Metaphorically speaking, a switching system is the glue that connects a UUT to a system’s test equipment. But all too often, it gets overlooked in the overall system design process. In reality, switching hardware may involve more potential solutions and compromises than any other part of the test setup.
The switching system often is the part of the test system that challenges engineers to do what they have to do well: Evaluate the options and come up with the most effective strategy that permits making meaningful measurements to show product compliance without spending any more capital or labor in developing it than required to accomplish the task.
Switches are a classic example of the need to make engineering compromises, including density, voltage and current ratings, path loss, bandwidth, cost, and configuration choices. Every aspect of the decisions made regarding the switching system has an influence on the test system. Even seemingly minor decisions like choosing interface connectors or picking a modular switching system vs. an integrated switching system affect the procurement cost and the amount of labor needed to get the system working.
One of the first choices the engineer typically makes is the platform to control the switching. Although this is not the first choice by necessity, for some it helps simplify product choices.
Pickering Interfaces supports the two most common platforms for switching systems: PCI eXtensions for Instrumentation (PXI) and LAN eXtensions for Instrumentation (LXI). Through our work, we’ve become familiar with the most common reasons for selecting one of these standards over the other. While there is no single answer to every switching need, there are questions you can ask once you understand the options.
Comparing the Platforms
The PXI modular platform has been around for more than a decade. The first products compatible with the increasingly popular LXI platform were introduced almost four years ago. Customers frequently ask us which platform is best. There is only one answer: It depends.
The reality is that the two platforms are simply different. The promoters of the two standards used different assumptions and starting points in creating their standards, but both have their place in the test and measurement industry, now and in the future.
The PXI Standard describes a mechanically modular platform that uses the PCI bus, both standard and PCI-e, on a defined backplane to control the modules. The modules have a fixed mechanical form factor with 3U being by far the most common.
The PXI chassis provides power from a common power supply with a baseline standard of performance. It has shared cooling and a backplane trigger bus system to exchange triggers between modules, a feature switching applications rarely need to exploit because of the inherently slow operation of relays.
A PXI-based system generally is Windows-centric. Use of other operating systems is possible, but the PXI specification only defines WIN32 drivers. Depending on the vendor, this may or may not be possible.
Most switching modules usually are simple register-based designs with little built-in intelligence. The control functions are dictated from the single central controller where all the software resides. This centralized architecture requires a fast backplane interconnection system to ensure that communications bandwidth does not slow down the central controller unduly.
LXI products are designed to be linked via Ethernet connections. There are no constraints on the physical size of these devices. Although most are designed to be mounted in racks, some may use a smaller format to suit the market they’re designed to serve.
Each LXI device has its own embedded controller, power supply, and cooling system. The LXI standard defines the way an LXI device behaves on the network so all LXI devices share a consistent look and feel. The Web server built into each device must provide at least a minimum set of configuration management support capabilities. The standard also requires LXI devices to be good neighbors in the presence of LXI devices and non-LXI devices from other vendors.
LXI systems can be small or very large. Their Ethernet connectivity permits the components of LXI-based systems to be widely distributed even over intercontinental distances. The IEEE 1588 timing protocol provides a common framework for systems in which timing synchronization is important.
Triggers can be LAN-based, or the system can use the wired trigger bus. Triggers can be controller-centric or operate peer-to-peer. Communications between LXI devices take place via a fast interconnect (Ethernet), but in most applications, the distributed nature of the data processing means that a fast interconnect is not essential to ensure fast system operation.
PXI vs. LXI
It is evident that the two standards use a different control model so they have quite different strengths and weaknesses. PXI focuses on a central processing model based on a PC, using products that are modular and reliant on a high-speed data bus for communicating with the central processor that provides the computing power needed to perform system functions. LXI uses a more distributed control and data-processing system that communicates via a reasonably fast interface—a model that has some similarities to GPIB.
For switching, speed is rarely an issue. At their best, mechanical relays take hundreds of microseconds to operate; at their worst, they can take many tens of milliseconds. When designing new modules for specific applications, the platform architecture chosen can impact switch system design.
For example, the physical constraints can make a difference. Modular structures are great for supporting relatively small switching systems or systems that require a broad mix of switch types. But eventually, for high-density requirements, even the cleverest design engineer will run out of space. At the other extreme, if a small and diverse switching system is needed, the controller and supporting hardware in LXI could be a heavy burden on cost.
To better understand the similarities and differences in switching applications for PXI and LXI platforms, these recent case histories illustrate how some of our customers addressed the selection process.
One recent customer’s test system required a 4×4 RF switch matrix with excellent performance at frequencies higher than 6 GHz. In these circumstances, the solution generally is to use mechanical microwave switches, typically SP6T, to construct a matrix from eight switches and 16 coaxial cables. We had existing solutions available in both PXI and VXI platforms. The customer had been using our VXI solution but was concerned about cost and ongoing availability.
