Switches Get No Respect

Because virtually every test application has some aspect that is unique, there is no such thing as a standard test setup. Instead, the test system integrator must determine the combination of instruments, test fixtures, and switching that best addresses the requirements.

From a top-down viewpoint, it’s important to choose the most appropriate switching system architecture for the job. For example, DC and relatively low-frequency signals often are handled by groups of either solid-state switches or conventional electromechanical relays configured as multiplexers or matrices. Some higher power applications are better served by arrays of individually controllable relays.

Taking a bottom-up approach, the switch’s electrical characteristics must closely match the signal. A prime example is the use of coaxial microwave relays for multigigahertz switching. Similarly, requirements such as operating rate and lifetime number of switching cycles must be satisfied. Solid-state switches excel at meeting both these specifications although small reed relays also may be appropriate and have the advantage of a very low on-resistance.

Switches are used in all kinds of automated test applications. A few comments from industry experts give some idea of just how ubiquitous switching is. Each statement reflects needs of the customer bases served by these companies:
•?”One area of growth is in hybrid switch applications that may require a combination of matrix, multiplexer, and high-current switching capabilities.” Mike Dewey, senior product marketing manager, Geotest-Marvin Test Systems
•?”Our VXIbus platform of switching products continues to enjoy success in large-scale ATE where the center spacing and depth of card assemblies allow us to achieve high channel count densities and a wide breadth of application coverage.” Tom Sarfi, business development manager, VTI Instruments
•?”We are currently seeing the greatest growth in wireless test, communications, and lab automation.” Nick Turner, president, Cytec
•?”The greatest potential for growth in the switching system market probably lies in providing solutions for medium- to high-density DUT applications such as latch-up, cable, and socket testing.” Jerry Janesch, business development manager, Multi-Application Products, Keithley Instruments

Available Switching Products

To address a large number of applications, many switches have a modular PXI or VXI form factor so they can be mixed and matched. For some users, this is the best solution. For others, a more tightly integrated switching system within a custom chassis may have benefits. Vendors of this approach often claim to provide lower cost and better signal integrity in large ATE systems.

There is a third, semicustom solution halfway between the other two. Because switching is needed in most test systems, it makes sense to integrate it in the same PXI or VXI chassis along with the required test instrumentation. Several proprietary switching systems are available that provide greater switch density than a straightforward VXI module designed for 1.2″ spacing.

ASCOR’s Series 4000 uses slim, nearly full-height switch boards with 0.5″ spacing. They are housed in a separate multiposition carrier that plugs into the actual VXI chassis. EADS North America Test and Services Adapt-a-Switch Modules have a similar approach, but these 1260 Series boards are about half as high as a usual VXI module, and up to six can fit in a carrier that occupies two C-size slots.

If you choose a semi- or full-custom switching system, although built to your detailed specification, it likely will be based on a switching system platform. Such a platform can be configured in an almost unlimited number of ways by populating it with the appropriate switches or modules. The major characteristics of several switching system platforms are presented in Figure 1.

Figure 1. Multidimensional Switching Platforms

This multidimensional view shows potential capabilities. Except for bandpass frequency, the categories are binary: the platform either supports the function or does not. Eight of the systems switch both RF and microwave frequencies. The ninth, the large Precision Filters custom matrix, has a 1-MHz bandpass but handles hundreds of signals.

Slight offsets in the position of one platform relative to others are intended to improve legibility, not to indicate relative merit. Including one type of platform doesn’t imply that a company only provides one. For example, RF/microwave switch platforms are shown for Agilent Technologies and Keithley, but both companies also make DC/low- frequency DMM/switching systems. Similarly, EADS and Universal Switching offer a number of switching systems and modules for signal speeds from DC through microwave.

PXI has not been listed as a category because most PXI switching systems consist of separate modules plugged into a mainframe. The platforms represented in Figure 1 are physically bigger to accommodate a greater number of low-frequency signal channels or large microwave switches.

A wide range of typical switching modules is shown in the comparison chart that accompanies this article. Here you can find detailed specifications that reflect company competencies such as high switch density or control via the PXI bus.

To view comparison chart click here.

Why Switching Is Changing

RF and Microwave Applications
According to Travis White, product manager for precision DC and switches at National Instruments (NI), “RF and microwave devices have continued to become more prevalent and extend to higher frequencies. As a result, engineers are incorporating more and higher frequency RF switching into test systems. Furthermore, switches are particularly important for reducing cost in RF test systems because typical RF instruments can be very expensive.”

Bob Stasonis, Pickering Interfaces sales and marketing manager, added, “In the communications world, an emphasis on bandwidth greater than 3 GHz and with better specifications than in the past is important. Factors driving the switch business include testing of multiple harmonics, switching multiple instruments to multiple UUTs with very low losses, and growing WiMAX usage.”

Some of the increased demand for RF/microwave switching has been defense related. Sheri DeTomasi, product manager at Agilent, explained, “Prior to the economic downturn, the wireless and military communications industries were growing. Applications included design verification and functional test of components that go into satellites, military radios, handsets, base stations, and other communications devices.”

According to Cytec’s Mr. Turner, the growth in communications systems is directly related to homeland security, the war effort, and updates to telemetry systems. “The amount of information being gathered is staggering, and a good communications switch can ensure that the data is collected and routed to the people that need it right away. The enormous growth in cell phone use, Wi-Fi, and radios operating at higher frequencies implies a need for more testing. Switching systems can drastically increase throughput without the need for additional staff,” he explained.

Regardless of the exact RF application, almost every test situation is unique. Jeff Lum, Giga-tronics CTO and ASCOR president, explained, “The interface between the signal sources, measuring instruments, and the DUT requires a switching system. It may only provide signal switching, or it may include some signal conditioning as well.

