ercome the effects of drift a number of precautions must be employed. The circuit should be run from a stabilized power supply. Changes in supply voltage cause the circuit parameters of some components to change. This is particularly true of some semiconductor devices like bipolar transistors. Coils should be wound as securely as possible and the tuned circuit should not be switched if at all possible. In addition to this the tuned circuit should be designed to have a high Q. Finally the whole oscillator circuit must be kept away from any sources of heat. 148 Newnes Guide to Radio and Communications Technology IF amplifier and filter After the mixer, the signals reach the IF stages. It is here that the majority of the gain and selectivity are provided. Most of today's sets utilize integrated circuits as the basic building blocks, and the actual circuits depend upon the particular IC being used. Some radios still use discrete components and a typical circuit is shown in Figure 5.32. Often two stages are used in series to give the required gain and selectivity. In this circuit the selectivity is provided by the coupling transformer. This is tuned using a ferrite adjuster and each one requires adjustment to the correct frequency. A number of standard intermediate frequencies are used. Frequencies of 455 or 465 kHz are widely used for medium and long wave broadcast radios. The low frequency means that it is possible to obtain sufficient selectivity with a minimum number of stages. Other frequencies are used for other purposes. A frequency of 10.7 MHz is a standard for VHF FM receivers, and this frequency is also used in many hand-held transceivers for mobile or on-site communica- tions. Frequencies of 1.6 and 9 MHz are also used in some high performance short wave communications receivers. + Vcc Output to next stage Figure 5.32 A typical IF amplifier stage using discrete components Receivers 149 The main filtering for the receiver is undertaken in the IF stages. It provides the selectivity that accepts stations on th