ACS

Adjacent channel selectivity measures how well a radio receiver can pick out a wanted signal while rejecting unwanted transmissions on nearby frequencies. In practical terms, it describes the receiver's resistance to interference from channels adjacent to the one being tuned. A poor ACS rating means strong signals from neighboring channels will degrade or block the target signal; good ACS means those neighboring transmissions have minimal impact.

ACS becomes critical in crowded spectrum environments. Mobile networks, two-way radio systems, and emergency services operate with channels spaced only 25 kHz, 12.5 kHz, or even 6.25 kHz apart. Without adequate selectivity, a high-power transmission on channel N+1 will swamp a weak incoming signal on channel N, making reception impossible. The metric is typically expressed in decibels, comparing the receiver's sensitivity on the target channel to its sensitivity when a strong signal is present on an adjacent channel.

Selectivity depends on receiver design and filtering. A sharp bandpass filter before the mixer improves ACS significantly, as does careful IF stage design. Modern digital receivers use software-defined filtering, which can achieve better adjacent channel performance than fixed analog filters of comparable complexity. Dual-conversion receivers, with two intermediate frequencies, generally offer superior ACS compared to single-conversion designs because the first IF stage can be tuned to reject nearby channels before mixing down further.

ACS differs from image rejection and other selectivity metrics. Image rejection addresses signals at the mirror frequency; ACS specifically concerns frequencies immediately adjacent to the desired channel. In cellular and commercial radio standards, ACS requirements appear alongside specifications for co-channel selectivity and blocking performance, each addressing a different interference threat. A receiver may have excellent ACS but poor blocking (vulnerability to very strong out-of-band signals), requiring separate design optimization for each parameter.

Field performance of ACS depends on antenna placement and signal strength distribution. In urban areas with multiple strong transmitters close together, weak receivers or antennas oriented toward nearby high-power sites will suffer more from adjacent channel interference than measurement on the bench suggests. System planners account for ACS when designing channel assignments and coverage strategies, especially in radio systems where mobile units operate across varied terrain and transmitter locations.