“Optimum performance requires careful design of the switching system to minimize path loss and other detrimental factors such as crosstalk and noise. A trade-off occurs between the ideal switch design for a specific DUT and the desire to have a universal switching system that can interface to multiple DUTs.” He concluded, “As a result, a new switching system often is needed for a new product, and this drives the growth in switching and signal conditioning solutions.”

Better Components
Regardless of frequency range, new and improved components have given switching system manufacturers greater opportunities for innovative designs. Agilent’s Ms. DeTomasi said that new PIN switches now handle up to 180-GHz signals and operate in less than 1 µs. PIN switches can have excellent port-to-port isolation, fast switching speed, and low insertion loss, but they must be used at frequencies greater than a few megahertz.

Keithley’s Mr. Janesch commented on the continuing trend to reduce component size. “With components such as relays now significantly smaller, manufacturers can pack more components on a single PCB, creating higher density switching solutions. For example, Keithley’s Model 3706 System Switch/Multimeter mainframe can support 576 two-wire multiplexer channels in a 2U-high enclosure.”

Cytec’s Mr. Turner said, “Advances in solid-state switching components have most likely had the greatest effect. There still are a lot of applications where electromechanical relays remain the best solution, but where solid-state switches can be used, they have allowed large application-specific configurations at a lower price and with longer life expectancy than relays.”

Similarly, Donald Chandler, president of Precision Filters, noted that the recent availability of reliable, highly integrated CMOS devices makes it much easier to design and manufacture larger switch systems.

Addressing both high- and low-frequency applications, NI’s Mr. White discussed new components as well as new sources of them. “While FETs and solid-state switches have been around for a while, their ON-resistance typically was high. In contrast, recent semiconductor advances have made possible devices with only a couple of ohms resistance. NI has used these parts to offer up to 544 crosspoints in a single PXI module.

“In addition to these advances,” he continued, “more high-frequency relays are available from an increasing number of vendors. As a result, NI now can provide a two-slot PXI module having a pair of 6×1 multiplexers with 26.5-GHz bandwidth and only 0.3-dB insertion loss.”

Switching System Selection

All manufacturers agreed that the most common mistake users make is not asking for advice when selecting switching products. VTI Instruments’ Mr. Sarfi said that switching often is an afterthought. System developers spend a lot of time choosing expensive stimulus/measurement equipment to ensure the highest accuracy. However, the switching in the test system can only degrade instrument performance, not enhance it. The goal of the system designer should be to make the switching as transparent as possible to the rest of the system.

Of course, the test system design involves more than selecting switching modules with good specifications. For higher frequency signals, the correct impedance must be maintained throughout the entire signal path. It can be particularly difficult to ensure signal fidelity when additional relays and cabling have been added to create larger switching networks.

Mr. Sarfi explained that sometimes people simply don’t start with the best overall architecture in mind. Cross- point matrices appeal to designers because of their flexibility, but without care, stubs can be introduced that degrade high-frequency performance. In these cases, a tree topology can provide better electrical performance even if it isn’t the most space-efficient configuration.

Pickering Interface’s Mr. Stasonis added that the increasing popularity of solid-state switching has added new parameters to be dealt with. Designers must understand the implications of voltage, current, hot or cold switching, power, and bandwidth, and now they have another type of switch to consider.

Agilent’s Ms. DeTomasi elaborated on these points. As the switch complexity increases, noise, crosstalk, and bandwidth become more important. When developing large switch matrices, for example, many people do not take into consideration how the bandwidth and crosstalk specifications are impacted as the matrix gets larger, which can affect measurement accuracy and repeatability. Using preconfigured switch matrices avoids these problems.

For RF and microwave switching systems, the VSWR, isolation, switch repeatability, and bandwidth are key specifications to ensure accurate and reliable measurements. Also, the impact of signal cable routing, connectors, and signal conditioning components on signal integrity must be considered.

Relay lifetime is greatly affected by the amount of power being switched. Agilent recommends removing or reducing the signal power prior to switch actuation to extend the life beyond the quoted minimum switch life. Input protection resistors also can protect switches against surge currents.

To achieve the highest performance, often a tightly integrated solution is better than a more distributed one. A modular switching system that also offers stimulus, measurement, and control modules simplifies functional test applications.

Cytec’s Mr. Turner summed up the manufacturers’ suggestions. “Communication is the key. Write down your specifications such as voltage, current, frequency, connector preference, and how the switches are to be controlled. Will the system be expanded in the future? Will the signals be hot-switched? Sketch the system configuration as you understand it, and share this information with vendors when discussing your switching requirements. Often, they can make suggestions that will help avoid problems and offer tips to keep cost and size at a minimum while maximizing performance.”

Summary

There really are a lot of things to consider when designing the switching part of a test system. The important electrical characteristics may get attention, but what about seemingly simple things like the connectors? They’re simple until you need to replace a cable and find that special connectors are both expensive and hard to obtain. This situation prompted Geotest’s Mr. Dewey to stress the importance of standardizing on the sub-D type connectors used on his company’s switching modules.

Plenty of help is available, such as a series of NI white papers that explains key specifications, connectivity options, and application examples. There’s also an online attenuation calculator for NI switches. And, because the reason you are using switches is to route critical signals, NI also has a number of low-level measurement resources including Webcasts and interactive tutorials.

It’s important to learn about switching systems, but obtaining a truly deep level of understanding may not be practical within a project’s time scale. As Cytec’s Mr. Turner explained, “Switching systems tend to be a niche business that covers an enormous array of applications. We are building systems to switch femtoamps one day, 40-GHz microwave signals the next day, and 5-kV the day after that. Every application is prone to completely different opportunities for mistakes. The easiest way to avoid these mistakes is to allow us to use our 25 years of experience to address your questions.”

May 2009

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