10 tips for building a switch system
Luke Schreier, Product Marketing Manager, Switching, National Instruments -- Test & Measurement World, 12/1/2005 2:00:00 AM
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| Routing databases like NI Switch Executive make it easier to manage hundreds or thousands of switch channels. |
Manufacturers of automotive and aerospace components face the challenge of developing complex test systems that must last for decades. As technologies converge in increasingly elaborate devices, a test system's capabilities may extend from basic voltage, current, and resistance measurements to audio, video, and RF measurements, and the tester may be expected to have a 10- to 20-year life span. These factors put a huge burden on the switch system to manage and route each pin on a device under test (DUT).
Most manufacturers have added capabilities to their switch systems that make it easier to implement advanced switching topologies with extended lifetimes. It is important to take advantage of these capabilities, but you also need to watch out for the traditional "gotchas" in switch-system development. Here are 10 tips that can help you develop a "future-proof" switch system that will last as long as you need it.
1. Rethink your choice of relay
In the past, armature relays were the only option for high-power signal switching, but they were physically large and had short life spans. These relays have improved during the past decade as new manufacturing techniques have reduced their sizes and increased their lifetimes to approach 1 billion cycles, but they are no longer your only option.
Reed relays have become an acceptable option for high-voltage switching and are typically an order of magnitude faster than armature relays. Solid-state relays and field-effect transistors (FETs), which have typically been used in infinite-life, low-power switching systems, are also another option for high-power systems with low on-resistance implementations.
Rethink what you knew previously about relays, and you are likely to find a better solution.
2. Understand safety considerations
Government programs typically require levels of certification for all commercial off-the-shelf hardware, and switch systems are no different. When selecting a switch system, consider only those products that comply with emissions, immunity, CE, and high-voltage safety (UL, VDE, and others) standards.
Also, when you need to connect switching signals to mains supply circuits (for example, wall outlets), pay attention to the module measurement category rating, or CAT rating, because supply circuits can have transients far above their working voltages. A CAT II rating is required for switching any 120-V/240-V wall outlet signal.
3. Decipher specifications accurately
Look past the marketing description of a product to find out how it compares to the competition. This is especially true in RF switching applications, where the term "bandwidth" is often interpreted differently. While 3 dB may be the right way to rate a digitizer's bandwidth, often 1 dB or 1.5 dB is more appropriate for the insertion loss of a switch (although not every vendor specs in that way). And even if a switch has excellent insertion loss, the voltage standing wave ratio (VSWR) or crosstalk spec may not be acceptable at the rated "bandwidth."
4. Use a routing database
Many switch systems can have hundreds of channels in each module. The system programming will be greatly complicated if you refer to each channel by its hardware name or a binary representation, and future maintenance will be almost impossible.
Use a database or look-up table to store route information and provide aliases for rows, columns, and channels. NI Switch Executive (figure) is one example of switch-management software that can store all of the channel aliases and routes in a system.
5. Take advantage of debugging options
Most vendors provide some simple way to close a relay without requiring you to write a program, whether it is through a Web interface, simple application software, or front-panel controls. Take advantage of these interfaces to verify functionality or to monitor relay states during execution. The shorter your "time to first click," the faster you can set up and debug a switch system.
6. Choose a mass interconnect
It can be aggravating to find the perfect switch configuration in the form factor you want only to learn you can't connect the signals to the DUT in the way you expected. Generally, the higher the switch density of the module, the more challenging the connectivity becomes. You can take advantage of commercially available mass interconnect solutions from vendors such as Virginia Panel and MAC Panel to improve the lifetime of your connectors (and the cleanliness of the connections).
7. Track relay lifetime
Mechanical relays often are rated in excess of 1 billion cycles. Instead of attempting to keep track of expected lifetimes through a calculator or ignoring the number of cycles completely, choose a vendor that includes relay lifetime counting in the switch hardware. Then, with a simple function call, you can find out how soon you will need preventive maintenance.
8. Maintain your switches
Once your relay approaches the end of its rated lifetime, how easy is it to replace? You should be able to easily swap out an old relay for a new one, so make sure your vendor provides the methodology to do this, especially on high-density modules that may have complicated assemblies.
9. Combine the switch and test platforms
The advantages of using relay modules in the same platform as measurement hardware are well-known: smaller physical size, common bus connectivity, and reduced cable lengths between instruments and switches. You can reduce large amounts of integration work by using a common platform such as VXI or PXI. VXI has long been successful as a modular instrument and switching platform, and the growth of PXI over the last five years ensures a common multivendor platform on which to build an ATE system.
10. Plan for the future
Whichever platform you choose, assume your test system will not last forever. You will likely need to either upgrade or augment portions of it through multiple product iterations, so make sure the switch vendor has the breadth of switch offerings to get you through future projects. You can lose valuable time learning how to set up, program, and maintain switch hardware, only to have to start all over again with a new vendor the next time.
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