Signal Switching Via Plug-In Cards
by Tom Lecklider, Senior Technical Editor
Automated testing provides a programmable
means of powering a DUT as well as connecting selected points to
signal sources and measuring instruments. Often, more signals
must be measured than there are instruments so a network of
switches is inserted to correctly organize and sequence through
the desired test configurations.
The types of switches used and their
interconnections are critical because they directly affect
signal integrity. Not surprisingly, relatively large armature
relays handle power switching at low frequencies, smaller reed
relays or FET and solid-state switches are used for
general-purpose applications up to a few hundred megahertz, and
conventional, large coaxial relays are found at RF and microwave
frequencies.
PXI 6U GX6616 High-Density Switch Matrix
Courtesy of Geotest-Marvin Test Systems
Designs based on FETs and solid-state
switches continue to improve performance and compete with reed
relays in more areas each year. Nevertheless, relays remain by
far the most common form of switch used in automated test.
Selecting the best switch form factor for a
test system may not be straightforward. On the one hand, if most
of the power and signal sources and measuring instruments are
VXI or PXI compatible, using one or more switch modules with a
matching form factor probably makes sense. You already need a
chassis so it can be very economical to use a slightly larger
one that also will accommodate the switches.
ATE companies such as Teradyne and EADS North
America Defense Test and Services Division often incorporate VXI
or PXI instruments in larger test systems. These form factors
are popular among integrators of both commercial and military
test systems because a wide range of functions is available and
interoperability is guaranteed among conforming devices.
On the other hand, depending on the switching
network required, you may find that overall performance
is lower or the cost is higher for a group of interconnected
modules than for
a custom solution. Several vendors provide proprietary switching
systems that can be configured exactly to fit your application.
For example, Cytec Sales Manager Nick Turner
said, "Although the company will continue to support ISA, VME,
VXI, PCI, compact PCI, and PXI switch cards as it has in the
past, Cytec’s emphasis in the future will be on building 19"
rack-mount systems. These generally are on a larger scale than
normally is cost-effective using many of the modular switch card
formats."
Agilent Technologies, Precision Filters,
KineticSystems, Pickering Interfaces, VXI Technology, and
Universal Switching provide several proprietary ranges of large
switching systems. It can be especially difficult to accommodate
a complex coaxial switching system in one of the smaller form
factors simply because of the size of the relays, cables, and
connectors. Also, there are signal integrity as well as size and
cost advantages associated with large switching matrices
constructed in a single chassis.
The growing popularity of Ethernet-based LXI
instruments has allowed manufacturers to separate form from
function to a large degree. LXI does specify certain indicators
and connectors for a device but not chassis size. Because they
are LXI compatible, proprietary switching systems made by
Agilent Technologies and Keithley Instruments might be good
solutions for your application.
These companies offer switching as part of a
larger system capability. For example, Agilent’s Model 34980A
Multifunction Switch/Measure Unit has a built-in 6½-digit DMM
and eight uncommitted slots that accept any of 20 different
plug-in modules. Similarly, Keithley’s Series 3700 System
Switch/Multimeter integrates a 7½-digit DMM into a six-slot,
2U-high, rack-mount chassis. These systems are claimed to be
typically lower cost than a PXI system with similar
capabilities, and the DMM is included.
Pickering Interfaces has approached the
switching problem from a different direction. Because the
company already had developed an extensive range
of PXI-compatible switching products, it created a series of LXI
modular switching chassis into which you
can plug a Pickering PXI 3U switching card. The chassis performs
the LXI-to-PXI translation and supports Ethernet communications.
VXI and PXI Switching Modules
The comparison chart accompanying this
article focuses only on VXI and PXI standards. VXI is now 20
years old, and a very large number of modules are available in
this format. PXI is 10 years old, but more than 1,000 different
modules have been introduced by various vendors.
To view the comparison chart click here.
Tom Sarfi, business unit manager at VXI
Technology, explained that VXI modules can be more
cost-effective than PXI, especially where large currents, high
voltages, or multiple RF circuits are involved. He said, "We can
accommodate 1,000-V multiplexers, up to 30-A switching, and
26.5-GHz relays in a reduced footprint at an overall lower cost.
The larger module area and greater center-to-center spacing
permit one module to support up to six SP6T 26.5-GHz relays."
Typical VXI modules are 6U high and offer a
much larger PCB area than the usual PXI 3U modules.
