Q.Explain the basic principle of superheterodyne AM receiver and explain function of each block.

The superheterodyne receiver converts every incoming station to a single fixed intermediate frequency (455 kHz for AM) by beating it with a local oscillator, and does most of the amplification and selectivity at that fixed IF. This gives uniform gain and selectivity across the whole band. …

[!TLDR]

Heterodyning converts every station to a fixed 455 kHz IF where most gain and selectivity are obtained; the blocks are RF amplifier, mixer + local oscillator, IF amplifier, detector, AF amplifier and loudspeaker, with AGC.

Basic principle: The superheterodyne receiver uses the principle of heterodyning (frequency conversion). The incoming radio-frequency signal $f_s$ is mixed with a locally generated frequency $f_{LO}$, producing a difference frequency called the intermediate frequency, $f_{IF}=f_{LO}-f_s$, which is kept fixed (455 kHz for AM) whatever station is tuned. All the important amplification and selectivity are then carried out at this single fixed frequency, so the receiver gives uniform performance across the whole band.

Function of each block:

1. RF amplifier: selects and amplifies the desired incoming RF signal from the antenna, improves the signal-to-noise ratio, and helps reject the image frequency.
2. Mixer (frequency converter): combines the RF signal with the local oscillator output and produces the difference frequency, the IF (455 kHz).
3. Local oscillator: generates a stable frequency $f_{LO}=f_s+455$ kHz. It is ganged with the RF tuning so that the difference always equals the fixed IF as the receiver is tuned.
4. IF amplifier: a fixed-tuned amplifier (455 kHz) that provides most of the receiver's gain and selectivity through sharply tuned circuits.
5. Detector (demodulator): usually a diode envelope detector; it recovers the original modulating (audio) signal from the amplified IF. …