Preselector

Lore & Background

A preselector typically is tuned to have a narrow bandwidth centered on the receiver's operating frequency. It passes the signal on its tuned frequency unchanged or only slightly diminished, but greatly reduces off-frequency signals. For any one frequency, using a larger tuning coil results in a narrower bandwidth and greater rejection of out-of-tune signals. The preselector's first input stage contains at least one RF amplifier with limited dynamic range; off-frequency signals waste part of that range, and overload can cause distortion or clipping. Extra filtering by the preselector limits the frequency range and power demands applied to later stages.

Preselectors are especially helpful at lower shortwave and mediumwave frequencies, where atmospheric and human-made noise grow more than exponentially louder as frequency drops below about 20 MHz. A good preselector can reduce a receiver's bandwidth to a narrower span than many general-purpose radios can manage on their own. Spectrum analyzers and other RF measuring equipment can incorporate switchable banks of preselector circuits.

A preselector may include extra features such as limiting input voltage to protect against static discharge or overload, providing a DC path to ground to drain static charge, or incorporating a small RF amplifier stage. However, for the noisy bands where a preselector is needed, an amplifier has no useful function. Ordinary preselectors contain no amplifier and are entirely passive devices; they are sometimes called 'passive preselectors' to emphasize this.

Reader's Guide

Preselectors are significant because they address a fundamental limitation of radio receivers: the finite dynamic range of the front-end RF amplifier. In crowded or noisy bands, off-frequency signals can overload the receiver, causing distortion and reducing performance. By filtering out these unwanted signals before they reach the receiver, a preselector allows the receiver to operate within its dynamic range and focus on the desired signal. This is particularly valuable on frequencies below 20 MHz, where both natural and human-made noise are intense.

The trade-off between bandwidth and signal strength is central to preselector use. Higher inductance gives narrower bandwidth and greater selectivity but slightly higher loss; lower inductance gives wider bandwidth and lower loss but less rejection of interference. Operators typically use a wide setting while searching for signals and then switch to a narrow setting to inspect an interesting signal closely. A preselector is distinct from an antenna tuner: the tuner matches impedances for efficient power transfer, while the preselector filters frequencies. Some circuits can perform both functions, but the purposes remain different.

Did You Know?

• Using a larger tuning coil for a given frequency results in narrower bandwidth and greater rejection of out-of-tune signals.
• For the narrowest bandwidth, a preselector is tuned using the highest inductance and lowest capacitance for the desired frequency.