In wireless communication devices, phase and amplitude distortion caused by power amplifiers has a direct impact on communication quality. In the latest communication system protocols, the most important measurement for analyzing power amplifier performance is the measurement error vector magnitude, or EVM. It measures the accuracy of the modulation, that is, the power amplifier transmits the information represented by the RF signals of different phases and amplitudes. The ability to observe the internals of the communication link through EVM measurements is the key to measuring transmitter performance. On the receiver side, the EVM measures the pros and cons of the receiver's demodulated transmission signal.
As various existing and new signal protocols and modulation methods are applied to emerging wireless communication standards, next-generation RF test instruments require new digital architecture implementations including software infinite power (SDR) to test new signals. Transmission mechanism. New instruments must have the flexibility to generate and analyze multiple types of modulated signals, and must be able to quickly switch between these modulation types. Therefore, new RF instruments must be able to quickly and accurately measure EVM metrics for many different modulation formats. In this paper, we will analyze how these new instruments accurately measure EVM for adequate characterization of RF amplifier performance.
RF power amplifierA simplified communication system is presented in which the input signal can be voice or data. Most modern systems digitally process all analog signals, so the communication system is virtually digital.
The power amplifier is the last stage of the signal transmitter. Any amplitude or phase distortion here directly affects the communication quality of the entire system.
For optimum performance, power amplifiers typically operate as much as possible at the maximum linear power output. Above the maximum linear output power is the gain compression region, which occurs when the power amplifier enters this compression region. Modulation methods such as OFDM are capable of generating signals with higher peak-to-average ratios. This forces the designer to "compensate" the average power operating point of the power amplifier to ensure that the peak power does not cause the amplifier to enter the gain compression region. For multipath signal modulation methods and multipath external environments, it is difficult to ensure that the power amplifier is far from the gain compression region.
However, the power amplifier is not the only component that affects the EVM. The transmitter's modulation module has amplitude and phase offsets as well as carrier leakage, all of which increase the EVM error. At the receiver side, the preamplifier, downconverter, and demodulator all affect the EVM error.
About EVMEVM characterizes modulation accuracy and is a key indicator of the quality of digital modulation in modern wireless communication systems. EVM is the ideal measurement component I (in-phase) and Q (quadrature phase) of the transmitted signal (referred to as the reference signal "R") and the amplitude of the I and Q components of the actually received measurement signal "M". Vector difference. The EVM is suitable for every symbol that is transmitted and received.
The quality of the signal can be observed by the EVM value, which is not characterization of measurement performance indicators such as eye diagram or BER. EVM is proportional to the bit error rate, but it is faster than the eye diagram or BER test and provides more information for observation.
EVM and signal-to-noise ratio (SNR) and signal-to-noise plus distortion ratio (SNDR) are also directly related. We can use the EVM to determine the actual errors introduced at different levels of the communication system, which can help designers find specific problems.
EVM measurementThe establishment of EVM measurements gives a typical EVM measurement setup. The device under test (DUT) is a power amplifier for transmitting signals conforming to the GSM/EDGE mobile communication standard. We tested the EVM performance of its EDGE modulation.
We use a vector signal generator (VSG) to generate RF signals with the desired frequency, amplitude and EDGE modulation. The RF signal is transmitted through the power amplifier to be tested and demodulated in a Vector Signal Analyzer (VSA), which is responsible for measuring and calculating the EVM.
The VSG and VSA reference frequency clocks are connected together. This approach eliminates the relative frequency error between the two instruments and greatly speeds up the measurement. The two instruments are connected to a computer via their LAN (LXI) or GPIB ports.
In this example, we will measure the EVM over the operating frequency range of the amplifier and the range of input power to analyze how the EVM of the power amplifier is affected by the frequency and input power magnitude.
The user interface of the new RF instrument is easily controlled by the mouse, the analyzer's touchpad or by computer remote control.
In this measurement example, the frequency is always maintained at 500 MHz, and the RF input power is varied from -40 dBm to -20 dBm in 0.1 dB steps. This will have 201 amplitude steps (ie measurement points), and each step measurement takes 200ms. The DC bias remains unchanged. The modulated signal is an 8PSK EDGE signal, and the average of 20 measurements is taken for each amplitude step when measuring the peak EVM.
Using a vector signal analyzer to test the relationship between EVM and input power gives detailed measurement results. The lower panel shows the amplifier gain vs. input power (blue line), which shows a nominal gain of approximately 19.5 dB. It begins to drop when the input power is -28~-30dBm. The amplifier gain is reduced by 1dB at an input power of -23.5dBm and by 3dB at -20dBm.
The above diagram shows the relationship between EVM and power. The red line that identifies the "Distorted Plot" is the EVM of the amplifier. Obviously, as the power amplifier enters the gain compression zone, the EVM drops rapidly. The EVM is less than 1% in the linear region. At a compression point of 1 dB, EVM grows to about 20%, and EVM increases to more than 40% at a compression point of 3 dB.
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