When compared to 10, five, or even one year ago, there are major differences in clock frequencies of computers and carrier frequencies of communications equipment for the products now being tested by ATE. Fortunately, there are differences in switching technology to accommodate these changes.
Sure, the enclosure that holds the switch modules may look the same, and some of the earlier modules may work in today’s systems. But when we look behind the front panels, we find new components to handle today’s requirements, even at microwave frequencies. Yes, including fiber-optics lines. And operation is faster.
All this has come just in time. Nick Turner, marketing manager at Cytec, noted, “The big growth in automatic switching over the past few years has been in communications. Five years ago, people didn’t need operation above 400 MHz, but now we sell equipment for switching up to 26 GHz.
“Future growth will be in wide-bandwidth fiber-optics and wireless systems,” he continued. “Wires just can’t handle the bandwidths that are needed. Standards such as Bluetooth, the new short-range 2.4-GHz communications technology, will open the door to a whole new array of devices, and they all must be tested.”
Unquestionably, switching speed is an issue, as stated by Kevin Leduc, product marketing manager at Racal Instruments. “People want to deliver more information faster, and they look to the switching system as a means for evaluating their systems. There is no other way to perform high-speed performance verification of such sophisticated equipment.”
New Switching Technology
In every aspect of switching development, we see new and exciting products. As Mr. Leduc pointed out, “Small light-switching units enable us to penetrate the broadband fiber-optics area. Cutting-edge technology and innovation are fueling phenomenal growth in this communications medium, and automatic testing of components now is enhanced by high-speed switching.”
Solid-state switching components also are making a big impact on the industry, especially in application-specific operations, noted Cytec’s Mr. Turner. “The use of switch chips in low-voltage, low-current data acquisition systems has made a big impact. Solid-state devices have taken over in digital communications, video, and low-power RF equipment. Armature relays also have improved. They are smaller, more reliable, faster, and less expensive, and they provide our best solution for high-power applications.”
Another area of change is the control bus. “People who wrote off VXI and VME as being too expensive are taking a new look now that CompactPCI is in the picture. Also, we see interest in the Ethernet LAN interface or the USB,” Mr. Turner concluded.
The designer can put many more switching elements in a given area now than 10 years ago, noted Shilin Jie, a switching engineer at Agilent Technologies. “This reduces system size and wiring complexity and allows more functionality to be added to the switch modules. It also decreases stray capacitance and improves accuracy,” he said.
Some industry leaders are enthusiastically greeting the standardization of software for switching systems and other applications. “The Interchangeable Virtual Instruments (IVI) Foundation is setting standards for drivers,” said Mr. Leduc of Racal Instruments. “These will provide interchangeability of products from multiple vendors and improve system performance.”
A Workable System Layout
All these new developments have not made switching-system layout any easier for you, but a step-by-step approach will help. When you design a switching system, start with a schematic drawing of what you need. This forces you to think the problem through, and the sketch is a handy document to use in talking to prospective vendors.
Your topology can be divided into simple switches, multiplexers, and matrices. The simple switch can be used in a wide range of functions, from applying power to forming a complex matrix. The common configurations are form A (normally open), form B (normally closed), and form C (double-throw). You can link these together to form binary switching networks of various types.
The multiplexer, the more versatile and most commonly used switch, connects one signal at a time. Typically, this is a make-before-break arrangement available in one-wire, two-wire, or three-wire configurations. It is the most economical switcher.
The crossbar matrix switch is the easiest to specify and allows any input or group of inputs to be connected to any output or group of outputs. The matrix requires many switch points, and this makes it the most expensive way to configure your system. However, if you need more than one instrument connected to the same test point at the same time, this may be the best way to do it.
After the basic layout is complete, review and explore ways to simplify the arrangement. This can reduce the cost of the system by as much as 50% in some cases and may decrease your product test time. “The ideal switching system should be completely transparent to the end user when it is functioning properly. The simpler the system, the easier it is to attain this goal,” said Mr. Turner of Cytec.
