Switching and Interface Solutions Provide Flexibility

Instrument reusability and flexibility are probably the most highly valued attributes of VXIbus-based test systems. VXI instruments selected to meet today’s test requirements most often are equally useful for tomorrow’s applications. New ones can be added and old ones reused because standardization permits old and new to easily “play” together.

Flexibility, the other major VXI virtue, is achieved in two ways. Signal routing and re-routing among instruments and with the UUT are facilitated through a vast range of VXI-based switching modules. And instrument-to-UUT interconnection flexibility is accomplished through a variety of off-the-shelf or custom interfacing adapters/fixtures.

Configuring an optimum switching arrangement for your application starts with the signal-routing topology definition, followed by a current/voltage/frequency

assessment and culminates in VXI switching-module selection. Choosing a suitable instrument-to-UUT interconnection scheme often is an easier task. It generally is dictated by the UUT’s form factor, quantities and type variations.

Switching Topologies

Before VXI, switching facilities usually were custom-designed and fabricated. But with VXI, this task has been greatly simplified. “VXI is a perfect host for all types of switches or switching systems and allows test engineers to easily configure test systems to meet specific requirements,” commented Bob Varo of ASCOR. Today’s VXI switch modules support all common topologies, including multiplex, scan, star and matrix configurations.

Multiplexers/Scanners

Multiplexers and scanners are similar in structure and resemble, in their simplest form, a single tree trunk with two or more branches. Scanners may sequentially connect a single source or measurement instrument to inputs or outputs of UUTs. Switch groups contained in VXI multiplexer or scanner cards often are reconfigurable into multiple trees to provide two-pole or four-pole instrument-to-UUT interconnections.

“The multiplexer/scanner topology is the most commonly used automated switch configuration,” said Norton Alderson, vice president of marketing at Universal Switching. A scanner allows only one device to be connected to the trunk at any time and a multiplexer formation permits multiple simultaneous connections.

“A typical multiplexer application may consist of a requirement to apply a combination of signals to UUT inputs and to measure the resulting outputs,” said Sally Aldrich, product marketing manager of VXI switches at Tektronix. “When several signals must be applied to multiple UUTs and measured concurrently, there often is a need for the switching module to perform as a scanner and multiplexer simultaneously.”

Star Switches

Star configurations are formed when one pole of several switches is tied to a common junction. This junction does not serve as a port but enables signal flow among multiple instruments, UUT ports or other switching facilities.

This arrangement is useful for high-frequency applications since it disconnects unused instruments or gateways, thereby achieving greater bandwidth by removing cable stubs. VXI cards containing nondedicated individual RF or microwave switches are often used to provide the star configuration.

Matrixes

The matrix-switch configuration offers the utmost interconnection flexibility. Using an X × Y interconnection scheme, it allows multiple channels of inputs (X) to reach multiple channels of outputs (Y). The matrix size is determined by the number of instrument ports to be routed, multiplied by the number of desired destinations.

Since the matrix can connect any Y line to multiple X lines and vice versa, exercise care to assure that incompatible devices, such as sensitive instruments and power sources, cannot inadvertently be connected to each other. The matrix size should be as small as possible, not only because of cost, but also the larger it is, the lower is its bandwidth.

A matrix-switching application example, provided by Sam Tsai of Hewlett-Packard, deals with testing 4-pair or 25-pair LAN cables to meet 100 MHz transmission specifications. Assuming a 4-pair cable, the system DMM measures resistance/foot on the first pair, while an LCR meter measures capacitance/foot on the second pair and a network analyzer measures crosstalk between the third and fourth pair. After completing the first set of simultaneous tests, the matrix reallocates the measurement resources until all tests have been performed on all pairs.

VXI matrix-switching cards are available from many suppliers for signal, power, RF and microwave applications. Combinations of these often provide multi-instrument-to-UUT interconnections in VXI-based ATE.

Combinations

While many inputs may be selectively connected to many outputs, not all signals usually are applied at the same time. For example, there may be a requirement to connect any of 64 inputs with any of 64 outputs but only four signals are used at the same time. In this case, using a combination of two matrices, one 64 × 4 and the other 4 × 64, provides substantial cost savings.

“A full-access matrix has a crosspoint for every I/O combination, namely 64 × 64 or 4,096 switches. But the 64 × 4 plus 4 × 64 arrangement requires only 256 plus 256 or 512 switches,” said Mr. Alderson. “This demonstrates that a multilevel topology can reduce the number of required crosspoints substantially; in this case, by more than 80%.”

