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In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier frequency 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'' or ''triple'' ''conversion'', respectively. ==Reasons for using IF== 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).〔F. Langford Smith (ed) ''Radiotron Designer's Handbook'', 3rd Edition (Wireless Press 1946) Page 99〕〔The 1946 ''Radiotron Designer's Handbook'' observes on page 159 that some short-wave receivers operate with an IF of 1600 kHz and that "At such a high frequency one or two additional IF stages are required are necessary to provide sufficient 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, to allow a relatively inexpensive coaxial cable to carry the signal to the receiver inside the building. Bringing the signal in at the original microwave frequency would require an expensive waveguide. A second reason, in receivers that can be tuned to different frequencies, is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build multistage amplifiers, filters, and detectors that can have all stages track in tuning different frequencies, but it is comparatively easy to build tunable 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 receivers. The main reason for using an intermediate frequency is to improve frequency selectivity.〔 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 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 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「intermediate frequency」の詳細全文を読む スポンサード リンク
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