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All about switching matrices.

A switching matrix used in a test system connects a signal on a particular column to an instrument on a certain row. Typically, there are more signals than instruments, so several voltage measurements might be made by successively connecting a DMM to different points in a circuit. The number of rows N and columns M defines the size of the matrix as NxM. By convention, rows are listed before columns.

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.

Control

Configuration

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.

Maintenance

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.

Other Considerations

Fidelity

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."

Expansion

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."

Summary

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."

Reference

(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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