Selecting Equipment
After system layout is optimized, start your equipment selection by picturing the ideal switch. This device has zero resistance and no degradation of the signal at your operating frequencies when it is closed. It has infinite resistance and no shunt capacitance when it is open, with complete isolation of the signal from all other signals. Add to those features small size, a long life, and a reasonable price.
The perfect switch doesn’t exist, of course, but you can look for the nearest equivalent. Keithley Instruments, for example, offers 75 or more types of switching modules, and one of them is likely almost perfect for your application.
Investigate before you buy. Paul Dhillon, executive vice president of VXI Technology, noted that engineers often ask these questions when evaluating a switching system:
How many closures per second can I expect? This includes software overhead that can be as much as five times the hardware settling time.
What choices do I have in modules? Select the optimum system architecture at the most reasonable price.
What are my programming and control options? The possible interfaces are GPIB, PCI, VXI, Firewire, and USB. Software includes Visual C, LabVIEW, and HP VEE.
How much space does the switching system occupy?
What are my cabling/wiring options to the UUT? Since most interfaces to the UUT go through the switch, this becomes a critical issue.
- How does the switching system protect my signal integrity?
Switching Systems
Switching Mainframe
The HP 3499A Switch/Control Mainframe is a full-rack-width unit that holds up to five plug-in switch modules routing up to 200 channels. With parallel drive capability, it opens or closes up to 50 switches simultaneously in 25 ms. With 19 modules from which to select, the system switches DC to 26 GHz, 1 mV to 250 V, and 1 mA to 5 A. The computer interface is RS-232-C or GPIB. $2,112. Agilent Technologies, (800) 452-4844, ext. 6699.
Video/RF Switch System
The VDX-Series Solid-State Switching System has 1 × 18 through 16 × 16 multiplexer or matrix configurations for high-speed video, T3, or low-power RF up
to 200 MHz. On each system, any input can be routed to any output or to all outputs. Impedance is 75 W with a 50-W buffer available. The 2U switch housing fits a standard rack. The control input can be RS-232-C, GPIB, LAN, USB, or a keypad. $2,900. Cytec, (716) 381-4740.
Wide-Bandwidth Card
The 7038 Multiplexer Card used in the 7001 or 7002 Switching Mainframe has three 1 × 4 multiplexers. It can be used as three switches or configured to make a 1 × 10 switch. The relays have less than 3-dB loss at 2 GHz in each signal path. Actuation time is less than 6 ms, and the switch can handle 10 W at 1.2 GHz. $1,495. Keithley Instruments, (800) 552-1115.
Switch Controller
The Option 01T Switch Controller installs in any 1260-Series Switch Module, including the 1260-100 Adapt-a-Switch™ carrier. It provides two-way communications with eight TTL trigger lines on the VXI bus. The controller has message-based and register-based control of up to 12 switch modules and verifies correct relay operation after each switching command. It accepts SCPI and IEEE 488-2 instructions. The response time is less than 9 µs. Call company for price. Racal Instruments, (800) 722-2528.
VXI-Based Switch Family
The Switch Modularity and Interface Platform (SMIP™) Modules operate on the VXI bus and are driven by VXIplug&play software. Three groups in the family switch DC to 4 GHZ, microwave from 1 GHz to 18 GHz, or fiber-optics from 780 nm to 1,650 nm. The first group includes power, matrix, coaxial, and general-purpose switches. Nonvolatile memory on each module stores a serial number, maintenance schedule, or other housekeeping data. Call company for price. VXI Technology, (949) 955-1894.
VXI Switch Matrix
The 3000-515 is a 16 × 12 switch matrix that suppports a worst-path bandwidth of more than 800 MHz and offers typical performance exceeding 1.1 GHz. The three-wide module is shipped with a VXIplug&play driver written in C++ under LabWindows/ CVI. Since the source code and the DLLs accompany the driver, you can recompile under LabVIEW. The matrix incorporates 50-
W coax cabling with matched impedance. The module also is available with 75-W
cabling. Call company for price. ASCOR, (510) 490-2300.
Copyright 2000 Nelson Publishing Inc.
February 2000