Switching augments instrument systems
Rick Nelson, Chief Editor -- Test & Measurement World, 9/1/2006
Switching Handbook: A Guide to Signal Switching in Automated Test Systems, 5th ed., Keithley Instruments (www.keithley.com), 2006. 190 pages. Free.
Do you need a switching system? Maybe not if your DUT has only a few I/O pins or you have the budget and lab space necessary to assign an individual instrument to each of the possibly hundreds of signal lines you need to drive or measure. In general, though, you'll find it desirable to employ switching systems to effectively deploy your test equipment in automated measurement applications.
But knowing that you need a switching system is far different from knowing which system to use. Switches can only approximate the ideal: infinite open resistance, zero closed resistance, zero switching time, and zero crosstalk with neighboring devices. Keithley Instruments' Switching Handbook is a concise compendium of the information you need for choosing wisely.
If you are new to switching, you'll find Chapter 1's overview of switch topologies (scanning and multiplex switching, for instance) and its discussion of special RF signal-switching requirements to be helpful. So, too, will you find useful Chapter 2's description of electromechanical, dry-reed, mercury-wetted-reed, and solid-state relays. Chapter 3 goes into detail on how to specify switching systems, covering topics ranging from insulation resistance to voltage standing-wave ratio. Chapter 4 explains how real-world uncertainties related to factors like contact potential, switching speed, and cold vs. hot switching can affect your measurement accuracy or switching-system performance.
An extensive section (Chapter 5) is devoted to describing switching system requirements based on the type of signal you want to switch (voltage or current, low frequency or RF, for instance) and the type of load (resistive or reactive) you intend to connect to your switch output. The subsequent chapter provides details on the wires and cables you'll use to connect your switch, instruments, and DUT.
The final chapter describes several specific applications: battery charge and discharge measurements, capacitor leakage tests, continuity tests, temperature scanning using thermocouples, and diode testing. It goes into detail about measurements specific to printed-circuit boards, accelerated cell-phone life tests, and VCSEL testing. The book also provides a handy glossary.
Like other free Keithley handbooks, this book includes vendor-specific examples, such as using the Keithley System 40 Microwave/RF switch system in the cell-phone life-test example. The book also provides an appendix describing the company's switch cards and modules, but it provides more than enough generic information to make it worthwhile regardless of where you get your switching systems.
