 is wider at higher Image signal Wanted rejected frequencies sufficiently signal rejected Figure 5.15 Image response is degraded at higher frequencies 132 Newnes Guide to Radio and Communications Technology At higher frequencies the frequency difference becomes a much smaller proportion of the frequency of operation. There are two main ways of improving the image response. The first is to make the RF tuning much sharper. If this is done then it is necessary to ensure that the RF tuning tracks the local oscillator very accurately. If the improved selectivity of the front end stages is achieved by using more than one tuned circuit, then these have to track one another as well as the local oscillator. Any misalignment of the circuits will cause the wanted signal to be reduced in strength and the image to increase. This can be difficult and for this reason multiple tracked tuned circuits are rarely used. A more satisfactory option is to increase the intermediate frequency. By doing this the frequency difference between the wanted signal and image is increased. This is the solution that is normally adopted these days because other various techniques are available which allow higher intermediate frequencies to be used. As image response is an important parameter in performance radio receivers it is a parameter that is often specified, particularly for high performance communications receivers. It is given as a certain number of decibels at a particular frequency. For example, it may be 50 dB at 30 MHz. This indicates that if signals of the same strength on the image and wanted frequencies were present at the input, then the image one would be 50 dB lower than the wanted one at the output of the receiver. The frequency at which the measurement is made has to be included because the figure varies with the frequency of operation. A typical figure for a modern short wave communications receiver would be 80 dB at most frequencies in the short wave spectrum falling to about 60 dB at the worst frequencies. Multiple conversion