cond. When transmissions such as single sideband are used different time constants are required. When using AM the carrier is present all the time, but this is not SO with single sideband. For single sideband the level of the signal varies according to the level of audio present at the transmitter, and it falls to zero when no sound is present. For an AGC system to be able to cope adequately with this type of transmission a dual time constant is needed. It should react very quickly when a new signal appears. In other words it must have a fast attack time. Typically figures of less than 0.1 seconds are used. The decay time must then take account of gaps in the speech or differing levels between syllables in words. Often decay times of a second or more are employed to prevent the AGC level varying unduly and increasing operator fatigue. Receivers 135 IF transformer To audio stages AGC line to RF and IF stages Figure 5.18 A simple detector giving a voltage output for automatic gain control To generate the voltage for the AGC a simple diode detector may be used. One like that shown in Figure 5.18 is often used, especially in medium wave broadcast sets. This performs the normally detection or demodulation function for the audio, but also generates a voltage which is filtered before being applied to the earlier stages of the set to control the gain. In more sophisticated sets a separate detector may be employed. This is likely to have different time constants for each mode being used. In many cases this may be linked to the mode switch on the set, or there may be a separate control on the front panel. Synthesizers Many receivers using frequency synthesizers boast such terms as 'PLL', 'Quartz', and 'Synthesized' in their specifications or advertising lit- erature. Frequency synthesizers are found in most of today's receivers. They are able to offer very high degrees of stability, they can operate over a wide range of frequencies, and they easily interface to microprocessor circuitry, enabling them to be controlled by th