission characteristic. For example, departures of 

 about 10 cycles per second in the crystal units of either branch will produce 

 variations in discrimination of about tlie same type and magnitude as those 



NORMAL INSERTION LOSS 



SERTIQN LOSS WHEN 

 NOUCTANCE or CRYSTAL 

 UNITS ARE IN ERROR 



-r" 



-4 -3 



FREQUENCY 



-2 -I C +1 +2 



IN KILOCYCLES FROM CARRIER 



Fig. 14.1. — The insertion loss characteristic of a crystal band-pass filter as affected 

 by deviations in the inductance of the crystal units. 



illustrated in Fig. 14.1 for departures in inductance. On the other hand, 

 if the crystal units of both branches exhibit equal departures the entire 

 transmission characteristic will be shifted by the frequency departure of the 

 crystal units, and there will be no loss in discrimination. 



Another way in which deviations in the properties of crystal units may 

 react on filter performance is illustrated by the schematic and curves shown 

 in Fig. 14.2. The schematic is the equivalent electrical circuit of a narrow 

 band filter, using two balanced quartz crystal units. The filter is designed 

 to provide a passed band of about 10 cycles per second with distortion of 

 less than 0.2 db. The insertion loss characteristics show that the desired 

 transmission can be obtained for various magnitudes of effective resistance 

 as long as the resistances in the series and diagonal branches are equal. 



