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Switching RF and Microwave signals.

Switching is central to most automated test setups, even for a single DMM that only measures voltage at a couple of test points. Switching performance can easily become the limiting factor when hundreds of signals and racks of equipment are involved. In all cases, a switching system must be designed to cause the least signal degradation consistent with the required measurements.

Of course, cost, size, and switching speed also are important factors. If they weren't, big, expensive relays rated for very high current might be the best choice for most DC and low-frequency switches. Large contacts have lower resistance so they would seem to have less effect on signal integrity.

This may sound like a good idea, but a closer look at the technical details quickly highlights several problems. Relay contacts made from silver, gold-plated silver, silver oxide, and tungsten are used depending on the characteristics of the signals to be switched.

According to a Tyco Electronics application note, "Silver cadmium oxide contacts have long been used for switching loads that produce a high energy arc. Silver cadmium oxide contacts are less electrically conductive than fine silver contacts, but have superior resistance to material transfer and material loss due to arcing. They do exhibit greater interface resistance between mated contacts ... and, like fine silver contacts, the silver in this alloy will oxidize and sulfate. Therefore, an are is necessary to keep these contacts clean." (1)

For low-level signals, gold-plated contacts are the best choice because sulfidation is minimal and a good electrical contact is made when the relay is closed. On the other hand, gold soon boils off contacts subjected to arcing so signal amplitude must be less than 15 V.

Tungsten often is used for very high-voltage switching contacts because its 3,380[degrees]C melting temperature provides good arc-erosion resistance. But, as the Tyco reference explains, "Tungsten may develop troublesome oxide films, especially when used as the anode contact in some DC applications. Therefore, tungsten is often used as the cathode contact, and a palladium alloy used as the anode contact. Such a combination also minimizes contact interface resistance and material transfer."

Instead of electromechanical relays, a FET-based design may be more suitable. Even tiny reed relays are relatively slow to switch compared to electronic speeds. If your application must switch at a high rate and have virtually unlimited life, FET switches could be the answer. The largest drawbacks of FET switches are their high ON resistance and limited current handling capability.

RF and Microwave Considerations

At frequencies where transmission lines are used, switches no longer can be adequately described just by their power-, voltage-, and current handling capabilities. In addition, the frequency range, isolation, insertion loss, and voltage standing wave ratio (VSWR) become important. These parameters are affected by contact material and size but greatly depend on the overall relay design. Specially constructed coaxial relays look very different from DC relays because of their high-frequency performance.

With an appropriate drive circuit, P-type intrinsic N-type (PIN) diodes act like a pure low resistance to RF signals and have been used for decades to switch frequencies greater than 1 MHz. Gallium arsenide (GaAs) FET and monolithic switches were developed more recently. Their speed and longevity advantages are similar to low-frequency FET technology but so too are their drawbacks such as higher ON resistance. Switch refinement continues with the benefit of colocation on the same die as related circuitry.

A Dow-Key Microwave switch selection guide lists the parameters shown in Table 1 for coaxial, PIN diode, and GaAs monolithic switches. It's likely that the GaAs column values have improved somewhat since the guide was published. In contrast, coaxial and PIN diode switches both are mature technologies and continue to be the most commonly used. (2)
Type of Switch       Coaxial         PIN Diode        GaAs
                    Low-Power        Low-Power     Monolithic

Switching Speed     1 to 20 ms      1 to 200 ns    1 to 200 ns

Frequency Range    DC to 40 GHz       >Octave        >Octave

Insertion Loss    0.05 to 0.8 dB   0.8 to 2.0 dB  0.8 to 2.0 dB

Isolation          80 to 110 dB     4 to 80 dB     40 to 80 dB

VSWR                1.1 to 1.5      1.5 to 2.0     1.5 to 2.0

Relative Cost           1x           2x to 10x      2x to 10x

Table 1 RF and Microwave Switch Technology Comparison

Courtesy of Dow-Key Microwave


As the table shows, nothing compares to an electromechanical coaxial relay for frequency range, insertion loss, and isolation. PIN diode switches have the advantage of size when the components are treated as part of a larger circuit board and, being electronic, longevity and speed.

Agilent Technologies' Boon-Khim Tan, operations manager test accessories operation, highlighted the differences among the electromechanical. PIN diode, and FET switches the company offers. "Electromechanical switches demonstrate low loss, superior distortion performance, and high power-handling capability but suffer from slow switching speed and a finite life. Solid-state PIN switches are used for applications requiring fast switching speed but have video feedthru that makes them unsuitable for testing sensitive devices," he said. "FET switches have very low video feedthru but slower switching speed and higher loss compared to PIN switches."

