EVM and ACPR Fully Automated Scan Test Based on LabVIEW

With the rapid development of wireless digital communications, more challenges have been raised for integrated circuit design and testing. In the product design stage, in order to ensure the collaborative working ability and compatibility of the RF and baseband chips in the system, the system needs to be rigorously tested for performance. However, the increasingly complex digital modulation technology often brings more pressure to the design team facing tight project deadlines. Therefore, designers must not only complete the system test in a short time, but also infer the possible causes of the problem from the test results as soon as possible. This paper proposes a fully automated scan test solution that can quickly and accurately measure the two key parameters of the digital communication system transmission link, EVM (Error Vector Magni rude) and ACPR (Adjacent Channel Power Ratio), so as to find the problem in the design at the first time.

1 Digital communication transmission link test

For digital communication system testing, most parameter indicators are completed in the frequency domain, which requires the control of spectrum analyzers and vector signal analyzers for measurement. Among them, measuring the quality of digital modulation is of great significance to ensure the normal operation of digital communication systems and accurate information transmission. The modulation indicators of digital communication systems mainly include EVM, phase error, IQ imbalance, etc.

EVM refers to the modulus of the vector difference between a certain instantaneous ideal reference signal and the measured signal. The Agilent Vector Signal Analyzer 89600 can provide fast, high-resolution spectrum measurement, demodulation, and time domain analysis to obtain EVM measurement results.

ACPR measures how much of the main signal energy of a communication frequency band leaks into adjacent frequency bands. It is also an important indicator of digital communication systems. Excessive power leakage can cause mutual interference between adjacent frequency bands. Usually, we are most concerned about the ratio of the signal power of the main frequency band to the power of the adjacent frequency band, which is measured by controlling the spectrum analyzer.

The transmission link scanning test of the digital communication system refers to the evaluation of the impact of changes in a certain parameter, such as gain, frequency, etc., on EVM and ACPR. This article takes the automatic power control scanning of the transmission link gain as an example. APC (Automatic Power Control) automatic power control scanning scans the gain of the power amplifier driver and up-conversion mixer in the transmission link. These control bits are set through specific register bits in the integrated circuit. LabVIEW accesses these registers on the chip through the SPI and I2C buses with a specific timing to implement read and write control functions to change the transmission link gain. The scanning test block diagram is shown in Figure 1.

LabVIEW controls the spectrum analyzer through the GPIB bus to measure ACPR; it uses ActiveX controls to control the Agilent 89600 vector signal analyzer to measure EVM parameters. This real-time control system can use TCP/IP and GPIB protocol functions to complete the two-way command transmission between the PC and the instrument. The front panel of the LabVIEW automatic scanning program is shown in Figure 2.

According to the order of the scan test operation, the panel is divided into two parts: the left side is to read the control register value of the linear attenuation of the transmission link power from the Excel table; the right side is to customize the instrument parameters to ensure higher measurement accuracy. Therefore, the entire LabVIEW program operation can be divided into 4 parts: read the register values ​​of the transmission link under different gain attenuation conditions from the Excel table; write these values ​​to the corresponding registers of the chip through the SPI bus to change the transmission link gain (power); then, automatically adjust the instrument settings and read the measurement parameters EVM and ACP from them; write the measurement data results in real time to the specified Excel file and store them for post-processing and analysis.

Excel Read.vi reads the register preset value from the specified row and column of the specified worksheet of the opened Excel file, and stores it in a two-dimensional data table buffer in LabVIEW. The advantage of this is that the preset value of APC can be corrected in time, making the test flexible. In this design, this action is triggered by the "Read APC data from Excel" button in Figure 2, and is processed using a LabVIEW event processing structure. [page]

SPI_ Write.vi and SPI_ Read.vi are used to program the PC parallel interface through LabVIEW, and the communication between the chip and the SPI three-wire control is completed. Among them, the parallel interface control is realized by the powerful I/O program module in LabVIEW as the basic operation unit.

2 Transmit link EVM automatic scanning

After completing the transmit link power configuration by changing the register value, you need to control the vector signal analyzer 89600 to adjust the instrument settings and read the scanned EVM data. After LabVIEW completes the initialization of the 89600, it needs to be configured as follows to ensure the accuracy of the EVM automatic test, as shown in Figure 3.

