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AM to PM Conversion

This describes AM-PM conversion basics in PA, its cause and mention its effect on constellation point of modulated signal.

AM to PM conversion is the measure of AM-PM distortion (nonlinear) in power amplifier (PA). This happens due to undesired amplitude changes due to temperature as well as power supply variations and multipath fading on the way of RF signal. This type of distortion more prominent in phase modulated signals such as QPSK, PM etc.

As shown in fig.1 X (t) should be as mentioned in equation 1, but due to nonlinear distortion it will be as mentioned in equation 2, here ψ[A(t)] contribute for AM-PM conversion to occur. AM-PM conversion usually results due to nonlinear capacitors in power amplifier circuit.
AM-AM/AM-PM conversion Equation
Fig.1 AM-AM/AM-PM conversion Equation
Unit of AM-PM conversion is deg/dB

QPSK constellation changes as shown in the fig. 2 due to AM-PM-conversion, but due to other impairments in the system such as AM-AM distortion, I-Q mismatch, phase noise etc. one cannot distinct whether the effect (constellation change) is due to which of the impairments.
AM-PM conversion

AM-PM measurement

Refer our page RF measurement tutorial which mentions test set up for AM-PM measurement.

Useful Link

AM-AM-conversion, Read More.


PA nonlinearity

RF and Wireless Terminologies