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Concept
Intermediate frequency
Résumé
In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done.
Conversion to an intermediate frequency is useful for several reasons. When several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors generally have higher gains so fewer stages are required. It's easier to make sharply selective filters at lower fixed frequencies.
There may be several such stages of intermediate frequency in a superheterodyne receiver; two or three stages are called double (alternatively, dual) or triple conversion, respectively.
Intermediate frequencies are used for three general reasons. At very high (gigahertz) frequencies, signal processing circuitry performs poorly. Active devices such as transistors cannot deliver much amplification (gain). Ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in satellite dishes, 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.
Source officielle
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