All about switching matrices.
Several terms are used to describe matrices. Crossbar or crosspoint switching is a general description that originated in electromechanical telephone switches that literally used groups of metal bars in both the X and Y directions. Depending on how the contacts were arranged between the NxM bars, more or fewer circuits could be simultaneously supported.
A matrix that allows every input to be connected to a separate output is described as non-blocking and can be very helpful in complex test systems. Jeffrey Lum, chief technical officer at Giga-tronics, clarified this definition, "For an IxPxO matrix, where I is the number of inputs, O is the number of outputs, and P is the number of simultaneous paths, when P = I, the matrix is nonblocking."
Clearly, when P is less than I, not all the inputs can connect to an output, and one or more inputs are blocked. As logical as this description appears, nonblocking and blocking are not used to mean the same things by all manufacturers. Some of the inconsistency is historical.
Nick Turner, CEO at Cytec, provided interesting background information, "Early on, when telephone companies were attempting to replace manual patch panels and switchboard operators with automated switching, the term blocking was used to describe a low-cost type of switch matrix that had a high probability of making a phone call without disconnecting any other call or giving the dialer a busy signal.
"The problem was one of accommodating perhaps 5,000 people that simultaneously needed to make a call from among a total of 100,000 telephones," he continued. "Rather than build a huge nonblocking matrix to handle any number of calls, the solution was to make a blocking matrix that only allowed up to 5,000 people to be connected at the same time. The more you reduced the switch count, the smaller and less expensive the system became, but with a higher probability that you would get a busy signal because a path was not available."
Some companies use blocking to refer to matrices that have sufficient switches to connect every input to an output, but you will need to disconnect all the paths to make changes to the connections. Other manufacturers use blocking to indicate that all inputs cannot be connected to outputs at the same time. Still other companies limit the number of simultaneous paths to one. So, although blocking implies a restriction, its severity is not standardized.
Nevertheless, as Mr. Turner commented, if your test system can be designed to run a test, reconfigure the switch, and run the next test, a blocking matrix may save lots of money compared to a nonblocking implementation. Mr. Lum added that blocking matrices often are used for high current switching. In these applications, blocking helps prevent unintended connection of multiple resources and devices.
Charles Greenberg, senior product marketing manager at EADS North America Test and Services, concurred, "Blocking matrices can offer improved performance over a crosspoint type nonblocking matrix in terms of shorter stub length for better AC characteristics and better expandability of its dimensions for the density and price. This technique is applied when multiple paths are required but not all possible paths from channels N to M are needed simultaneously," he concluded.
The comparison chart accompanying this article lists many matrices from several companies. The terminology used to describe each product is consistent with the datasheet or information provided by the manufacturer.
Further Matrix Flavors
Mathematically, a sparse matrix is mostly filled with zeros. Accordingly, the term is used to describe a switching matrix that allows any row to connect to any column, but only one or a few of each at a time. By definition, a sparse matrix is blocking. For one circuit, the configuration can be implemented as a pair of back-to-back multiplexers. The first multiplexer selects one of N inputs, and the second multiplexer directs that input to one of M outputs.
The National Instruments (N3) PXI-2593 and SCXI-1193 RF Switches can be configured as any 18-terminal or any 36-terminal sparse matrix, respectively. Figure la represents the routing possibilities within the PXI-2593 module. Paths cannot cross red lines but can cross black lines. Only one path can be drawn through any closed space on the diagram. Figure 1b shows how four simultaneous paths are supported. However, using the module resources in this way blocks terminals COM1, 1, 3,5,6,7, 11, 13, 14, and 15.
[FIGURE 1a OMITTED]
[FIGURE 1b OMITTED]
Another phrase that describes a matrix is full access, which many companies use to mean that an input can be routed to any or all outputs. This definition has several implications depending on the frequency range and application. For low-frequency signals driving high impedance loads, inputs can be distributed to multiple outputs with little degradation. Simultaneously connecting a power supply to several matrix outputs often is required. In a semiconductor test application, many pins may need to be grounded to isolate a specific diode when measuring IC protection diodes.
Giga-tronics' Mr. Lum explained the full-access term with reference to a non-blocking matrix, "Nonblocking allows any resource to connect to any device, but a resource can connect to one device at a time. In a full-access matrix, a resource may be shared with multiple devices."