First, activate Trace B of the 89600 display spectrum, as shown in Figure 4. And command its vertical axis to automatically adjust to ensure that the power spectrum is in the appropriate position on the instrument display.

Next, activate the band power measurement mode (BandPower) and set the left and right boundary frequencies of the band power according to the "Bandwidth" parameter set on the front panel. At this time, LabVIEW can accurately read the power of the carrier through the Band-PowerResult property node.

The band power value is of vital importance for adjusting the instrument's Range parameter and ensuring EVM accuracy. The Range parameter adjusts the input signal range of the analog-to-digital converter in the instrument. If its value is too large, it will inevitably cause serious distortion of the input signal and deteriorate the EVM parameter; if the Range value is too small, the EVM parameter will be too sensitive to the introduction of noise, which will also lead to inaccurate test results. A large number of actual test results show that when the Range parameter value is set 3 dB larger than the band power value obtained by the above measurement, the accuracy of EVM can be guaranteed. In addition, since the band power unit in LabVIEW programming is dBm, and the Range parameter unit is the voltage peak value Vpk, the program needs to use the corresponding algorithm to convert units when performing automatic Range adjustment, as shown in the block diagram at the end of the second line in Figure 3.

After completing various configurations, it is necessary to read the corresponding test results such as EVM. This is done by temporarily creating a text file "TempTable.TXT" to read the measurement parameter result table in Trace D in 89600, and importing it into LabVIEW to store it as an array variable. To read the measurement parameters, just specify the subscript where the parameter is located and read the parameter. As shown in the last line of the block diagram in Figure 3, subscripts 6, 8, and 18 point to the parameter parameters EVM, phase error, and phase error peak, respectively. Finally, the data is written and stored in a CSV data file through LabVIEW for data processing and analysis.

3 Transmit link ACPR automatic scanning

Before measuring ACPR, the power of the transmission link needs to be configured and the spectrum analyzer needs to be manually adjusted to the ACP test mode. However, the difference is that this measurement needs to be performed by controlling the spectrum analyzer through the GPIB bus or TCP/IP protocol using SCPI instructions through the VISA interface. The block diagram of LabVIEW is shown in Figure 5.

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The outermost part of the program is a While loop and an event structure to select which test mode to trigger. In the ACPR scan test mode, the scan is implemented through a For loop, and the number of times is determined by the number of rows in the APC preset value table. A sequence structure is nested in the For loop to implement step-by-step operation control. In the 0th and 1st frames, the power attenuation configuration of the transmission link is completed by changing the chip register, and the second frame implements measurement and stores data.

Instrument access in LabVIEW is achieved through the VISA interface. After specifying the address of the instrument, you can send SCPI commands through the VISA write module and read the feedback information of the instrument through the read module.

First, we need to mark the peak power of the carrier. The "DISP:WIND:TRAC:Y:RLEV 8" command in Figure 5 sets the reference power of the spectrum analyzer's vertical axis to 8 dBm. This will push the spectrum graph down in the instrument display interface for easy comparison with the following operations: use marker Marker1 to find the peak in the spectrum and read it out.

Next, the same command is required to set the vertical axis reference power to -6 dBm. This is because during the entire scanning process, the power of the transmitting link attenuates from 0 dB to -76 dB. When the attenuation is large, the carrier signal amplitude is already very small and may even be submerged by the noise floor. This requires the spectrum of the entire instrument to be increased again to ensure the accuracy of the instrument's ACPR calculation.

Finally, the instrument measurement results are stored in the LabVIEW array through the "FETC:ACP?" instruction, and the parameters to be read are pointed to by subscripts and stored in the CSV data file.

4 Test Results and Analysis

The correctness of the proposed method is verified by testing the EVM and ACPR of the transmission link of the TD-SCDMA communication system based on RDA8206. The measured scanning results are shown in Figure 6 and Figure 7.

The actual test shows that the modulation quality can still be guaranteed when the transmission link power is attenuated to -50 dB, so the EVM scan can intuitively see the deterioration of the modulation quality of the transmission link of the digital communication system and analyze the causes of the problem.

ACPR scanning can be used to analyze the interference caused by the power leakage of the carrier signal into the adjacent frequency band. The method proposed in this paper can improve the test efficiency by 60% compared with manual operation while ensuring the measurement accuracy, giving full play to the advantages of automated instrument testing.