As technology evolves and new standards emerge, engineers face an ever-increasing array of challenges. These challenges only increase when testing at RF and microwave frequencies or when high performance measurements are required. While these challenges will vary depending on the technology, standard and even application in question, for device test engineers and managers at least one thing remains the same: the desire to test designs more confidently and start making measurements on those designs more quickly. Lofty goals indeed, but that’s where the idea of modular instrumentation, via standards like AXIe and PXI, comes into play. Modular instrumentation not only offers space and cost savings, flexibility and scalability, but the high performance and speed needed to ensure fast and accurate measurements.
Clearly the concept of modular instrumentation is nothing new, but advances in PC technology today are allowing modular instrumentation to deliver even higher performance and speed. These improvements are enabling a solutions-based approach to measurement instrumentation that promises to more effectively solve engineers’ real world problems. These AXI- or PXI-based “modular solutions” not only help engineers make their measurements more confidently, but speed time-to-first-measurement (TTFM) as well. Let’s take a closer look.
WARNING: TECHNICAL CHALLENGES AHEAD
A critical task for device test engineers is to assess whether a given test solution meets their needs. Unfortunately, this process can take a tremendous amount of time and may not always return the desired result. Consider, for example, that a power amplifier (PA) design company has a customer — a mobile device or mobile RF module manufacturer — with very specific performance and throughput requirements for specific measurements. The company needs to find a solution to start making these measurements quickly and confidently, yet it can’t afford to waste precious time evaluating test solutions due to tight design cycles and other customer commitments.
The PA company may even need to perform envelope tracking measurements for its customer. These measurements rely on the performance of a single instrument to generate the proper waveform (e.g., an arbitrary waveform generator) and on tight multi-instrument control to ensure the power envelope start and servo-loop control are time-synchronized and triggered correctly. Building and evaluating such a test solution could take months, and there’s no guarantee it would meet the company’s requirements once finished.
Figure 1 Illustrates the gain in productivity when using a modular solution approach.
For the PA design company, and others like it, modular instrumentation can be a highly advantageous solution to their challenges, offering the benefits of size, density, backplane speed and measurement speed. Traditionally, engineers have relied on a software defined radio (SDR) to leverage these modular benefits. With this approach, test vendors essentially provide the engineers with a blank slate to develop a test system. The downside, however, is that the engineers may not be aware of all of the adjustments that must be made to the system or the optimization required to get the best possible performance. Just as critical, given today’s increasingly shorter time-to-market cycles, modularity via an SDR is simply not enough.
Defining Modular Solutions
A more compelling option to this dilemma is a modular solution, sometimes referred to as a reference solution. The modular solution provides the same basic hardware functionality as the SDR with an added layer of measurement application software. It comprises multiple modular AXIe or PXI test instruments that come with software drivers, measurement algorithms and sample programs. The result is a test system that provides a much more tailored solution to a specific application. Unlike the SDR, the modular solution has already been optimized by the test vendor, leaving the engineer free to focus on getting the job done, rather than having to make system adjustments (see Figure 1).
The modular solution’s framework is built on standard application programming interfaces (API), common reusable libraries and a software infrastructure. The software provides the automation, managing the solution’s resources so that its various modular instruments can work together seamlessly. This combination allows instruments to be combined to achieve a smaller granularity of measurement blocks, while simultaneously optimizing system performance.
Ideally the modular solution’s framework will feature an open and flexible software solution that is not dependent on the programming environment. This allows engineers to automate the measurements in whatever programming language they prefer (e.g., .NET, C#, LabVIEW or MATLAB) and easily optimize the speed or performance of their test solution.
With a modular solution, the instrumentation is properly conditioned to make measurements, collect data in the correct format with the correct algorithm, then sent for analysis to be received and processed. However, system interdependencies or unwanted measurement conditions could cause the modular solution to generate incorrect or inaccurate results. The user might not even know a result is inaccurate. For example, a PA under test could be sent into a mode where gain compression is occurring, meaning the signal is becoming clipped and the amplitude measured at the output is lower than predicted or modeled in the design phase of the amplifier.
Figure 2 A modular solution's software can handle measurement complexity to ensure accurate results.
Fortunately, the modular solution’s software effectively deals with issues related to measurement complexity without the engineer ever needing to understand them. One way in which it may do this is through use of a real time fast Fourier transform (FFT) capability that is built into a modular instrument, such as a vector signal analyzer. Basically, once a signal is captured and prepared for analysis, the FFT capability in the signal analyzer measures the adjacent channel power (ACPR). Before generating the result, the filter shape is applied to the FFT bins to optimize the result without sacrificing any measurement performance (see Figure 2).
