 of transmission as they need a certain bandwidth to accept all of the signal. To obtain a much better performance in terms of the ultimate rejection and required bandwidth, a number of crystals are used together. One popular circuit is called a half lattice filter, and this is shown in Figure 5.33. The response curve for this is far more even as shown in the diagram, although there is some ripple. To achieve the wider bandwidth the resonant frequencies of the two crystals are spaced apart slightly. As a rough guide the bandwidth of the filter at the -3 dB points is approximately 1.5 times the difference in the resonant frequencies of the crystals. Two important factors of any filter are the rate at which the response falls away and the ultimate rejection. These can both be improved by adding further stages to the filter. A two pole filter, i.e. one with two 150 Newnes Guide to Radio and Communications Technology Impedance Frequency Figure 5.33 A half lattice crystal filter circuit crystals like that shown in the diagram, is rarely sufficient to meet the requirements in a receiver. Accordingly six or eight pole versions are more common. Quartz crystals are not the only way in which filters with a better performance than LC filters can be made. A variety of different substances exhibit the piezoelectric effect. A number of ceramics exhibit it, although it is not combined with such a sharp mechanical resonance. Even SO they give a much better performance than their LC counterparts. Ceramic filters operate in exactly the same way as quartz filters, the signal being linked to the mechanical resonances in the substance. As the resonance is less sharp, the bandwidths of these filters is wider and the shape factor is poorer. Filters made from these ceramics are much cheaper to manufacture than quartz ones, and as a result they find widespread use in many areas. Many millions are used each year in broadcast receivers where they are ideal in terms of cost and perform- ance. They are never used in performance sets as the p