AM-PM Distortion

The core mechanism of AM-PM distortion is the variation of the amplifier's insertion phase as a function of the instantaneous input power. Unlike an ideal linear amplifier with constant phase shift, a nonlinear PA exhibits phase compression or expansion near saturation. This is primarily caused by the voltage-dependent capacitance of the transistor's junction (e.g., the gate-source capacitance in a FET), which varies with the signal envelope and alters the device's S-parameters in real-time.

In digitally modulated signals, AM-PM conversion manifests as a signal-dependent phase rotation of the constellation points. Higher-amplitude symbols experience a different phase shift than lower-amplitude ones, causing the constellation to warp asymmetrically. For QAM and APSK schemes, this results in an increased Error Vector Magnitude (EVM) and a higher Bit Error Rate (BER) at the receiver, as the decision boundaries become distorted.

While AM-AM distortion is the dominant cause of spectral regrowth, AM-PM distortion contributes significantly to asymmetric spectral regrowth in the adjacent channels. The phase nonlinearity mixes with the amplitude nonlinearity to generate intermodulation products with complex phase relationships. This asymmetry in the upper and lower sidebands is a key signature of memory effects interacting with the static AM-PM characteristic, complicating linearization efforts.

AM-AM and AM-PM distortion are inherently coupled in physical devices. As a PA approaches its 1 dB compression point (P1dB) and saturation, both the gain and the phase shift change simultaneously. A complete behavioral model must capture both effects. The complex gain representation combines them into a single complex-valued function of the instantaneous envelope, where:

• The magnitude represents AM-AM (gain compression)
• The phase represents AM-PM (phase distortion)

AM-PM distortion significantly increases the difficulty of digital predistortion (DPD). A memoryless AM-AM correction can be implemented with a simple Look-Up Table (LUT) indexed by input magnitude. However, correcting AM-PM requires a complex-valued predistorter that applies a phase rotation in addition to a gain adjustment. When memory effects are present, the phase correction must also account for the signal's envelope history, requiring full Volterra series or memory polynomial models with complex coefficients.

AM-PM distortion is characterized by measuring the phase shift versus input power using a vector network analyzer (VNA) or a vector signal analyzer. Key metrics include:

• Degrees per dB: The rate of phase change with input power, typically specified in °/dB near the amplifier's operating point.
• Total phase variation: The peak-to-peak phase shift across the dynamic range of the signal.
• AM-PM coefficient (kp): A parameter in the Saleh model that quantifies the phase nonlinearity as a function of the normalized input amplitude.