Receiver selectivity characterizes the immunity of the receiver to radio signals on adjacent frequencies

Selectivity of a receiver describes its ability to reject radio signals on adjacent (non-carrier) frequencies. In other words, selectivity characterizes the ability of the receiver to extract the desired signal from interference.

Any radio signal has a radiation spectrum of a certain bandwidth. Additionally, during transmission, harmonics — multiples of the fundamental frequency — are generated. An ideal receiver with perfect selectivity would accept a radio signal of a specific frequency within a set passband while rejecting all others, regardless of their strength. In real receivers, complete suppression of signals outside the passband is impossible, so selectivity is defined as the degree of attenuation of signals outside the passband.

Selectivity is typically expressed as the relative attenuation of a signal at a given tuning offset, measured in dB. The higher the value in dB, the better the receiver’s interference immunity, which increases communication range under electromagnetic interference conditions.

Receiver selectivity is determined by input band-pass filters and intermediate frequency (IF) filters. The steeper the IF filter’s frequency response, the better the selectivity.

Specifications often list selectivity for the adjacent channel (attenuation of an interfering signal 10 or 25 kHz above or below the main signal) and for the image channel (suppression of the frequency symmetrical to the IF). Image channel selectivity applies only to superheterodyne receivers.

Methods of Improving Receiver Selectivity

Adjacent channel selectivity is mainly determined by the IF stage. However, a strong adjacent channel interference can reduce sensitivity and cause intermodulation, as the RF input circuits and mixer of the receiver are insufficiently protected from interference. To address this, band-pass filters are used at the receiver’s input, or sensitivity is intentionally reduced by using an attenuator.

Selectivity depends on the number of stages in the RF amplifier and the quality of resonant filters in each stage. Increasing selectivity generally involves adding more tuned circuits and amplifier stages, which raises cost and complexity. Therefore, the required selectivity is determined at the receiver design stage according to its intended application.

A dual-conversion superheterodyne design provides significantly better selectivity than a single-IF design. High-end imported radios built with superheterodyne circuits and dual conversion can achieve selectivity of 60–75 dB, while direct conversion receivers on the RDA chip typically have about 40 dB.

For broadcast FM radio receivers, GOST standards specify: for class 1 receivers, sensitivity should drop by at least a factor of 200 at a 10 kHz detuning; for class 2 — at least a factor of 20; for class 3 — at least a factor of 10.