Difficult signal selection

There is little disagreement among switching vendors that PXI is a very useful test system platform. It is an especially good choice if a small to medium amount of switching is needed in combination with several instrument modules. However, when a large amount of switching is needed, vendor opinions significantly diverge.

PXI solutions

Manufacturers that offer a substantial number of PXI switching modules, such as National Instruments (NI), Marvin Test Solutions (MTS), and Pickering Interfaces, use special approaches to create large systems. MTS is one of the few companies that have implemented 6U PXI modules. Figure 1 shows the GX7016 GENASYS 6U switching subsystem complete with a MAC-panel SCOUT receiver. As Mike Dewey, director of marketing, explained, “We have found that the flexibility of PXI, particularly the 6U format, has allowed us to accommodate a wide range of capabilities, negating the need to move to an alternate architecture ….

Figure 1. GX7016 GENASYS 6U switching subsystem
Courtesy of Marvin Test Solutions

“By leveraging the flexibility and size of the 6U PXI platform, we have been able to build large switching systems that feature internal cross-connect buses, offer high bandwidth and high density, and have very minimal internal wiring for connection to a mass-termination interface,” he said. “Using the 6U PXI architecture, we have been able to eliminate the limitations associated with 3U PXI switching systems—namely limited PCB real estate and front-panel connector space and the need to use wiring to support cross-connect, inter-card buses.”

Scalability and the tight synchronization that PXI provides between relays and instruments were highlighted by NI’s Mike Watts, product engineer-modular systems. He said, “Test engineers need a platform that allows them to scale from systems with a small number of switches up to larger systems while reusing a common set of hardware and software. The NI SwitchBlock occupies multiple PXI slots and uses an integrated analog bus with optional PCB expansion bridges to build scalable large switching matrices without external wiring. Many NI PXI matrix switch modules also offer matrix expansion via cables to neighboring switch modules.”

Pickering Interface’s Shaun Fuller, switching product manager, added, “For larger requirements in PXI, Pickering Interfaces offers a range of BRIC high-performance scalable large switching solutions covering matrix, multiplexer, and fault-insertion configurations. These BRICs … are designed to minimize the cost and complexity of cable assemblies to the device under test and instrumentation as well as maximize reliability and performance through isolation relay switching to internal analog busing housed within the module on a high-performance screened backplane.”

For many large systems, high-density PXI switching is appropriate. As Cytec’s Nick Turner, company CEO, commented, “PXI is a wonderful and popular format for setting up test systems. The high-speed backplane and wide variety of instruments, interfaces, and switch modules make it ideal for general-purpose test scenarios. If you have a PXI test platform and you need a small amount of switching, it’s very convenient and easy to just add a PXI switch module to the system.”

Modular, but not PXI

“However, if your switching needs become more complicated,” Turner continued, “it often is debatable whether PXI switching is the best fit. And if your switching needs involve certain types of signals, have limited connector options, or are extremely cost sensitive, it may be almost impossible to accomplish your task using PXI.”

Turner listed five characteristics that make applications poor candidates for PXI-based solutions:

Signal level: high/low level voltage, high current/power
Connectors: density, durability, cost
Microwave: size and cost
Features: LED indicators, manual controls
Size: large number of circuits, especially high power/current or microwave

The typical 3U PXI module severely constrains the real estate available for a switching solution. If you need to switch 5,000 VAC or perform high-pot tests at up to 25 kV, PXI’s small size and 0.8-inch center-to-center spacing work against you. Within one module, little distance is available between components, modules, and connector pins.

In short, Turner said that Cytec’s most successful market areas have been “… in rack-mount switching systems that are outside of the low-level, general-purpose applications. While many companies that build switching do it as an accessory for other products, Cytec has made it our specialty, and for a lot of the applications we deal with, bus systems in general do not lend themselves to switching applications. Rack-mount enclosures give us the flexibility to use the best relays, connectors, and features to provide a system that will last indefinitely and be easy to support and maintain.”

Many large switching systems have proprietary modular architectures to provide adequate configurability as well as maintainability. Control may be via a standard data/communications protocol, but the modules are electrically and mechanically compatible only with the manufacturer’s chassis. For example, one of Cytec’s CXM microwave systems, shown in Figure 2, can house up to four CXM/4×2-T-L-2.92 microwave matrix modules to allow a large variety of configurations.

