FM Transmission and Reception

  
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The original f-m transmitters manufactured by the General Electric Company were the direct f-m type employing the regular electronic automatic frequency control system similar to that discussed in connection with the CBS transmitter. The f-m multiplication system in these original transmitters employed two triplers for a total frequency multiplica
FM Transmission and Reception - schematic

tion of 9. In the new f-m transmitters the General Electric Company employs a completely different system. The new system is one of indirect f. m. where p. m. is the initial type of modulation. By using the proper audio frequency correction network the p-m signal is made equivalent to an f-m signal. The method of obtaining the initial amount of phase deviation is new. In these transmitters the famous Phasitron tube is employed as the modulator and the operation of the transmitter is based upon this tube. Before we analyze the complete exciter and modulator system, let us first study the Phasitron tube and see how it works. The Phasitron tube enables us to obtain wide phase deviations, equivalent to a peak frequency deviation of about 175 cycles, from a crystal controlled source of about 200 kc. The tube itself is quite a complicated structure so far as regular electron tubes are concerned. It more nearly belongs to the class of the cathode-ray tube than to any other type, although its size is comparatively small. A cutaway pictorial view of the Phasitron tube is shown in Fig. 4-13, while Fig. 4-19 provides a more technical cutaway view of the functioning elements together with a simplified diagram of the circuit elements to which it is wired. In Fig. 4-19 all the parts shown are definite physical parts, except the so-called electron disk which is formed by a constant stream of electrons emanating from the cathode and...



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