RF power amplifier if 100 per cent modulation is to be achieved. In other words if the RF amplifier consumes 100 watts, then the audio amplifier must be able to deliver 50 watts. The power audio signal is then used to modulate the supply to the final RF power amplifier and in this way modulate the RF signal. If any further increase in the power level of the transmitter is required after the modulation has been applied, the RF amplifier must run in a linear mode, typically class A or a variant of class A to prevent distortion of the modulation. These amplifiers are often called linear amplifiers in view of their mode of operation. The final stage of the transmitter, like that of the Morse transmitter, is to pass the signal through the matching and filtering circuits. Again these are required to reduce the levels of spurious signals to acceptable limits. Single sideband transmitter There are a number of different methods in which a single sideband signal can be generated. The first and most commonly used employs a filter to remove the unwanted sideband. A second method uses phasing techniques to eliminate the unwanted elements in the signal. The filter method is virtually a reverse of the signal path followed in a typical receiver. This is the most common method used as it is capable of giving excellent results. It is also very convenient for use in transmitter- receivers (transceivers) because many of the circuits can be used in both the transmit and receive paths, saving on production costs. The outline of an SSB transmitter using the filter method is given in Figure 6.12. The first block in the transmitter is the carrier oscillator. This is normally crystal controlled because its frequency stability is critical to maintain the correct frequency relative to the filters used later. Audio signals from the microphone are amplified and processed as necessary and applied with the carrier to a balanced mixer circuit. The Transmitters 197 SSB output to frequency conversion circuits Carrier Balanced Filter oscillator mix