 500 kHz Input signal 2.0 MHz 500 kHz IF Mixer amplifier and filter Oscillator signal 1.5 MHz Local oscillator Figure 5.9 Using a mixer to change the frequency of a signal 128 Newnes Guide to Radio and Communications Technology Amplitude Local oscillator signal Intermediate frequency Unwanted Wanted image signal signal Frequency Figure 5.10 Signals either side of the local oscillator can enter the IF stages signal represented the difference between the incoming signal minus the local oscillator. However, a signal representing the local oscillator minus the incoming signal can also pass through the filters. Take the same example again where the local oscillator frequency is 1.5 MHz. An incoming signal of 1.0 MHz also gives an output at 500 kHz. It can be seen that signals which can enter the IF stages are at frequencies equal to the local oscillator frequency plus the intermediate frequency and the local oscillator minus the intermediate frequency as shown in Figure 5.10. It is obviously undesirable to have two signals on totally different input frequencies which are able to pass through the intermediate frequency filters and amplifiers. Fortunately it is relatively easy to remove the unwanted or image signal to leave only the required one. This is achieved by placing a tuned circuit in the radio frequency stages to remove the signal as shown in Figure 5.11. From the diagram it can be seen that this tuned circuit does not need to reject signals on adjacent channels like the main intermediate frequency filters. It is only necessary to reject the image signal. This happens to fall Response of Amplitude RF tuning Local Wanted oscillator signal signal Intermediate frequency Unwanted image signal Frequency Figure 5.11 A tuned circuit used to reject the unwanted image signal Receivers 129 at a frequency equal to twice the intermediate frequency away from the wanted signal. In the case of the example this is 1 MHz away from the wanted signal. This is compared to stations on adjacent channels that are rejected by the main