“Using a very large matrix simplifies the engineer’s design task but also may increase costs and reduce performance that a good combination of switch architectures would provide,” concurred Mr. Varo. “The multiplexer can reach many UUT points, the star switches can isolate and selectively interconnect instruments while the matrix, as an intermediate switch, simultaneously routes many signals.”

To accommodate extended switch topologies, some companies favor a multimodule approach. “ASCOR has one multimodule design that holds up to 11 switch cards within a six-slot VXI module,” said Mr. Varo. “Using two of these modules, we designed a switching system with a 48 × 10 × 408 matrix and star switches to achieve 10-MHz performance.”

A set of different individual switch cards also can be integrated via VXI backplane facilities controlled by the local bus. “The local bus conveys commands from the first card in a system, acting as a master unit, to all other switch cards in a system,” said Todd Nash, marketing manager at Racal. “The first card typically is message based while all others are not. This reduces cost and allows the group of cards to act like a system rather than individual cards.”

The SurePath™ Switching Family from Tektronix uses a single VXI interface and slave-control daughter board. “The daughter board is installed on the left-most SurePath switching module and controls up to 12 additional modules via the local bus,” explained Ms. Aldrich. “Subsequent units are placed to the right of the first module and are controlled and monitored by the master via the VXI local bus.”

Selecting Switch Elements

Arriving at an optimum architecture for your switching application and selecting appropriately configured cards are the first steps. The switching elements on the card also must handle the signal amplitudes, bandwidth, crosstalk and power levels your application may require. Table 1 provides an overview of typical performance levels.

There is no such thing as a perfect switch, but some of the apparent shortcomings may not be performance impediments. “For instance, the relatively high on-resistance of high-frequency solid-state elements may not be a detriment,” Mr. Alderson commented. “Most often, you do not even see the on-resistance since other items (amplifiers, power dividers) may be interjected between the switching element and your UUT connections.”

Nevertheless, the potential shortcomings must be considered when determining which type of switching element is best for your application. According to Ms. Aldrich, the major limiting factors of each type include:

Reed-Relay—offers a typical contact life of 10⁷ operations compared to an infinite contact life for solid-state relays.

Coax—suffers from decreasing RF power-handling capability when frequency increases.

“VXI lends itself very well to support switching of general-purpose analog and digital signals. However, applications entailing high power, high frequencies or extremely low-level signals may present problems requiring special attention,” cautioned Nick Turner, sales manager at Cytec.

“Power switching can be problematic in different ways, depending on whether the requirement is for high voltage, high current or both. Mercury reed relays will switch ±500 V and up to 2 A as long as the total power is kept below 50 W. But they must operate in a vertical position, making them unusable in applications where the switch modules are installed horizontally,” he said.

“High-voltage reed relays will switch 1,000 V and are not position sensitive. However, they only will handle 10 W of total power so you need to worry about in-rush currents which could cause premature failures,” Mr. Turner continued. “Armature relays will handle currents of 10 A and up to 2,500 VA of total power, but their size and the necessary connectors limit the number of relays you can place on a single module. High-frequency applications are almost always limited by the need for a quality coaxial connector; and while BNC connectors are cost-effective, you can’t put many on the front of a VXI module.

“For extremely low-level signals, many factors other than the switch must be considered. VXI chassis often use switching supplies and cooling systems that make the supply voltage extremely noisy. This noise can be easily transmitted through the relay drive coils and coupled to the signal lines, making it difficult to perform extremely low-level measurements,” Mr. Turner concluded.

VXI-to-UUT Interconnection

While signal-routing flexibility in the VXI system is a function of its internal switching facilities, external routing adaptability is determined by the interface. Three basic VXI-to-UUT interfacing schemes exist: direct VXI-to-UUT wiring, remote interconnect adapters, and interconnect assemblies mounted on or adjacent to the VXI mainframe.

Direct VXI-to-UUT Wiring

In almost all cases where in the VXI ATE tests only a few types of UUTs and a limited number of interconnections are required, running direct cables between the instruments and the UUT may offer the most cost-effective solution. “Users of benchtop VXI systems usually prefer this connection,” said Gregory Davis, product marketing manager of integration services at Tektronix. “This scheme may have frequency-response and noise-immunity challenges, especially in configurations that require more than just a few cables. It also has the longest setup times for switching from testing of one type of UUT to another.”

