Power, Linearity, and Distortion Intermodulation and Spurious Informational

What causes AM to PM conversion in an amplifier and how does it affect system performance?

AM-to-PM conversion is the phenomenon where changes in the input amplitude of an RF signal cause changes in the output phase. It is an inherent property of nonlinear amplifiers and directly degrades signal quality: (1) Mechanism: in a transistor, the output capacitances (Cds, Cgd) are voltage-dependent (they vary with the drain voltage swing). As the signal amplitude increases: the average capacitance changes, shifting the output phase. The transconductance (gm) also varies with the input amplitude, creating a phase shift proportional to the input level. Additionally: biasing effects (the operating point shifts with signal level), thermal effects (the junction temperature changes with power dissipation, affecting the device parasitics), and nonlinear junction capacitances all contribute. (2) Measurement: AM/PM is expressed as the phase change per dB of input power change. Typical values: Class A amplifier: 0.5-2°/dB near P1dB. Class AB amplifier: 2-5°/dB near P1dB. GaN HEMT: 1-3°/dB (GaN has lower AM/PM than GaAs due to higher voltage operation). In the small-signal region (far from compression): AM/PM ≈ 0 (the phase is constant). Near compression: AM/PM increases rapidly. At and beyond P1dB: AM/PM can reach 5-15°/dB. (3) Effect on system performance: EVM degradation: the AM-to-PM conversion rotates the constellation points by an angle proportional to their amplitude. Outer points (high amplitude) rotate more than inner points. This creates a "spiral" distortion in the constellation. For 64-QAM with AM/PM = 3°/dB: the outer points may rotate 5-10°, causing EVM > 5%. Phase-modulated signals (e.g., 8-PSK): particularly sensitive to AM/PM because the information is carried in the phase. Even small phase errors directly corrupt the data. Spectral regrowth: AM/PM contributes to spectral regrowth (adjacent channel power) alongside AM/AM compression. AM/PM and AM/AM together determine the total ACLR.

Category: Power, Linearity, and Distortion

Updated: April 2026

Product Tie-In: Amplifiers, Filters, Connectors

AM-to-PM Conversion

AM-to-PM conversion is often overlooked compared to AM/AM compression, but it can be the dominant source of EVM degradation in some PA topologies.

DPD Correction of AM/PM

Digital predistortion corrects both AM/AM and AM/PM simultaneously: the DPD lookup table stores both the gain correction (inverse of AM/AM) and the phase correction (inverse of AM/PM) for each input amplitude level. The DPD applies the inverse AM/PM: if the PA rotates the phase by +3° at a given amplitude, the DPD pre-rotates by -3°. The net phase error is zero. DPD can typically correct AM/PM to within ±0.5° (limited by the DPD model accuracy and the look-up table resolution).

AM/PM Parameters

AM/PM: degrees phase change per dB input
Class A: 0.5-2°/dB near P1dB
Class AB: 2-5°/dB near P1dB
GaN: 1-3°/dB (lower than GaAs)
DPD correction: ±0.5° residual

Common Questions

Frequently Asked Questions

Is AM/PM more important than AM/AM?

For most applications: AM/AM dominates. But for phase-sensitive signals (8-PSK in GSM/EDGE, constant-envelope modulations with phase information): AM/PM can be the binding constraint. For wideband signals (OFDM): both AM/AM and AM/PM contribute to EVM. The relative importance depends on the modulation format and the PA technology.

How do I measure AM/PM?

Method 1: VNA with power sweep: measure S21 phase vs input power. The phase change per dB is the AM/PM coefficient. Method 2: VSA measurement: transmit a known modulated signal, measure the output constellation, and extract the AM/PM characteristic from the angle vs amplitude relationship. Method 3: two-tone test: the phase asymmetry between the upper and lower IM3 products is related to the AM/PM.

Does AM/PM affect radar systems?

Yes. In pulse-Doppler radar: AM/PM in the transmit PA modulates the pulse phase based on the pulse amplitude (which varies across the pulse due to the pulse shape). This creates spurious spectral lines in the Doppler filter output, which can mask weak target returns. The AM/PM requirement for radar PAs: typically < 1°/dB across the pulse dynamic range.

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