Band Pass Filter, Band Elimination Filter and Phase Simulating 

 Network for Carrier Program Systems 



By F. S. FARKAS, F. J. HALLENBECK, F. E. STEHLIK* 



A paper In- Leconte, Penick, Schramm and VVier' discusses the system aspects 

 of 8-kc program circuits over carrier facihties and outMnes the functions of several 

 filters and networks. This paper describes in detail two of the filters and one 

 network. These are: 



1. The channel selecting crystal band pass filter used at program terminals 

 of all broad-band carrier systems, 



2. The band elimination filter which blocks the program at branching points 

 on type K carrier systems, 



3. The network used at branching points on type K carrier systems to simu- 

 late the phase shift of the band elimination filter. 



Channel Selecting Crystal Band Pass Filter 



AN IMPORTANT component of the modulator-demodulator circuit at 

 " the carrier program terminal is the band pass filter which selects the 

 lower side band resulting from modulation of the audio frequency program 

 material with the 88-kc carrier. This step of modulation locates the pro- 

 gram frequencies in their allotted position in the carrier frequency spectrum 

 of the standard broad-band terminal. 



System flexibility requires that long program circuits be established by 

 tandem connections of carrier links. A link consists of a transmitting and 

 a receiving carrier program terminal connected by the appropriate trans- 

 mission medium. The original objectives were based on a ten-link carrier 

 circuit. This means that each terminal must introduce no more than five 

 per cent of the total allowable system distortion. Assuming the band filter 

 introduces the major part of the terminal distortion it is seen that the re- 

 quirements placed on each band filter are extremely severe. 



One of the transmission objectives of the system is to transmit audio 

 frequencies as low as 50 cps. Hence the band filter must transmit the 

 wanted carrier frequency sideband to within 50 cps of the carrier and must 

 suppress the unwanted sideband beginning at 50 cps above the carrier. 

 This sharp cut-ofif and the need for low distortion in the pass band requires 

 the use of filter elements with so little dissipation that the only possibility 

 of realizing the desired performance is by the use of quartz crystal elements. 



In addition to suppressing the unwanted sideband above the carrier the 

 filter must also provide suflkient discrimination above and below the pass 



* Phase simulating network by F. S. Farkas. 

 Band elimination filter by F. J. Hallcnbeck. 

 Band pass filter by F. K. Stehlik. 



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