In contrast, Agilent Technologies and Universal Switching call an NxM matrix with N paths formed from back-to-back multiplexers or multiport switches a full-access blocking matrix. Offering yet a different definition, Keithley Instruments' Paul Meyer, senior staff technologist, said, "A blocking matrix allows the connection of a single input to a single output. Therefore, only one signal path is active at any given time. A nonblocking matrix allows simultaneous connection of multiple input/output signal paths."
To create multiple copies of an RF or microwave signal, a power splitter must be used. Each of the splitter outputs then can be selected by an output.
Dow-Key Microwave's Sara Nazemzadeh, an applications engineer at the company, explained, "Nonblocking solutions such as a fan-out matrix provide more flexibility because any input can fan out to multiple outputs simultaneously and divide the input power to multiple switch outputs. ... The trade-offs are low isolation between output ports connected to the same input and restricted bandwidth caused by limitations of the power dividers/combiners. This approach is best suited for applications with limited bandwidth such as fan-out downlink and receiver sites and fan-in and transmission sites."
Buffer amplifiers only operate in one direction, so matrices that include them aren't bidirectional. In contrast, matrices built only with electromechanical relays are bidirectional. Passive power splitters generally are bidirectional, working as either splitters or combiners. Active power splitters usually are not bidirectional.
Most test applications don't intentionally connect more than one input to a single output although this is what is meant by reference to RF signals fanned-in to a power combiner. For the kinds of matrices usually encountered in test applications, manufacturers provide configuration software that helps you define which switches should be open or closed for each test.
One of the software program's capabilities is identification of possibly damaging switch combinations, such as two power supplies connected together or a signal connected to a power supply. Sometimes, hardware interlocks are used as an additional safety precaution when high voltages are involved. However, in most cases, it's up to you to ensure that only the desired connections occur.
Cytec's Mr. Turner said the company "has always taken the attitude that the engineer using the system knows what he wants to do and can write protection into the software if desired. We do provide both front-panel LED indicators and full status feedback so the state of any relay can be checked before proceeding to the next configuration, and we encourage people to make use of these protection features when they write test code."
Switching Systems Comparison Chart Company Matrix Model Size Type Agilent Technologies 34934A Module for Signal www.agilent.com 34980A LXI 87606B Coaxial Matrix RF Switch Microwave M9128A 3U PXI RF Cytec LXA/128 19" Rack-Mount Signal www.cytec-ate.com CXM/128 19" Rack-Mount RF Microwave HXV Series 19" Rack-Mount High Voltage Dow-Key Microwave 3202 19" Rack-Mount RF www.dowkey.com 3U Height 4101-10/10 19" Rack-Mount RF 4U Height Microwave EADS North America Test 1260-43 VXI C High Density and Services Signal www.ts.eads-na.com 1450D 1800 Series Signal Plug-In Module Geotest-Marvin Test GX6377 3U PXI Signal, Systems Power www.geotestinc.com GX6384 3U PXI Signal GX6264 6U PXI Signal Giga-tronics 3000-45 C-Size VXI RF www.gigatronics.com Module 8000 Series 19" Rack-Mount Microwave Keithley Instruments 7174 Series 707B Low Current www.keithley.com Module Signal 7072-HV Series 707B High Voltage Module 3730 Series 3706 High Density Module Signal National Instruments