Use aluminum foil in troubleshooting
Kenneth Wyatt- July 23, 2012When assessing a new prototype for the first time, I'll first pull off all the I/O cables so I can initially obtain a baseline emissions profile of just the product enclosure. This allows me to characterize the enclosure leakage directly without the effect of cable emissions.
Some poorly-designed products will invariably have multiple gaps, seams and apertures for keypads and LCD displays. Prior to radiated emissions testing, I'll examine the seams and gaps with a near-field probe and use a marking pen to record the dominant leakage frequencies and the measured lengths of the gap. If the gap or seam is more than 1/20th of a wavelength at the dominant frequency, than it's possible it will cause emissions. Of course, the worst-case is if the gap or seam length is 1/4 wavelength and then it becomes an efficient antenna.
The usual tool for dealing with seams and gaps is copper tape, but what about the apertures used for LCD displays or keypads? In that case, I use sheets of aluminum foil to cover the display or front panel keypad. If, however, the product has so many seams, gaps and apertures that copper tape doesn't help, I'll resort to covering the product completely with foil, as shown in the picture. This will also tell you if the line cord or power connections are the culprit and you can focus on that first. Many times, it's easier and more efficient to start with a completely shielded system and open up one area at a time with an Exacto knife, while watching the emission level.
Sometimes (occasionally out of desperation) it's more efficient to cover a product in aluminum foil to better assess emissions by eliminating variables. See photo.
Another use for aluminum foil is for subsystem shielding (obviously, being careful to avoid shorting connections. By first covering the sub-system, cable or PC board with plastic Saran wrap, or equivalent, and then covering with foil, you can potentially isolate internal couplings that may be transferring energy outside the product enclosure.
Aluminum Foil Saves the Day - Here's an interesting real-world application where aluminum foil saved the day for my son and his team of science students. The team had concocted a rather complex "Rube Goldberg" machine that demonstrated several physics principles. This machine was entered in a contest with other schools at the local university here in Colorado Springs.
Part of the machine depended on a simple 555 timer circuit. When they ran it through its cycle prior to the judging, however, the timer appeared to be intermittent. Sometimes it would work, but mostly, it triggered too soon, ruining the cycle of events. I realized that something was interfering with the unshielded timer circuit and also realized there were three large mobile phone towers just outside the building where the judging was to occur.
We had only a few minutes to come up with a solution, so thinking quickly, we ran over to a candy machine, bought several Hershey chocolate bars and removing the wrapper, carefully peeled away the thin foil. We took the pieces of foil, and carefully wrapped them around the timer circuit, and finally got it to work reliably. We celebrated later by eating the chocolate!