Q.What is the need of intermediate frequency?

Superheterodyne Radio Receiver

Tuning a radio across a whole band is hard if every amplifier stage must be re-tuned for each station. The superheterodyne ("supersonic heterodyne") design solves this elegantly: instead of amplifying at the received frequency, it first shifts every station down to one fixed intermediate frequency (IF), then does all the heavy amplifying there.

How the frequency shift works

The incoming signal at frequency $f_s$ is fed into a mixer along with a local oscillator running at $f_{LO}$. Mixing (heterodyning) two frequencies produces sum and difference frequencies. The receiver keeps the difference:

As you tune, $f_{LO}$ is changed together with $f_s$ so that this difference stays constant. Standard fixed values are 455 kHz for AM and 10.7 MHz for FM.

Why a fixed IF is so powerful

• The IF amplifier is optimised once, for a single frequency — giving high, uniform gain and sharp selectivity for every station across the dial.
• Adjacent-channel rejection and sensitivity improve because filtering is done at one carefully chosen frequency.

The signal chain …