ACRR

Adjacent-channel rejection ratio measures a receiver's ability to accept a wanted signal while rejecting an unwanted signal on a neighboring frequency. It is the ratio of the wanted signal level to the unwanted signal level, both measured at the receiver input, when both produce equal output power. The result is expressed in decibels. A receiver with poor ACRR will suffer interference from transmitters operating on nearby channels; one with good ACRR can coexist peacefully in crowded spectrum.

The test itself is straightforward in principle but demands precision in practice. A test signal at the desired frequency is set to a reference level, typically producing a defined output. Then a second signal is introduced on an adjacent channel, and its level is increased until the output power equals that of the wanted signal alone. The ratio between these two input levels is the ACRR. For example, if a wanted signal at 100 mW produces the same output as an unwanted signal at 10 mW on the next channel, the ACRR is 10 mW divided by 100 mW, or 0.1, expressed as minus 10 dB.

Selectivity and the receiver front end

ACRR is fundamentally a question of filter selectivity. The receiver's input filter, intermediate frequency stages, and detector all shape how sharply it rejects off-channel signals. A narrow filter with steep skirts gives better ACRR; a broad filter passes more adjacent-channel energy. In modern digital receivers, software-defined filtering can achieve very high ACRR figures by applying narrowband algorithms after initial analog stages, whereas older superheterodyne designs were limited by the quality of their IF transformers and ceramic filters.

Adjacent-channel performance becomes critical in dense frequency allocations. Land mobile radio systems operating in 25 kHz or 12.5 kHz channel spacing, cellular networks with narrower allocations, and emergency dispatch systems all depend on maintaining adequate ACRR to prevent crosstalk and lost communications. A degraded ACRR from component aging, antenna coupling, or poor shielding can render a receiver unreliable in its operating environment long before it fails a sensitivity test.

The term reflects the radio engineer's practical concern: not how loud a signal the receiver can hear in isolation, but how well it can hear the signal you want when everyone around you is transmitting too. ACRR appears alongside adjacent-channel power ratio (ACPR), which measures a transmitter's leakage into neighboring channels, in the same way that transmitter and receiver selectivity are mirror concerns in spectrum management.