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An EMC troubleshooting kit - part 1a (emissions)

Kenneth
Wyatt -June 01, 2012

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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 (http://www.emcesd.com/ or http://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 http://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 (http://www.saelig.com/) and Newark Electronics (http://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 (http://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!

Related postings:

Troubleshooting Kit - Part 1a (Emissions)

Troubleshooting Kit - Part 1b (Emissions)

Troubleshooting Kit - Part 2 (ESD Immunity)

Troubleshooting Kit - Part 3 (Detecting ESD)

Troubleshooting Kit - Part 4 (Radiated Immunity)

Troubleshooting Kit - Part 5 (Summary & Parts List)

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