Keysight Technologies Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules Application Note
Introduction In applications as diverse as radar, radio astronomy and wireless communications, developers are using antenna arrays to enable beamforming. This technique provides benefits including enhanced spatial selectivity and greater resistance to interference. To meet today s most challenging scenarios, the latest designs use active electronically steered arrays (AESA). When characterizing these advanced arrays, it s important to minimize any phase errors caused by the test system: better cross-channel phase coherence means greater precision in measurements and, ultimately, in beam positioning. In today s competitive business environment, it s also essential to reduce overall test times. With high channel-count systems, there are two options: sequential measurements with a few test channels plus external switching, or simultaneous testing with many parallel measurement channels. For years, it has been possible to implement simultaneous testing using several large, benchtop vector network analyzers (VNAs). More recently, solutions based on modular AXIe and PXIe hardware are making it possible to implement time-saving parallel testing in less space without sacrificing measurement performance. To meet these challenges, Keysight has created a reference solution that provides a flexible starting point for system creation. The system hardware includes high-speed, multi-channel AXIe digitizers that can be arranged into a maximum of 104 parallel measurement channels. An alternative approach is based on one-slot, two-port PXI VNAs that can provide up to 30 parallel measurement channels. This app note provides a brief description of the measurement problem before outlining the two system configurations and comparing the speed and performance of those solutions.
03 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Summarizing the Problems Individual transmitter/receiver (T/R) modules can be integrated into linear- or planararray antennas. To characterize such a system, testing is focused on characterization of each T/R pair and the list of required measurements is long. Transmitter Gain and voltage standing wave ratio (VSWR) versus frequency Magnitude and phase versus frequency Gain compression versus frequency at maximum transmission power Pulse profile: phase and amplitude; pulse width; rise and fall times Receiver Gain and VSWR versus frequency Noise figure versus frequency Magnitude and phase versus frequency Spectrum: harmonics, spurious & intermodulation Other essential information includes timing delays, current (peak and average) and module status. As arrays grow and element counts increase, the first problem is the increase in total test time if the system relies on traditional techniques. Beamforming adds another challenge because it depends on precise shifting of the phase of each transmitter to create constructive or destructive interference and thereby steer the beams. Because this technique requires extremely high precision, there are two important implications for system testing: Calibration and measurements require phase-synchronous sampling across all input channels Measurements require high-resolution sampling and fast frequency switching In these scenarios, the use of sequential or limited-simultaneous techniques makes it difficult or impossible to create cross-channel measurements or comparisons.
04 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Outlining Two Possible Solutions Keysight reference solutions encapsulate the required combination of hardware, software and measurement expertise through programming examples to provide a proven starting point for specific applications. In this case, the starting point is the multi-channel antenna calibration (MAC) reference solution, which is based on the M9703A AXIe 12-bith high-speed digitizer/wideband digital receiver. The MAC reference solution includes the essential components of a narrowband antenna-calibration test system. You can use, modify and enhance the system, as needed, to meet specific test requirements. System software includes example test code written in C# that is provided in the form of a.net class library. The code is designed specifically to collect data from an antenna under test (AUT) and compute the cross-channel magnitude and phase data versus frequency. Built-in capabilities in include the following: Set up digital downconverters and receiver channels Perform magnitude and phase measurements Add cross-channel correlation factors Export measurements for post-processing with 89600 VSA software, MATLAB or other applications You can build on the example code using Microsoft Visual Studio or NI LabVIEW to customize data collection and processing. The MAC reference solution is one of three possible approaches: the other two are based on benchtop vector network analyzers (VNAs) or modular PXI VNA instruments. The benchtop VNA-based solution can provide a maximum of five measurement channels, requiring time-consuming sequential measurements for large phased arrays. For that reason alone, the remainder of this application note focuses on the other two approaches, which enable parallel measurements. Either parallel architecture provides scalable channel count: the digitizer- and VNAbased configurations support simultaneous testing on up to 104 or 32 test ports, respectively. The reference solution also encompasses alternatives for downconversion, local oscillator (LO) and RF/microwave test stimulus.
