Frequency-Modulation Radio-Telephone Transmitters

A block diagram of a frequency-modulation transmitter is shown in Fig. 28. The modulation is accomplished in most transmitters at a carrier frequency of several hundred thousand cycles and with a fairly low maximum frequency deviation. Also, modulation is accomplished at a low power level using receiving-type tubes; this is a decided advantage of frequency modulation. The frequency multipliers increase both the frequency and the frequency deviation to the correct values, and the power amplifier increases the power level to the proper amount. Because the frequency-modulated wave has constant amplitude, the class C power amplifier operates at constant load. At the very high frequency of approximately 100 megacycles, special tubes and circuits are used. Input and output tuned circuits of the "coaxial-cable type" instead of inductors and capacitors are common.

Although this section has been devoted to frequency-modulation telephone transmitters, it should be mentioned that frequency-modulation radio telegraph transmitters also are used extensively. These employ what is called² frequency-shift keying or FSK (see also page 335).

Figure 28. Block diagram of a frequency-modulation radio-broadcast transmitter using the Phasitron tube. For block diagrams of other such transmitters see chart "Frequency Modulation Systems," supplement to Electronic Industries, April, 1946.