Microwave switches are not the easiest components to fit into a PXI chassis. They are bulky and take a lot of current from the 12-V supply. Our PXI solution took up 10 slots of a PXI chassis, more than half the available space in the largest 4U/5U chassis, and had to use high-efficiency switches to avoid overloading the limited +12-V supply. It is difficult to support the switches in PXI, which adds to the cost in the mechanical design. There also is the overhead cost associated with a very underused PXI chassis.
The customer selected an LXI device that we designed with an 8×4 matrix in 2U of rack height. The design, simpler to implement because of the increased space and power supply freedom, resulted in a substantial reduction in the cost of the matrix compared to a comparable PXI solution. In this example, the mechanical freedom of LXI allowed better use of the available height and depth of the case, reduced the rack space requirement, and permitted a matrix size (8×4) twice the size that the PXI solution would support.
A customer who needed to test video products using a baseband signal illustrates another case where LXI proved to be the most appropriate switching choice. This application required a matrix that allowed eight video baseband signals to be directed to up to 48 products as part of a quality test. The eight video streams were switched between the different products while they were being life tested. Space was at a premium so a compact form factor was essential.
Figure 1. PXI-Based 48×8 Video Matrix Requiring a Number of External Cables and Occupying 5U of Rack Space
With PXI solutions, the normal approach would be to link smaller matrices together using coaxial cables and loop-through connections on the matrix. For example, we could have configured the matrix using four Pickering matrix modules with 32 interconnecting cables
However, there simply was not enough room to support a 48×8 matrix while maintaining the required crosstalk and bandwidth performance. In this application, PXI would have been both a cumbersome and expensive approach, and a 4U-high PXI chassis typically requires at least 5U of rack space to allow for airflow for cooling the chassis.
Fragmented approaches like this also are more complex to manage because the modules are individually programmed so they do not behave like a single matrix but rather like a collection of matrices stitched together. The user has to decide what connections need to be made to close a particular crosspoint in the matrix.
Figure 2. LXI-Based Video Matrix Occupying 1U of Rack Space
Instead of using this fragmented approach, we designed a video matrix that packed a 48×8 solution into 1U of rack space (Figure 2). The increased board area available for the switching components and PCB tracking allowed the elimination of virtually all the coaxial cables, replaced by PCB traces and simple board interconnect systems.
The module behaved exactly as the customer wanted it to—as a 48×8 matrix—without the need to control fragmented individual modules as would be the case in PXI. In this application, it was the LXI device’s capability to support larger PCBs that was crucial because it simplified control, reduced space requirements, and minimized costs.
Testing in an Airframe
Whether measured from nose to tail or from wing tip to wing tip, an airframe is a pretty big UUT. In this case, our customer needed to test cable runs installed in an airframe. In addition to performing insulation testing between cables, the customer had to check end-to-end continuity for high or open resistance paths.
The test was accomplished by installing a tester and switches at one end of the cable run and a switching system at the other end that could loop the connections back to the tester. Although PXI modules were available with sufficient voltage ratings, the customer wanted to run the switching system at the far end of the airframe from the same location at which the tester was installed.
Although that can be awkward with a PXI-based system, an Ethernet cable or a wireless interconnect makes such an arrangement straightforward. We offer a PXI-based chassis that supports our PXI modules in an LXI-compliant environment. It was more effective to use LXI because of its capability to control switching functions at a distance over a network cable.
The LXI chassis provided all the Web browser support that the LXI standard required down to the level of the PXI modules installed in the chassis. The customer was able to develop the system using PXI hardware and transfer it to LXI with almost no additional effort. In this case, the critical issue was control at a distance over a network cable.
LXI Isn’t Always the Best Solution
When comparing LXI to PXI for specific applications, all the advantages aren’t always on LXI’s side. The PXI platform comes into its own with systems that use relatively diverse and compact switching and compact instrumentation from multiple vendors.
But there also are many circumstances where LXI offers a better switching solution than PXI. LXI often may be the best solution for systems that need large switching architectures or control at a distance or that must include large components. In instrumentation applications as well, systems requiring the highest parametric performance may best be configured using LXI. In all likelihood, however, many test systems will continue to be hybrids of LXI, PXI, and other platforms.
About the Author
David Owen is the business development manager for Pickering Interfaces. Previously, he held key engineering, product management, and strategic marketing positions with Marconi Instruments, then IFR. Mr. Owen currently is chairman of the technical committee for the LXI Consortium and a participant in the technical committee for the PXISA. He also is the innovator of more than 10 patents in the field of RF signal generation and analysis and a graduate of Hatfield Polytechnic with a degree in electrical and electronic engineering. Pickering Interfaces, Stephenson Rd., Clacton-on-Sea, Essex, CO15 4NL U.K., +44 (0) 1255 687900, e-mail: David.Owen@pickeringtest.com