Even More Channels in the Same Space

by Tom Lecklider, Senior Technical Editor

The best engineering solutions provide an optimum trade-off among several conflicting factors, and high-density switch module design is no exception. The form factor is an important starting point because, regardless of anything else, a switch module must meet the relevant specifications.

VXI and 6U PXI continue to be used for large switching applications because of their installed base and size. Similarly, AXIe provides a large board area and may become popular for switching modules although it is too early in the standard’s adoption to tell. In contrast, the smaller 3U PXI footprint more severely constrains a design.

For a large switching system, choosing the most suitable form factor has complexity and cost implications. LXI has been adopted by several vendors because its lack of fixed dimensions allows the design to conform to the application. Formats with fixed dimensions may require multiple modules and chassis as well as expensive cabling. An LXI implementation with fewer partitions avoids much of this expense.

Click here for Switching Systems Comparison Chart

According to Anirudh Narayanan, applications engineer at VTI Instruments, “Our LXI switch cards have the same center spacing and height as a 3U Eurocard but with added depth. This generally is not a concern for ATE applications where rack height has a high premium. By conforming to the 3U height and increasing the depth of our boards, we can offer much better performance and signal integrity without sacrificing density.”

Mr. Narayanan elaborated, saying that a larger board area not only supports more relays, but it also facilitates better signal routing. Track widths can be increased for higher current-carrying capacity and track separation made greater to reduce crosstalk and increase voltage capacity. In addition, shield planes more effectively ensure signal isolation.

Nevertheless, many high-density fixed-format switch modules are available, and it is instructive to understand their capabilities and trade-offs. The comparison chart that accompanies this article lists many modules from several companies.

Technologies

Solid-State

FET switches provide a virtually unlimited number of operations but have higher ON resistance and limited voltage range. VTI Instruments’ Mr. Narayanan said the company’s EX1200-3048S FET-based 48-channel switch was used as part of a battery test solution. The customer’s pervious test system was limited to 60 V, which restricted the number of batteries that could be tested in series. The 250-V rating of the EX1200-3048S allowed more batteries to be simultaneously tested, improving throughput.

In this application, the FET switch 8-Ω ON resistance probably is not a concern because voltmeters typically have extremely high input resistance. However, in an application that needed to switch current, the switch dissipation could be a limiting factor. It generally is the case that FETs with a high-voltage rating also have a higher ON resistance.

Figure 1. NI PXI 2536 3U Switch Module Courtesy of National Instruments

Solid-state switching really does support high density although with important trade-offs. For example, National Instruments’ PXI 2534 and 2536 3U PXI Modules both use solid-state switches but with different characteristics. The 2534 has 256 switches and can switch 55-V levels and carry 1-A currents. In contrast, the 2536 shown in Figure 1 has 544 switches but is limited to 12 V and 100 mA.

Jordan Dolman, product manager for NI switches, commented, “As test engineers turn to solid-state and FET-based switching to improve system reliability, the triggering and synchronization features of PXI remove operating-system overhead and capitalize on the fast switching speed.”

Pickering Interfaces also uses FET-based switching in some modules. Bob Stasonis, sales and marketing manager at the company, explained, “We have applied FET switching where it makes sense. In most applications, we find that reed relays work better, but we are in the process of introducing a series of FET-based RF switches and attenuators with 6-GHz bandwidth. They have consistent insertion loss and VSWR as well as density and speed that reed relays can’t match.”

Reed Relays

A wide range of capabilities also is found in reed relay-based switching. Very small reed relays have correspondingly small contact areas, which limit their power-handling capability to a few watts. Several of the reed-based modules listed in the comparison chart are rated from 0.5 A to 1 A and 10 W to 15 W maximum. On the other hand, specially constructed small reed relays can have very good RF characteristics.

Giga-tronics’ Chief Technical Officer Jeffrey Lum commented, “The cost penalty of very small reed relays is lower voltage, current, and power ratings, but the benefit is higher frequency performance. The ultra-small relays have surpassed 7 GHz with some being usable at 10 GHz. Giga-tronics ASCOR has built large high-frequency matrices based on small reed relays. We don’t use FET switches because of path resistances and variation with temperature.”