NI PXI-2533 3U PXI Signal www.ni.com NI 2811 A/B Module for PXI Signal SwitchBlock NI PXI-2593 3U PXI RF Pickering Interfaces 60-550 LXI Signal www.pickeringtest.com 40-585 3U PXI Signal 60-770 LXI RF Universal Switching SS219A-1206-50A 2U 19" System RF www.uswi.com Microwave VTI Instruments EX71HD LXI Microwave www.vtiinstruments.com 1U Height EX1200-4003 EX1200 LXI Signal Module SMP6122 SMIP//[TM] RF Series Module Company Matrix Model Circuits Relay Type Agilent Technologies 34934A Quad 4x32 Reed www.agilent.com 87606B 3x3 Coaxial 2x4 1x5 M9128A 8x12 -- Cytec LXA/128 128 Form A www.cytec-ate.com nonblocking Form C CXM/128 128 SPDT to SP12T coaxial HXV Series 128 Form A nonblocking Form C Dow-Key Microwave 3202 6x6 Solid www.dowkey.com 6x12 State 12x12 4101-10/10 10x10 Coaxial Bidirectional EADS North America Test 1260-43 3x8x24 Reed and Services www.ts.eads-na.com 1450D Dual 4x36 -- Geotest-Marvin Test GX6377 13 Relays, Form A Systems Dual 16x2 www.geotestinc.com Form C GX6384 Dual 32x2 Form A Dual 32x4 64x4 GX6264 Multiplexer Form A with Matrix Mode Giga-tronics 3000-45 Dual Wire Shielded www.gigatronics.com 4x64 8000 Series Up to 256 Coaxial Relays Keithley Instruments 7174 2x8x12 DPST www.keithley.com 7072-HV 8x12 DPST SPST 3730 2x6x16 DPST National Instruments NI PXI-2533 4x64 Solid-State www.ni.com NI 2811 A/B 8x21 Form A Reed NI PXI-2593 Sparse Any Form A 18-Terminal Form C Pickering Interfaces 60-550 Up to 4,096 Form A www.pickeringtest.com relays 40-585 64x4 Form A 60-770 up to 32x16 Coaxial Universal Switching SS219A-1206-50A 6x6 Coaxial www.uswi.com VTI Instruments EX71HD Up to 12 SPDT through www.vtiinstruments.com Switch SP6T Modules EX1200-4003 Dual 2x4x16 DPST 2x8x16 2x4x32 SMP6122 6x2x2 Form C Company Matrix Model Connectors Switching V Bandpass Agilent Technologies 34934A 2x78-pin [+ or -] www.agilent.com D-sub 100V 20 MHz 87606B SMA -- 20 GHz M9128A Snap-on SMB -- 300 MHz Cytec LXA/128 various 2 kV max www.cytec-ate.com 10 MHz CXM/128 SMA, N, BNC -- DC to 40 GHz HXV Series Binding Post, 5kV SHV 10 MHz Dow-Key Microwave 3202 BNC -- www.dowkey.com 800 to 2,500 MHz 4101-10/10 SMA or N -- DC to 18 GHz EADS North America Test 1260-43 IDC 220 V DC and Services 6x34 pin 250 V AC www.ts.eads-na.com 2x20 pin >75 MHz 1450D 160-Pin DIN 300 V >65 MHz Geotest-Marvin Test GX6377 78-Pin D-sub 200 V DC Systems -- www.geotestinc.com GX6384 78-Pin D-sub 170 V 35 MHz typ GX6264 78-Pin D-sub 100V 20 MHz typ Giga-tronics 3000-45 -- 200 VDC www.gigatronics.com 70 MHz 8000 Series N, SMA, 2.9 -- mm 40 GHz Keithley Instruments 7174 3-lug triax 200 V www.keithley.com 30 MHz 7072-HV 3-lug triax 1,300 or 200 >4 MHz 3730 2x50-Pin D-sub 300 V screw-term 27 MHz opt National Instruments NI PXI-2533 68-Pin 55 V www.ni.com Male SCSI >1.5 MHz NI 2811 A/B 96-Pin 150 V SCSI 15 MHz NI PXI-2593 Push-On 150 V MCX >500 MHz typ Pickering Interfaces 60-550 78-Way 300 V www.pickeringtest.com Male D-Type >5 MHz 40-585 78-Way 300 V Male D-Type > 4 MHz 60-770 SMA -- >2.5 GHz Universal Switching SS219A-1206-50A SMA -- www.uswi.com 18 GHz VTI Instruments EX71HD SMA -- www.vtiinstruments.com 26.5 GHz EX1200-4003 104-pin 250 V HD D-sub 45 MHz SMP6122 26-pin 100V Coaxial >1.0GHz Company Matrix Model Breakdown V Switching I Insertion Impedance Loss Agilent Technologies 34934A -- 0.5 A www.agilent.com -- -- 87606B -- -- 1.0 dB@ 20 50 [OMEGA] GHz M9128A -- -- 3.0 dB @ 300 50 [OMEGA] MHz Cytec LXA/128 2 kV max to 8 A www.cytec-ate.com -2 dB -- CXM/128 -- -- <2dB @ 18 GHz 50 [OMEGA] HXV Series 5 kV 3A 0.4 [OMEGA] -- Dow-Key Microwave 3202 -- -- www.dowkey.com -- 50 [OMEGA] 4101-10/10 -- -- 5 dB @ 18 GHz 50 [OMEGA] EADS North America Test 1260-43 1,000 V 2 A and Services <1 dB@ 10 MHz 50 [OMEGA] www.ts.eads-na.com 1450D 1,000 