 5.51, then there is the possibility of Filter response too wide allowing interference through Amplitude Unwanted signal Wanted signal Unwanted signal Frequency Filter too narrow not allowing all of the Amplitude wanted signal through Wanted signal Frequency Correct filter response, allowing all the wanted Amplitude signal through Wanted signal Frequency Figure 5.51 Filter bandwidths must match the transmission bandwidth Receivers 167 picking up unwanted signals. On the other hand, if it is too narrow then part of the wanted signal will be rejected and this will result in distortion. It is therefore necessary to use filters of the required bandwidth. Typically medium wave AM transmissions need about 9 or 10 kHz, and the standard for short wave AM transmissions is about kHz while for SSB a bandwidth of 2.7 kHz or thereabouts is usually used. The VHF FM broadcasts normally require a bandwidth of about 200 kHz to ensure the whole of the signal is received, and other types of transmission all have their own requirements. Other types of transmission will occupy different bandwidths and filters will be required to accommodate the required bandwidth. As the bandwidth of the filter in the IF stages of the receiver determines the close in selectivity of the receiver, it is necessary to be able to specify its performance accurately. One of the main aspects of the performance of a filter is its pass-band. This is the band of frequencies that the filter allows through. In an ideal world, it would allow through signals within the pass-band, and totally reject all others that fall outside the pass-band as shown in Figure 5.52. Filter response Pass-band Stop-band Stop-band Frequency Figure 5.52 An ideal filter response The response of a real filter like one used in a receiver to give it its selectivity is different and is shown in Figure 5.53. From this it can be seen that the response curve does not change instantly from accepting signals to rejecting them. Also it does not have an infinite rejection of unwanted signals. Never