Preselector

A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector is an adjustable 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.

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, wideband software-defined radio receivers, ordinary consumer-market shortwave and AM broadcast receivers – particularly with receivers operating on frequencies where static is pervasive – below 10~20 MHz (lower-frequency half of the short waves, and all of medium waves, long waves, and longer wavelengths). Sometimes faint signals that occupy a very narrow frequency span (such as radiotelegraph or 'CW') can be heard more clearly if the receiving bandwidth is made narrower than the narrowest that a general-purpose receiver may be able to tune; likewise, signals which individually use a fairly wide span of frequencies, such as broadcast AM, can be made less noisy by narrowing the bandwidth of the signal, even though making the span of received frequencies narrower than was transmitted will sacrifice some audio fidelity. A good preselector often can reduce a radio's receive bandwidth to a narrower frequency span than many general-purpose radios can manage on their own. A preselector typically is tuned to have a narrow bandwidth, centered on the receiver's operating frequency. The preselector passes through the signal on its tuned frequency unchanged (or only slightly diminished) but it greatly reduces or removes off-frequency signals, cutting down or eliminating unwanted interference.

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, wasting part of its dynamic range on unwanted and 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 overload are nonlinearity (tonal "distortion") and ultimately clipping ("buzz"). 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 is loud interference from nearby, high-power 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 the desired signals within the preselector's pass-band.

Preselect filter bank Spectrum analyzers, heavy-duty network analyzers, and other RF measuring equipment can incorporate switchable banks of preselector circuits individually similar to preselector circuits in conventional radios, that reject out-of-band noise at the frequencies being analyzed. Automatically switched and tuned filter banks can likewise be incorporated into various high quality, general purpose, broadband receivers.

Multifunction preselectors A preselector may be engineered with extra features, so that in addition to attenuating interference from unwanted frequencies it can provide additional services which may be helpful for a receiver:

It can limit signal input voltage to protect a sensitive receiver from damage caused by static discharge, nearby voltage spikes, and overload from nearby transmitters' signals. It can provide a DC path to ground, to drain off noisy static charge that tends to collect on the antenna when dry or dusty wind or snow blows across its metal surface. 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 in particular, for the typical noisy frequency bands where a preselector is needed, an amplifier in the preselector has no useful function. On the other hand, when an antenna preamplifier (preamp) is actually needed, it can be made "tunable" by incorporating a front-end preselector circuit to improve its 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. Ordinary, regular preselectors (that are just preselectors) contain no amplifier: They are entirely passive devices. A standard, ordinary preselector, with no amplifier, sometimes has the word "passive" prefixed – hence "passive preselector" means "standard preselector". Since preselectors are normally "passive", adding the redundant word is pedantic, but even so, emphasizes to those only familiar with tunable preamplifiers that the "passive" preselector has no internal amplifier and does not require any power source. In the noisy longwave, mediumwave, and shortwave bands where preselectors are typically used, when used with "modern" (post-1940) receivers they function with no noticeable loss of signal strength.

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