Designing Ideas

A recent Design Idea entitled "Dither a power converter's operating frequency to reduce peak emissions" has drawn several heated comments from readers regarding the wisdom—or lack thereof—of reducing the EMI signature of an SMPS (switched-mode power supply) by smearing it over a broad swath of spectrum.

Interference production by consumer electronics and electrical appliances has existed since the dawn of the radio age. Certain early-vintage incandescent lamps produced RF that interfered with reception, as did arcing thermostats, neon advertising signs, electric razors, and just about everything that drew juice from the ac line. Some noise sources didn't have to plug in—for instance, automobile-ignition noise plagued television reception in the immediate post-WWII era.

What's new in SMPS design—and, for that matter, in BPL (broadband over powerline) data transmission, frequency-hopping communications equipment, and baseband or extremely wideband systems—is their common tendency to disperse signals and noise over a range of frequencies. We won't see SMPSs go away any time soon, and consumer electronics continue to get smaller and lighter, not to mention cheaper.

Most consumers don't understand how the products they purchase actually work, nor do they appreciate why products don't work as expected when a newly purchased device radiates interference. The average consumer is neither capable of making, nor offered, an intelligent choice between well-shielded product A or marginally shielded product B, which costs $50 less.

My own experience involved a mysterious noisemaker that obliterated a weak AM radio station I enjoyed. The culprit turned out to be a Magnavox VCR that produced wavering noise peaks at multiples of approximately 800 KHz, but only when it was switched off. Fortunately, the VCR developed a mechanical problem, and I had the pleasure of scrapping it and salvaging a few components for other projects.

As long as consumer-electronics prices continue to decline, designers will feel pressure to reduce manufacturing costs, including reduction or elimination of expensive EMI-suppression filters and shields. If a product can only meet its cost goals by using a frequency-smeared SMPS to squeak past EMI regulations, then smear it will.

I'd like to see several things happen. First, let's educate the public that the electromagnetic spectrum represents a natural resource that gets polluted just as air, water, and soil do.

Second, I'd like every EE's college education to include a mandatory course in practical electromagnetism, with an emphasis on interference production and reduction.

Third, I'd like to read your opinions. Should EDN continue to publish Design Ideas that cite spread-spectrum power conversion? Would the design technique in question be acceptable if the power supply were packaged in a shielded, filtered enclosure? Do we really need all of the wireless gimmicks and gimcracks that are reaching the market? At what point does individual convenience and profit override the value of a clean common resource?

The foregoing polemic leaves little space for exploring the latest batch of Design Ideas, but if you're looking for a low-cost bit-error-rate (BER) tester (or an earlier version), or are interested in using DMA to speed up a waveform generator, or you need a snazzy high-performance, high-frequency current source, seek no further.

And if you'd like to explore some quirky EMI problems and their solutions, check out the "Banana Skins" section of the Compliance Club's online site.

73 for now.

—Brad