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A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector blocks trouble-causing out-of-tune frequencies from passing through from the antenna into the radio receiver (or preamplifier) that otherwise would be directly connected to the antenna. == Purpose == A preselector improves the performance of nearly any receiver, but is especially helpful to receivers with broadband front-ends that are prone to overload, such as scanners and ordinary consumer-market receivers. A preselector typically is tuned to have a narrow bandwidth, centered on the receiver's operating frequency. The preselector passes through the signal it is tuned to, only slightly reduced, but attenuates other signals, diminishing unwanted interference. A preselector can be engineered so that in addition to attenuating interference from other frequencies, it will protect a sensitive receiver from damage caused by static input, voltage spikes, and overload from signals from other, nearby transmitters. However, a preselector does ''not'' remove interference on the ''same'' frequency that it and the receiver are tuned to. Extra filtering can be useful because the first input stage (''front end'') of receivers begins with at least one RF amplifier, which has a limited capacity (dynamic range). Most RF amplifiers amplify ''all'' radio frequencies delivered to the antenna connection. So off-frequency signals constitute a wasteful load on the RF amplifier. The amplifier circuits also have a limit to the amount of incoming RF energy they can handle without overloading. When the front-end overloads, the performance of the receiver is severely reduced, and in extreme cases can damage the receiver.〔George Cutsogeorge. (2014). ''Managing Interstation Interference with Coaxial Stubs and Filters'', (2nd ed.); (1st ed. 2009). Aptos, CA: International Radio Corporation.〕 In situations with noisy and crowded bands, or where there are strong local stations, the dynamic range of the receiver can quickly be exceeded. Extra filtering limits frequency range and power demands that are applied to all later stages of the receiver, only loading it with signals within the preselected band. Tunable antenna preamplifiers (''preamps'') often incorporate a front-end preselector circuit to improve their function. The integrated device is ''both'' a preamplifier and a preselector, and may correctly be referred to with either name. This ambiguity sometimes leads to confusion. ''Passive'' preselectors that have no power and no internal amplifier work quite well with modern receivers with negligible loss to the signal, and preamps do not need a preselector when fed from a narrow-band source, such as a tuned-resonance loop antenna. 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「preselector」の詳細全文を読む スポンサード リンク
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