How can selectivity be achieved in a communications receiver?

Selectivity in a communications receiver refers to its ability to isolate a specific signal frequency while rejecting signals at other frequencies. Employing a preselector is an effective method to achieve selectivity because it allows the receiver to filter unwanted frequencies before the signal is amplified and processed further.

A preselector typically consists of a bandpass filter that is tuned to the desired frequency range. It functions as an initial filtering stage, allowing only signals within a narrow band of frequencies to pass through while attenuating those outside that range. This helps to ensure that only the relevant signals reach the subsequent stages of amplification and demodulation, improving overall selectivity and reducing the risk of intermodulation distortion from strong out-of-band signals.

In contrast to using a preselector, other methods may not provide the same level of selectivity. For example, adjusting the gain of the RF amplifier can improve the signal strength but does not filter out unwanted signals. Increasing the noise floor would actually degrade the receiver's performance, resulting in a poorer signal-to-noise ratio. While a postselector can refine selectivity after initial processing, it is generally more effective to achieve selectivity upfront with a preselector.