# F.M., A.M. and S.S.B. transmitters

In an FM transmitter, the carrier frequency is changed by the modulating signal. The change in carrier frequency is a function of the amplitude of the modulating signal. Deviation is the alteration in carrier frequency, and is proportional to the amplitude of the modulating signal. Modulation index is given by the ratio-

When both the frequency deviation of the carrier and the frequency of the modulating signal are at their maximum values, the modulation index is termed the deviation ratio.

The normal layout of an FM transmitter is as Fig. 18.1

In phase modulation, the modulator is positioned between the buffer amplifier and frequency multipliers.

In an AM transmitter, the carrier amplitude is changed by the modulating signal such that the amplitude of the modulated carrier wave varies in sympathy with the amplitude of the modulating signal. The modulation depth (modulation factor) is important. A value greater than unity indicates over-modulation, and its effect is to produce harmonic distortion. Percentage modulation is simply the modulation depth multiplied by 100.

When two frequencies f1and f2 are mixed, the output is xf1 ± yf2, where x and y are whole numbers (integers).

A sideband generator consists of a balanced modulator in which is mixed AF from the speech amplifier, and RF from the carrier oscillator. The balanced modulator suppresses the carrier present in AM transmission. Both sidebands are then fed to a filter which removes one of the sidebands. Multi-band operation is achieved by mixing the output from a VFO with the output from the filter. The power amplifier of an SSB transmitter is normally class AB or class B, and must not be overdriven.

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