Editing Intermediate frequency (section)

==Justification==
Intermediate frequencies are used for three general reasons.<ref name="Army_1952"/><ref name="Rembovsky_2009"/> At very high ([[gigahertz]]) frequencies, signal processing circuitry performs poorly. Active devices such as [[transistor]]s cannot deliver much amplification ([[Gain (electronics)|gain]]).<ref name="Langford-Smith_1941"/>  Ordinary circuits using [[capacitor]]s and [[inductor]]s must be replaced with cumbersome high frequency techniques such as [[stripline]]s and [[waveguide]]s. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in [[satellite dish]]es, the microwave downlink signal received by the dish is converted to a much lower IF at the dish so that a relatively inexpensive [[coaxial cable]] can carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive [[waveguide]].

In receivers that can be tuned to different frequencies, a second reason is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build multistage [[amplifier]]s, [[Electronic filter|filters]], and [[detector (radio)|detector]]s that can have all stages track the tuning of different frequencies, but it is comparatively easy to build tunable [[Electronic oscillator|oscillators]]. Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same fixed frequency: the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed as was necessary in the early [[tuned radio frequency receiver]]s (TRF). A more important advantage is that it gives the receiver a constant bandwidth over its tuning range. The bandwidth of a filter is proportional to its center frequency. In receivers like the TRF in which the filtering is done at the incoming RF frequency, as the receiver is tuned to higher frequencies, its bandwidth increases.

The main reason for using an intermediate frequency is to improve frequency [[selectivity (radio)|selectivity]].<ref name="Langford-Smith_1941"/> In communication circuits, a very common task is to separate out, or extract, signals or components of a signal that are close together in frequency. This is called [[Filter (signal processing)|filtering]]. Some examples are: picking up a radio station among several that are close in frequency, or extracting the [[chrominance]] subcarrier from a TV signal. With all known filtering techniques the filter's [[Bandwidth (signal processing)|bandwidth]] increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency. [[FM broadcasting|FM]] and [[television broadcasting]] with their narrow channel widths, as well as more modern telecommunications services such as [[cell phone]]s and [[cable television]], would be impossible without using frequency conversion.<ref name="Dixon_1998"/>