Preselector

A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. 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. A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. 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. 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 shortwave and AM broadcast receivers. However, a preselector is of no benefit for receivers or preamps if they are fed from a narrow-band source, such as a tuned small loop antenna. A preselector typically is tuned to have a narrow bandwidth, centered on the receiver’s operating frequency. 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. However, a preselector does not remove interference on the same frequency that it and the receiver are both tuned to. Extra 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). Most radios’ front ends amplify all radio frequencies delivered to the antenna connection. So off-frequency signals constitute a load on the RF amplifier, using part of its dynamic range for unused signals. “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. When the front-end overloads, the performance of the receiver is severely reduced, and in extreme cases can damage the receiver. 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 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. A 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. It can also incorporate a small radio frequency amplifier stage to boost the filtered signal. None of these extra conveniences are necessary for the function of preselection, and for typical use, amplification in the preselector is unnecessary. Antenna preamplifiers (preamps) can be made “tunable” by incorporating a front-end preselector circuit to improve their performance. The integrated device is both a preamplifier and a preselector, and either name is correct. This ambiguity sometimes leads to confusion – conflating preselection with amplification. An ordinary preselector is called a “passive” preselector to clearly specify that it requires no power supply and has no internal amplifier. “Passive” preselectors typically work quite well with modern receivers, with no noticeable signal-loss. With all preselectors there is some loss at the tuned frequency; usually, most of the loss is in the inductor (the tuning coil). Tuning the preselector for narrower bandwidth (or higher Q, or greater selectivity) increases this loss. Most preselectors have separate settings for an inductor and (at least) one capacitor. So with at least two adjustments available to tune to just one frequency, there are often a variety of settings that will tune the preselector to a frequency in its middle-range.