WLAN Baseband Tx Module Testing using High-Speed DAQ Cards Cheng-Tao Hu, Senior R&D Manager, ADLINK Chris Ni, Product Manager, ADLINK Companies have developed high-speed data acquisition (DAQ) cards and claimed that they are applicable for military radar, supersonic, digital broadcast signal analysis, or inkjet cartridge system testing applications. DAQ cards have sampling rates of 20-100 MS/s, a bandwidth of 30-60MHz, and the ability to provide simultaneous software-selectable multigroup analog signal inputs to meet the demands of these applications. However, published examples of DAQ cards employed in these applications are scarce. For this reason, this article will use ADLINK Technology’s PXI-9820, their latest high-speed DAQ card, as a model to detail possible uses in wireless LAN (WLAN) development and all aspects of mass production testing equipment. Why WLAN? Several of Taiwan’s leading wireless broadband solution providers predict a huge growth for 2005. Gemtek, forecasted approximately 90% of notebook computers sold in 2005 will include WLAN modules. They go on to state that the 802.11g will replace the standard module, 8.02.11a+g, in the second quarter with an expected output of 20-25 million units for the entire year. CyberTAN Technology has also stated 25 million units will sell in 2005. And Global Sun Technology, after merging with Cameo, has estimated that it will sell eight million units. These figures suggest an enormous growth potential for WLAN products and the largest market share will be won by the company that bring their products to market the quickest. Therefore, companies must ensure two things: 1. Engineers can effectively resolve possible issues during the development stage and that their designs pass through comprehensive verification processes. 2. The production lines must use adequate testing equipment to ensure both product quality is the best possible and production capacity meets the demand. However, companies are faced with difficult business decisions the cost and effectiveness of existing testing platforms. Companies, chip designers and system manufacturers, for example, must decide whether or not to invest the funds to purchase a large quantity of equipment for use in R&D departments and production lines that test one WLAN card per minute. The trend of chip and equipment manufacturers is moving toward enhancing test lab facilities and utilizing the latest equipment to simulate RF network environment to test the dependability and scalability of networking products and solutions. This article is centered on ADLINK’s latest low-cost and flexible PXI-9820 high-speed DAQ card, suitable for mass-produced WLAN transmitter module real-time error vector magnitude (EVM) testers aimed at chip designers and systems manufacturers. System Structure: The module has three major components: a WLAN transmitter, a high-speed DAQ card with controller, and a software module interface for EVM calculation and analysis. 1. WLAN Transmitter: a. Wireless card (802.11a) + card bus: WLAN transmitter module. b. Analog device instrument (ADI) evaluation board: Converts differential signals I+, I-, and Q+, Q- to single-ended output circuit signals I and Q. 2. High-Speed DAQ Card and Controller: a. ADLINK PXI-3800: 1.6GHz Intel® Pentium® M PXI controller with real-time signal calculations (see Figure 1). b. ADLINK PXI-2506: 3U, 6 slot PXI portable chassis (see Figure 2). c. ADLINK PXI-9820: 3U PXI 65MS/s, 14-bit digitizer with 128MB SDRAM on-board for I-Q signal acquisition (see Figure 3). 3. EVM Calculation and Analysis Software Module Interface a. ADLINK’s in-house wireless card signal control program: Controls constant generation and transmission of WLAN card frames. b. ADLINK’s in-house real-time I-Q signal analysis program: Analyzes discrete fast Fourier transforms, 64-QAM, EVM calculations, and more. Figure 1. ADLINK PXI-3800 Controller. Figure 2. ADLINK PXI-2506 Portable Chassis. Figure 3. ADLINK PXI-9820 High-Speed DAQ Card. As shown in the test system block diagram of Figure 4, ADLINK’s PXI-3800 controller executes a WLAN card signal control program where the card bus triggers the WLAN card to send continuous Tx test signals. Output signals from the network card are differential signals I+, I- and Q+, Q-. Because the DAQ card uses two-channel single-ended inputs, an Analog Device Instrument (ADI) evaluation board converts the signal from differential to single-ended. Baseband I-Q signals are sent to the PXI-9820 and analyzed using ADLINK’s in-house real-time I-Q signal analysis program to perform discrete Fourier transforms (DFT) and EVM. Figure 4. Baseband Transmitter Test System Block Diagram. Figure 5. Complete Baseband Transmitter Test System. Principle: The IEEE 802.11a specification defines WLAN transmission/reception principles (see Figure 6). The physical layer (PHY) utilizes orthogonal frequency division multiplexing (OFDM) techniques to merge the majority of the various frequency carriers into one signal to complete the transmission. The transmitter (Tx) uses inverse fast Fourier transformations (IFFT) to modulate the signal before transmitting each frame. I-Q modulation (I - in-phase, Q - quadrature) is then used to separate I-Q signals. Finally, a RF circuit is used to up convert signals from baseband frequency to 5GHz band for transmission. The receiver (Rx) then down converts the RF signals to the baseband, demodulates I-Q signals, and uses discrete fast Fourier transforms (DFT) to convert each frame back to its original state. For practical high-speed DAQ applications, the WLAN circuit and its signal processing can be simplified in the following manner: 1. The RF circuitry is omitted for direct acquisition and analysis of baseband signals. 