Nevertheless, because of changes in technology, many functions
that previously required a complete VXI C module now are
available in the PXI format. In addition, PXI also offers a 6U
size, and this format is especially useful for large switching
solutions such as matrices that benefit from having more
elements on one PCB.
PXI 3U NI PXI-2535 544-Crosspoint FET Matrix
Courtesy of National Instruments
Competition between the two standards is influenced by the test systems already used within a
company. For example, much of the early adoption of VXI was
among military contractors, and new modules still are being
developed for these end users.
Charles Greenberg, senior product marketing
manager at EADS, said, "In 2007, we demonstrated a new suite
of [VXI] switching cards and software that can run legacy ATLAS
Test Program Sets (TPS) from four widely used military test
systems: the U.S. Army IFTE V6, the U.S. Navy CASS, the U.S. Air
Force ESTS, and the U.S. Marine Corps TETS/Viper/T. All four
were run from a single test system, the ARGCS ATS-1
demonstrator."
He explained that each program required
slightly different switching configurations so each card had to
be able to morph itself into the correct configuration to meet
the specifications for any given program. This was accomplished
by using small switch modules that could be reconfigured as
required. For example, CASS needed control of a coaxial
multiplexer down to a 2:1 level. The IFTE and ESTS TPS worked
with only 4:1 granularity and TETS only 8:1 for most of the
signals.
The software used a common IVI switch
interface but separate translation wrappers for each TPS. The
IVI interface then could drive the reconfigurable switch card to
create the required set of multiplexers. Although a switching
card only switches, the concept of satisfying several
instrumentation requirements with one piece of hardware is in
the same spirit as the broader synthetic instrument initiative.
EADS provides many types of VXI and PXI
switch cards as well as proprietary cards that are used with
carriers such as the Model 1260-100X Adapt-a-Switch®
Carrier. It accommodates up to six switch cards but occupies
only two VXI slots.
The X-Series Carrier is split both vertically
and horizontally to house up to four plug-ins, some containing
two switch cards. VXI Technology has a similar SMP1200 Carrier.
In contrast, ASCOR’s Series 4000 Carriers occupy from three to
eight VXI slots and house from six to 17 switching cards. The
cards are 10.75" high, extending nearly the full height of a VXI
6U slot, but mount on a 0.5" or 0.6" pitch compared to the
standard 1.2" VXI slot pitch.
Carriers are available for both PXI and VXI
systems and essentially are subchassis preconfigured to
interconnect a number of smaller switch cards. In addition to
allowing a level of mix-and-match customization among types of
switches, they electrically and physically concentrate the
connections from several cards so that fewer main chassis slots
are required.
In an attempt to reduce the number of modules
needed, Geotest-Marvin Test Systems has introduced the Model
GX6377 Multi-Function Relay Card. Mike Dewey, the company’s
senior product marketing manager, commented, "This product
supports several channels of 10-A switching, 2-A form A and form
C switching, and two 16x2 configurable relay matrices, all in a
single-slot 3U module. The result is a switch module that offers
customers the means to configure a test system using only one
instead of several switch cards. The test system can be more
compact and potentially simpler as well."
Signal Integrity
Switching module manufacturers all are
benefiting from the reduced size of modern components. EADS’s
Mr. Greenberg cited the availability of relays as small as 25%
the size of previous models with the same voltage, current, and
DC power switching capabilities. In addition, highly integrated
FPGAs and relay drivers are allowing more channels than ever to
be packed into a switching product.
Of course, merely mounting hundreds of relays
on a PCB doesn’t make a switching module. Mr. Greenberg referred
to new EDA software that gives designers advanced modeling tools
to help optimize bandwidth, crosstalk, and isolation. He added
that the larger VXI PCB area can improve switching-channel
density and may be more appropriate for higher frequency and
higher power applications. In his view, PXI is better suited to
lower power and smaller applications.
ASCOR’s President Jeffrey Lum explained the
design principles his company uses, "We place relays on a PCB as
part of a transmission line. The signal paths are designed as
50-Ω lines with the return path matched to the signal line to achieve
high bandwidth. The analog, digital, and chassis grounds are
kept separate to preserve bandwidth and increase isolation from
conducted ground noise. This treatment is continued up to the
front panel with return-path connections included on the same
connector as the signal connections."
High-Density Connections
The switching module manufacturer only has
control of that product’s hardware design. Connectors are
provided on the module, but it’s up to you to cable between the
switch and the other elements in the overall test system.
