An EMC troubleshooting kit - part 1a (emissions)
If you’re an EMC engineer or independent EMC consultant, you’ve probably amassed a collection of troubleshooting tools and accessories. Once you build up some expertise in this area, you may find yourself in demand - especially, if there are many projects or client jobs, ongoing. In most cases, the project can come to you for testing or troubleshooting, but in a few cases, you might find it expedient to do the troubleshooting where the project/prototype (or client job) is located. I’d like to show you what I’ve done to consolidate all this gear into one roller case, such that on a moments notice, I can dispatch myself right to where the problem is. You consultants may find this especially intriguing.
The basic idea for the roller case was borrowed from my good friend and fellow consultant, Doug Smith (www.emcesd.com or www.desertlabrat.com will also get you there). I’m using a Pelican 1510 roller case with the optional pocket insert for the lid. You can often find these discounted at www.pelicancase.com. The case is not only sturdy and waterproof, it’s lockable and will fit in the overhead bin of the larger aircraft. Although, I should note that it may be more difficult to get it through inspection as “carry on” these days. I usually just check it as luggage and I’ve had no trouble in most places of the world. I do include a special note inside addressed to “TSA” explaining in general what all the junk is for.
Figure 1 - The complete EMC troubleshooting kit on display.
The one travel story I’ll relate is when I had finished some training in Penang, Malaysia. I had dropped the case off on the ramp leading to the big x-ray machine at the terminal and when the security guys saw the resulting image, I could see their eyes open up wide. They regained their composure and asked me to unlock it and then very politely asked what was in the case. I told them (as politely) it was just equipment I used to train their engineers. This seemed to satisfy them and we were able to pass through OK. Of course, Penang is a hotbed of engineering and geek activity, so I suspect the contents weren’t that unusual.
So, what’s included in my troubleshooting kit? In this installment (Part 1), I’d like to run through the radiated and conducted emissions tools I use. I’d love to learn what you use, as well. Perhaps we could trade ideas and get some discussion going?
Spectrum Analyzer - Now, dragging an HP 8566 spectrum analyzer around the lab or to client locations is not the most practical thing to do. But a good spectrum analyzer is the heart of most troubleshooting efforts. After some diligent searching on the Internet a few years ago, I was able to find a truly hand held spectrum analyzer that covers 1 MHz to 2.7 GHz and that costs under $2000 (Model PSA2701T). There’s also a lesser-priced model that tunes from 150 kHz to 1.3 GHz selling for about $1400 (Model PSA1301T). You can see it in the picture above next to the “bow tie” antenna. These are manufactured by a company virtually unknown in the U.S.; Thurlby Thander, a test equipment manufacturer in England. The units are marketed in North America by Saelig (www.saelig.com) and Newark Electronics (www.newark.com). Saelig has slightly better prices. Specified amplitude accuracy is ±1.5 dB and the displayed average noise level is -93 dBm at RBW = 15 kHz. There are three RBW settings (none are EMI BWs, however), amplitude readings may be dBm or dBuV and there are two markers as well as averaging and “max hold” settings. Your can also save and recall instrument setups and store screen captures for documentation purposes. There’s much more in the way of features. A full review is posted on my web site.
Probes - The other main items you’ll need for troubleshooting, of course are close field probes (e- and h-field) along with a current probe. All these may be constructed in your lab and I’ve written several recent articles describing the details.
Figure 2 - Several DIY h-field probes.
E- and H-Field Probes - The close field probes are constructed from semi-rigid coax (see photo above). The simplest design for an h-field probe merely takes the center conductor, bends it around and solders it to the shield in a loop. The larger the loop diameter, the more sensitivity, but less resolution. Because these probes are geometrically unbalanced, a small ferrite choke clamped around the coax near the connector will help keep common-mode currents from flowing along the outside of the shield and disrupting your measurements. The e-field probe is constructed by stripping away a small part of the outer shield. It’s wise to cover the probes with an insulating material, such as “tool dip” (red coating above) to avoid shorting the product under test. Currently, for a “quick and dirty” probe, you may also use a length of standard coax cable. Strip out the center conductor, bend it around in a loop and solder it the the braided shield (as in the “micro” loop pictured above). I’m currently using a low-cost set of close field probes from Beehive Electronics (www.beehive-electronics.com). The set of three h-field probes and one e-field probe is just $295, ordered from their web site. I reviewed these in a recent issue of my EMC newsletter (new subscribers will receive links to the back issues).
Antennas - For assessing radiated emission, I use some simple TV antennas positioned about a meter, or two, away from the product under test. They should be located close enough to observe an easily-readable signal. You’ll find that any hunk of metal large enough to pick up the harmonics from the product under test will work just fine for troubleshooting purposes. Be sure to fix it in place with tape to eliminate additional variables in your measurement. Connect up your analyzer and commence your troubleshooting assessment of the product. You’re looking for relative changes in harmonic amplitude. Is it “better”, “worse” or “no change” from the baseline? My rule of thumb is that changes less than 2 dB are probably measurement or setup error. Changes higher than that are likely real.
Figure 3 - Some of the simple antennas used for troubleshooting radiated emissions, based on "off the shelf" TV antennas.
Current Probe - The current probe is the other most-used accessary and I describe how to make one in a couple recent articles. You may use a large toroid core or a medium-sized snap-on ferrite choke. Wind a few turns around the toroid or choke and solder a coax connector (epoxied to the core/choke). See figures below for samples. I’ve compared the performance of these probes to commercial grade probes and they are amazingly useful up to about 200 MHz. For the purposes of troubleshooting, you really don’t require great accuracy - just knowledge that the fix you’re trying is (again) “better”, “worse” or “no change” than the baseline measurement. Of course, commercial probes are calibrated and can clamp around cables under test - a major advantage.
Figure 4 - The current probes pictured were photographed prior to the installation of a simple e-field shield constructed from copper tape. Wrap the tape around the windings, leaving a narrow slot around the inside of the toroid to allow h-field coupling.
The troubleshooting kit also includes the usual copper tape, cables, an assortment of capacitors, resistors, inductors, ferrite chokes, etc. If you were to purchase everything new, the whole kit might run $3000, or so - more if you purchase commercial probes. In the next installment, we’ll go through a few useful immunity tools I carry around!
Please reply to this post with other things you use for troubleshooting. I’d love to compare ideas!