"The original wireless local area network standard was mainly used to provide low data transmission rate wireless connection for wired broadband connection for web browsing and e-mail sending and receiving. With the evolution of time, the new 802.11 wireless protocol can provide higher data transmission rate for new applications. The current 802.11ac WLAN standard can provide data transmission rate of up to 867mbps in a single RF channel and up to 6.93gbps in MIMO channel. The 802.11n standard has been extended to such a high 802.11ac data transmission rate due to the use of more immediate bandwidth (up to 160MHz), more MIMO channels (up to 8), and high-density modulated constellation (up to 256qam).
Power amplifier test
Power amplifier (PA) is a key component in WLAN transmitter circuit, because PA performance will affect wireless coverage, data transmission rate, capacity and battery life. The goal of any transmitter PA is to use as little DC power as possible to produce sufficient linear RF output power. When the output power increases to the gain compression region of the amplifier, PA performance can dominate the transmitter performance at the WLAN system level due to PA nonlinear distortion. Mobile devices and wireless hotspots usually transmit RF output power between 100MW (+ 20dbm) and 1W (+ 30dBm), and the PA must be able to generate sufficient power with minimum nonlinear distortion. For PA testing, a complete set of IEEE 802.11ac specific transmitter verification tests includes:
● spectrum mask
● spectrum flatness
● peak power
● center frequency error
● symbol clock frequency error
● center frequency leakage
● error vector magnitude (EVM)
This paper will further expand EVM testing, which is a comprehensive and widely used technology for PA testing. EVM is a test used to quantify the performance of digital communication channels and provide error measurement between the captured coded data characters and the ideal position in the I / Q constellation. Root mean square EVM is a comprehensive measurement, which will be reduced by any defect in RF signal or equipment. Therefore, for WLAN transmitter design, PA needs acceptable EVM effect within the complete operating range of its output power and channel frequency. Since 802.11ac includes 256qam cluster with 2.5% (- 32dB) EVM specification limit, PA linearity and corresponding EVM action necessary conditions are more stringent than the early 802.11 standard, while the PA EVM action in 802.11n is limited to about 3%, while the PA EVM action in 802.11ac is limited to about 1.5%. In addition, the new 256qam signal modulation has a higher peak to average ratio (PAR) and also increases the PA in the 802.11ac transmitter design and its necessary linear output power.
Figure 1: test equipment architecture for PA test.
Figure 1 shows a typical test equipment architecture for PA test using Zec instrument z8201rf test kit. Typical equipment lists include:
● z8651 6GHz vector signal analyzer (VSA), 80 or 160MHz analysis bandwidth can be selected
● z8751 6GHz vector signal generator (VSG), 250 or 500MHz adjustable bandwidth can be selected
● z5211 200ms / s arbitrary waveform generator
● lb480a USB power meter of Ladybug technology can be selected as required
● PXI / pxie chassis and host computer
● cables, directional couplers and attenuators
Since the input and output power of PA are set and measured by VSG and VSA, USB power meter and relevant directional coupler can be selected as required. The power meter provides more accurate correction measurements of PA input and output power measured by the directional coupler on the DUT. VSA and VSG can usually be accurate to < 0.5dB, while the power meter can be accurate to < 0.1dB. When used for attenuator and power meter configuration, the correction factor for directional coupler must be corrected in advance.
PA EVM
A typical PA EVM test will measure the output power of EVM relative to pa through many test frequencies. Figure 2 shows the actual measured data curve measured by a typical PA EVM test using the z8201rf test kit. These curves show that all five 80MHz 802.11ac channel frequencies tested are applicable to PA in the input power range of 30dB. The actual PA output power is measured using a power meter, and this data is used as the horizontal axis coordinate of the curve in Figure 2. In this test, there are a total of 150 test points with 5 channel frequencies and 30 power. One advantage of PXI / PIXE highly integrated test equipment architecture is fast data transmission and processing speed. In 150 test cases, the total measurement time can be greatly reduced compared with other test equipment with interfaces such as LAN or pgib. The z8201rf test suite and zprotocl WLAN Software provide example coding of setting and operation optimization for 802.11ac test, which can ensure that each EVM test can be completed within 20ms.
When discussing the actual PA test data shown in Figure 2, it can be found that the EVM decreases at high output power. As the PA output power increases to its gain compression region, nonlinear distortion will appear and cause the EVM to increase. This EVM power scanning test identifies the linear power region of PA, which is a key factor in the design of WLAN transmitter. It should be noted that in order to achieve the critical value of 802.11ac lower than 1.5% EVM, this specific PA can achieve + 10dBm linear output power; Although this pa is designed for 802.11n transmitters and works well, its linear output power will not be enough for 802.11ac transmitter design without additional linearization technology such as digital predistortion.
