530 BELL SYSTEM TECHNICAL JOURNAL 



The filter is then transferred to a resonance bridge and the impedance 

 looking into either end of each section, with the opposite end open- 

 circuited, is adjusted to series resonance at a specified frequency by 

 means of the condenser Cr. This adjustment primarily controls the 

 shape of the loss characteristic of the filter in the transmission range. 

 The resistance at resonance is recorded for later reference. 



In some types of filters, adjustment must be made to secure the 

 correct absolute capacitance between certain points in the filter rather 

 than to obtain desired attenuation peaks or resonances. A capacitance 

 bridge is employed for this purpose. 



In all of these adjustments, test leads connecting the filter to the test 

 set play an important part. Shielding, balance, capacitance to ground, 

 dielectric loss, stability and other characteristics of the leads must be 

 carefully controlled or compensated by adjustments within the test 

 sets in order to meet precision requirements of the order of ± 0.01 

 per cent. 



After adjustment, in the case of the channel filter, the individual 

 sections are connected through a resistance pad selected to complement 

 the values of resistance measured during resonance adjustment. Uni- 

 formity of overall transmission loss, regardless of manufacturing varia- 

 tions in components, is secured by this means. 



The completely wired filter is now placed in a copper shell and 

 hermetically sealed with solder, except for an inlet and an outlet vent. 

 In order to remove vestiges of moisture which might afifect the crystals 

 or other components during service life, a current of air of less than 3 

 per cent relative humidity is then passed through the filter for 12 hours 

 and the vents are sealed off. 



Final test consists of measurements of transmission loss at a series 

 of frequencies in the transmission and attenuation bands of the filter, 

 using equipment similar to that on which the peaks were adjusted. 

 The variety of product which must be tested with these facilities de- 

 mands maximum flexibility and minimum set-up time. This require- 

 ment is met with plug-in terminating impedances, pads, leads, etc., and 

 with oscillators and detectors tuning continuously over a wide range of 

 frequencies. Several filters of the same type are normally tested 

 simultaneously, all being measured at one frequency before the next 

 frequency is set up. Contact fixtures for particular types are provided 

 when justified by quantity requirements, in order to facilitate the 

 transfer of test leads from one filter to the next. 



Transmission loss characteristics of channel filters under various 

 conditions are shown in Fig. 14. The solid curve illustrates a normal 

 filter. The loss in the passband is approximately 5.6 db, with distor- 



