Question

With the help of a functional block diagram. Explain the operation of a superheterodyne AM receiver.

Answer 1

The functional block diagram of AM receiving system of superheterodyne type is shown in figure.
(i) RF amplifier : The RF amplifier uses a tuned parallel circuit. The radiowaves from various broadcasting stations are intercepted by the receiving antenna and are coupled to this stage. This stage selects the desired radiowave and enhances the strength of the wave to the desired level.
(ii) Mixer and local oscillator : The amplified output of RF amplifier is fed to the mixer stage, where it is combined with the output of a local oscillator. The two frequencies beat together and produce an intermediate frequency (IF). The intermediate frequency is the difference between oscillator frequency and radio frequency. The output of this section is always equal to the intermediate frequency $455kHz$.
For example, if $600kHz$ station is tuned, then local oscillator will produce a frequency of $1055kHz$ and consequently the output from the mixer will have frequency of $455kHz$. By achieving this fixed intermediate frequency, the amplifier circuit in such receivers can be made to operate with maximum stability, selectivity and sensitivity.
(iii) IF amplifier : The output of the mixer circuit is fed to the tuned if amplifier. This amplifier is tuned to one frequency (i.e., $455KHz$) and is amplified.
(iv) Detector : The output from the IF amplifier is coupled with input of a detector. The audio signals are extracted from the IF output. Usually a diode detector circuit is used because of its low distortion and excellent audio fidelity (reproducing ability).
(v) AF amplifier : The detected AF signal is usually weak and so it is further amplified by the AF amplifier. Then, the output signal from the amplifier is fed to the loud speaker, which converts the audio signal into sound waves corresponding to the original sound at the broadcasting station.