2. The I-Q demodulation circuit is implemented using two ADI evaluation boards. 3. Rx frame synchronization and sampling will not be discussed. We have set a simple threshold value following the single-ended I-Q signals so that receivers will find the symbol boundary before demodulating sub-carriers. 4. Detailed signal processing techniques will also not be discussed (data descrambler, convolutional encoders, data interleaving, normalize average power, windowing functions, etc.). Figure 6. WLAN Tx/Rx Operations Each time a frame structure, similar to Figure 7, is transmitted, the preamble components (including two short and two long symbols) are modulated using binary phase shift keying (BPSK). Signal and data components are modulated using 64-QAM. The length of data is arbitrary. Figure 7. Frame transmission structure. Test Method: Test Signal Measurement: The job of the test system is to test baseband signals at a specific location (the testing point of Figure 6). Two sets of test points, I+, I- and Q+, Q-, are separately connected after the circuit performs guard interval (GI) addition. These two signal sets are differential signals of I and Q. Using an ADI differential signal single-ended output circuit, we send the I and Q signals to a PXI-9820 digitizer in a single-ended, two-channel mode. The PXI-9820 sampling rate is set at 60MS/s with a resolution of 14 bits. Middle trigger triggering mode is selected. Test Signal Generation: The transmitter baseband signal frame is generated by ADLINK's in-house WLAN card signal controller program. The program will continuously send frames. The preamble and symbol sequences, including ten periodic short (8s total) and two periodic long (8s total) training sequences, of each frame are sequentially generated in accordance to the training symbol in the 802.11a specification. Data length and content are arbitrary. Frame time intervals are also arbitrary. For this test, Data length is set as 4096±n periods with arbitrary time intervals. Baseband Signal Analysis: Using correct triggering mode settings, the PXI-9820 can accurately sample data at the start point of each frame. Afterwards, the data of the entire frame is sent to the memory of the PXI-3800 controller. The PXI-3800’s computing power carries out real-time calculations on all the data, in addition to executing the following operations on the entire preamble and data components: 1. Convert the individual single-ended I and Q signals into a single complex signal, I+Qi. 2. Using 80 points as one unit, discard the first 16 points of the cyclic extension in each symbol and compute the FFT on the last 64 points. 3. Calculate first 4 units, then demodulate the FFT result using BPSK (two short and two long training sequences). 4. Repeat steps (2) and (3), analyze continuous data units, and execute 64-QAM and constellation operations. 5. Calculate the signal EVM for a quantified reference value of transmission quality and system design. An EVM is defined as: M Z (k ) R(k ) 2 EVM dB 20*log10 k 1 M R(k ) 2 k 1 where Z is the test signal, R is the ideal signal, M is the test symbol number, and k is the sample number. Test Results: Figure 8 shows ADLINK’s I-Q signal analysis software interface. The green signal in the upper portion represents the I signal. The red signal in the lower portion represents the Q signal. The periodic waveform at the left of each signal are the short and long preamble signal sequences. The random waveform to the right is data. The lower left window, labeled “I/Q Vector for PLCP preamble (BPSK),” displays the results of the preamble after BPSK encoding. The lower right window, labeled “I/Q Vector for Data (64-QAM),” displays the constellation diagram of the data after 64-QAM encoding. After processing this frame, the system then captures the next frame in real-time. Figure 8. Test results. Conclusion: This process of developing systems for a specific application shows how quickly low-cost and practical test equipment can be designed. It can also easily be manufactured in large quantities by selecting a DAQ card with the right specifications together with a fully-functional computer and adding a small team of engineers to develop the software interface. By merely improving PHY wireless digital signal processing capabilities, operators can then use this system to verify Tx PHY system design performance or Rx signal processing quality. Adding a vector signal generator (VSG) will enable the ability to estimate Tx-Rx hardware design performance that can be applied to the production line for product baseband performance verification. And by adding up and down converter circuitry to the system, it will function like a regular WLAN product tester. The immense business opportunities for the WLAN companies mentioned above, including chip designers and system manufacturers, will weigh heavily on their investment in R&D design verification and production test equipment. When next generation products are developed (such as multiple input, multiple output (MIMO) for WLAN or ultra wide band (UWB)), will companies abandon costly verification and test equipment and invest, yet again, in next generation equipment? The WLAN product testing system outlined in this article not only reduces development time but also provides a low-cost, scalable, and easily producible solution for engineers and assemblers that is upgradeable for next generation products. In fact, the same concept can be applied to TFT-TV, set-top boxes, and the telecom industry. Selecting the right data acquisition card is key to building a cost-effective testing system.
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