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Why Test Nonlinear Power Amplifier Performance?

2022-08-23  |  5 min read 

An essential component in modern RF communication chains is the power amplifier (PA). A critical determining factor of transmission quality and battery life, PAs provide needed power to antennas. To satisfy 5G demands, designers need to optimize PA performance. However, the millimeter-wave (mmWave) carrier frequencies, wide bandwidth signals, and complex modulation schemes required to implement 5G impose stringent component error vector magnitude (EVM) qualifications. The significance of PA characterization lies in the following three key reasons:

  • verification by designers that the amplifier meets performance specifications
  • determination by customers that the PA meets their system demands
  • assignment of market value to the device based on its performance

Why characterize the nonlinearity of power amplifiers?

While test parameters for robust amplifier characterization abound, the most critical for PA modeling are EVM and adjacent channel power ratio (ACPR). EVM and ACPR quantify PA nonlinearity, data that designers need for the following reasons:

  • nonlinear response directly affects signal demodulation and bit error rate
  • nonlinearities cause spectral regrowth, creating interference in frequency bands outside the channel of interest

For PAs stimulated with wideband input signals, nonlinearity causes in-band and out-of-band distortion products. Correction of linear distortion is straightforward, but nonlinear effects are harder to address. Designers must quantify the PA’s nonlinearity to ensure that the device meets performance specifications and follows strict 5G EVM and ACPR standards. Essentially, “good” linearity indicates that the PA will amplify the input signal without adding distortion. EVM represents in-band amplifier distortion, and ACPR represents out-of-band distortion.

Figure 1. In-band distortion gets buried under the signal

What is EVM, and why is it important?

EVM is the industry-standard FOM for evaluating the in-band distortion of a communication system. The error vector is the vector difference between the ideal reference signal and the measured signal at a given time. Nonidealities will distort the received and transmitted signals, so quantifying modulated signal quality requires EVM measurements. Modulation standards, such as 802.11ac and 5G New Radio (NR), set the minimum acceptable EVM level. As standard stringency increases, PA designers want to provide standard-specific EVM to their customers.

What is ACPR, and why is it important?

ACPR, adjacent channel power level, and adjacent channel leakage ratio all describe the ratio of transmitted power on the assigned channel to the power received in an adjacent radio channel after a receive filter. These FOMs measure how much one channel may interfere with another. ACPR is the industry-standard FOM for quantifying out-of-band distortion characteristics. ACPR measurements hold particular importance for the UMTS and LTE standards. PA characterization is critical for designers, customers, and the market at large to understand a device’s value and performance quality. While linear performance characterization gives basic amplifier behavior information, designers need accurate nonlinear characterization to fully understand a PA’s efficiency and standard compliance.

Conclusion

Two key nonlinear characterization measurements are EVM and ACPR. While these measurements have been important for previous communication system generations, because of steep increases in system complexity, they function as the benchmarks for 5G NR standard compliance. Through these measurements, design engineers access critical nonlinear amplifier response data. This data helps engineers understand their PA’s performance limitations and market value.