Mr. Tan's comment about video feedthru is another way of saying that the switch control signal corrupts the output. Generally, it's the edges of the switch control gate that feed through capacitively and add to the actual RF being switched. This causes distortion to the leading and trailing edges of an RF burst. If the edge speed of the control signal is sufficiently fast compared to the RF frequency, the resulting interference can be filtered. On the other hand, the control signal feedthru amplitude can be relatively large, which is the reason for Mr. Tan's caution with regard to sensitive devices.

Relay Reliability

Relay contact life is most affected by arcing. Different contact materials are at a range of minimum voltages between about 10 V and 15 V, so one way to avoid the problem is to limit the voltage levels being switched. If this is impractical, try to synchronize signal generation with relay operation.

The term cold switching describes switching a zero-level signal. After the switching action has completed, the signal level can be returned to the required value.

For example, as part of a test setup, a signal generator could be driving an amplifier with the output selectively distributed to several loads through a relay network. Cold switching could be achieved by programming the generator to output zero volts, then selecting the appropriate relay path, and finally programming the generator to output the full-amplitude signal. At least an order of magnitude difference in operational life typically results from cold vs. hot switching.

Relay cycle counting is related to reliability. Relays eventually wear out and must be replaced when contact resistance has increased beyond a certain limit, usually not because something actually broke in the mechanism. As logical as this sounds, once a relay has been connected as part of a switching system, it's disruptive to isolate it to measure the contact resistance and simply impractical to do so for hundreds of relays in a large multiplexer or matrix. Pickering Interfaces makes the argument that cycle counting is an indirect and potentially misleading way to determine when a relay should be replaced.

According to David Owen, business development manager at the company, "... relay operation counting ... [doesn't] take account of the switching load conditions--factors that have an impact of three or four orders of magnitude on relay life. BIRST [built-in relay self-test] will help avoid unnecessary service operations and help guarantee peace of mind in terms of test system reliability."

The datasheet for a BIRST-equipped relay matrix states. "No supporting test equipment is required to run a BIRST test; simply disconnect the UUT from the module's user connector, launch the supplied BIRST application software, and the tool will run a diagnostic test that will find all relays with contacts welded closed or with high (open) contact resistance. It makes it simple for systems integrators to diagnose the cause of switching failures in a system."

At present, BIRST capability is built into a number of PXI low-frequency relay-based matrices such as the 40-527 64x2 matrix with 15-MHz bandwidth. It is not clear if this technology is compatible with the company's RF and microwave switching products although Mr. Owen's comments about the inadequacies of cycle counting and the need to account for the effects of the load also apply to high-frequency relays.

Switch Modules

As in low-frequency applications, RF and microwave switching must be integrated with the rest of the test system. For PXI and VXI systems, this often is done by choosing switch modules with the same form factor as the majority of the test instrumentation.

National Instruments (NI) offers a range of PXI-based RF switches including a transfer switch and various multiplexers with 26.5-GHz bandwidth. Space is limited in 3U PXI products, and the maximum complexity is found in Model PXI-2596, a dual 6x1 multiplexer built as a two-slot-wide module (Figure 1).

[FIGURE 1 OMITTED]

At lower frequencies, the relays are smaller, and greater circuit density is possible. For example, the Model PXI-2548 is a 2.7-GHz quad SPDT switch in a single-width size. All of the 26.5-GHz modules have 50-[omega] impedance as do many lower frequency models although several 75-[omega] units also are available at 2.5 GHz.

Travis White, the company's product manager-precision measurements and switching, said, "NI switches based on the PXI platform offer onboard hardware scanning and integrated triggering, which enable RF engineers to significantly reduce test times. A scan-list can be loaded onto PXI switch hardware and successive routes actuated based on hardware triggers from other PXI instruments such as RF vector signal analyzers and generators or a variety of other RF, DC, and digital instrumentation.

"Because this whole process is hardware-timed, all software delays are eliminated, thereby saving precious milliseconds on each test point--over thousands of points, test times can be substantially reduced," he continued. "We've seen this technique provide the most benefit to engineers working in production test environments where RF generators and analyzers must generate the same sweeps on many devices under test in as short a time as possible."

