ACPR

Adjacent channel power ratio measures the amount of RF energy a transmitter leaks into frequency bands immediately next to its assigned channel. It expresses the ratio of power in an adjacent channel to power in the transmitter's own channel, typically given in decibels. ACPR is a key compliance metric for radio equipment operating in shared spectrum, where interference from one channel to its neighbors can degrade service quality or violate regulatory limits.

A transmitter's modulated signal occupies a certain bandwidth around its center frequency, but the signal does not stop abruptly at the channel edges. The shoulders of the signal, its spectral skirts, extend outward and overlap slightly into adjacent channels. ACPR quantifies this spillover. In a 5 MHz wide channel, for example, the lower adjacent channel power ratio (LACPR) measures energy in the 5 MHz band just below, while the upper adjacent channel power ratio (UACPR) measures the band just above. Most regulators specify a minimum ACPR: values might be 40 dB to 60 dB depending on the radio service.

Where ACPR matters most

ACPR becomes critical in dense spectrum environments where channels sit tight together. Cellular systems, WiFi, and land mobile radio all operate in crowded bands where one device's poor ACPR harms neighbors. Power amplifier linearity, modulation filter design, and the digital waveform shape all affect ACPR. A sharp, steep transmit filter reduces spillover but costs design complexity and cost; a gentle filter is cheaper but risks regulatory failure or field complaints. Burst transmitters in TDMA systems may show worse ACPR during the turn-on and turn-off transients than during steady state.

Poor ACPR is often traced to inadequate output filtering, power amplifier compression, or aggressive modulation schemes that pack data densely but produce wide spectral skirts. Manufacturers test ACPR in the factory using calibrated RF test equipment and a spectrum analyzer or vector signal analyzer. The measurement requires precise frequency alignment and clean test conditions to avoid false results from environmental interference.

The term adjacent is sometimes extended beyond immediate neighbors. Alternate channel power ratio (ACPR measured two channels away) is occasionally specified for high-power systems. However, the bare term ACPR almost always means the nearest channel.