Remote Interconnect Assemblies

An interconnection scheme, consisting of a set of cables semi-permanently affixed to the VXI system and terminated in one or several separate UUT mating fixture(s), sometimes is used. It is appropriate when a set of interconnected UUTs or a variety of similar but not identical UUT types must be tested with a common set of instrument resources. It also is used for in-process tests when automated quick UUT connect/disconnect is required.

Integrated Interconnect Assemblies

A UUT mating fixture may be mounted directly on a VXI mainframe or a rack containing VXI equipment. The fixture may be equipped with edge-card or other UUT mating connectors or support a bed-of-nail type interface. If the VXI system will test a variety of vastly different UUTs, a quick-connect-and-disconnect interface arrangement is needed.

The common approach uses a set of mating assemblies, one containing a set of connectors wired to the VXI instruments, the other containing mating connectors wired directly or indirectly to the UUT. The first usually is referred to as the receiver, and the second as the interface test adapter (ITA). The ITA may be an integral part of the functional or in-circuit test mating fixture.

Many receiver/ITA arrangements are available. “Mac Panel offers several interface solutions mounted directly to the rack or at a location designated by the user,” said Anthony Sedberry, manger sales and marketing at Mac Panel. “They provide up to 4,800 I/O paths and use contact types ranging from low-current (3.5 A per pin) 96-position (DIN)-style connectors to high-frequency contacts carrying 35-GHz signals.

“The receiver/ITA does not have to be in front of the equipment, although this arrangement keeps cable length short, which is good for high-speed applications. A convenient interconnection and access method is provided by a hinged receiver,” Mr. Sedberry continued. “This implementation allows you to hinge the unit forward and to have total access to the rear of the interface and all instrumentation. The hinged format is the most versatile and most popular design.”

Many receiver/ITA suppliers incorporate convenience and safety features in their products. These include assembly mating aides and microswitches in the receiver to prevent the presence of electrical potential on exposed contacts when the ITA is not installed and engaged.

Trends

The continuing trend toward achieving higher test throughput for UUTs using ever-increasing frequencies will accelerate the demand for VXI switch modules and higher performance interconnections. “Also, contract manufacturing and flexible production spurs the growing need for reuse of test equipment across multiple UUT types,” said Mr. Davis. “These factors drive the industry toward less use of direct interconnect and higher use of mainframe-mounted and remote interconnection schemes.”

More attention is being paid to the economics of using a receiver/ITA vs other interconnect solutions. Lisa MacMaster of TTI Testron re-emphasized that selecting the optimum interconnect scheme for your application is primarily a business decision depending on these factors:

What are your throughput requirements?

How much time does your current test setup take?

How many products require similar test instruments?

What are the signal transmission quality requirements?

What are the acquisition, maintenance and repair costs?

The increased use of receivers has prompted the establishment of several standards such as VPP-8, developed by the VXIplug-and-play consortium. “VPP-8 extends the open architecture of VXI out to the receiver,” said Tom Wultich, product marketing manager of VXI mainframes at Tektronix. “It establishes a defined standard mainframe-receiver interface to provide mainframe-to-receiver interchange compatibility. When you buy a VPP-8-compatible mainframe and receiver, you can choose to port the receiver to any other VXIplug-and-play mainframe in the future.”

Another standardization effort is being undertaken by the Receiver/Fixture Interface Alliance (RFI), which had its first meeting last September. The alliance wants to use formal and de facto standards to define a system specification leading to an open multivendor hardware architecture for receiver/fixture interfaces.

An initial RFI architecture proposal was developed by AMP, Mac Panel and VXI Associates and all three companies offer products conforming to initial RFI standards. In addition to the first three companies, the RFI Alliance sponsors include ASCOR, ATTI, Lockheed and Teradyne.

The continuing standardization efforts and additional switching and interfacing equipment will provide new and more economical VXI-to-UUT implementation choices for achieving instrument reusability and flexibility.

VXI Test Products

Programmable Microwave Switch

Module Includes Embedded CPU

The VXI-RMR410 VXIbus Register-Based Switching Module offers DC to 18-GHz switching with four independent bidirectional 1 × 10 relay sections in a double-wide module. The relay sections are individually controlled and monitored by an embedded CPU. The transmission loss is 70 dB at 18 GHz. The relay contacts are rated for 40 W at 18 GHz and >100 W at 1 GHz. Universal Switching, (818) 785-0200.