The highest current rating found in reed relays used for test and measurement applications is about 2 A. Above 2 A, most switch modules use electromechanical relays that are more robust. Nevertheless, specially designed reed relays can provide very high-voltage performance as described by VTI Instruments’ Mr. Narayanan.

“Reed relays typically offer better isolation compared to electromechanical relays,” he explained. “This is because dry reed relays are hermetically sealed with inert gases which have strong dielectric properties. The company’s EX1200-2007A, a 48-channel, 1,000-V multiplexer, is a good example of a reed relay being used to achieve high-voltage and high-density switching.”

Electromechanical Relays

Electromechanical relays have larger, heavier contacts and are capable of withstanding high inrush currents. Generally, electromechanical relays also require higher actuating current, which can be a limitation. FET switch modules typically allow all or most of the switches to be activated simultaneously because so little power is needed.

A large reed relay-based matrix may limit simultaneous actuation to a smaller quantity. For example, the Pickering Interfaces Model 40-540/541/542 Ultra High Density Matrix Module supports up to 528 crosspoints but allows only 100 relays to be activated simultaneously.

This factor is a concern for DC and low-frequency operation but not for RF. Multiple connections to any single signal should be avoided at RF frequencies because attaching additional circuits changes the load impedance, resulting in reflections and distortion.

For optimum insertion loss and reflection performance for Pickering’s Model 40-726A 12x8 RF Coaxial Matrix, only one crosspoint is set in any one row or column. According to the company, multiple crosspoints could seriously degrade RF performance.

Electromechanical relays remain the best method of switching microwave signals up to 40 GHz. However, reed relays are making significant progress in this area as discussed by Mike Dewey, senior product marketing manager at Geotest-Marvin Test Systems. “Probably the biggest advancement in reed relays has been improved performance into the gigahertz range. Compared to the older coaxial structure switches, new reed-based switches offer high bandwidth with a much smaller footprint,” Mr. Dewey said.

Even at low frequencies, the capacitance associated with unused sections of large matrices can cause problems. Giga-tronics refers to destubbing routines that disconnect unused portions of matrices to optimize signal integrity. Similarly, Agilent Technologies’ Model 34934A Matrix Module features row-disconnect relays to eliminate the capacitance of matrix modules not used for a particular connection.

Subchassis

Several companies provide subchassis carriers that plug into a standard VXI or PXI chassis. The switch cards that fit the carriers are designed with a very low profile, and typically six cards can fit in the space occupied by four standard modules. The carriers provide interconnections between the switch cards that eliminate much if not all external wiring. This is especially useful when creating large matrices.

NI’s SwitchBlock occupies four PXI slots and accommodates up to six switch cards. In addition, an expansion bridge can be positioned between adjacent SwitchBlock Carriers to connect the two analog buses and facilitate matrix expansion.

The company’s Mr. Dolman explained, “This modular approach to building a large matrix also makes it possible to mix and match relay cards of different sizes to optimize relay usage. For example, if only a handful of instruments requires 2-wire switching, then only one 2-wire relay card could be added to the matrix. Similarly, if only certain tests require 16 simultaneous connections, this can be met by one relay card without increasing the number of rows for the entire switch matrix.”

The Pickering Interfaces BRIC™ Carriers occupy from two to eight PXI slots. There are two series: The 40-560/561/562A Multi Slot Matrix Module cards use reed relays and switch up to 150 V, 1 A, and 20 W. The 40-566A 2-A Module is available in both four- and eight-slot versions and based on electromechanical relays. Both subchassis provide more flexibility and higher density than standard 3U PXI cards.

Gigatronics offers the 4000 Series VXI switch card carriers together with compatible 4000 and 4500 Series cards. These carriers also increase density by supporting card spacing on smaller-than-standard centers but in addition feature 500-MHz backplane interconnections. According to the company, this means that you can connect cards within the carrier with no loss in bandwidth.

Custom Chassis

Although LXI-based instruments may be any size, most can be rack mounted and have adopted standard vertical increments. VTI Instruments’ EX1200 Series Mainframes are good examples, accommodating from two to 16 measurement modules within chassis ranging from 1U high and half-rack wide to 3U and full rack. The modular instruments must be used with the proprietary EX1200 Series Mainframes.

