Troubleshooting Radiated Emissions: Three Case Studies

Radiated emissions (RE) are often the number-one cause of compliance failures for most electronic products. This article describes simple troubleshooting steps and tools to isolate most RE issues. Three case studies are described showing how low-cost probes and instruments may be used to help the EMC or product development engineer troubleshoot a product right in the R&D lab. By characterizing and resolving the RE prior to compliance testing, the chances are much greater for a successful compliance outcome.
Radiated emissions (RE) are typically the toughest compliance issue for most electronic products. Because emissions limits are established worldwide, products that don’t meet the limits may not be placed on the market. The best way to achieve compliance is through proper product design, but often these design techniques are not taught in universities, nor are these techniques fully understood by many experienced engineers. The result is that, many products lack the proper EMC design and therefore are unlikely to meet limits for RE. In this article, I’ll describe the basic troubleshooting steps involved and show a few of the tools and probes I use. Then, I’ll take you through three case studies that demonstrate the troubleshooting philosophy and specialized probes and instruments used to reduce RE to meet compliance limits.


Background Theory

In order to better understand RE and how to troubleshoot your product, let’s review how harmonics are created, and then describe differential-mode (DM) and common-mode (CM) currents and how they get generated. General design techniques are mentioned, but specific design practices are a subject for another paper.

A periodic square wave (Figure 1) may actually be represented as a series of more basic signals called “basis functions”. Assuming the rise and fall times of the square wave are straight up and down (zero rise/fall time), an infinite number of harmonically related basis functions, or sine waves, are required. Digital circuitry today uses rise and fall times of sub-nanoseconds, which can generate harmonics of several hundreds to thousands of MHz.

Figure 1: An infinite number of harmonically-related basis functions (sine waves) will create an ideal square wave.

DM currents and their associated radiation are caused by digital signals (and their harmonics) traveling through circuit loops. The larger the loop, the stronger the fundamental and harmonic emissions will be. We want to minimize the area of any circuit loops through use of signal or power return planes, typically by use of multi-layer circuit boards. For low-cost products, multi-layer boards may not be feasible, so other design techniques must be used to minimize these loops.

To reduce emissions, the area of the loop may be decreased. This is an important point to keep in mind during circuit layout. Placing a crystal oscillator (one common source of harmonics and resulting emissions) close to the circuitry that requires its signal is a good design practice. Likewise, the use of multi-layer boards with full signal or power return planes serves to reduce the loop area substantially.
Now let’s consider CM currents and how they are generated. How current may travel the same direction through both the signal and signal-return wires in a system is not necessarily intuitive. Referring to Figure 3, note that due to finite impedance in any grounding system (including circuit board signal/power return planes), there will be a voltage difference between any two points within that return plane. This is denoted by VGND1 and VGND2 in the figure. This difference in potential will drive CM currents through common cabling or circuit traces between circuits or sub-systems. These CM currents may be generated on circuit boards or within sub-systems inside product enclosures. In addition, unbalanced geometries - for example, different lengths or path routings for high-speed differential pairs - can create CM voltage sources that drive associated CM currents. Because the current phasors are additive, the resulting radiated phasor may be quite large compared to those generated by DM currents, which are opposite in direction. Therefore, CM emissions tend to be more of an issue than DM emissions.

Figure 2: General model for DM current generation. Note that the resulting phasors from the wires are subtractive.

Figure 3: General model for CM current generation. Note that the resulting phasors from the wires are additive.

So, how do these large DM loop areas and CM sources get generated? One major issue I run into constantly is that the return plane (power or signal) often contains gaps or slots, forcing the return DM signal out around the lowest-impedance path, thus creating a large radiating loop; this and the fact that unbalanced geometries or common-impedance couplings can create CM voltage sources. It’s no wonder PC boards can generate a lot of harmonic emissions. The solution to most EMC problems is to control the path of current! Namely, both power and signal return currents must be well defined.

So, how do you tackle a product with high radiated emissions?


