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BELL SYSTEM TECHNICAL JOURNAL 



Other modulation products. The desired voice-frequency band is then 

 separated from these products by the receiving low-pass filter. 



In addition to performing the function of selecting desired and re- 

 jecting undesired currents, a filter, if operating in parallel with another 

 as in the case of the directional filters or the line filters, should ofifer a 

 high impedance to the transmitted currents of the other and thus 

 prevent an excessive drain of these currents. Since these filters are 



Fig. 17 — Filter (with cover cut away) showing general method of assembly. 



designed for operation in parallel, either filter operated without the 

 other would have somewhat different electrical characteristics. 



The economies and reduction in size of these filters as compared to 

 those of the Type D system are due to several factors. Improved 

 paper condensers having the required stability are used. Compared to 

 mica condensers, these condensers are less costly and smaller for a 

 given capacity. Moreover, at a very slight increase in cost paper 

 condensers are used which will withstand 1000-volt line surges. Most 

 of the condensers of the Type D system were designed for only 500- volt 

 protection. 



New types of coils with molybdenum permalloy cores having the 

 necessary stability and low hysteresis losses are employed instead of 



