riLTERS FOR CA RK/ER .'iVSTEMS 207 



17% below what it would be with a fully plated crystal element having the 

 same inductance. This method of capacitance reduction was used on the 

 six low-inductance crystal elements in each section. Another step in 

 minimizing the unwanted capacitances was to design the retardation coils 

 which connect to the terminal ends of each lattice to have as little capacitance 

 as possible. Finally precautions were taken to keep the wiring capacitances 

 to a minimum and the air condensers used inside the lattice for adjustment 

 purposes are of special design having a minimum capacitance of only 

 0.5 mmf. 



The resonant frequencies of each of the twenty-two crystal elements must 

 be adjusted to the desired nominal frequencies within very close toler- 

 ances. On the ten high-impedance crystal elements the tolerance is ±2 cps 

 while on the 12 low-impedance crystal elements the tolerance is ±5 cps. 

 This precise frequency adjustment is accomphshed by careful grinding of 

 the length of the quartz plate. 



The equivalent inductance of each of the 22 quartz crystal elements is 

 required to be within two per cent of its nominal value. This specification 

 is met primarily by close dimensional tolerances in the manufacture of the 

 quartz plate. Any small adjustments which are necessary to meet this 

 requirement are done by the aforementioned method of isolation of a small 

 amount of plating from near the end of the quartz plate. 



The four fixed retardation coils are adjusted to be within two per cent 

 of their nominal inductance values. The variations from nominal are par- 

 tially absorbed in the filter adjustment procedure where the coils are tuned 

 with their associated variable capacitors to give the desired resonance fre- 

 quency. The fixed mica capacitors are manufactured to be within 0.5 

 per cent of the desired nominal value. The three adjustable retardation 

 coils are constructed to permit an inductance variation of five per cent on 

 either side of their nominal values. This is done by moving a permalloy 

 core in the field of the coil. Adjustment of these coils in the filter is accom- 

 plished by tuning them with their associated precision capacitor to give the 

 desired resonance frequency within ±25 cps. This type of adjustment 

 procedure gives the correct LC product. The correct L/C quotient is 

 obtained also since C is accurate to ±0.5 per cent. The two resistors at 

 each end of the filter compensate for the dissipation in the end retardation 

 coils and thus restore the terminating impedance to the value required for 

 optimum filter performance. 



Each lattice of crystal elements and capacitors is a four-terminal bridge 

 which is adjusted for maximum bridge balance at a particular frequency 

 by means of the variable air capacitors in two of the arms. The precision 

 of inductance adjustment of the crystal elements insures that the other 

 peaks of attenuation will be sufficiently close to their nominal locations. 



