ng in both cases is equal Frequency to the intermediate frequency Figure 5.50 Reciprocal mixing This can be explained by considering the receiver when it is tuned to a strong signal. The local oscillator mixes with the wanted station to produce phase noise which falls within the pass-band of the receiver filters. If the receiver is tuned off channel by a given amount, for example 10 kilohertz, then the station can mix with the local oscillator phase noise 10 kilohertz away from the main oscillator signal as shown in Figure 5.50. If the incoming station signal is very strong the reciprocal mixing effect can mask out weaker stations, thereby reducing the effective sensitivity of the set in the presence of strong off-channel signals. Filters There is a wide variety of different types of filter which are found in today's receivers. LC, crystal, ceramic, audio filters and more can all be found. Each one has its own advantages and disadvantages, and by careful use of the correct type in particular positions in the set, excellent selectivity can be obtained. The basic selectivity of a set is provided in the IF stages. Here the stations on adjacent channels are rejected, and the performance of the filters here will determine the performance of the whole set. Often several filters placed at different points in the set provide the selectivity. However, in most sets one high quality unit is used to provide the majority of the selectivity. In multi-conversion sets this filter is placed at the last intermediate frequency where the frequency is generally the lowest. In this way it is possible to achieve the most effective filtering. 166 Newnes Guide to Radio and Communications Technology Selectivity and filters Each filter has a finite bandwidth, and this must be taken into consideration when selecting a filter for a receiver. As transmissions have a finite bandwidth which depends upon the type of transmission in use it is necessary to match the filter bandwidth to the transmission. If the filter is too wide as shown in Figure