In my never-ending quest to search out useful reference books on EMC, I recently ran into the 3rd edition of “The ARRL EMC Book” (ISBN 9780872590915). First published in 1999, the new 3rd edition was released in 2010. For those unfamiliar with the ARRL (Amateur Radio Relay League), this is the national organization representing amateur radio operators (“hams”) in the U.S. Most hams are members of the league, which also publishes a range of useful operating, design and general radio reference books. Because hams are allowed to operate their two-way radios at up to 1.5kW, on occasion, this may be the cause of local interference to poorly-designed or poorly-shielded consumer products. Thus, several years ago it was decided to publish a reference book on RFI (radio frequency interference). In this article, I’ll describe the most important content and why you might want to buy it. More…
There are a number of engineering tools that work well for simulating EMC scenarios. One of these is MathCad, a tool that’s been around for a number of years. The neat thing about MathCad is that you can define a set of equations in free-form layout and plot out the analysis in several engineering-type graphs, including logarithmic and polar. These plots are easily copied and may be pasted into documents or technical papers.
As an example, I’ve calculated the harmonic content of a square wave given user-defined fundamental frequency, rise times, pulse widths and duty cycles. It’s very instructive to run through several scenarios prior to building hardware. By changing any of the user-defined parameters, you can quickly judge the outcome. More…
There are times when an increase in harmonic content can’t completely be explained by circuit or PC board design. If you’ve already done a good EMC design and are still getting radiated emission problems, then perhaps resonances in the product enclosure are, in effect, amplifying the internal harmonics. This internal amplification can cause a myriad of mysterious couplings internally to your product with resulting radiated emissions.
Any metal structure can become resonant if driven by a noise source. For example, I’ve seen the tines on a microprocessor heat sink resonate in the 2+ GHz region. More commonly, you’ll discover resonant modes created by the product enclosure. For example, for a rectangular enclosure, we have:
Where: epsilon = material permittivity, mu = material permeability and m, n, p are integers. Cavity resonance can only exist if the largest cavity dimension is greater, or equal, to one-half wavelength. Below this cutoff frequency, cavity resonance cannot exist. In this configuration (where a < b < c), the TE011 mode is dominant, because it occurs at the lowest frequency at which cavity resonance can exist.
The resonant frequency of the circular cavity is 1.225 GHz, very close to the calculated 1.274 GHz.
To read more about constructing a simple demonstration of resonance, click here…
After noticing the continued banter and discussions regarding the use of guard traces in both the EMC and SI discussion forums over the past months, I decided to consult a couple experts on the subject – Howard Johnson and Eric Bogatin. I summarized their thoughts in my latest blog posting on the Test & Measurement World web site: http://www.tmworld.com/blog/The_EMC_Blog/41806-Guard_Traces_Use_Em_or_Not_.php. I invited both to add any additional comment, if they wished. Feel free to add to the discussion.
Guard traces are typically grounded at both ends to the signal reference plane. There are certainly situations where guard traces can help. For example, for low-frequency audio – especially for two-sided board designs, guard traces can reduce crosstalk by an order of magnitude. However, on modern-day high-frequency digital designs, guard traces may help, but only if implemented correctly.
Crosstalk occurs when the magnetic lines of force pass from the aggressor trace under the victim trace. In other words, the lines of force must encircle the victim (Faraday’s Law). As the distance between the aggressor and victim traces increases, the coupling decreases, as you might expect. Henry Ott, in his latest book, Electromagnetic Compatibility Engineering (2009), summarizes succinctly when he states, “crosstalk between adjacent microstrip traces is proportional to the square of the trace height divided by the square of the separation distance.”
There’s a great article by SI expert, Howard Johnson, explaining how guard traces work. For more detailed info on the pros and cons of guard traces, please refer to the T&M World link above.
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