Guide to Radio and Communications Technology Whatever the use of the receiver, the same basic principles apply and the same basic building blocks are used. A receiver performs two main functions. The first is to remove the modulation from the radio frequency signal. This process is called demodulation or detection and gives signals at audio frequencies that can be amplified by an audio frequency amplifier and passed into head- phones or a loudspeaker. The other is to provide selectivity. With the vast number of stations on the radio frequency bands today it is necessary that the receiver is able to tune in the station on the required frequency and reject the others. If this function is not performed well then a number of different stations will be received at one time making it very difficult to copy any of them. Amplification is another important function that radios provide. The signals picked up by the antenna are very weak, and need to be amplified to a large degree if they are to be heard in a loudspeaker or passed to some other form of unit like a modem where data is extracted. There are a number of different methods of performing some of these functions. As a result there are a number of different types of format for receivers. Obviously some are much simpler than others. The simplest is a crystal set. This is the most basic type of receiver, and while it is not used for any serious listening these days it shows some of the basic principles required for a radio receiver. Crystal set The crystal set has been in existence for very many years, often being built by hobbyists these days as a first construction project. The simple circuit of a very basic set is shown in Figure 5.2, and from this it can be seen that very few components are used. Naturally the simplicity reflects the level of performance which can be achieved, and more complicated and effective designs can be built if required. In the circuit L1 and VC1 act together to form a resonant circuit. This performs the function of accepting signals on and