Figure 2. Custom microwave chassis with up to four 4×2 matrix modules
Courtesy of Cytec

VTI Instruments’ EX1200 Series, which includes instrumentation functions in addition to switching, was described by Tom Sarfi, director product development at the company. He said that this is a proprietary modular platform with larger format PCBs than 3U PXI. One benefit of the bigger modules is higher level switching—from 1 A to 16 A and from 100 V to 1,000 V. In addition, the increased PCB area supports custom features such as a built-in attenuator on the EX1200-3824 module that allows 50-V signals to be used with a 10-V maximum input digitizer. Chassis sizes include 1U half rack and full rack with horizontally mounted modules and a 3U full-rack unit with vertically mounted modules (Figure 3).

Figure 3. EX1200 Series modules in a 3U chassis
Courtesy of VTI Instruments

Astronics Test Systems’ 1256L LXI-based switching system houses up to 12 of the company’s Adapt-a-Switch modules in a 2U-high rack-mountable chassis. The company’s Greg Riley, senior business development manager, commented on the benefits of using a separate switching chassis together with PXI instrumentation. He said, “The Adapt-a-Switch Series includes many different topologies such as discrete relays, multiplexers, and matrices. It also supports a variety of signal types including single-ended, differential, six-wire, small signal, general-purpose, high-power, high-voltage, and high-frequency and RF at output impedances of 50 Ω and 75 Ω. This leaves room in the PXI chassis for instruments that need to be tightly synchronized and provides a single point of control for all of the switching.”

Figure 4. Model S2561F solid-state matrix
Courtesy of Universal Switching

Universal Switching’s Model S2561F matrix, shown in Figure 4, is a 5U-high rack-mountable modular platform with up to 1,024×1,024 capability using multiple chassis. Each module provides 32 single-ended inputs or outputs switched via solid-state relays and accessed via 68-position DSUB connectors. In addition, the three-stage matrix configuration is completed by mid-stage cards inserted behind the hinged front panel. Optional I/O connector adapter panels accommodate multipurpose requirements such as combinations of TTL, PCM, analog audio, and video signals. The panels can be remotely located relative to the switching chassis.

Jesse Woolridge, senior application engineer at the company, described the automation or replacement of patch panels, a popular use of the S2561F.

“Operationally, there are advantages to computer control over a physically manual process and eliminating mechanical fatigue of the cables and attached hardware. Electrically, the primary advantage is transparency to attached equipment. A large channel-count nonblocking baseband matrix allows for a variety of signals to pass through in whatever state they may be in.

“For some cases, signals might be in an analog telemetry form, or in others, it may be a degraded digital signal requiring additional processing before reaching a digital receiver,” he continued. “With minimum impact to the original waveform, the S2561F can deliver a waveform in its original quality. This would allow a degraded signal to be switched to a bit-sync, switched again to a digital receiver, and that digital signal could be switched again to a recorder.”

The Precision Filters Model 464kC also is a high-density, very large solid-state switched matrix. According to Doug Firth, sales manager at the company, “The system is ideal for switching instrumentation signals sourced from different reference potentials while breaking ground loops—something a relay system cannot do—since every input is buffered by a high-CMRR, high-impedance balanced differential input. The built-in go, no-go test feature automatically verifies the setup by checking each programmed input-to-output connection through direct input-to-output measurement.”

Firth continued, “Many PXI solutions require external busing cables. Precision Filters’ 464kC topology optimizes busing of signals inside the chassis by using internal connection planes. Another advantage that separates the 464kC is its built-in self-test (BIST) capability. BIST routines measure switch-system parametrics (gain, offset, crosstalk, open/short) in situ to verify performance, providing full test reports.” Like Universal’s S2561F, the 464kC has a three-stage architecture. However, the 464kC modules each handle 16 channels of input and output signals rather than 32 channels of one or the other.

Giga-tronics has developed the proprietary GT-4600 switching system to overcome the limitations of previous attempts at a “universal” system. The company’s Robert Waldeck, vice president of business development, said that several large companies such as Teradyne and Northrop Grumman had been using a so-called universal switching system for years. However, he explained, “The problem with [the concept] has always been to overcome the bandwidth limitations of the analog signals traveling across the backplane and the performance compromises that it entails …. The other big problem we were looking to solve was scalability. Many customers are looking to create a next-generation test system that can be scaled down to support a single product line or scaled up for a service center system that can test any of their products. In standard cabled systems, this is impossible to accomplish because of the need to put the large harness in the small system and the cost and performance penalties that entails.”