Figure 2: PA EVM vs. output power.
Dynamic EVM
Battery life and power consumption are important considerations in the design of system level WLAN transmitter. Because a large part of the total system DC power is consumed by PA transmission, it is necessary to adopt a variety of technologies to reduce the use of PA power. Relative to DC power consumption, many PAS provide adjustable DC supply voltage to optimize RF output power, and most pas can be powered off or disabled when not in use to save power, such as when receiving or between packets during transmission. In order to maximize power efficiency, PA must have fast on and off switching time. Figure 3 shows the PA enable (PA EN) correlation timing and RF signal of PA under the pulse condition of 50% duty cycle captured by oscilloscope. Note that the adjustable delay between PA en pulse and RF signal is set to 2.0 in this test equipment μ s。 When the time interval between PA en and RF signal is the smallest, DC power benefits, but short delay will aggravate the instantaneous effect of RF signal.
Figure 3: time domain curve of PA enable (yellow) and RF pulse (blue).
Because the power supply / off operation of PA can cause transient and thermal effects and reduce the performance of transmitter, it is often necessary to measure another index called dynamic EVM. The dynamic EVM is measured by applying a square wave pulse to the PA en to simulate the actual dynamic working environment of the transmitter. The decrease of dynamic EVM is due to the transient response of PA affecting the initial header of the packet and causing defective channel estimation. The research shows that the dynamic EVM with 50% duty cycle square wave is more unsuitable for PA en than the static EVM (with 100% duty cycle).
Using the test equipment shown in Figure 1, the dynamic EVM test is fully automated by using the PXI / pxie system. All clock synchronization of dynamic EVM measurement can be realized by using PXI / pxie backplane trigger and clock signal. The block diagram of Fig. 1 shows the z5211 arbitrary waveform generator (AWG), which generates PA en pulses with adjustable voltage size, pulse width, pulse delay and repetition rate. The actual PA test data is shown in Figure 4. Below + 18dbm output power, the dynamic EVM is worse than the static EVM. For this particular PA, dynamic EVM is better than static EVM above + 18dbm output power. As mentioned earlier, because dynamic EVM can measure PA performance in actual pulse operation mode, this typical PA dynamic EVM measurement is very important for transmitter design considerations.
Figure 4: PA dynamic EVM and duty cycle.
Digital predistortion
At high output power, improving linearity in PA is a challenge. Digital predistortion (DPD) is a technology to eliminate distortion essentially through digital signal processing technology. For combined VSA / VSG test systems, software tools can simplify and automate DPD, such as z8201 RF test suite. In essence, the software model measures the nonlinearity of pa through VSA and forms an opposite operating state applied to VSG. When DPD compensation is completed, the predistorted VSG RF signal is applied to the PA of the effectively linearized PA output.
Some 802.11ac WLAN transceiver chipsets use DPD technology to improve PA linearity. In order to quantify the improvement that will be achieved in circuits with DPD, the test equipment must be able to perform DPD during PA characteristic analysis. Together with the z8201rf test suite and zprotocl WLAN Software, the DPD software tool of Zec instrument and the corresponding sample code provide a fast and easy method to evaluate the DPD of PA or transmitter design. Because the DPD algorithm requires VSG / VSA instruments to capture multiple adjacent channels, DPD applications require a wide measurement bandwidth such as the z8201rf test suite.
Fig. 5 shows the improvement of leakage on adjacent channels caused by nonlinear distortion of DPD when PA works in its nonlinear region. It is also important that EVM improvement can be achieved using DPD (as shown in Figure 6). Both figures depict the actual data obtained by using zprotocol WLAN and DPD software to z8201rf test suite.
Figure 5: reducing PA adjacent channel leakage using DPD.
Figure 6: improving PA EVM with DPD
Test equipment
For 802.11ac test, the background noise, phase noise, crossover distortion and in band spurious signal of the test equipment must be minimized to avoid reducing the measured PA EVM performance. Figure 7 shows the effect of test equipment residual EVM on the measured PA DUT EVM.
Figure 7: effect of test equipment residual EVM on the measured DUT EVM.
As discussed earlier, the z8201rf test suite in Figure 8 consists of a 6GHz VSG / VSA combination with a measurement bandwidth of up to 160MHz. In addition to wide measurement bandwidth, the z8201rf test suite also provides low noise and distortion necessary for 802.11ac device characteristic analysis and testing.
Figure 8: z8201 PXI or pxie RF test kit.
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