What is AM/PM Distortion?
AM/PM distortion is a form of signal degradation where changes in input signal amplitude cause unwanted changes in the output signal phase. It is a critical non-linearity effect in RF power amplifiers, where increasing signal power not only compresses the amplitude (AM/AM distortion) but also shifts the phase of the output signal. In modern digital modulation schemes like 256-QAM used in 5G NR, even small phase errors can significantly degrade error vector magnitude (EVM) and bit error rate.
How Does AM/PM Distortion Work?
AM/PM distortion is caused by voltage-dependent capacitances and non-linear transconductance within semiconductor devices (GaN, GaAs, LDMOS) used in power amplifiers. As the input power level changes, the internal junction capacitances shift, altering the phase delay through the device. This creates a power-dependent phase rotation that distorts the constellation diagram. AM/PM conversion is characterised by measuring the phase difference between output and input as a function of input power level, typically expressed in degrees per dB.
Use Cases
Power amplifier characterization for 5G base stations, DPD algorithm development accounting for both AM/AM and AM/PM effects, EVM budget analysis for high-order modulation, mmWave front-end module design, and linearization of Doherty and envelope-tracking amplifier architectures.
3GPP / Standards Reference
3GPP TS 38.104 (NR BS radio transmission), 3GPP TR 38.817 (General aspects for BS RF for NR).
Related Terms
AM/PM | EVM | PAPR | ACPR | OFDM
Learn More
This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
