to next Ferrite rod stage antenna Oscillator Figure 5.28 Circuit of a typical self-oscillating mixer stage Local RF input oscillator IF port Figure 5.29 A high performance diode mixer 146 Newnes Guide to Radio and Communications Technology Local oscillator To IF input stages Figure 5.30 Mixer using a dual gate MOSFET Active mixers have a conversion gain. Of these, the circuits based around FETs generally give the best performance because of the characteristics of the devices. They give lower levels of unwanted or spurious signals. Often the circuits use what are called double balanced circuits, but those using a single dual gate MOSFET like that shown in Figure 5.30 perform well and are used in many hi-fi tuners and other radio receivers. In these circuits the signal is applied to one of the gates and the local oscillator or switching signal is applied to the other. The output is taken from the drain or source in the usual way. Local oscillator The performance of the local oscillator is very important, particularly where frequency drift is concerned. Today frequency synthesizers are often used. As they normally have a crystal oscillator or crystal oven as their reference, their stability is usually very good. Many older receivers Receivers 147 have LC tuned variable frequency oscillators like that shown in Figure 5.31. These oscillators are prone to drift in frequency. This is generally caused by changes in temperature causing the values of some of the components to alter slightly. Even very small changes will give noticeable amounts of drift. If the oscillator drifts then the set will need to be retuned. This is not usually a problem from long and medium wave sets, but where frequencies are higher and modes of transmission like SSB are used the stability of the oscillator is of great importance. Stabilized supply High Q coil securely wound on former To buffer amplifier to reduce the effects of loading Oscillator circuit mounted away from sources of heat Figure 5.31 An LC tuned variable frequency oscillator To ov