DELIVERING CRITICAL ADVANTAGES
The two main advantages to utilizing a modular solution are shorter TTFM and greater measurement confidence. Essentially, all the engineer has to do before a modular solution is ready to make measurements, is connect its mainframe and modules and power on the system. The entire process takes just a few hours – a far cry from the days and weeks it would typically take to set up a solution that had not been optimized with sample programs.
Modular solutions also provide test engineers with confidence that they no longer have to spend time optimizing system test solutions they are not familiar with. As modular products and modules become even more complex and difficult to optimize without the right expertise in RF test, especially when placed together in a system, this benefit will become all the more critical. Essentially, modular solutions will offer engineers the only way to get their first measurements quickly.
Figure 3 This RF PA characterization and test reference solution utilizes Keysight PXI modules with LXI and third-party products. It is configurable to test devices without code changes for first level evaluation.
Figure 4 Before (top) and after (bottom) images show how Keysight's MAC modular Reference Solution manages multiple GB worth of data inside its custom database of I&Q samples. The MultichannelArrayCalibration.exe process size (memory) went from just 10s of KB (top) to greater than 3 GB (bottom).
MODULAR SOLUTIONS IN ACTION
Modular solutions can be used in virtually any area where a modular platform would prove beneficial, such as in manufacturing or in market segments requiring multiple channels (e.g., 5G wireless multiple-input-multiple-output antenna system designs). Another prime example is the design validation and production testing of PA or radio modules (see Figure 3), where it is important to quickly develop metrics grade test stations to meet the fast delivery requirements of wireless device manufacturers.
One real-world example is an AXIe-based modular solution for multi-antenna calibration (MAC) that features four analog-to-digital converters with eight-channel AXIe digitizer modules, with precisely controlled on-board clock distribution networks. This configuration enables aggregation of a large number of channels, more easily scaling to 104 channel phase-coherent MAC configurations. Tuned modules can also be aggregated easily. Additionally, the MAC solution utilizes a PCIe backplane that supports up to 600 MB per second data offload rates. Support for .NET ensures there are tools and support for 64-bit architectures to optimize the code to compile into a 64-bit dynamic link library (DLL). Fast transfer and management of large multi-GB measurement records up to 16 GB are performed using a 64-bit .NET class library (see Figure 4). The modular solution also features software specially designed to make cross-channel amplitude and phase measurements that can be modified to meet application-specific requirements.
The key advantage to using a modular solution such as this is the ability to make measurements across many coherent channels concurrently. The modular solution can also provide substantial increases in calibration throughput if supported by the antenna architecture and/or test range (Rx/Tx). Another advantage comes from its use of fast frequency switching sources, which provide additional throughput improvements, assuming the antenna can quickly settle as the stimulus steps to the various tones used in the calibration.
Various options can be added to modular solutions such as this to support the different needs of device test engineers and managers. Suppose, for example, that a company performing multi-antenna calibration on large multi-channel arrays requires a solution that reduces the calibration time by an order of magnitude while still maintaining the excellent sensitivity (dynamic range) of a traditional network analyzer for narrow bandwidths. As investment protection, the same hardware should be programmatically adjustable to support the use of wider bandwidths for calibrations using modulated signals (by trading off some reduction in sensitivity). These needs can be met with just a programmable digital down-converter (DDC) option running inside a modular digitizer’s onboard FPGA. The MAC solution automates control of this DDC across multi-channel/multi-blade configurations to provide optimal phase-coherent sensitivity measurements, depending upon the application requirements.
THE BOTTOM LINE
While the use of an SDR offers engineers one way to access the benefits of a modular test system, modular solutions provide a much more compelling option now that PXI and AXIe can provide the performance necessary to meet the most challenging requirements in RF, microwave and high speed digital tests. Not only do they boast the benefits commonly touted by modular instrumentation, they are also tailored to specific real world applications. This enables a shorter TTFM with the capability to efficiently deal with issues related to measurement complexity without an engineer intervening. Since the test vendor optimizes the measurements, engineers can more confidently focus on just doing their job. These advantages, coupled with the growing need for smaller footprint test systems and optimized application-focused solutions, are sure to make modular solutions the ideal option for any engineer currently using modular instruments.
