e required channel, and rejects those on the next channel. The filtering can take many forms dependent upon the receiver circuitry. One of the most basic is based around LC tuned circuits. They consist of inductors and capacitors and are often in the form of tuned transformers used to couple the individual stages together as shown in Figure 5.32. In many older style portable sets used for broadcast reception two or three of these transformers are used to give the required degree of selectivity, although they do not provide the required levels of performance for many other applications. A further disadvantage of LC filters is that they require alignment. transformers are used then each one has its own ferrite tuning core. This is required SO that the resonant frequency of the circuit can be tuned to exactly what is required. These have to be tuned to the correct frequency during the production of the set. Normally each tuned circuit is slightly offset from the others SO that the correct bandwidth is achieved. If they were all placed on exactly the same frequency then the bandwidth would be too narrow. When better performance is required there are a number of options that can be taken. One is to use a filter based around one or more quartz crystals. These crystals have very high levels of Q and accordingly they are able to make exceptionally selective filters. Being more expensive than simple LC tuned circuits they are not used in low cost broadcast sets, but they are often found in more expensive specialist receivers where very high levels of performance are required. The operation of quartz crystals depends upon the piezoelectric effect. This links the electrical circuit to the very high Q mechanical resonances that these crystals exhibit. As a result quartz crystals possess Qs of many thousands and they are able to provide very high levels of selectivity. A single crystal on its own does not give an ideal response curve. The response is not symmetrical, and has a very narrow peak. This is not ideal for all types