This page discusses  Power Amplifier Linearity Measurement Methods, covering techniques like IMD (intermodulation distortion) measurement, NPR (noise power ratio) measurement, and CF (Crest Factor) measurement (or peak-to-average power ratio measurement).

Introduction

    GaN technology is rapidly advancing, with wideband RF  Power Amplifiers (PAs) being the most popular GaN devices used in 5G networks.  5G systems demand very stringent linearity.  Traditional methods are often insufficient to accurately predict real-world  amplifier linearity.  The following methods offer simpler, more accurate, and less error-prone alternatives to traditional tools.

    Amplifiers are characterized by various parameters, including gain, bandwidth or frequency response, power output, linearity, efficiency, and noise figure.

    Amplifier Linearity Measurement Method-1 | IMD Measurement

    Linearity: This is determined by measuring the output power with a uniform increase in input power until the amplifier enters the compression region.

    One key parameter is P1dB, which indicates the point where the output power no longer follows the input power linearly.  In other words, a 1 dB change in input power results in less than a 1 dB change in output power.

Figure-1: P1dB and IP3 curve    The measurement setup for IMD measurement is shown in the block diagram below. It consists of two signal generators whose outputs are combined using an RF power combiner and fed to the Device Under Test (DUT). The output is measured using an RF Spectrum Analyzer.

Figure-2: IMD measurement setup    Besides the desired outputs (at F1 and F2), the setup generates other products, which are harmonics or IMD products of the two input signals (F1 and F2). The major consequences arise from 2f1-f2 and 2f2-f1 as they are near to desired frequencies(f1,f2) in the frequency spectrum.

Figure-3: IMD products

    Let’s understand the Third Order Intercept (TOI) Point. This is the point where each 1dB increase in input power results in a 3dB increase in output power. Hence, a slope of 3 is achieved. The third-order intercept point is the result of third-order IMD products, such as 2w1-w2 and 2w2-w1, as shown in the frequency spectrum.

    The table below lists the third-order intermodulation output frequencies resulting from the application of two input frequency channels (f1 and f2).

Table-1: IMD products

    The graph below shows various points, including P1dB at the input and output, and TOI points at the input (IIP3) and output (OIP3). Each 1dB increase in P1dB results in a 3dB increase in 3rd order IM (Intermodulation) power. The TOI point is a figure of merit that characterizes an RF receiver’s tolerance to multiple RF signals outside the passband of interest. The TOI points on the X-axis and Y-axis are designated as IIP3 and OIP3, respectively.

    Refer to IIP3 versus OIP3 for a description of the difference between IIP3 and OIP3.