Band Pass Filter, Band Elimination Filter and Phase Simulating Network for Carrier Program Systems

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 program 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 transmission 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 requirements 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-off 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 sufficient discrimination above and below the pass * Phase simulating network by F.