theless many filters achieve very good results. The first point which needs to be noted on any filter is its pass-band. This is the bandwidth over which the filter accepts signals. The limits of the pass-band are normally taken to be the points where the response has fallen by 6 dB. For a filter used in a communications receiver to receive an AM signal on the short wave bands the bandwidth may be 6 kHz. 168 Newnes Guide to Radio and Communications Technology Real filter response Pass-band 60 dB Stop-band Stop-band Frequency Figure 5.53 A typical filter response The stop-band is also of interest. For this the bandwidth for a given attenuation is taken. The normal value for this is 60 dB although again other values may be taken. As a result the bandwidth and attenuation levels should be specified. Typically a filter might be specified as having a stop-band bandwidth of 6 kHz at -60 dB. The rate at which the filter reaches its final attenuation is of interest. Even though it may have the correct pass-band, if it does not reach its final attenuation fast enough, it will let interference through. The shape factor is a measure of the rate at which it reaches its stop-band. This is taken as the stop-band bandwidth divided by the pass-band bandwidth. Again the levels of attenuation must be given. Taking the figures already used as an example the shape factor would be 2.2:1 at 6/60 dB. It is also noticed that the filter response is not flat. From the diagram it can be seen that there is ripple within the pass-band, and also in the stop- band. Pass-band ripple is generally only a few decibels and this is often quoted in the specification for the filter. Typical values for the filter may be one or two decibels. The stop-band ripple is also of importance and can easily be tens of decibels. Filter also introduces a certain amount of loss. This can be important for circuit designers because the effect of any losses has to be taken into consideration when calculating signal levels at certain points in the circuit. It may be in 