.7 Block diagram of a direct conversion receiver Receivers 125 number of functions. First of all it amplifies the signals prior to passing them into the mixer. The choice of the level of gain is important, because too much gain can cause the mixer to overload when very strong signals are present. The tuning is also important. If it was not present then signals from a very wide range of frequencies would appear at the input to the mixer and this again might cause the mixer to overload. This is particularly important where very strong signals may appear at fre- quencies far away from the ones being used. Finally the presence of the radio frequency amplifier helps prevent the local oscillator signal from reaching the antenna. The amplifier acts as another stage of isolation reducing it to an acceptable level. The local oscillator circuit is crucial to the performance of the radio. It must be capable of tuning over the range that the radio is required to cover and the design and construction of the oscillator is crucial. One of the prime requirements is that it should not drift. If it does then the set will need to be retuned at intervals if the same station is required. Response Bandwidth typically less than 1 kHz (a) A band-pass audio filter response, Frequency suitable for Morse reception Response Cutoff frequency often about 3 kHz (b) A low-pass filter response, Frequency suitable for SSB reception Figure 5.8 Band-pass, low-pass filters in a direct conversion receiver 126 Newnes Guide to Radio and Communications Technology Once through the mixer the signals enter the audio amplifier. This section may require some filtering. Off-channel signals will produce high frequency interference. This can be reduced by employing an audio filter. For transmissions like Morse a narrow band-pass type of filter with a response like that shown in Figure 5.8(a) can be employed. As the name suggests this will just allow through a narrow band of signals and is particularly useful for removing other signals which may appear in the au