Editing Superheterodyne receiver (section)

==Multiple conversion==
[[File:Double-conversion superheterodyne receiver block diagram.svg|thumb|upright=2.0|Double conversion superheterodyne receiver block diagram]]
{{anchor|Double|Triple}}To overcome obstacles such as [[image response]], some receivers use multiple successive stages of frequency conversion and multiple IFs of different values. A receiver with two frequency conversions and IFs is called a ''dual conversion superheterodyne'', and one with three IFs is called a ''triple conversion superheterodyne''.

The main reason that this is done is that with a single IF there is a tradeoff between low [[image response]] and selectivity. The separation between the received frequency and the [[image frequency]] is equal to twice the IF frequency, so the higher the IF, the easier it is to design an RF filter to remove the image frequency from the input and achieve low [[image response]]. However, the higher the IF, the more difficult it is to achieve high selectivity in the IF filter. At [[shortwave]] frequencies and above, the difficulty in obtaining sufficient selectivity in the tuning with the high IFs needed for low image response impacts performance. To solve this problem two IF frequencies can be used, first converting the input frequency to a high IF to achieve low image response, and then converting this frequency to a low IF to achieve good selectivity in the second IF filter. To improve tuning, a third IF can be used.

For example, for a receiver that can tune from 500&nbsp;kHz to 30&nbsp;MHz, three frequency converters might be used.<ref name="Carr_2002"/> With a 455&nbsp;kHz IF it is easy to get adequate front end selectivity with broadcast band (under 1600&nbsp;kHz) signals. For example, if the station being received is on 600&nbsp;kHz, the local oscillator can be set to 1055 kHz, giving an image on (-600+1055=) 455&nbsp;kHz. But a station on 1510&nbsp;kHz could also potentially produce an image at (1510-1055=) 455&nbsp;kHz and so cause image interference. However, because 600&nbsp;kHz and 1510&nbsp;kHz are so far apart, it is easy to design the front end tuning to reject the 1510&nbsp;kHz frequency.

However at 30&nbsp;MHz, things are different. The oscillator would be set to 30.455&nbsp;MHz to produce a 455&nbsp;kHz IF, but a station on 30.910 would also produce a 455&nbsp;kHz beat, so both stations would be heard at the same time. But it is virtually impossible to design an RF tuned circuit that can adequately discriminate between 30&nbsp;MHz and 30.91&nbsp;MHz, so one approach is to "bulk downconvert" whole sections of the shortwave bands to a lower frequency, where adequate front-end tuning is easier to arrange.

For example, the ranges 29&nbsp;MHz to 30&nbsp;MHz; 28&nbsp;MHz to 29&nbsp;MHz etc. might be converted down to 2&nbsp;MHz to 3&nbsp;MHz, there they can be tuned more conveniently. This is often done by first converting each "block" up to a higher frequency (typically 40&nbsp;MHz) and then using a second mixer to convert it down to the 2&nbsp;MHz to 3&nbsp;MHz range. The 2&nbsp;MHz to 3&nbsp;MHz "IF" is basically another self-contained superheterodyne receiver, most likely with a standard IF of 455&nbsp;kHz.