Useful Troubleshooting Tools

Troubleshooting Kit

Several years ago, I assembled an EMC troubleshooting kit in a portable Pelican model 1510 roller-case that can be wheeled right to an engineer’s workbench or into your client’s facility. Contents include a handheld spectrum analyzer1, a broadband preamplifier, small DIY antennas, various probes and other accessories. Other useful items for your troubleshooting kit include the usual ferrite chokes, aluminum foil, copper tape, power line filters, signal filters and various values of resistors and capacitors. Figure 4 shows the contents of the case.

Figure 4: Contents of the EMC troubleshooting kit. I can probe for various RE problems, as well as test for ESD and radiated immunity. This is based on a Pelican 1510 roller-case.

Antennas

The antenna you select is not really that critical for troubleshooting purposes. As long as its fixed in length and fixed in place on the bench, you’ll receive consistent results. During troubleshooting, it’s more important to know whether the fix is “better”, “worse” or “no change”; as long as the test setup doesn’t change, the results should be believable.

I use a couple inexpensive television antennas available through most electronics parts suppliers. These include a pair of television “rabbit ears” and a UHF “bowtie” with TV balun to match 50-ohm coax (Figure 5). If the troubleshooting workbench is non-metallic, I’ll extend the antenna to approximate resonance (if possible) and tape it down to the bench with duct tape. If the bench is metallic, I support and position it some distance above the bench. Try using a test distance of about a meter from the EUT – closer if the emissions are too low to see clearly. Sometimes I find a low-noise, wide-band preamp between antenna and analyzer helps.

Figure 5: Examples of DIY antennas for radiated emissions troubleshooting. The television “rabbit ears” is resonant from 65 to 200 MHz depending on how the elements are extended, while the bowtie resonates well from 300 to 800 MHz. I installed an inexpensive 300 to 75-ohm television-style balun to better match the 50-ohm coax to the bowtie.

If ambient signals from broadcast radio, television, mobile phones and two-way radio services interfere with observing the product harmonics, you may need to bring the antenna closer or set up the troubleshooting measurement in a basement or building interior away from outside windows.

Probes
Useful probes include E-field, H-field and current probes. All are easily constructed or are available from several manufacturers2. A simple E-field probe may be made by extending the center conductor about 0.5 cm from a section of semi-rigid coax or high-quality flexible coax and then attaching a coax connector to the other end. H-field probes may be fashioned by looping the center conductor of a coax cable around and soldering it to the shield to form a small loop of 0.5 to 5 cm in diameter - the larger the loop, the more sensitivity. A more sturdy H-field probe design uses semi-rigid coax to form the loop. Be sure to add a small ferrite choke on the probe input coax to reduce CM currents due to the unbalanced geometry. Beehive Electronics makes a low-cost set of E- and H-field probes and, because the H-field probe design is balanced, it does not require the ferrite choke. Depending on the diameter of your H-field probe, you may need to use a broadband preamplifier between the probe and analyzer.3


General Troubleshooting Steps

Locating Internal Sources

Using an E-field or H-field probe, identify the high-harmonic sources and circuit traces, and determine potential coupling paths to slots, seams or cables (Figure 6).

Figure 6: Use of simple H-field probes to locate emission sources.

Once the potential sources are mapped, you’re ready to start applying fixes. You should generally start with the lower harmonics and work upwards. Often, lower-frequency sources will cause significant high-frequency harmonics, depending upon the rise time. Sometimes adding a simple low-pass filter to power or signal traces can reduce emissions dramatically.


Cables

Check your cables next, as CM currents often couple into them resulting in radiation. Try unplugging all the cables and then plug each in one at a time to find any that are radiating. Remember that there may be more than one bad cable! Snapping a ferrite choke around the base of the cable near its chassis connector may help as an interim fix. I’ve found that most cable emissions are very likely due to poor grounding to the enclosure at the I/O connector.

These CM currents on cables may also be measured with a current probe. Clamp the probe around the cable in question and move it back and forth to maximize the readings, then tape it in place while you apply potential fixes. I made my own current probes (Figure 7), but the advantage of commercial probes is that they can open up and snap around a cable, rather than having to be threaded on.

