True software instrument set to revolutionize RF test
Ideally a software instrument comprises code that defines the instrument, which runs on a common silicon fabric. Such an instrument is completely programmable and can reuse common code blocks that represent certain functions. Other code blocks can be added to integrate other software instruments. Software instruments are the code and vice versa. Completely customizable, software instruments can be integrated and configured as needed to address a specific test scenario. Code is simply loaded onto the silicon as required.
Two key tends are also pushing for faster test, those of increasing complexity and data rates. Complexity increases the number of test scenarios that need to be covered thereby increasing test times. Faster data rates are making it harder to find anomalies such as glitches. The need for faster and more intelligent test is required to address these needs.
Standards are also posing problems. Firstly, there are too many of them and new standards keep emerging. Engineers are faced with maintaining interoperability with earlier standards while trying to deal with the latest and emerging standards. Further, engineers are being pushed to develop devices and systems while the standards are still being laid down. This parallel development requires test vendors to be ahead of the standards curve, a position that is extremely difficult to maintain. Designers still have to wait for test vendors to come out with a box or module to address their requirements. Some are forced to get into the business of building custom test instruments, a time consuming process that detracts from the core project goals.
With a true software instrument, engineers could customize the instruments needed to a specific task and load the code onto a silicon fabric. Software reuse would reduce the programming burden significantly and still allow test to be faster, take less space and be specific to the requirements of the design flow.
To address today’s test bottlenecks in RF, National Instruments claim to offer the first true software instrument, the NI PXIe-5644R vector signal transceiver (VST). Not only does this instrument bring Moore’s law to the test world but it can be customized down to firmware. Engineers can ‘touch the pins’ through code.
The instrument features a software-centric architecture and represents a new era in which engineers and scientists can use LabVIEW to tailor open, field-programmable gate array (FPGA)-based hardware for their specific needs.
The vector signal transceiver is a new class of instrumentation that combines a vector signal generator (VSG) and vector signal analyzer (VSA) with FPGA-based real-time signal processing and control into a single PXI modular instrument. A user-programmable FPGA allows custom algorithms to be implemented directly into the hardware design of the instrument. This software-designed approach allows a VST to have the flexibility of a software-defined-radio (SDR) architecture with RF instrument class performance. Figure 1 illustrates the difference between traditional approaches to RF instrumentation and a software-designed approach with a VST.
Ideal for testing the latest wireless and cellular standards such as 802.11ac and LTE, the VST features up to 6.0 GHz frequency coverage and 80 MHz instantaneous RF bandwidth, more than 10 times faster measurements than comparable solutions, and can easily be expanded to support multiple input, multiple output (MIMO) configurations or parallel testing in a single PXI chassis. Further, engineers can transform the vector signal transceiver into a different instrument or enhance its existing functionality using LabVIEW system design software.
Testing 802.11ac
VSTs offer both the fast measurement speed and small form factor of a production test box combined with the flexibility and high-performance expectation of instrument-grade box instruments. The transmit, receive, baseband I/Q, and digital inputs and outputs all share a common user-programmable FPGA, which reduces complexity, boosts measurement speed and increases flexibility in how the instrument is used. This gives the VST the ability to test standards such as 256 QAM 802.11ac with an error vector magnitude (EVM) of better than -45 dB (0.5%) at 5.8 GHz. Compared to the current industry ‘gold standard’ this represents an improvement of 3 db in EVM, along with an improvement in measurement speed by 20 fold.
Testing power amplifiers
To accurately calibrate a PA, a power-level servo feedback loop is used to determine the final gain. Power-level servoing captures the current output power with an analyzer and controls the generator power level until desired power is achieved, which can be a time-consuming process. In simplest terms, it uses a proportional control loop to swing back and forth in power levels until the output power-level converges with the desired power. A VST is ideal for power-level servoing because the process can be implemented directly on the user-programmable FPGA, resulting in a much faster convergence on the desired output power value (Figure 2).
The software instrument approach implemented in the VST reduces the test time of typically 5 seconds per measurement in this case to around 5 ms. This represents an improvement of three orders of magnitude by just moving the instrument into an FPGA (Figure 3).
2×2 MIMO channel emulation
In this test setup, engineers can now program fading models used to simulate air interference, reflections, moving users, and other naturally occurring phenomenon that can hamper an RF signal in a physical radio environment into the FPGA to implements a real-time radio channel emulator.
Figure 4 shows a 2×2 MIMO radio channel emulator implemented using two VSTs in LabVIEW. Settings for the fading models are shown on the left and in the center of the screen. The resulting RF output signals from the fading models were acquired with spectrum analyzers and are displayed on the right. These spectral graphs clearly show the spectral nulls that have resulted from the fading models.
Conclusion
VST software is built on LabVIEW FPGA and the NI RIO architecture, and features a multitude of starting points including application IP, reference designs, examples, and LabVIEW sample projects. These starting points all feature default LabVIEW FPGA personalities and prebuilt FPGA bit-files to get started quickly — bringing unprecedented levels of customization to high-end instrumentation.
LabVIEW is well suited for FPGA programming because it clearly represents parallelism and data flow, so users who are both experienced and inexperienced in traditional FPGA design can productively apply the power of reconfigurable hardware.
The VST represents a class of instrument that is truly software designed, with capabilities limited only by the user’s application requirements—not the vendor’s definition of what an instrument should be. As RF DUTs become more complex and time-to-market requirements become more challenging, this level of instrument functionality shifts control back to the RF designer and test engineer.
The PXIe-5644R delivers typically over 10 times faster measurements than comparable solutions and can replace multiple traditional instruments at a fraction of the cost and size, while consuming under 60 W of power. By bringing true software instrumentation to the market National Instruments has set off a revolution in how engineers approach test in the RF world.
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