A superheterodyne receiver is a receiver that maximizes both sensitivity and selectivity. Normally, these are conflicting goals, and in a single stage, you can only get one, b
…ut not both. The difficulty is that you cannot build a high gain RF stage with enough selectivity to reject adjacent signals. The superheterodyne process solves this problem. First, the RF stage is built for high gain, but it is not very selective. It might have some tunability, i.e. selectivity, but that is not its primary objective. Second, there is a local oscillator that is tuned to a frequency some distance away from the primary signal's carrier. In a standard AM receiver, that distance is 455 KHz; in a standard FM receiver, that distance is 10.7 MHz. Third, the amplified RF signal and the local oscillator are mixed together. Recall from Fourier analysis that when you mix two signals together you get four signals; the two original signals, their sum, and their difference. We are interested in their difference. Basically, the output of the mixer contains the original signal downshifted to either 455 KHz or 10.7 MHz. Fourth, we have an IF stage that is tuned around 455 KHZ or 10.7 MHz. The IF stage is highly optimized for operation at one range of frequencies, and can therefore be highly selective. Usually, the IF stage is three bandpass filters, each tuned slightly differently, so that the end result is a bandpass filter that is wide enough to pass the entire bandwidth of the original signal, 20 KHz for AM and 100 KHz for FM. Fifth, we have the demodulator. At this point, we retrieve the signal and either just amplify it (AM) or further decode it for stereo (FM). The end result is that we got sensitivity in the RF stage and selectivity in the IF stage, along with tunability in the local oscillator. That is the superheterodyne process. A very common type of radio receiver. The output from a simple radio aerial will be of the order of a microvolt or so, and the output desired is a few volts. So an amplification of a million or so is needed. This is achieved by multiple stages of amplification - each generally of less than 100 times. And we need to be able to tune over a range of 3:1 for the Broadcast Band, and perhaps greater for some other bands. For technical reasons concerning the mechanics of tuning, this is more easily achieved at a single frequency - typically about 0.5 MHz. The alternative would be to have the same number of stages of amplification but each with its own tuning. Mechanically much more difficult. So, the incoming signal from the aerial is mixed in a mixer stage, along with an internally generated signal. [This mixer stage is a non-linear process, and as a result, produces a complex output, of both the input frequencies, the sum of them, and the difference between them. ] The difference frequency is selected - the 0.5MHz above. So the mixer stage is followed by several intermediate stages of amplification operating at the 0.5 MHz, - where most of the amplification takes place. This is then followed by a demodulator (detector) stage, which separates the audio signal from the carrier signal. And followed in turn by an audio amplifier stage which feeds the loudspeaker. The 'hetrodyne' part of the name refers to the mixing of the signals at the mixer stage. For radios operating at higher frequencies than the broadcast, the Intermediate stage will commonly operate at 10MHz or so. (MORE) 