V 2 A <0.3 dB @ 10 50 [OMEGA] MHz Geotest-Marvin Test GX6377 320 V DC 10, 2, 0.5 A Systems see comments www.geotestinc.com -- <0.2 [OMEGA] GX6384 210 V 0.5 A -- <0.1 [OMEGA] GX6264 -- 0.5 A -- <0.2 [OMEGA] Giga-tronics 3000-45 -- 0.5 A www.gigatronics.com 1 dB @ 50 MHz 50 [OMEGA] 8000 Series -- -- <0.6 dB @ 26.5 50 [OMEGA] GHz Keithley Instruments 7174 -- 1 A www.keithley.com <0.2 dB typ @ -- 1 MHz 7072-HV -- 0.5 A 0.1 dB typ @ 1 -- MHz 3730 -- 1 A -- -- National Instruments NI PXI-2533 -- 1 A www.ni.com -- <1.4 [OMEGA] NI 2811 A/B 800 V Pulse 1 A -- <1.0 [OMEGA] NI PXI-2593 -- 0.5 A <2.4 dB @ 500 50 [OMEGA] MHz Pickering Interfaces 60-550 >300 V 2 A www.pickeringtest.com -- <1.0 [OMEGA] 40-585 >300 V 2 A -- <1.0 [OMEGA] 60-770 -- -- <3 dB @ 2.5 50 [OMEGA] GHz Universal Switching SS219A-1206-50A -- -- www.uswi.com <0.5 dB @ 4 50 [OMEGA] GHz VTI Instruments EX71HD -- -- www.vtiinstruments.com <0.6 dB @ 26.5 50 [OMEGA] GHz EX1200-4003 -- 2A -- <0.5 [OMEGA] SMP6122 -- 0.5 A <3 dB @ 1 GHz <1.0 [OMEGA] Company Matrix Model Carrying I Switched W VSWR Isolation Agilent Technologies 34934A 0.5 A -- www.agilent.com -- -- 87606B -- 1 W avg 1.9:1 max @ >70 dB @ 20 GHz 20 GHz M9128A -- -- 1.2:1 @ 300 80 dB @ 300 MHz MHz Cytec LXA/128 to 10 A to 2,500 VA www.cytec-ate.com -- 60 dB @ 10 MHz CXM/128 -- -- typ <1.5:1 60 dB @ 18 GHz @ 18 GHz HXV Series 5 A 200 W -- 60 dB @ 5 MHz Dow-Key Microwave 3202 -- -- www.dowkey.com 1.8:1 max 55 dB input-to-output 4101-10/10 -- 40 W 2.0:1 @ 18 60 dB @ 18 GHz GHz EADS North America Test 1260-43 2 A 60 W and Services -- >40 dB @ 10 MHz www.ts.eads-na.com 1450D 2 A 60 W -- >50 dB @ 10 MHz Geotest-Marvin Test GX6377 10, 3, 1.2 20 W Systems A see www.geotestinc.com comments -- -- GX6384 0.5 A 10 W -- -- 1.0 A 10 W GX6264 -- -- Giga-tronics 3000-45 1.5 A 10 W www.gigatronics.com -- <-40 dB Crosstalk @ 50 MHz 8000 Series -- 15 W 1.6:1 max @ >55 dB @ 26.5 26.5 GHz GHz Keithley Instruments 7174 2 A -- www.keithley.com -- 1E14 [OMEGA] 7072-HV 1 A -- -- 1E12 [OMEGA] 3730 2 A 70 W -- 1E10 [OMEGA] National Instruments NI PXI-2533 1 A 55 W www.ni.com -- 2E9 [OMEGA] NI 2811 A/B 1 A 20 W -- >10 dB @ 10 MHz NI PXI-2593 1 A 10 W <1.8:1 @ -- 500 MHz Pickering Interfaces 60-550 2 A 60 W www.pickeringtest.com -- 1E9 [OMEGA] 40-585 2 A 60 W -- 1E9 [OMEGA] 60-770 -- 0.5 W 1.7 @ 2.5 >75 dB typ @ 2.5 GHz GHz Universal Switching SS219A-1206-50A -- 100 W @ 2.5 GHz www.uswi.com -- >75 dB @ 4 GHz VTI Instruments EX71HD -- -- www.vtiinstruments.com <1.6:1 @ 55 dB @ 26.5 26.5 GHz GHz EX1200-4003 2 A 60 W -- >50 dB @ 10 MHz SMP6122 0.5 A 10 W <1.5:1 @ >55 dB @ 1.3 1.3 GHz GHz Company Matrix Model Switching Time Life Expectancy Agilent Technologies 34934A 0.35 ms -- www.agilent.com 87606B <15 ms 5E6 operations M9128A -- -- Cytec LXA/128 2 ms 1E8 www.cytec-ate.com operations CXM/128 10 ms 1E6 operations HXV Series 5 ms 1E7 @ rated load Dow-Key Microwave 3202 1 [micro]s Unlimited www.dowkey.com 4101-10/10 100 ms max 1E6 EADS North America Test 1260-43 5 ms typ 1E5 @ rated and Services load www.ts.eads-na.com 1450D 5 ms typ 1E6 @ half load Geotest-Marvin Test GX6377 10, 3, 0.5 ms 1E7 Systems see comments operations www.geotestinc.com low level GX6384 0.1 ms 5E6 @ rated load 0.7 ms 5E6 20 V, 0.5 A load GX6264 Giga-tronics 3000-45 -- -- www.gigatronics.com -- -- 8000 Series 15 ms 1E6 Keithley Instruments 7174 1 ms 1E8 cold www.keithley.com switching 7072-HV <15 ms 1E7 cold swithcing 3730 -- >1E8 -- operations no load National Instruments NI PXI-2533 0.72 ms typ Unlimited www.ni.com NI 2811 A/B 1 ms 4E6 20 V, 1 A NI PXI-2593 10 ms typ 3E5 30 V, 0.3 A load Pickering Interfaces 60-550 3 ms typ >1E5 rated www.pickeringtest.com load 40-585 3 ms typ >1E5 rated load 60-770 3 ms typ >1E6 rated load Universal