ACIR

Adjacent channel interference ratio measures the power of a transmitted signal that leaks into the frequency bands immediately next to the one you are licensed to use. A transmitter operating on 460 MHz will inevitably radiate some energy at 460.025 MHz and 459.975 MHz, or whatever the channel spacing happens to be. ACIR quantifies this spillover as a ratio, typically expressed in decibels (dB), comparing the power in the adjacent channel to the power in the primary channel.

The ratio is measured at the receiver input, after the signal has passed through the transmitter's output filter and the receiver's input filter. Poor ACIR means a strong signal on one channel can degrade or block reception on a neighboring channel, even when both receivers are working correctly. This is especially critical in dense radio systems, such as commercial land mobile radio networks, cellular base stations, and maritime communication bands, where channels are spaced closely to maximize spectrum efficiency.

Causes and typical values

ACIR performance depends on transmitter spectral mask (how well the modulation is contained), the sharpness of the output filter, and receiver selectivity. A typical specification for a professional VHF or UHF transmitter might be 60 to 70 dB; modern digital modes and tighter filtering can achieve 70 to 80 dB or better. A poor result, say 40 to 50 dB, indicates filter degradation, phase noise in the oscillator, or nonlinear amplification.

ACIR is distinct from adjacent channel leakage power (ACLP), which measures absolute power levels rather than ratios. Regulators like the FCC and ISED specify both limits to protect shared spectrum. A transmitter may meet ACLP limits in an isolated test but still create harmful ACIR in the field if the receiving antenna and filters do not provide adequate rejection.

In digital systems, waveform design and baseband filtering heavily influence ACIR. Rectangular pulse modulation performs poorly; Gaussian or root-raised-cosine filtering reduces out-of-band energy significantly. Time-division duplex systems, which use the same frequency for transmission and reception at different times, are especially vulnerable to ACIR problems because transmitter and receiver share antennas and nearby circuitry.