se a frequency synthesizer to give the stability and coverage without the need for band switching. When mixers are used it is very important to remove the unwanted mix products. If not they will pass through the later stages of the transmitter and it will be difficult to remove them all in the final stages of filtering. As such it is good practice to place good filters after each mixer to ensure that the unwanted signals are removed as soon as possible after they are generated. Apart from converting the signal to its final frequency the signal also needs to be amplified to the required level. In view of the fact that the signal carries analogue modulation the amplifier must preserve the nature of the modulation and must be linear. Any distortion will result in the audio becoming distorted as well as the signal occupying a wider bandwidth and causing interference to others. The second method of generating a single sideband signal is to use phasing techniques. Although not widely used today it had a number of advantages as a cheap and easy method of producing an SSB signal in some applications in the past. A block diagram for this method of SSB generation is shown in Figure 6.14. The audio and carrier are split into components shifted by 90 degrees and applied to balanced modulators. The outputs from these are combined, at which point one sideband is reinforced while the other is cancelled out. The reverse sideband can be selected by transposing the audio or carrier phasing. Using this method of SSB generation it is possible to generate an SSB signal at the operating frequency. However, it is necessary to maintain very accurate control of the amplitude and phase of the signals if cancellation of the reverse sideband is to be achieved. Even a 1 degree change in the phase of either the RF or audio signals will reduce the sideband suppression to 40 dB if a perfect match of the amplitude of both signals is maintained. Similarly mismatches in the levels will cause the suppression of the unwanted signals to be reduced. T