NI supports switch configuration and management through the company's Switch Executive software program. It allows engineers to graphically configure large route groups spanning multiple switch modules. The routes can be easily validated and a group of multiple relay closures programmed with a single function call.

Dow-Key Microwave provides a large number of microwave components as well as a few PXI modular switches. One of these, Model 14F3S-1/4x4 is an 18-GHz reconfigurable bidirectional north-locking matrix with four inputs and four outputs. It occupies four PXI chassis slots and comes with five external semirigid cables interconnecting appropriate ports. By removing and/or rearranging the cables, the module can be reconfigured as separate sets of SPDT, DPDT, and SP4T switches.

The relays have gold-plated beryllium copper contacts, switch in a maximum of 25 ms, and under cold-switching conditions provide a 1E6-cycle operating life. An included software application facilitates individual relay coil control and features GUIs for Lab VIEW, Lab Windows/CVI, and Visual Basic users.

In contrast to PXI and VXI, LXI has the advantage of not being tied to a specific module shape or size. For larger switching networks, this makes possible a more compact solution than one based on interconnecting several smaller modules, each with limited functionality. It also improves the control software both by making programming straightforward as well as simplifying the hardware-software interaction.

In addition to standard form factors, proprietary switch subassemblies that can be mixed and matched within a larger carrier are available. One example is the VTI Instruments EX71HD High-Density 26.5-GHz Microwave Switch, a 1U-high rack-mountable LXI Class A enclosure that accepts a variety of front-pluggable switch building blocks.

Electromechanical relay topologies include SPDT, SP4T, SP6T, and transfer switches (DPDT), all with 1E6 lifetime and SMA connections. Any combination of up to 12 building blocks can be housed in a single chassis. Atypical configuration is shown in Figure 2. A flexible API supports IVT and Linux, exclude lists, relay cycle counting, and storage of up to 128 relay state configurations.

[FIGURE 2 OMITTED]

Switch Assemblies

Interconnecting cables also affect RF and microwave test system performance. For this reason, several manufacturers provide switching assemblies in a chassis complete with interconnections. System performance is specified at the accessible terminals taking into account all the intermediate paths. For complex RF or microwave switching functions, this approach could well be the least expensive way to achieve guaranteed results.

For example, Cytec has the CXM range of multiplexers and matrices with from two to 128 switch points per 19" rack-mount chassis and a life expectancy of at least 1E6 cycles. Steve Moore, a manager at the company, commented, "We also offer multichassis systems with up to 2,048 switch points. A bandpass of DC to 8 GHz is typical but extends to 40 GHz in some cases. SMA connectors are standard although many other types are available. Control is via GPIB, RS-232, or Ethernet."

Four versions of the Pickering Interfaces type 60-800 LXI Multiplexer are available with 4, 8, 12, or 16 6x1 multiplexer banks with 20-GHz bandwidth. Terminated versions with up to six six-channel multiplexers and 26.5-GHz bandwidth also are offered. A model with 16 banks of 6xl multiplexers and 40-GHz bandwidth completes the series.

Bob Stasonis, the company's sales and marketing director, explained, "The variety of choices we give the customer differentiates Pickering Interfaces from our major competitors. We've noted that coaxial relay manufacturers continue to improve product life, insertion loss, and isolation. And, isolation is becoming more important as multiple protocols are transmitted on a microwave signal," he continued. "These protocols need to be isolated from other signals in a switching system to ensure correct communications."

Agilent offers several routes to complex RF switching assemblies. Customers who intend to build their own switching solutions can base them on the L4490A/91A RF Switch Platform, which consists of a 2U-high (90A) or 4U-high (91 A) box with a built-in driver and power supply. These LXI-compliant chassis come with software drivers for most common programming environments and 3-D models to support the design and documentation of RF cable layout. Alternatively, a full custom design and construction service is available to produce a switch matrix based on the customer's exact requirement (Figure 3).

[FIGURE 3 OMITTED]

A similar capability up to 40 GHz exists with Giga-tronics' ASCOR Series 8000 Microwave Switch products. Available in a variety of sizes, these are self-contained designs with a built-in power supply and an interface card to control up to 256 relay coils. Chassis can be mounted in half- or full-width racks and range in height from 2U to 7U. Switches plug in from the front to facilitate easy installation and repair.

Walt Strickler, the company's vice-president of business development, switching solutions, explained, "Although most designers are familiar with common SPST, SPDT, and multithrow switches, they may not have encountered the star switch. A four-position star switch can be routed from any position to any other. Without a common, the switch is more balanced: The transmission line lengths are equal and stubs are minimal, resulting in better RF performance."

