by Bryan Bergeron, Editor
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
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