his limits the bandwidth of operation quite considerably. The generation of a 90 degree Transmitters 199 90 phase Balanced shift modulator SSB output to frequency 90 phase conversion shift circuits Adding or summing Audio circuit input Oscillator Balanced modulator Figure 6.14 Phasing method of SSB generation phase shift over the complete audio bandwidth requires some innovative circuit design. Often high tolerance components are required to achieve this. Another solution is to use a Gingell polyphase shift network. Using this it is possible to obtain the correct phase shift using standard tolerance components. In view of the ease with which the filter method can be implemented, and the flexibility of frequency synthesizers to give tuning, the phasing method of SSB generation has few advantages to offer. Accordingly it is not widely used today except in some limited applications. Frequency modulation transmitter Frequency modulation is used for a variety of applications from wideband FM broadcasting to handheld transceivers using narrow-band FM. There are a variety of circuits for producing an FM signal, and these depend on the type of modulation, whether it is narrow bond or wideband and the circuits being used. A simple FM transmitter may take the form shown in Figure 6.16. Here the modulating signal is applied directly to the oscillator. This can be accomplished as shown in Figure 6.17. The audio signal applied to the 200 Newnes Guide to Radio and Communications Technology Figure 6.15 A hand-held FM transceiver Microphone Audio Oscillator amplifier and modulator Frequency Power Matching multiplier amplifier and filtering and buffer Figure 6.16 A basic FM transmitter circuit changes the capacitance of the varactor diode. As the crystal is operating in its parallel resonant mode its frequency can be changed by external components. In this way the frequency of oscillation can be changed, if even by a relatively small amount. FM transmitters are usually used for frequencies above 30 MHz and as a result crystals ra