Infrared receivers

Sometimes in mobile radio stations, in amateur radio stations, with non-exploratory tuning superheterodyne infrared-type receivers are used. In such receivers do not need a tunable preselector (only the local oscillator is rebuilt) - this is their main advantage. The block diagram is the same as for the superheterodyne receiver.

Figure 2.11.

Instead of a tunable preselector, combinations of upper and lower filters are often used, or broadband filters of concentrated selection.

Since the input stages of such a receiver are broadband, there is a danger of overloading their active elements with extraneous signals and interference — this is the main disadvantage of receivers of this type. Overload can be reduced by using cascades with a large dynamic range in the signal and intermediate frequency amplifiers.

The intermediate frequency is chosen above the maximum frequency of the signal, and the frequency of the local oscillator is even higher. Therefore, the local oscillator operates at sufficiently high frequencies, and increased demands must be placed on its stability - this is the second drawback of such a scheme. These requirements can be met if a frequency synthesizer with a reference quartz oscillator is used as the local oscillator. It is also desirable to use quartz filters in the HRO. In receivers of this type, it is also possible to use double frequency conversion - in this case, the requirements for UPCH filters are reduced.