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This ambiguity sometimes leads to confusion – conflating preselection with amplification.Īn ordinary preselector is called a “ passive” preselector to clearly specify that it has no internal amplifier and requires no power supply. The integrated device is both a preamplifier and a preselector, and either name is correct. None of these extra conveniences are necessary for the function of preselection, and for typical use, an amplifier in the preselector serves no helpful purpose.Īntenna preamplifiers (preamps) can be made “tunable” by incorporating a front-end preselector circuit to improve their performance. It can also incorporate a small radio frequency amplifier stage to boost the filtered signal. Multifunction preselectorsĪ preselector can be engineered so that in addition to attenuating interference from unwanted frequencies, it will perform other services which may be helpful for a receiver: It can limit input signal voltage to protect a sensitive receiver from damage caused by static discharge, nearby voltage spikes, and overload from nearby transmitters’ signals. Extra filtering by the preselector 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. In situations with noisy and crowded bands, or where there are strong local stations, the dynamic range of the receiver can quickly be exceeded. When the front-end overloads, the performance of the receiver is severely reduced, and in extreme cases can damage the receiver. “Limited dynamic range” means that the amplifier circuits have a limit to the total amount of incoming RF signal they can amplify without overloading, symptoms of which are nonlinearity and ultimately clipping. So off-frequency signals constitute a load on the RF amplifier, using part of its dynamic range for unused signals. Most radios’ front ends amplify all radio frequencies delivered to the antenna connection. Įxtra filtering can be useful because the first input stage (“front end”) of receivers contains at least one RF amplifier, which has power limits ( dynamic range). The preselector passes through the signal on the frequency it is tuned to unchanged (or only slightly diminished) but it reduces or removes off-frequency signals, cutting down or eliminating unwanted interference. Frequency response curves for a simple preselector with tuning capacitor setting of 10, 30, 100, or 300 pFĪ preselector typically is tuned to have a narrow bandwidth, centered on the receiver’s operating frequency. signal strength trade-offĪ 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 shortwave and AM broadcast receivers. The preselector is a band-pass filter that blocks troublesome 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. Circuit of a very simple preselectorĪ preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. For the type of gearbox, see preselector gearbox.
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