Microwave Switch Module

Covers DC to 18 GHz

The 1260-66 message-based C-size RF VXIbus dual-slot switch card contains up to six SP6T relays. The power-handling capability per channel extends to 70 W at 4 GHz, 40 W at 12.4 GHz or 30 W at 18 GHz. Direct SMA front-panel connections ensure high signal integrity. Relays can be interconnected to create large switch matrices. This high-density card is suitable for applications with large RF switch requirements such as communications system testing. Racal Instruments, (800) 722-2528.

New VXI Switching Modules

Feature 10-A Capacity

The VX4351 and VX4381 High-Current VXI Switching Modules extend the power-handling capability of the company’s SurePath™ routing and switching technology from 2 to 10 A. The VX4351 is a 40-channel, 10-A single-pole single-throw switch module. The VX4381 is a 4 × 4, 10-A, dual 1-wire switch matrix. Both modules incorporate interchangeable external socketed relays for quick replacements. SurePath modules use an innovative relay-control architecture to reduce cost, improve reliability, simplify programming and speed test-program execution. Tektronix, (800) 426-2200, press 3, code 1008.

Interface Products Provide

Integrated Solutions

The VPC 90 Series Interface Systems provide a spectrum of interface solutions for common as well as special VXI-to-UUT interconnection requirements. Products include receivers, interchangeable test adapters, connector modules, tools and a variety of pre-wired interconnect adapters for use with VXI cards, mainframes and cabinets. The interconnect adapters supply easy access to and facilitate insertion and removal of individual VXI modules without disturbing unrelated connections. Virginia Panel, (540) 932-3300.

VXI Switch Modules

Available in Many Styles

VXI C-Size and B-Size Switch Modules are offered in a variety of configurations, including multiplexers, matrices and discrete relays. They accommodate message- or register-based operation and contain reed switches, power relays, coaxial or solid-state switches. Status feedback is provided. LabVIEW and LabWindows drivers are supported. Cytec, (800) 346-3117.

Modular Kits and Receivers

Provide Flexible Interfaces

The VG Series of products includes receivers, fixture kits and adapters. The VG Series Receiver provides a VXI system mass interconnect which handles up to 4,080 contact points. Modular contact blocks accommodate coax, minicoax, power, shielded or unshielded leads, or combinations of these. Benchtop or rack-mount configurations are available. The VG Series is VXIplug-and-play compliant. TTI Testron, (800) 262-4894.

RF Switch Module Operates

At 16/32-b Path Speeds

The 3000-64 VXIbus RF Switch Module occupies two slots and is offered in a single configuration containing six 1 × 4 DC to 4-GHz switches. The unit features the VXIMAX™ 16/32 VXIbus interface so it can operate at VXIbus data path speeds of 16 bits or 32 bits. The 3000-64 is the latest addition to the company’s line of VXIbus switching and digital modules for industrial, medical, scientific and government automatic test applications. ASCOR, (510) 490-2300.

Relay Modules Feature

Switching Speed of <1 ms

The ITI Series 114 and 117 VXIbus Discrete-Switch Modules contain 64 1 × 1 and 96 1 × 1 relays, respectively. To help conserve internal VXI chassis power, an external DC voltage source may be used to power the relays. The switching speed is less than 1 ms. I/O access is provided through standard high-density sub-miniature D connectors. Information Transfer, (201) 251-7700.

Relay Matrices Offer

Multiple Topologies

The HP E1465A, HP E1466A and HP E1467A C-Size, 1-Slot VXI Modules are register-based and provide 16 × 16, 4 × 64 and 8 × 32 matrix switching facilities, respectively. The three units use identical switching hardware, but a unique terminal block arrangement establishes alternative matrix configurations. The matrix expansion is facilitated via a chaining cable which allows you to interconnect rows and columns on different modules. The maximum voltage rating is 200 V and the minimum bandwidth is 10 MHz. Hewlett-Packard, (800) 452-4844.

Interface System Enables

Use of Short Cable Length

The Mac Panel Series 120 Pull-Thru Interface System mounts on any plug-and-play compliant VXI mainframe. It facilitates direct VXI instrument-to-module interconnections and ensures signal routing using minimal cable length. Mac Panel, (910) 861-3100.

Table 1.

Copyright 1997 Nelson Publishing Inc.

May 1997