05 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Examining the AXIe digitizer-based solution This version of the reference solution can be configured with a maximum of 16, 40 or 104 channels, depending on which AXIe chassis is selected: 2-, 5- or 14-slot models are available from Keysight. Figure 1 shows a block diagram of this configuration: although this is a 16-channel system, only eight measured signals are shown. 16 receiver channels RF source Phase center from antenna array Rx ref Splitter Frequency down convertor LO source Figure 1. This simple implementation includes a two-slot AXIe chassis loaded with two eight-channel digitizers. At the lower left, the receiver reference signal and seven outputs from the AUT are connected to the inputs of two quad downconverters (M9362A). Each IF signal is connected to one input of an eight-channel digitizer (M9703A) as the starting point for the various measurements. At the upper right, the RF source a Keysight UXG agile signal generator provides a known test stimulus that is fed into the antenna horn. At the lower right, the other source a Keysight MXG analog signal generator provides a common LO that is connected to each quad downconverter, ensuring phase coherency across all measured channels. Heuristically, making eight or 16 measurements in parallel should provide a speed advantage compared to a sequential series of pairwise measurements. With this known hardware configuration and the example test code, we can determine the actual measurement speeds.
06 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Figure 2 provides a schematic illustration of steps and states across an array of measurement channels, test frequencies and AUT states. In this case, the UXG can switch frequencies in, which is much faster than the system can switch antenna states (e.g., phase and polarization). State 1 State 2 Ch 17 Chs 815 Chs 16z f1 M S1 Sn f2 f3 f4 fn Digitizer re -arm = DUT state switching time >? f1 f2 f3 f4 fn Measurement interval UXG frequency switching time Figure 2. This approach to AUT measurements uses nested loops that cycle through all test frequencies for an operating state before moving to the next state. This is the typical stepped sine testing technique that walks through a series of continuous-wave (CW) signals and captures a block of data at each frequency. Here s how it works: 1. Set the phased array to its first operating state (phase/polarization). 2. Using its list mode, step the UXG through frequencies f1 through fn. Frequency-switching time is with the UXG operating in list mode. 3. At each frequency, acquire a block of data on all 16 channels simultaneously. This duration is long enough to capture 16 cycles of the CW signal at its lowest value (f1) and with a 640 ns sampling interval (i.e., a sampling rate of 1.56 MSa/s). This integration time ensures comparable sensitivity at all frequencies. In post-processing it will be necessary to apply a window function to reduce the effects of discontinuities at the beginning and end of the data block. Discontinuities will occur if the UXG s CW output and the digitizer s acquisition process do not start in perfect alignment. 4. After measuring all frequencies, set the phased array to its next state and repeat steps 2 and 3. This example assumes for AUT state switching. 5. Repeat steps 2 through 4 until all states have been tested. The example measurement times assume the M9703A AXIe digitizers are operating at a sample rate of 1.56 MSa/s (I & Q) and are using a 1 MHz bandwidth; it also assumes use of the digital downconverter (DDC) capability.
07 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Examining the PXI VNA-based solution In this configuration, the solution is configured with PXI VNA instruments that provide a complete two-port vector network analyzer in a one-slot module. Currently, the maximum supported configuration is 16 modules mounted in a single 18-slot PXI chassis (Figure 3). 1 Figure 3. Loading an 18-slot chassis with a maximum of 16 PXI VNA instruments provides a powerful solution for simultaneous testing of phased-array T/R modules. The maximum configuration provides a total of 32 test ports that can be used to perform 30 parallel measurements of the phased-array system (Figure 4). Port 1 is the stimulus, port 2 is the reference, and ports 3 through 32 are the measurement channels, all measured relative to port 2. Each module has LO input and output ports and these are daisy chained to ensure ultra-stable phase alignment. New PXI-VNA 32-port Module #17 Module #2 Module #1 Ref Stimulus Tests Measurement setup (ratio): Port 3/Port 2 Port 4/Port 2 Port 31/Port 2 Port 32/Port 2 30 Test channels Parallel simultaneous (30 measurement channels) Figure 4. The stimulus from port 1 is connected to the antenna horn (blue line) and up to 30 measurements are made simultaneously (red bracket), all referenced to port 2. 1. If more channels are needed, then external switching will be required. In this configuration, each bank of 30 measurements will be simultaneous; cross-comparisons between measurement sets will not be valid.
