In wireless communication systems, phase and amplitude distortions caused by power amplifiers significantly affect the quality of signal transmission. To evaluate the performance of these amplifiers, one of the most critical metrics used in modern communication protocols is the Error Vector Magnitude (EVM). EVM quantifies the accuracy of the modulation process, reflecting how well the power amplifier transmits RF signals that represent different phases and amplitudes. By analyzing EVM, engineers can gain insight into the internal behavior of the communication link, making it a key parameter for assessing transmitter performance. On the receiver side, EVM helps determine the quality of the demodulated signal, ensuring reliable data reception.
As new signal protocols and advanced modulation techniques continue to evolve, next-generation RF test instruments must support flexible digital architectures, such as Software Defined Radio (SDR), to handle complex signal generation and analysis. These instruments need to be capable of quickly switching between various modulation formats and accurately measuring EVM across multiple standards. In this paper, we explore how modern RF instruments efficiently measure EVM to provide accurate characterization of RF amplifier performance.
A basic communication system consists of an input signal, which could be voice or data. Most contemporary systems digitize all analog signals, effectively making the entire system digital. The power amplifier, located at the final stage of the transmitter, plays a crucial role in maintaining signal integrity. Any distortion introduced here directly impacts the overall communication quality.
To achieve optimal performance, power amplifiers are typically operated near their maximum linear output. However, when the input power exceeds this level, the amplifier enters the gain compression region, leading to nonlinear distortion. Modulation schemes like OFDM generate signals with high peak-to-average power ratios, forcing designers to adjust the operating point of the amplifier to avoid compression. In multipath environments, maintaining a safe distance from the compression region becomes even more challenging.
It's important to note that EVM is not solely influenced by the power amplifier. The transmitter’s modulation module can also introduce amplitude and phase offsets, as well as carrier leakage, which contribute to EVM errors. Similarly, on the receiver side, components like the preamplifier, downconverter, and demodulator can affect EVM values.
EVM is a fundamental metric that measures the accuracy of digital modulation in wireless systems. It calculates the vector difference between the ideal I (in-phase) and Q (quadrature) components of the transmitted signal (referred to as the reference signal "R") and the actual measured I and Q components of the received signal ("M"). This measurement is performed for every transmitted symbol, providing a comprehensive view of signal quality.
Unlike other performance indicators such as eye diagrams or bit error rate (BER), EVM offers faster and more detailed insights into signal integrity. While EVM is proportional to BER, it provides more actionable data for troubleshooting. Additionally, EVM is closely related to the signal-to-noise ratio (SNR) and signal-to-noise plus distortion ratio (SNDR), allowing engineers to identify specific issues within the communication chain.
To measure EVM, a typical setup involves a device under test (DUT), such as a power amplifier designed for GSM/EDGE standards. A vector signal generator (VSG) creates the desired RF signal with specific frequency, amplitude, and modulation characteristics. The signal is then passed through the DUT and analyzed by a vector signal analyzer (VSA), which computes the EVM.
The VSG and VSA share a common reference clock to eliminate frequency offset errors and speed up measurements. Both instruments connect to a computer via LAN or GPIB ports, enabling remote control and automation. In this example, EVM is measured across the amplifier’s operating frequency range and input power levels to assess its impact on performance.
The user interface of the RF instrument is intuitive, supporting mouse control, touchpad navigation, or computer-based remote operation. During the test, the frequency is fixed at 500 MHz, while the RF input power is varied from -40 dBm to -20 dBm in 0.1 dB steps, resulting in 201 measurement points. Each step takes 200 ms, and the average of 20 measurements is taken for each power level to determine the peak EVM.
Using a vector signal analyzer, detailed results show the relationship between EVM and input power. The lower graph displays the amplifier’s gain versus input power, showing a nominal gain of approximately 19.5 dB. The gain begins to decrease when the input power reaches -28 to -30 dBm, dropping by 1 dB at -23.5 dBm and by 3 dB at -20 dBm.
The EVM vs. power plot illustrates the amplifier’s performance. As the power increases and the amplifier enters the gain compression region, EVM rises sharply. In the linear region, EVM remains below 1%. At a 1 dB compression point, EVM increases to about 20%, and at a 3 dB compression point, it exceeds 40%. This clearly demonstrates the strong correlation between EVM and amplifier linearity.
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