er Microphone Audio amplifier Figure 6.12 Block diagram of an SSB generator using the filter method advantage of using this type of mixer is that the input signals are suppressed at the output. This generates a double sideband suppressed carrier signal. In other words the carrier, being one of the input signals, is suppressed by this circuit. Once this signal has been generated, it is passed into a filter to remove the unwanted sideband. The filter will typically have a level of selectivity sufficient to pass the required audio bandwidth. As single sideband is usually used for communications purposes the filter will be relatively narrow, typically the bandwidth will be about 2.7kHz or slightly more. The position of the carrier frequency relative to the filter pass-band is important. It is necessary to ensure that the wanted sideband is not attenuated while ensuring that no undue amounts of the unwanted sideband are passed through. Normally the filter and carrier are positioned as shown in Figure 6.13. Response Filter response Frequency Position of carrier Figure 6.13 Frequency of carrier and filter relative to one another 198 Newnes Guide to Radio and Communications Technology It is normally found that the filter will give additional attenuation to the carrier, enabling high degrees of attenuation than the mixer alone can provide. For most transmitters the carrier will be suppressed by at least 40 dB as will the unwanted sideband. The single sideband signal is generated on a fixed frequency. Most transmitters will need to be used on a variety of frequencies. To enable this to be achieved the superhet principle is used in the reverse way to that employed in a receiver. The fixed frequency single sideband signal is mixed with a variable frequency local oscillator to give a mix product on the required frequency. As in the case of a receiver there are a number of methods of achieving this. A single variable frequency oscillator on its own is unlikely to give sufficient stability and therefore virtually all sets now u