Depending on the types of connectors and number of channels
involved, this may not be entirely straightforward.
VXI C2-Size SM7000N Microwave Switch Base Unit
Courtesy of VXI Technology
Geotest’s Mr. Dewey commented, "When a switch
card is selected, the buyer often has placed little emphasis on
connecting it to other instruments or the UUT. Many PXI 3U
switch cards will use a high-density SCSI style connector to
provide lots of I/O in a small space. But trying to interface to
this connector is a problem.
"Interfacing to these cards requires you to
purchase special cables or breakout boards, which drives up the
cost and complexity of the test system," he cautioned. "You
really need to think about how easy or difficult it will be to
interface to a switch card. This is the reason why Geotest
switch products only use sub-D style connectors. We want to make
sure that users can easily and reliably fabricate interfacing
cables for our switch products and don’t need to buy special
cables."
National Instruments (NI) Switches Product
Manager Jaideep Jhangiani also commented on a module’s
connectivity: "Occasionally, the connectors and cables limit
switch module functionality. Mid-frequency modules rated for
20 MHz to 100 MHz, for example, often are used with cables that
have a much lower frequency response. In such a case, the cable
is a bottleneck for the entire switch system.
"Users should be aware of the challenges
associated with building high-density switching solutions," he
continued. "They need to understand the different connectivity
options available for PXI modules. These include cable fixtures,
front-mounting and external terminal blocks, ribbon cables,
backshell/connector kits, and discrete cables. Knowing the pros
and cons of these connectivity options helps users choose the
best switching modules for their systems."
NI offers more than 20 RF multiplexer,
general-purpose, and matrix PXI switch modules ranging in
bandwidth from 500 MHz to 26.5 GHz. These products feature relay
count tracking and low insertion loss at the specified
bandwidth. Because relays are mechanical and eventually wear
out, knowing the number of closures a relay has performed helps
users to estimate the remaining lifetime and avoid unexpected
system downtime.
Many very dense modules such as NI’s
544-crosspoint PXI-2535 and 2536 matrix switches are based on
FETs rather than relays. FETs provide unlimited lifetime,
unlimited simultaneous crosspoint connections, and switching
speeds as high as 50,000 crosspoint connections/s.
Summary
Switching solutions have been developed for
almost any application you may have. This means that developing
an automated test system involves an element of choice. And, as
in most engineering work, initial decisions can have a large
effect on system flexibility, cost, and performance.
VXI and PXI switching modules are featured in
the comparison chart, and many more models are available. On the
other hand, neither of these formats may be the best for your
test system.
If parts of the test setup are separated by
large distances, an LXI system may be more appropriate. Ethernet
with IEEE 1588 timing can synchronize widely separated
instruments to within tens of nanoseconds. LXI instruments also
can communicate on a peer-to-peer basis, which generally is
faster than involving a central controller in each transaction.
Perhaps the required type of signal
conditioning or current or voltage handling capability only
exists in one format. In that case, you need to determine
whether it is cost-effective to develop your entire test system
in that format or if a hybrid approach makes more sense.
Determining the best overall system design can be difficult when
switching is involved.
Except for compact matrices and small circuits such as individual coaxial switches or one- or
two-level multiplexers, custom switching chassis may provide
better performance than circuits assembled from either VXI or
PXI modules. For very large circuits such as big matrices,
custom switching chassis also may cost less and have a smaller
footprint.
VXI Technology’s Mr. Sarfi commented, "Larger
switch systems that bridge together many modules using extensive
cabling can introduce added insertion loss, capacitance, and
propagation delays, which ultimately affect the integrity of the
signals passing through the system. Analysis prior to build
helps to determine how much this will affect test reliability."
However, electing to use a larger custom
switching system involves other considerations. You must
integrate switch control with your overall test program so an
amount of software development may be necessary. Also, future
test system capabilities must be supported. How readily can the
switching network be modified and expanded as your needs change?
How much will additional engineering development cost to
accommodate the changes?
Many of these factors are reduced or
eliminated if a test system is constructed within a single
format such as PXI or VXI. All the cards conform to a common
standard, are guaranteed to work together, and plug into the
same chassis. The job of test system development can focus more
readily on testing the DUT rather than on distracting hardware
and software incompatibility issues. And, you have the choice of
modules from a large number of manufacturers rather than being
tied to a proprietary solution with limited availability of
functions.