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.
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??-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??-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.
To view the comparison chart click here.
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.”
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® 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 16×2 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.
ASCOR's President Jeffrey Lum explained the design principles his company uses, “We place relays on a PCB as part of a transmission line. The signal paths are designed as 50-Ω lines with the return path matched to the signal line to achieve high bandwidth. The analog, digital, and chassis grounds are kept separate to preserve bandwidth and increase isolation from conducted ground noise. This treatment is continued up to the front panel with return-path connections included on the same connector as the signal connections.”
High-Density Connections
The switching module manufacturer only has control of that product's hardware design. Connectors are provided on the module, but it's up to you to cable between the switch and the other elements in the overall test system. Depending on the types of connectors and number of channels involved, this may not be entirely straightforward.
Geotest's Mr. Dewey commented, “When a switch card is selected, the buyer often has placed little emphasis on connecting it to other instruments or the UUT. Many PXI 3U switch cards will use a high-density SCSI style connector to provide lots of I/O in a small space. But trying to interface to this connector is a problem.
“Interfacing to these cards requires you to purchase special cables or breakout boards, which drives up the cost and complexity of the test system,” he cautioned. “You really need to think about how easy or difficult it will be to interface to a switch card. This is the reason why Geotest switch products only use sub-D style connectors. We want to make sure that users can easily and reliably fabricate interfacing cables for our switch products and don't need to buy special cables.”
National Instruments (NI) Switches Product Manager Jaideep Jhangiani also commented on a module's connectivity: “Occasionally, the connectors and cables limit switch module functionality. Mid-frequency modules rated for 20 MHz to 100 MHz, for example, often are used with cables that have a much lower frequency response. In such a case, the cable is a bottleneck for the entire switch system.
“Users should be aware of the challenges associated with building high-density switching solutions,” he continued. “They need to understand the different connectivity options available for PXI modules. These include cable fixtures, front-mounting and external terminal blocks, ribbon cables, backshell/connector kits, and discrete cables. Knowing the pros and cons of these connectivity options helps users choose the best switching modules for their systems.”
NI offers more than 20 RF multiplexer, general-purpose, and matrix PXI switch modules ranging in bandwidth from 500 MHz to 26.5 GHz. These products feature relay count tracking and low insertion loss at the specified bandwidth. Because relays are mechanical and eventually wear out, knowing the number of closures a relay has performed helps users to estimate the remaining lifetime and avoid unexpected system downtime.
Many very dense modules such as NI's 544-crosspoint PXI-2535 and 2536 matrix switches are based on FETs rather than relays. FETs provide unlimited lifetime, unlimited simultaneous crosspoint connections, and switching speeds as high as 50,000 crosspoint connections/s.
Summary
Switching solutions have been developed for almost any application you may have. This means that developing an automated test system involves an element of choice. And, as in most engineering work, initial decisions can have a large effect on system flexibility, cost, and performance.
VXI and PXI switching modules are featured in the comparison chart, and many more models are available. On the other hand, neither of these formats may be the best for your test system.
If parts of the test setup are separated by large distances, an LXI system may be more appropriate. Ethernet with IEEE 1588 timing can synchronize widely separated instruments to within tens of nanoseconds. LXI instruments also can communicate on a peer-to-peer basis, which generally is faster than involving a central controller in each transaction.
Perhaps the required type of signal conditioning or current or voltage handling capability only exists in one format. In that case, you need to determine whether it is cost-effective to develop your entire test system in that format or if a hybrid approach makes more sense. Determining the best overall system design can be difficult when switching is involved.
Except for compact matrices and small circuits such as individual coaxial switches or one- or two-level multiplexers, custom switching chassis may provide better performance than circuits assembled from either VXI or PXI modules. For very large circuits such as big matrices, custom switching chassis also may cost less and have a smaller footprint.
VXI Technology's Mr. Sarfi commented, “Larger switch systems that bridge together many modules using extensive cabling can introduce added insertion loss, capacitance, and propagation delays, which ultimately affect the integrity of the signals passing through the system. Analysis prior to build helps to determine how much this will affect test reliability.”
However, electing to use a larger custom switching system involves other considerations. You must integrate switch control with your overall test program so an amount of software development may be necessary. Also, future test system capabilities must be supported. How readily can the switching network be modified and expanded as your needs change? How much will additional engineering development cost to accommodate the changes?
Many of these factors are reduced or eliminated if a test system is constructed within a single format such as PXI or VXI. All the cards conform to a common standard, are guaranteed to work together, and plug into the same chassis. The job of test system development can focus more readily on testing the DUT rather than on distracting hardware and software incompatibility issues. And, you have the choice of modules from a large number of manufacturers rather than being tied to a proprietary solution with limited availability of functions.