rime source of filtering, although they are sometimes included where a small degree of filtering is used. However, as the performance of these filters is being improved all the time, they are likely to be found increasingly. Ceramic filters are available with centre frequencies that range from a few kilohertz up to ten megahertz and more and they possess bandwidths ranging from around 0.05 per cent up to 20 per cent. A simple ceramic resonator consists of a disc with electrodes plated on opposite faces. These items perform in a similar way to a single quartz crystal. More commonly used is a resonator with one of the electrodes separated to form two isolated parts. This forms a three electrode network that can be incorporated into a circuit such as that shown in Receivers 151 Ceramic filter Figure 5.34 A circuit using a ceramic filter Figure 5.34. They are widely used with integrated circuit IF strips where they are the only filter providing the selectivity. Demodulation The signals that appear at the output of the intermediate frequency amplifier and filter section of a receiver consist of a radio frequency type of signal. In other words they consist of a carrier with the required modulation superimposed. To remove the modulation, a circuit called a demodulator is required. While these circuits are often termed demodu- lators, the name detector is also used almost interchangeably to describe them as well. The type of demodulator and hence the circuitry of the demodulator will depend upon the type of modulation which is being used. Often different demodulators are required for different modes of transmission. When a receiver needs to be able to demodulate a variety of types of signal, different detectors are switched in. 152 Newnes Guide to Radio and Communications Technology AM demodulation The simplest type of detector is a simple diode detector like that shown in Figure 3.4. This type of detector is widely used in many receivers, although it can give relatively high levels of distortion for some applications. I