quency response curve capacitors running to ground. The filter has a cutoff point where the response has fallen by 3 dB. This is chosen to be above the frequency of operation. The response of the filter can be controlled to a degree by changing the relative values of the components. Factors such as in-band ripple, the initial roll-off, phase characteristic and SO forth can all be varied, and names are given to the different types of filter. A Butterworth filter gives the flattest in-band response, for example. The values are not simple to derive manually, but they can be obtained either from books that contain tables of values that can then be scaled for the required frequency and impedance, or by using a computer program. These filters are often described as having a certain number of poles. In general terms a filter has a pole for each capacitor or inductor it contains. In other words the filter shown in Figure 6.7 has seven poles. The greater the number of poles the faster the filter response rolls off beyond its cutoff point. Unfortunately it is not possible to produce the perfect filter and this means that small levels of unwanted signals will always be present. However, they must be reduced to a level where they will not cause any undue interference to other users. When analysing a signal for harmonics or other spurious signals a spectrum analyser is used. This gives a plot of amplitude normally on a logarithmic (decibel) scale against frequency, enabling signals to be seen and analysed. A diagram of a typical plot from a spectrum analyser might look like that shown in Figure 6.8. In this example it can be seen that the main signal falls at a level of -10 dB, and the next strongest signal is at -40 dB. This means that the second signal is 30 dB below the main one. When stating the level of a spurious signal the levels are not normally given in absolute levels, i.e. a given number of watts or milliwatts. The more usual way is to relate them to the level of the wanted signal. In other Transmitters 191 Frequen