Switching SS219A-1206-50A <50 ms >5E6 www.uswi.com operations VTI Instruments EX71HD <15 ms >1E6 www.vtiinstruments.com operations EX1200-4003 <3 ms 5E5 full load SMP6122 <5 ms 1E5 full load Company Matrix Model Comments Agilent Technologies 34934A Configurable: dual-wire www.agilent.com 4x32, 8x32, 4x64; single-wire from 4x32 to 4x128, 16x32; switchable 100-[OMEGA] input protection resistors; row bypass switching to disconnect unused rows; used with 34980A switch/measure unit 87606B One of several 87xxx Series self-contained switch modules; used in L4490A/91A Switch Platform to build custom matrices M9128A PXI module compatible with cPCI (J1), PXI-1, PXIe Hybrid slots; with IVI-COM, ICI-C, and LabVIEW drivers Cytec LXA/128 User-configurable, modular www.cytec-ate.com switch chassis; mercury-wetted, low thermal, armature, high current, high voltage relays; front-panel LED display, full status readback; GPIB, RS-232, and LAN I/O CXM/128 Switch chassis supports 16 to 256 switch points; accommodates splitters and attenuators; GPIB, RS-232, and LAN I/O HXV Series Applications include high-voltage switching and hipot testing; matrices formed from modular multiplexers or discrete relays; GPIB, RS-232, and LAN control Dow-Key Microwave 3202 Solid-state nonblocking, www.dowkey.com full-fanout switching system; buffered inputs +11 dBm 1-dB compression, +25 dBm TOI; RS-232, LAN I/O; removable hard drive; redundant power supply; front panel manual override 4101-10/10 CANBus-based matrix; RS-232, GPIB, LAN, CANBus I/O; front-panel override; optional removable hard disk, touch screen EADS North America Test 1260-43 Blocking matrix expandable and Services up to 288x10x1152 in VXI www.ts.eads-na.com chassis by 10-wire bus; with 5 terminations per 8x24 matrix 1450D Direct connection to instruments via 1830 backplane signal raceway; expandable to 4x324; 8x18 version 1450F with 80-MHz bandwidth Geotest-Marvin Test GX6377 Multifunction PXI relay Systems card; specifications listed www.geotestinc.com in order for 5 high current, 8 form A or C relays, and dual 16x2 matrix; with 32-b DLL driver libraries GX6384 Configurable high-density matrix; -1 version dual 32x2 or 64x2, -2 version dual 32x4 or 64x4; nonblocking GX6264 Configurable scanner/multiplexer with 8 differential groups, each with a separate output, and separately 4 universal buses; matrix mode connects the universal buses to multiple channels within each group Giga-tronics 3000-45 One of many 3000-Series www.gigatronics.com matrices, expandable in 4x32 increments; supported by GT-8300A chassis with 4 slots, LAN and GPIB I/O, integral resource manager 8000 Series Custom half- or full-rack switch assemblies; easily replaceable front-mounted switch modules; SCPI commands; GPIB, USB, and LAN I/O; can include related microwave components Keithley Instruments 7174 Very low leakage <100 fA; www.keithley.com signal and guard switched at each crosspoint; intended for semiconductor l-V and C-V signals; expandable to 8x72 or 12x60 7072-HV Combination of 2 rows 1,300 V, 4 rows general-purpose 200 V, 2 rows CV signals 200 V; SPST with common ground used for CV rows; semiconductor parametric test application; expandable 3730 With latching relays; includes analog backplane connection relays for expansion up to 6x96; on-board memory stores relay closures National Instruments NI PXI-2533 Solid-state switch matrix; www.ni.com 256 crosspoints configured as 4x64 nonblocking matrix; scan list architecture with external trigger; with NI-SWITCH IVI driver NI 2811 A/B One of four plug-in cards for SwitchBlock PXI carrier; up to 8,800 crosspoints possible in a PXI chassis; row-column and column-column supported; analog bus extension via expansion bridge cards NI PXI-2593 