Understanding AM to PM Conversion in Power Amplifiers

This article describes the basics of AM-PM conversion in Power Amplifiers (PAs), its causes, and its effect on the constellation points of modulated signals. AM to PM conversion is essentially a measure of AM-PM distortion (a non-linear effect) within a power amplifier.

This distortion occurs due to unwanted amplitude changes. These changes can be triggered by things like temperature fluctuations, power supply variations, and even multipath fading affecting the RF signal as it travels. This type of distortion becomes particularly noticeable in phase-modulated signals, such as QPSK, PM, and similar formats.

As shown in Fig. 1, ideally, X(t) should behave as described in equation 1. However, due to non-linear distortion, it ends up behaving as described in equation 2. In this scenario, ψ[A(t)] is what contributes to the AM-PM conversion. AM-PM conversion often arises from non-linear capacitors within the power amplifier circuit.

Fig.1 AM-AM/AM-PM conversion Equation

The unit used to measure AM-PM conversion is degrees per dB (deg/dB).

The QPSK constellation will change, as illustrated in Fig. 2, due to AM-PM conversion. However, in a real-world system, other impairments like AM-AM distortion, I-Q mismatch, and phase noise also affect the constellation. Because of this, it can be difficult to isolate and determine whether a specific constellation change is solely due to AM-PM conversion or a combination of factors.