Signal switching via plug-in cards.

Automated testing provides a programmable means of powering a DUT as well as connecting selected points to signal sources and measuring instruments. Often, more signals must be measured than there are instruments so a network of switches is inserted to correctly organize and sequence through the desired test configurations.

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The types of switches used and their interconnections are critical because they directly affect signal integrity. Not surprisingly, relatively large armature relays handle power switching at low frequencies, smaller reed relays or FET and solid-state switches are used for general-purpose applications up to a few hundred megahertz, and conventional, large coaxial relays are found at RF and microwave frequencies.

Designs based on FETs and solid-state switches continue to improve performance and compete with reed relays in more areas each year. Nevertheless, relays remain by far the most common form of switch used in automated test.

Selecting the best switch form factor for a test system may not be straightforward. On the one hand, if most of the power and signal sources and measuring instruments are VXI or PXI compatible, using one or more switch modules with a matching form factor probably makes sense. You already need a chassis so it can be very economical to use a slightly larger one that also will accommodate the switches.

ATE companies such as Teradyne and EADS North America Defense Test and Services Division often incorporate VXI or PXI instruments in larger test systems. These form factors are popular among integrators of both commercial and military test systems because a wide range of functions is available and interoperability is guaranteed among conforming devices.

On the other hand, depending on the switching network required, you may find that overall performance is lower or the cost is higher for a group of interconnected modules than for a custom solution. Several vendors provide proprietary switching systems that can be configured exactly to fit your application.

For example, Cytec Sales Manager Nick Turner said, "Although the company will continue to support ISA, VME, VXI, PCI, compact PCI, and PXI switch cards as it has in the past, Cytec's emphasis in the future will be on building 19" rack-mount systems. These generally are on a larger scale than normally is cost-effective using many of the modular switch card formats."

Agilent Technologies, Precision Filters, KineticSystems, Pickering Interfaces, VXI Technology, and Universal Switching provide several proprietary ranges of large switching systems. It can be especially difficult to accommodate a complex coaxial switching system in one of the smaller form factors simply because of the size of the relays, cables, and connectors. Also, there are signal integrity as well as size and cost advantages associated with large switching matrices constructed in a single chassis.

The growing popularity of Ethernet-based LXI instruments has allowed manufacturers to separate form from function to a large degree. LXI does specify certain indicators and connectors for a device but not chassis size. Because they are LXI compatible, proprietary switching systems made by Agilent Technologies and Keithley Instruments might be good solutions for your application.

These companies offer switching as part of a larger system capability. For example, Agilent's Model 34980A Multifunction Switch/Measure Unit has a built-in 6 1/2-digit DMM and eight uncommitted slots that accept any of 20 different plug-in modules. Similarly, Keithley's Series 3700 System Switch/Multimeter integrates a 7 1/2-digit DMM into a six-slot, 2U-high, rack-mount chassis. These systems are claimed to be typically lower cost than a PXI system with similar capabilities, and the DMM is included.

Pickering Interfaces has approached the switching problem from a different direction. Because the company already had developed an extensive range of PXI-compatible switching products, it created a series of LXI modular switching chassis into which you can plug a Pickering PXI 3U switching card. The chassis performs the LXI-to-PXI translation and supports Ethernet communications.

VXI and PXI Switching Modules

The comparison chart accompanying this article focuses only on VXI and PXI standards. VXI is now 20 years old, and a very large number of modules are available in this format. PXI is 10 years old, but more than 1,000 different modules have been introduced by various vendors.

Tom Sarfi, business unit manager at VXI Technology, explained that VXI modules can be more cost-effective than PXI, especially where large currents, high voltages, or multiple RF circuits are involved. He said, "We can accommodate 1,000-V multiplexers, up to 30-A switching, and 26.5-GHz relays in a reduced footprint at an overall lower cost. The larger module area and greater center-to-center spacing permit one module to support up to six SP6T 26.5-GHz relays."

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Typical VXI modules are 6U high and offer a much larger PCB area than the usual PXI 3U modules. Nevertheless, because of changes in technology, many functions that previously required a complete VXI C module now are available in the PXI format. In addition, PXI also offers a 6U size, and this format is especially useful for large switching solutions such as matrices that benefit from having more elements on one PCB.

Competition between the two standards is influenced by the test systems already used within a company. For example, much of the early adoption of VXI was among military contractors, and new modules still are being developed for these end users.

Charles Greenberg, senior product marketing manager at EADS, said, "In 2007, we demonstrated a new suite of [VXI] switching cards and software that can run legacy ATLAS Test Program Sets (TPS) from four widely used military test systems: the U.S. Army IFTE V6, the U.S. Navy CASS, the U.S. Air Force ESTS, and the U.S. Marine Corps TETS/Viper/T. All four were run from a single test system, the ARGCS ATS-1 demonstrator."

He explained that each program required slightly different switching configurations so each card had to be able to morph itself into the correct configuration to meet the specifications for any given program. This was accomplished by using small switch modules that could be reconfigured as required. For example, CASS needed control of a coaxial multiplexer down to a 2:1 level. The IFTE and ESTS TPS worked with only 4:1 granularity and TETS only 8:1 for most of the signals.

The software used a common IVI switch interface but separate translation wrappers for each TPS. The IVI interface then could drive the reconfigurable switch card to create the required set of multiplexers. Although a switching card only switches, the concept of satisfying several instrumentation requirements with one piece of hardware is in the same spirit as the broader synthetic instrument initiative.

EADS provides many types of VXI and PXI switch cards as well as proprietary cards that are used with carriers such as the Model 1260-100X Adapt-a-Switch[R] Carrier. It accommodates up to six switch cards but occupies only two VXI slots.

The X-Series Carrier is split both vertically and horizontally to house up to four plug-ins, some containing two switch cards. VXI Technology has a similar SMP1200 Carrier. In contrast, ASCOR's Series 4000 Carriers occupy from three to eight VXI slots and house from six to 17 switching cards. The cards are 10.75" high, extending nearly the full height of a VXI 6U slot, but mount on a 0.5" or 0.6" pitch compared to the standard 1.2" VXI slot pitch.

Carriers are available for both PXI and VXI systems and essentially are subchassis preconfigured to interconnect a number of smaller switch cards. In addition to allowing a level of mix-and-match customization among types of switches, they electrically and physically concentrate the connections from several cards so that fewer main chassis slots are required.

In an attempt to reduce the number of modules needed, Geotest-Marvin Test Systems has introduced the Model GX6377 Multi-Function Relay Card. Mike Dewey, the company's senior product marketing manager, commented, "This product supports several channels of 10-A switching, 2-A form A and form C switching, and two 16x2 configurable relay matrices, all in a single-slot 3U module. The result is a switch module that offers customers the means to configure a test system using only one instead of several switch cards. The test system can be more compact and potentially simpler as well."

Signal Integrity

Switching module manufacturers all are benefiting from the reduced size of modern components. EADS's Mr. Greenberg cited the availability of relays as small as 25% the size of previous models with the same voltage, current, and DC power switching capabilities. In addition, highly integrated FPGAs and relay drivers are allowing more channels than ever to be packed into a switching product.

Of course, merely mounting hundreds of relays on a PCB doesn't make a switching module. Mr. Greenberg referred to new EDA software that gives designers advanced modeling tools to help optimize bandwidth, crosstalk, and isolation. He added that the larger VXI PCB area can improve switching-channel density and may be more appropriate for higher frequency and higher power applications. In his view, PXI is better suited to lower power and smaller applications.