208 BELL SYSTEM TECIINICA L JOURNAL 



To obtain maximum loss at the filter peaks it is necessary to secure a 

 conductance balance in each lattice section as well as a susceptance balance. 

 This can be done if care is exercised in the choice of materials used in fabri- 

 cating the crystal elements and capacitors which appear inside the lattice. 

 In this case the crystal element insulators and dielectrics consist of glass, 

 mica, quartz and clean dry air or vacuum while the air capacitors use glass, 

 ceramic and air for their insulators and dielectrics. If these materials are 

 clean and dry they have very low conductance and do not influence the 

 bridge balance. A complete discussion of the effects of impedance un- 

 balances on crystal lattice performance has been given in a recent paper by 

 E. S. Willis." 



To further insure that dirt and moisture will not influence its performance 

 the filter is adjusted, tested and hermetically sealed in an air conditioned 

 room where the relative humidity does not exceed 40 per cent. Since 

 manufacture started about the beginning of 1946 several hundred of these 

 filters have been made and are functioning satisfactorily in the telephone 

 plant. 



Band Elimination Filter at Branching Points 



When broad-band carrier systems are equipped for the transmission of 

 a carrier program channel, it is frequently necessary to provide between 

 carrier terminals intermediate or branching points at which the program 

 may also be received. If only receiving facilities are involved, rather simple 

 bridging arrangements can be provided. However, program network needs 

 often require a more flexible arrangement at the branching point so that a 

 line may be cleared of the program originating at one terminal and a new 

 program introduced for transmittal toward the next terminal. 



To do this without affecting the message channels also being trans- 

 mitted on the line, a filter has been developed to block the program channel 

 already on the line while freely transmitting the message channels. With 

 this filter in the circuit the high-frequency line between the branch point 

 and the following terminal is free of program frequencies and the program 

 originating at the branch point may be sent toward that terminal. 



Since the program channel occupies frequency space near the center of 

 the 12-channel message group, the remaining message channels appear above 

 and below the program frequencies. Therefore the blocking filter at the 

 branching points must be of the band elimination type. The circuit em- 

 ploying this filter may be designed to block either at line frequencies or at 

 basic group frequencies. The latter method, of course, requires that a 

 demodulation process be provided to translate line frequencies to basic group 

 frequencies before the blocking filter is inserted in the circuit. 



