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The projects in this issue of Nuts & Volts address a range of interests, from rocketry instrumentation and weather, to thermal management and instrument calibration standards. Although the relevance of standards is most evident in Doug Malone’s article on building a voltage reference, each of the projects is a tribute to the necessity of standards in the design, construction, operation, and maintenance of electronic circuitry and instrumentation.
In electronics, standards — which are sometimes expressed in terms of compatibility — range from the thread and diameter of nuts and bolts, logic families, and computer busses, to communications and low-level signal levels. Fortunately for readers, the authors have assumed the responsibility of determining which hardware and software combinations provide the best results. You don’t have to decide between resistor-transistor logic (RTL), transistor-transistor logic (TTL), or complimentary metal-oxide semiconductor logic (CMOS) chip sets, Bluetooth or WiFi wireless communications, or between a PIC and a STAMP microcontroller.
A wireless weather station, for example, may incorporate sensors calibrated to international standards of barometric pressure, temperature, and humidity. These inherently analog signals are typically interfaced to digital hardware and associated software. The cascade of national and international standards involved — from the physical property measured to the signal levels in the microcontroller or microprocessor — is invisible to the casual experimenter. The IEEE alone is responsible for over 1,300 standards in telecommunications, information technology, and related fields (see standards.ieee.org). In addition to decreasing development time and costs, standards increase product quality and safety, and provide a modicum of protection against obsolescence.
Several important standards are readily accessible, even if only indirectly. For example, the National Institute of Standards and Technology (NIST) broadcasts standard time and frequency signals over the web, telephone network, and radio. NIST maintains the primary standard for frequency and time intervals with a Cesium Fountain Atomic Clock in Boulder, CO (see tf.nist.gov/cesium/fountain.htm). You can access time based on this standard over the Internet, accurate to within 0.2 seconds, at www.time.gov. The NIST radio station WWV, known to most radio amateurs and shortwave listeners, broadcasts time signals at 2.5, 5, 10, 15, and 20 MHz. If you have one of those ‘atomic’ clocks or watches, the synchronizing signal is from NIST station WWVB, which broadcasts continuously at 60 kHz.
The Cesium time standard is an example of an intrinsic standard, in that it is based on a constant of nature, as opposed to an experiment conducted in a specific environment. A voltage standard based on a battery, for example, is an extrinsic standard. For this reason, in 1972 NIST moved from a definition of the standard volt based on the Weston cell to the Josephson Volt Standard, which is based on electrical properties of the Josephson junction — two superconductors linked by a non-conducting barrier. While a standard based on a reproducible, solid-state, cryogenic superconductor has been a boon to industry, it is still beyond the reach of individuals. Hence, the value of Doug’s article. Doug’s article, albeit an extrinsic standard, is an affordable source for a standard volt.
Given the readily available, inexpensive digital multimeter, a standard volt may seem superfluous. After all, for $30, it’s possible to obtain a six-digit DMM with a built-in frequency counter. If you own a Fluke, HP, or other quality DMM, you know that part of what you paid for is accuracy over time and with changes in the operating environment. A $30 meter might be accurate to within a tenth of a volt out of box, but six weeks later, in the heat of summer or cold of winter, a measurement might be off by a half volt. Although all meters require recalibration, it’s more likely that the $30 DMM will require more frequent recalibration than a quality DMM. A handy voltage standard, even if it is extrinsic, enables you to recalibrate your inexpensive DMM or oscilloscope.
As you prepare to build one of the projects described in this month’s Nut & Volts, pause for a moment to consider the myriad standards involved, and the limitations of your multi-digit digital test equipment. Understanding the underlying standards can not only make your debugging more productive, but you’ll appreciate the lengths developers have gone through to insure component and system compatibility. NV
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