Agilent’s 34980A Mainframe accepts up to eight modules and has an integrated DMM. A range of functions is available in addition to switches so you may be able to completely address your application through a suitable choice of modules. The 34934A Matrix Module includes expansion cables to minimize wiring between multiple modules when creating large matrices.

Keithley Instruments’ Model 3706 DMM/Switch Mainframe also uses proprietary modules to provide more complete application solutions. In this case, the 3706 Analog Backplane eliminates external wiring and supports column expansion while preserving signal integrity.

Universal Switching’s single-ended S2561E Switching System is a good example of a modular approach to large matrices. The DC-to-125-MHz frequency response accommodates analog telemetry, audio, video, and TTL logic signals in a 32 x 32 to 256 x 256 switching matrix within a single 5U chassis. Multiple chassis can be combined to build a 1,024 x 1,024 maximum size matrix. Fanout supports one input to any one output, to several, or to all.

The solid-state circuitry is partitioned into VSI2561D-S25 Input Modules and VSO2561D-S25 Output Modules, each handling 32 channels. Up to eight of each type can be plugged into the chassis. Mid-stage switches that connect the inputs and outputs are built into the chassis and provide routing redundancy. In addition, dual redundant power supplies and dual redundant controllers are available. A range of optional I/O connector panels adds flexibility for system integration.

Figure 2. GX6325 6U PXI Switch Module Courtesy of Geotest-Marvin Test Systems

Applications

A primary use for high-density switching is in ATE systems. Geotest’s Mr. Dewey described a large application solution based on the company’s Model GX6325 6U Switch Module (Figure 2), “Our customer needed a switch system offering more than 7,000 form-C 2-A relays. In the 3U PXI format, the system would have required 20 chassis. We provided a 6U-based system with only five chassis and a total of 95 modules. The result was lower cost, less space, fewer interconnects, and the convenience of rugged D-sub connectors.”

Agilent Technologies’ Sheri De­Tomasi, product marketing manager at the company, explained a typical use of high-density switching “in design verification and manufacturing applications in the consumer, automotive, medical, and aerospace/defense electronics industries. In these applications, the device under test will have many pins where simulated inputs need to be provided while the outputs are measured to validate the product functionality. A high-density switch matrix is used to simultaneously route multiple signals. Also, high-density multiplexers are used to successively connect multiple points on a DUT to a single instrument like a DMM,” she concluded.

Keithley’s Jerry Janesch, senior market development manager, discussed how the company’s 3706 DMM and Model 3722 dual 1x48 Multiplexer Cards were used for calibration/verification of industrial control equipment. “For this application, they needed to pack hundreds of two-pole switching channels into a single 2U rack space. With this combination, they accommodated 576 channels without sacrificing signal integrity,” he said.

Summary

Before designing a high-density switching solution, make sure you understand all the parameters. The test system form factor may be the best place to start. Either the required switching can be supported at a reasonable cost, or if not, perhaps a hybrid approach is more appropriate.

The switch architecture may be the next factor to consider. Do you really need a large matrix, or will an application-specific arrangement of multiplexers work as well at lower cost? And, if you are switching RF or microwave signals, how does the proposed approach deal with stubs? How consistent is the insertion loss or VSWR across channels?

Even at low frequencies, the chosen switch must operate well within its specifications to remain reliable. For high-density reed- and FET-based switching, you need to explore the various voltage, current, and power trade-offs.

There are many switch modules available, so the challenge is not in finding a good technical solution. Rather, it is a matter of choosing the optimum solution among several possibilities.

 

 

Continuing to expand our switching and distribution product offerings, the new LS1601A offers a fresh new alternative to system engineers. The 3RU rack mountable unit has a unique "open window" front panel design that allows up to sixteen plug-in modules to be installed with both forward and rear facing connectors, controls, and indicators.

A growing selection of digital and analog modules are available for this cost effective solution. Modules install from the rear of the LS1601A and can be mixed and matched to meet various user configurations.

Optionally, a plug-in LXI certified 10/100 Ethernet module can be installed to remotely monitor system health, and to control certain module types such as switches.