 131 signals are rejected. In the same stages the signals are amplified to the required level and passed into the mixer or multiplier circuit. Here they are mixed with the output from the local oscillator to convert them to the intermediate frequency. As the mixer performs the action of changing the frequency of the signals it is sometimes called a frequency changer. The signals then pass through the intermediate frequency stages where they are amplified and filtered. The majority of the signal amplification takes place in these stages and the filtering to remove signals on adjacent channels occurs here. In fact the selectivity performance of the whole receiver is dependent upon the filter present in these stages. Accordingly its performance determines that of the whole receiver. As the frequency of the IF stages is fixed, it is possible to design and manufacture highly effective filters. Finally the signals are demodulated to reconstitute the original signal, and assuming the radio is to be used for audio purposes, the signal is amplified to bring the signal to a suitable level for presenting to the earphones or a loudspeaker. Overcoming image response problems The problem of receiving an image signal is one of the major drawbacks of the superhet receiver. Sufficient selectivity needs to be placed into the front end circuits to be able to reduce any unwanted image signals to a sufficiently low level. Unfortunately a receiver covering a wide range of frequencies will have a much greater problem at the top end of the frequency range than at the bottom. The reason can be deduced from looking at the RF tuned circuits. As the circuit is tuned, the bandwidth increases in proportion with the frequency. As the frequency doubles SO does the bandwidth. As the difference between the wanted signal and the image remains constant at twice the intermediate frequency it means that the RF tuning will reject the image signal much less at higher frequencies as shown in Figure 5.15. Image signal Bandwidth of filter not sufficiently