Editing Heterodyne (section)

===Superheterodyne receiver===
[[Image:Superheterodyne receiver block diagram 2.svg|thumb|upright=1.6|Block diagram of a typical superheterodyne receiver.  <span style="color:red;">Red</span> parts are those that handle the incoming radio frequency (RF) signal; <span style="color:green;">green</span> are parts that operate at the intermediate frequency (IF), while <span style="color:blue;">blue</span> parts operate at the modulation (audio) frequency.]]

An important and widely used application of the heterodyne technique is in the [[superheterodyne receiver]] (superhet). In the typical superhet, the incoming [[radio frequency]] signal from the antenna is mixed (heterodyned) with a signal from a local oscillator (LO) to produce a lower fixed frequency signal called the [[intermediate frequency]] (IF) signal. The IF signal is amplified and filtered and then applied to a [[detector (radio)|detector]] that extracts the audio signal; the audio is ultimately sent to the receiver's loudspeaker.

The superheterodyne receiver has several advantages over previous receiver designs. One advantage is easier tuning; only the RF filter and the LO are tuned by the operator; the fixed-frequency IF is tuned ("aligned") at the factory and is not adjusted. In older designs such as the [[tuned radio frequency receiver]] (TRF), all of the receiver stages had to be simultaneously tuned. In addition, since the IF filters are fixed-tuned, the receiver's selectivity is the same across the receiver's entire frequency band. Another advantage is that the IF signal can be at a much lower frequency than the incoming radio signal, and that allows each stage of the IF amplifier to provide more gain. To first order, an amplifying device has a fixed [[gain-bandwidth product]]. If the device has a gain-bandwidth product of 60&nbsp;MHz, then it can provide a voltage gain of 3 at an RF of 20&nbsp;MHz or a voltage gain of 30 at an IF of 2&nbsp;MHz. At a lower IF, it would take fewer gain devices to achieve the same gain. The [[regenerative radio receiver]] obtained more gain out of one gain device by using positive feedback, but it required careful adjustment by the operator; that adjustment also changed the selectivity of the regenerative receiver. The superheterodyne provides a large, stable gain and constant selectivity without troublesome adjustment.

The superior superheterodyne system replaced the earlier TRF and regenerative receiver designs, and since the 1930s most commercial radio receivers have been superheterodynes.