 intermediate frequency filters that may be a few kilohertz away, or in some cases less. Tuning the receiver is accomplished by changing the frequency of the local oscillator. If it is moved up in frequency by 100 kHz then the frequency of the signals which are received will be 100 kHz higher. In the example used earlier, if the local oscillator is 100 kHz higher at 1.6 MHz then signals will be received at 2.1 MHz, assuming the higher frequency signals are wanted and those at 1.1 MHz are rejected as the image. If the local oscillator and hence the receiver frequency is varied, it is necessary to ensure that the RF tuning also moves at the same rate. In this way the RF tuning will be set to the correct frequency and this will ensure the wanted signal passes through without being attenuated. It also ensures that the image signal is fully rejected. To ensure that this occurs the tuning for the local oscillator and RF circuits must be linked SO that they track together. In many older sets ganged tuning capacitors were used like that shown in Figure 5.12. Here the two sections of the capacitor are mechanically linked. In this way the two tuned circuits can be varied by the same degree at the same time. In more modern receivers electronic methods of tuning are used employing varactor diodes, and these circuits are designed to track at the same rate to maintain the correct circuit conditions. Figure 5.12 A ganged tuning capacitor 130 Newnes Guide to Radio and Communications Technology Basic superhet receiver Having looked at the superhet principle the various blocks can be put together to form a complete radio as shown in Figure 5.13. The signals enter the radio frequency circuits from the antenna where the required band of frequencies is selected and those which might give rise to image Mixer demod- amplifier amplifier and filter ulator amplifier Local oscillator Figure 5.13 Block diagram of a basic superhet Figure 5.14 A high quality communications receiver that uses the superhet principle (courtesy Icom UK) Receivers