High Accuracy EVM Measurements for a Power Amplifier Using the PNA-X Series Network Analyzer - Keysight
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The PA is the biggest contributor to the quality of the RF chain in communication systems. Linearity is extremely important in systems with high peak-to-average-power (PAPR) signals, as in modern wireless standards where OFDM is used for the digital modulation scheme, because the nonlinear response of the RF chain directly impacts error in the demodulation, resulting in poor bit-error-rate. Also, spectral regrowth created by nonlinearity can create interference in other bands. Thus, maintaining linearity of the RF chain is critical for the quality of wireless communication.
The PA is also known as a contributor to the power consumption for wireless communication systems. Especially in the user equipment (UE) side, power consumption or power efficiency of the PA directly impacts the quality of service for wireless communication because a PA with poor efficiency consumes limited battery life quickly.
Because there is a trade-off between linearity and efficiency, one of the challenges for the PA industry is to design the PA to be linear as well as efficient, with demanding and emerging operating conditions such as mmW carrier frequency and extremely wide signal bandwidth. Digital predistortion (DPD) is a method of linearizing the PA by predistorting the input signal with the inverse of the distortion contributed by the PA, resulting in a linearized signal at the output of the PA.
To quantify PA nonlinearity under a modulated stimulus condition, EVM is commonly used as a Figure of Merit (FOM) for the in-band characteristics, and Adjacent Channel Power Ratio (ACPR) is used for the out-band characteristics. In this application note, a novel method of characterizing PA nonlinearity under a modulated stimulus condition is introduced.