High-frequency multiplexer/matrix module; dimensionally flexible sparse matrix 2x16, 4x14, or 9x9; scan list architecture with external trigger; equal path lengths; with NI-SWITCH IVI driver Pickering Interfaces 60-550 LXI matrix with up to 4,096 www.pickeringtest.com relays in 1U enclosure; up to 512x8 in 64x8 increments; daisy-chain expansion for large matrices; built-in relay self-test 40-585 One of a family of matrices 128x2 (40-584), 32x8 (40-586), 16x16 (40-587); 256 crosspoints; with built-in relay self-test; VISA/IVI drivers 60-770 Family of nonblocking RF matrices 16x(8, 16, 24, or 32); internal termination of unused inputs; loop-thru expansion to max 32x16 Universal Switching SS219A-1206-50A Blocking matrix; G2 series www.uswi.com configurable to 10x10; hot-swappable redundant power supplies; RS-232-C, RS-422A, RS-485, GPIB, LAN I/O VTI Instruments EX71HD LXI Class A mainframe; www.vtiinstruments.com accepts wide range of microwave front-pluggable switches; scan list; exclude list; relay operation counter EX1200-4003 2-wire configurable matrix; expandable to 4x192 or 8x96 in EX1200 mainframe; if power interrupted, all relays return to their normally open state SMP6122 One of several SMIP//switch modules; no unterminated stub effects
Mr. Greenberg explained that the EADS switches often are driven by smart controllers, which afford additional opportunities for monitoring switch combinations. "Exclude/include lists and paths can be defined to prevent the creation of potentially damaging paths. And when safety is an issue, interlock features are provided,"' he said. "For example, the 1220 family of 16-A LXI switch plug-ins is provided with an emergency shutdown input that allows the user to manually or automatically open all power switches."
Keithley's Mr. Meyer said, "Model 3706 Switch System/Multimeter modules have hardware interlocks designed to keep the switching modules disconnected from the system backplane. To engage the hardware interlocks, the user must provide a low resistance path between the two interlock pins. This path routes a 5-V power source to an on-board interlock relay that enables power to the backplane relays."
It's easy to get caught up in test program implementation details, but before buying any switching solution, you may wish to consider how it behaves when the power unexpectedly fails. Many matrices are based on latching relays. When powered normally, latching relays can be a great advantage because they only require a short pulse to change states. On the other hand, should power fail, the last configuration will be retained until power again is applied.
In the event that power was lost coincident with applying pulses to the relays, you have no way to guarantee what state the relays will be in when power is next applied. Nonlatching relays require constant power to keep them activated, but should the power be lost, they all will revert to their nonactivated state. For most matrices, this is an open circuit.
Relay operation counting is built into many matrices. Because relays have a finite life, knowing how many times a relay has been operated can help improve reliability. But, it's not as simple as replacing any relay that has reached more than a certain number of make/break cycles.
Two wear-out factors are at work in a relay. One is the mechanical degradation that occurs from repeated opening, closing, and flexing of the contacts and supporting structure. The purely mechanical lifetime for some relays is listed as >100 million operations. Contact pitting and contamination are much more often the causes of relay failure and may occur after only 100,000 operations or even earlier. It all depends on the relationship between the relay and the signal it switches.
