 sets The idea of the superhet radio has many advantages over the other types of set which were previously used. It gives better selectivity, it is possible to easily include several types of demodulator which can be switched in as required, it is capable of being made very sensitive, and it can have a whole host of facilities included into its design. However, in its basic form it does have a number of disadvantages. In this first instance its image rejection may be poor, especially at high frequencies as previously described. If the intermediate frequency is increased to improve this then the selectivity may suffer. To overcome some of these problems multiple conversion receivers may be used. By using more than one conversion the signal may be stepped down to the final IF in two stages. The most obvious method of Receivers 133 Demod- amplifier Mixer Mixer ulator amplifier Variable frequency Crystal local oscillator oscillator Figure 5.16 A double conversion superhet with variable frequency first oscillator achieving this is to adopt a system as shown in Figure 5.16. Here a variable frequency oscillator is used for the first conversion and a crystal oscillator for the second conversion. In this scheme the first conversion takes the signal down to a higher intermediate frequency to increase the difference between the wanted signal and the image. A second conver- sion using a fixed frequency oscillator is used to convert the signal down to the second intermediate frequency stage. This oscillator is on a fixed frequency and is usually based around a quartz crystal for convenience and stability. As the first intermediate frequency is at a higher frequency than before, a major improvement in image response is seen. By keeping the final intermediate frequency stage at a suitably low frequency, then the selectivity performance of the receiver is maintained. Automatic gain control A circuit which is standard on all superhet radios these days is called an automatic gain control (AGC). Occasionally this circuit is called 