08 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Similar to the digitizer-based example, this configuration walks through a series of test frequencies and AUT states. In this case, the fastest test time is achieved by setting a single frequency and incrementing through the entire set of AUT states before advancing to the next frequency. Figure 5 shows a conceptual diagram of this approach: the VNA per-point acquisition time () and frequency-switching time (100 µs) are actual values; the AUT state-switching time of is an estimate. f1 f2 Channels 1 through 32 ST1 ST2 ST3 ST4 STn 100 µs ST1 ST2 ST3 ST4 STn AUT state-switching time () PXI VNA frequencyswitching time Figure 5. This alternate approach provides simultaneous acquisition on all channels for each AUT state as it steps through a series of test frequencies. Calculating and comparing total test times To provide a meaningful comparison, let s look at the estimated test times using 32 channels with the digitizer- and VNA-based solutions. With each one, we ll assume the following: 1,000 AUT states with state-switching time 30 discrete test frequencies 1 MHz IF bandwidth Using the process illustrated in Figure 2, calculating the total test time for the digitizer-based system uses the following equation: {[(t dwell + t freq-switch ) * (# of test frequencies)] + (greater of t re-arm or t state-switch )} * (# of states) Substituting the actual values yields the following: {[( + 0.12 µs) * 30] + } * 1000 = 0.306 s total test time Referencing the process illustrated in Figure 5, calculating the total test time for the VNA-based system uses the following equation: {[(t acquisition + t state-switch ) * (# of states)] + [t freq-switch * (# frequencies 1)]} * (# of test frequencies) Substituting the actual values yields the following: {[( + ) * (1000)] + [100 µs * (30 1)] * 30 = 0.237 s total test time Thus, with 30 or fewer measurement channels, the VNA-based approach is the faster alternative. When more channels are needed, the digitizer-based solution will be faster because total test time will remain 0.306 s. The ability to measure more channels with the VNA-based system requires the use of an automated external switching solution that multiplexes the source, reference and measurement channels to the additional AUT ports. This will increase the total test time by an amount equal to (0.237 s test time plus switching time) multiplied by (one minus the number of additional test banks).
09 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Comparing solution performance & capabilities In addition to test time, measurement performance and solution capabilities are also worth considering. Table 1 compares three key specifications for the two parallel methods. Table 1. There are noteworthy performance differences between the eight-channel digitizer and two-port PXI VNA. Description AXIe digitizer PXI VNA Receiver noise floor at IF bandwidth 10 Hz 115 dbm 108 dbm 1 khz 99.3 dbm 88 dbm 1 MHz 80.5 dbm 55 dbm Cross-channel phase variance < 1 deg < 1.63 deg Data buffer 300 Mpts 1.28 Mpts Focusing on solution capabilities, the following items are worth considering: As implemented here, both solutions rely on the reference solution software for calibration and calculation of cross-channel magnitude and phase values. At 1 MHz IF bandwidth, the digitizer-based solution provides greater sensitivity because it has a lower noise floor ( 80.5 dbm versus 55 dbm). The PXI VNA includes a source so does not require an external source such as the UXG. Output power varies by frequency but the maximum is +10 dbm for the PXI VNA (typical) and the UXG (specified). The digitizer-based solution is configured to be used primarily as an array-calibration system. The VNA-based system can also be used to perform full vector signal analysis on a wide range of system components.
10 Keysight Accelerating the Testing of Phased-Array Antennas and Transmit/Receive Modules - Application Note Conclusion Because a typical antenna array contains more than eight T/R modules, parallel testing on multiple test channels is the fastest and most efficient approach. The Keysight MAC reference solution provides a flexible starting point based on eight-channel AXIe digitizers. The alternative approach is to assemble a system based on Keysight s one-slot, two-port PXI VNA instruments. For up to 30 channels, the VNA-based solution is the faster, smaller and less-complex alternative. For arrays that need up to 104 test channels, the digitizer-based MAC reference solution is the fastest choice. It also provides superior measurement sensitivity at a 1 MHz IF bandwidth (a common requirement). Related Information Brochure: Multi-Channel Antenna Calibration, Reference Solution, literature number 5991-4537EN Brochure: Solutions for Transmit/Receive Module Test, literature number 5991-3765EN Brochure: RF PA/FEM Characterization & Test, Reference Solution, literature number 5992-0071EN Data sheet: M9703A AXIe High-Speed Digitizer/Wideband Digital Receiver, literature number 5990-8507EN Brochure: PXI Vector Network Analyzer Series, literature number 5992-0098EN Brochure: UXG Agile Signal Generator N5193A, literature number 5992-0091EN Technical Overview: X-Series Signal Generators, literature number 5990-9957EN Symposium Paper: Simplifying Your TR Module-Test Solution and Improving Throughput Symposium Paper: Reducing Phase Errors and Test Times for Phased Array/Horn Antennas
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