In so-called cold switching, current and voltage are very low or zero so that relay lifetime approaches the mechanical limit. After the new switch configuration has been established, the signal is applied. In contrast, hot switching simply interrupts the signal and often results in arcing and contact wear. Nevertheless, as mentioned in Keithley's Switching Handbook, a relay designed to handle large currents may require a certain amount of switching current to keep the contacts clean. (1)
Relay operation counting allows you to balance relay usage. For example, a test program may work correctly but unnecessarily open several relays between test steps only to immediately close them again for the next test. A review of relay counts can highlight this kind of problem, allowing simple but beneficial test program changes.
Because the switched signal plays so great a part in determining a relay's lifetime, several manufacturers have developed built-in relay test capabilities. Much more comprehensive than counting the number of operations, these test routines find relays stuck open or closed as well as those with high contact resistance.
The NI Switch Health Center included with the NI SwitchBlock incorporates an integrated relay test and can access relay count information. The test algorithm identifies each relay as functional, stuck open, stuck closed, or unverified because of board faults that prevent thorough testing. Pickering's built-in relay self-test (BIRST) tool offers similar functionality.
RF and microwave matrices typically maintain a 50-[OMEGA] impedance and route signals on transmission lines. This means that when a switch pole is open it must be terminated to avoid creating reflections, and many matrices include terminating resistors for this reason.
The need for relatively large electromechanical switches at microwave frequencies increases the difficulty of maintaining signal fidelity through multiple levels of switching. Mike Dewey, senior product marketing manager at Geotest Marvin Test Systems, favors blocking matrices for this reason. "[Multiplexers, star switches, and blocking matrices] all generally offer better bandwidth and crosstalk performance compared to a general matrix topology. For higher frequencies of interest or if you are looking for a controlled impedance path, these types of topologies probably are a better choice," he explained.
Further elaborating on matrix construction, VTI's Jon Semancik, director of marketing, said, "Anytime a crosspoint matrix is used, path stubs introduce reflections. However, tree-based matrices do not create stubs and are well suited to RF and microwave switching. VTI's modular switching systems, such as the LXI EX1200 Series, are scalable so that larger matrices can be constructed by adding more matrix modules. We break the modules into multiple smaller matrices by using stub-breaking relays to improve frequency performance."
The easiest matrices to expand are those that have been designed with an internal analog bus. NI's SwitchBlock, Keithley's Model 3706, VTI's EX1200, Pickering's BRIC, the EADS 1800 Series, and Giga-tronics 4000 Series matrices are examples of multimodule designs. Mechanically, most of these products comprise an outer chassis into which the various matrix modules are plugged. This approach has the advantage of creating a large matrix with only one system address but the disadvantage of proprietary modules.
There are other expansion possibilities. Geotest's Mr. Dewey observed, "One way to expand is simply to daisy-chain the X or Y side of the matrix. However, performance of the overall matrix can be limited. A better way is to use a matrix with bypass and matrix disconnect relays that significantly reduce stubbing effects and help to maintain signal bandwidth."
Agilent's Ms. DeTomasi explained how these kinds of capabilities are implemented in the company's 34934A Matrix: "This matrix is specifically designed for expansion with minimal impact on signal integrity. It features row expansion cables to minimize wiring between multiple modules. Additionally, to keep the capacitance low, the 34934A uses row-disconnect relays to disconnect the matrix modules not being used."
A very wide range of matrices is available to address almost any application. Of course, the challenge is to find the optimum solution from among the many that are possible. It can be complicated.
Bob Stasonis, sales and marketing manager at Pickering Interfaces, cautioned that matrices share the same constraints associated with all relay applications. Some requirements are best addressed by solid-state switching, some by electromechanical relays, and yet others by reed relays. To these considerations you must add the specific needs of the test setup. How many simultaneous paths are used? How can the paths be partitioned? What different types of signals must be accommodated?
Mr. Stasonis concluded, "Selecting a matrix is a balance of specifications, form factor, project life, and budget. For all these reasons, customers unfamiliar with the available choices should work closely with the matrix manufacturer of their choice."
(1). Switching Handbook; A Guide to Signal Switching in Automated Test Systems, 5th Edition, Keithley Instruments, 2006, pp. 4-6.
by Tom Lecklider, Senior Technical Editor
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|Date:||Nov 1, 2010|
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