428 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



tuning slug associated with each inductor. In the voice frequency units, 

 which use toroidal molybdenum permalloy core inductors, these same 

 threaded inserts accept the machine screw which supports the inductors. 

 The associated capacitors and resistors support themselves from their leads 

 after connection to the unit terminals. Carrier and voice frequency units 

 are illustrated in Fig. 7. 



Carrier Frequency Units 



There are seventeen designs of carrier frequency filters: twelve channel 

 band filters (each comprising two identical units) for use in the terminal 

 equipment, a high-pass input filter (two different units) and a low-pass 

 group modulator output filter (two different units) for use in High-Low 

 repeaters, a low-pass input filter (one unit) and a broadband group modu- 

 lator output filter (three different units) for use in Low-High repeaters, and 

 a low-pass group modulator filter (one unit) for use at low group trans- 

 mitting terminals. 



The channel band filters are designed to utilize the mutual inductance 

 between inductors, the bandwidth being a function of the coupling factor, 

 and are schematically all alike. Channel separation by means of filters is 

 required only at the receiving terminal. It was decided to do this separation 

 only in the high frequency range. The use of one range required only 12 

 channel filter designs instead of 24, with resultant lower costs because of 

 the doubhng of the demand for each design. The upper frequency range 

 was chosen: (1) because less mutual inductance is required and, since this 

 causes the inductors to be farther apart, better control of the mutual in- 

 ductance value can be realized; and (2) because it reduced capacitance 

 values and resultant cost. The designs are such that the corresponding 

 inductor windings are identical for all twelve channels, while the distance 

 between windings, which controls the coupling factor, and the associated 

 capacitors are different for each channel. Modifications made on standard 

 duo-lateral type winding machines have made it possible to eliminate any 

 adjustment of the coupling factor, which is held to J% limits by dimen- 

 sional control only. 



The high- and low-pass filters are of conventional configurations. The 

 effect of mutual inductance in these circuits is to degrade performance by 

 causing excessive distortion in passbands, displace attenuation peaks and 

 limit otherwise realizable loss in the attenuating band. In order to utilize 

 the same assembly methods as for the channel filters and to avoid the need 

 for shielding schematically adjacent inductors, a small inductor is used to 

 annul the unwanted mutual inductance. This inductor has two identical 

 windings with nearly perfect coupling, so that the self inductance of each 