The GT-4600 is based on a modified AXIe form factor with a 16-channel differential analog bus on the backplane. Using a large format such as VXI or AXIe provides room for more switched circuits so fewer modules are required. In addition, better signal fidelity can result because there is more room for properly designed transmission lines to carry signals on and off the board. GT-4600 modules include a cable-less interface area that directly mates with the test receiver. Eliminating cables and connections improves signal fidelity and overall reliability.

In addition to PXI switch modules, Pickering also designs larger switching systems based on LXI. The Model 65-110 wideband matrix originally was developed to monitor Large Hadron Collider signals at CERN and covered in a previous switching systems special report: “As described in the product user manual, ‘the 65-110 is a modular matrix with a Y-axis size of either eight or 16. Between one and 13 matrix cards can be fitted, each having an X-axis size of eight. This gives a maximum matrix configuration of 104 connections on the X axis and 16 on the Y axis.’ A variable X-size was necessary because different numbers of sensors were available at the various monitoring locations.”¹

The company’s Fuller said, “CERN required the capability to select up to 16 from a maximum 104 signals available for digitizing at each monitor location. These analog signals had frequency content to many megahertz, with potential for considerable level deviation from the different monitors. These factors put constraints upon the allowable crosstalk between channels as well as bandwidth.”

Not modular

Keysight Technologies has long been involved in RF/microwave switching, making both the separate switches as well as switching systems for use from DC to 26.5 GHz or 40 GHz. Marty Leeke, market development manager, discussed both the standard and custom large switching systems made with the company’s RF and microwave switch components. In addition to specific multiplexer and matrix products, Keysight offers the 2U-high L4490A and 4U-high L4491A RF/microwave switch platforms for up to 32 or 64 switches, respectively. Leeke stressed the proven track record and reliability of the switches, confirming that the systems are LXI-based at least partly because the components are large compared to the typical PXI module size.

VTI’s Sarfi discussed a customer’s selection of a VTI LXI-based 2×8 microwave switching solution rather than one based on PXI. He said, “Our EX7000 series is an LXI platform designed for microwave switching applications with standard products which can be modified for custom solutions.” The 1U-high EX71HD accepts up to 12 miniature multipole microwave switches mounted through the front panel.

Sarfi continued, “[The] … solution embedded our EX7000-OEM LXI command/control interface with a base mechanical architecture. By embedding the cabling, we were able to deliver a fully integrated 2×8 switch in a 2U footprint. Furthermore, our LXI command/control interface embedded the smarts that take into account the wiring between switch modules so the customer can make end-to-end path-level calls without having to make calls to discrete relays.”

Pickering’s Model 60-553, shown in Figure 5, also is LXI-based but features higher 1,024×4 density and compact size with more typical 2-A, 150-VDC/100-VAC, 60-W electrical specifications. As described in the company’s press release, “The 60-553 LXI switching matrix is designed with a dual bus arrangement that allows the matrix to be divided into two smaller independent matrices, for example, two matrices of 512×4. The dual bus design features isolation relays; this also allows submatrices to be isolated from the main matrix when there are no closures on that submatrix, minimizing capacitive loading and maximizing bandwidth of the matrix. They also can be linked together by cables to form larger matrices and supplied as partially populated matrices from 256×4 in increments of 128×4.”

Figure 5. Model 60-553 LXI-based matrix
Courtesy of Pickering Interfaces

The Astronics 1257A is an RF and Microwave Switching platform that is available in 1U to 4U heights and supports commercial off-the shelf switches in many different topologies. Similar to VTI’s EX71HD, the 1257A-1 will accept up to 12 relays cans in a 1U foot print.

A range of solutions

For very large switching systems, several approaches are possible. If the signals have amplitudes between about 1 V and 100 V and frequencies up to a few megahertz, most of the high-density PXI- and VXI-based solutions probably are suitable. These use subchassis to provide higher density by mounting special switch cards on closer-than-standard spacing.

Even subchassis are limited in the number of circuits provided when compared to very large rack-mount matrix products. Many of these units are LXI-based with modular hardware. This means that they can be expanded to some maximum size within the same chassis by adding switching modules. And, like some PXI switch modules, many of these products use solid-state switches with unlimited lifetime.

Nevertheless, as Cytec’s Turner pointed out, if your signals are not relatively easy to deal with because of power, voltage, current, or frequency, the best solutions generally need more space than available in a 3U PXI module. As Pickering’s Fuller commented, “Not only can [increased real estate] be used to create larger or higher current solutions, but also to optimize performance; for example, larger components with higher switching capacity can be used to maximize reliability, or increased spacing [can be used] to significantly improve parameters such as crosstalk.”

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