MANUFACTURE OF QUARTZ CRYSTAL FILTERS 



525 



pregnated and potted in wax. A molded jacket with protruding fins, 

 placed around the core, reduces the capacitance from windings to core 

 and improves the uniformity of the windings. The coils are adjusted 

 to within ± 1 per cent for inductance and 2 per cent for inductance 

 unbalance by removal of excess turns, all adjustments being made at 

 low frequency. Figure 7 shows a coil at this stage of manufacture. 



The coil is then potted in a copper can and a cover soldered in place. 

 Final test simulates actual service conditions. The coil is resonated 

 with an external variable condenser at the operating frequency for 

 which it is designed. 



Condensers 



Nearly all crystal filters contain condensers shunted across the 

 crystal elements. These condensers must meet stability requirements 

 similar to those already mentioned in connection with coils. 



One form of fixed condenser, used where small values of capacitance 

 and high stability are required, is illustrated in Fig. 8. Silver is fused 



Fig. 8 — Silvered glass condensers used in crystal filter applications 

 where high stability is required. 



to the inside and outside of a glass tube by applying a coating of silver 

 paste and firing the tube in an oven. A gap is left uncoated on the 

 outer surface near the open end and leads are soldered to the silver on 

 both sides of the gap. The capacitance is then adjusted to the re- 

 quired value, within approximately ±1.5 mmf., by scraping off a portion 

 of the silver coating. Capacitances up to 80 mmf. are realized by this 

 means. Two condensers may be combined in a single unit, as shown 

 at the right in Fig. 7. The completed condenser is dipped in varnish 

 to protect the silver from corrosion. 



Pairs of such condensers, matched to each other within 0.4 mmf., are 

 required in some types of crystal filters. This precision is achieved by 

 manufacturing a quantity of condensers of the correct nominal capaci- 

 tance and sorting them into close-limit groups after final measurement. 



