EMC: THE PROBLEM
Wireless sensor networks by their nature are designed to be
used with other circuits and assemblies — be they sensors, actuators, or
computers — or installed with other systems, such as personal digital assistants
(PDAs), cellular telephones, medical devices, consumer electronics, and
industrial equipment. An important part of their design, therefore, is their
ability to work well in a variety of electromagnetic environments, consisting of
both intentional and unintentional radiation, as well as in the presence of
noise on their supply, ground, and signal lines. In addition, most electronic
products must meet regulatory requirements limiting their radiated emissions
below a specific limit — in the United States, this requirement is in Part 15 of
the Federal Communications Commission (FCC) regulations — so that other devices
near them may achieve at least a modicum of interference protection.
An example of this type of EMC issue is the consumer television
receiver. Bringing an NTSC television receiver near the antenna of a shortwave
radio receiver can result in interference every 15.750 kHz up to 20 MHz or more;
these "birdies" are harmonics of the receiver's 15.750-kHz horizontal oscillator
that drives the electron beam of the picture tube across the screen. As will be evident, the waveform of the
horizontal oscillator can be a particularly troublesome one; it is a sawtooth
waveform with a necessarily sharp transition as the electron beam retraces its
position after filling in a line on the screen.
The effect of the television receiver on the shortwave radio
receiver has been termed "desensitization" — a reduction in apparent sensitivity
of the shortwave receiver, caused by the television interference. A general
definition of desensitization could be "a reduction of apparent receiver
sensitivity, caused by interference from nearby sources." (Interference may also
affect transmitters, by corrupting, adding noise, or otherwise degrading a
transmitted signal.) While the television receiver is a practical example, in
modern systems desensitization usually occurs due to the presence of digital
waveforms associated with microcontrollers, and the high-energy,
fasttransitioning waveforms associated with switching power
converters.
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