EADS North America Test and Services also has long been involved in RF and microwave switching. Charles Greenberg, senior product marketing manager, noted, "As an instrumentation and integration company, EADS North America Test and Services is capable of integrating our standard switching control platforms with switches and other devices, using the latest available technology to meet industry needs. As an example," he continued, "we can build a switching matrix to expand a two- or four-port VNA up to N ports for S-parameter measurements of multiport devices. If needed, we can integrate technology to completely calibrate each switch path seamlessly."

In addition, the company provides full custom design and fabrication with cabling, amplifiers, attenuators, directional couplers, and an I/O panel. The systems have a smart software interface that can control switches individually or in groups of user-defined paths. Features include automatic scanning, relay cycle counting, exclude lists to prevent inadvertent unsafe connections, and a predefined power-on state to ensure a safe configuration at startup.

Keithley Instruments offers several RF and microwave switching systems with bandwidths up to 18 GHz. The Model 7116-MWS also provides up to 40 low-frequency channels in the same chassis. The rack-mount system comprises the 16-channel 7020-MWS Microwave Switch and the 7001 Switch System Mainframe with a spare card slot.

The five RF relays are connected by semirigid cables to implement the 1x16 multiplexer function but can be reconfigured as five independent 1x4 multiplexers by removing the cables. More than 30 types of cards are available to expand the switching capabilities of the 7116-MWS to include low-frequency, general-purpose, optical, or additional RF up to 2 GHz.

One of the newer high-frequency products from Universal Switching is the Model 10948B DC-18 GHz Four-Channel A/B Transfer Switch System. A transfer switch has a DPDT configuration. In one position, signal A connects to port 1 and signal B to port 2. In the other position, the signals and ports swap. Although a unit that allows individual or ganged control of four transfer switches could be handy in many applications, the datasheet highlights back-up switching in high-reliability communications systems.

Because of its intended use in critical applications, the rack-mount 1U-high 10948B includes redundant power supplies as well as redundant system control interfaces and specially chosen relays to improve reliability. Complete control and status monitoring are possible from the front panel or the dual remote interface. Six models are available with dual RS-232-C, dual RS-422-A, dual RS-485, and Ethernet plus an RS-232-C, RS-422-A, or RS-485 interface.

Summary

Virtually all of the RF and microwave switching modules and systems discussed in this article are based on relays because they have the best over-all performance. Wide bandwidth including DC, low loss, and good isolation are especially attractive specifications in a test system. PIN diode and GaAs FET switches cannot offer this combination of features although these technologies have advantages in other types of applications.

In spite of the many factors affecting individual relay performance, RF and microwave switching system design doesn't have to be difficult, but it does need to be thorough. Obviously, the correct type of relay must be chosen to work with specific types of signals. Details such as contact material and relay construction can be avoided by working with well-defined switching modules and components from reputable sources. After all, your job is to develop a test system, not to reinvent the parts used to build one.

References

(1.) "Relay Con tact Life," Application Note 13C3236, Tyco Electronics, 2000.

(2.) "Design Guide Microwave Switches and RF Coaxial Relays," Dow-Key Microwave, 2001.

by Tom Lecklider, Senior Technical Editor
FOR MORE INFORMATION               Enter www.rsleads.com/005ee-XXX

Agilent Technologies  L4490A/91A RF Switch Platform             176

Cytec                 CXM Series Microwave Coaxial Switching    177
                      Systems

Dow-Key Microwave     14F35-1/4x4 Reconfigurable Bidirectional  178
                      Nonblocking Matrix

EADS North America    Configurable Microwave and Optical        179
Test and Services     Switching Platform 1257

Giga-tronics          ASCOR Series 8000 Microwave Switch        180
                      Systems

Keithley Instruments  7116-MWS Microwave Switch                 181

National Instruments  Nl PXI-2596 Dual 6x1 Multiplexer          182

Pickering Interfaces  60-800 LXI Multiplexer                    183

Universal Switching   10948B A/B Transfer Switch System         184

VTI Instruments       EX71HD Microwave Switch                   185
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Title Annotation:SWITCHING SYSTEMS; Tyco Electronics Corp.
Author:Lecklider, Tom
Publication:EE-Evaluation Engineering
Geographic Code:1USA
Date:May 1, 2010
Words:3232
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