Figure 7: Examples of DIY current probes. These photos were taken prior to installing the E-field shield by wrapping a layer of copper tape over the windings, leaving a small gap around the inside of the probe. Fourteen (14) turns of Teflon-insulted wire wound around a Würth Electronik #74270097 ferrite core (4W620 material) was used, which is useful from 10 to 1000 MHz.

Slots and Seams

Once any cables issues are addressed, its time to probe for leakage through slots or seams in the chassis. The length of the slot or seam is important. The worst-case is when the slot/seam is 1/4-wavelength long at the harmonic in question. I use a permanent marking pen to record the areas of leakage and frequencies of concern from every seam/slot on the enclosure. Once these are marked, I’ll cover them with copper tape and re-measure the RE levels. Keeping an eye on the levels, I’ll start removing the tape piece-by-piece to determine which slots or seams are actually causing problems.

Case Studies

#1 Industrial Alarm System

This first project consisted of an extensive alarm and access control system for large industrial or government buildings. There were several different control units with associated highly secure, door-access keypads and remote switches and sensors. It was all to be interconnected with RS-485 control cabling – up to several hundred feet apart. It also had to comply with FCC Class B limits.

Taking a look at the major system components quickly revealed the major issue – the RS-485 and LAN cables were penetrating the shielded enclosures and were radiating badly. The other issue turned out to be splits in the power and signal return planes with signal traces crossing over these splits, which created CM current sources. I’ve found both issues to be fairly common.

Fortunately, the client had a screen room available, so ambient signals were not as much an issue. Because cable emissions were the dominant source, I ended up using a current probe around the cable under test for most of the board-level troubleshooting.

Figure 8: General configuration of the major sub-assemblies showing the RS-485 cable penetrating the shielded enclosure. Also shown is the current probe and portable spectrum analyzer used to probe potential sources.

As previously mentioned, the other very common issue with many products I evaluate is high-speed circuit traces crossing over splits in signal return or power return planes. In the case of the alarm system, many of the circuit boards included RJ-45 LAN connectors and the associated 20 MHz PHY oscillator trace crossed several of these splits. This forced the return signal out into a large loop area, which caused radiated emissions and coupled CM currents onto both the RS-485 and LAN cables.

Figure 9: Section of the circuit board showing several splits or gaps in the signal and power return planes.

Ultimately, I designed a simple, low-pass L-C filter (ferrite choke and capacitor) for each RS-485 wire. Low-pass filters were installed on all the on-board dc-dc power supplies and large ferrite chokes were added to the penetrating cables. Once the circuit boards were re-laid out to eliminate the splits, the client was able to achieve its FCC-Class B emissions goal. This was a case where some simple EMC and system design in the front end of the design would have done wonders.


#2 Torque Measurement System

This case study was of a self-contained, computer-controlled torque measurement system used for determining the force required to remove soft drink bottle caps. In this case, the embedded OEM Windows PC controller with touch-screen LCD display was radiating at several frequencies from 90 to 200 MHz.

Since most of the radiation appeared to be coming out the front of the controller through the LCD display, we disassembled and found several issues. Because this was an OEM component, there was little that could be done internally except to install retrofit bonding and gasketing. It also turned out the controller/LCD display assembly was “floating” as mounted to the shielded enclosure. Finally, when the video display cable was probed, a very high CM current was observed at the dominant radiated frequency of 95 MHz.

Figure 10: Copper tape was added to bond the front bezel and rear panel on the LCD module.

Here’s where copper tape comes in handy. Most LCD displays I’ve seen are comprised of two metal case halves – a front bezel and rear shield cover. For whatever reason, these are rarely connected. In addition, this LCD assembly was isolated from the sub-chassis of the controller. During the troubleshooting phase, I added copper tape to bond the LCD case halves together and copper tape (later replaced with finger-stock) between the LCD module and sub-chassis. We then copper-taped the sub-chassis to the main shielded enclosure. “Floating” metal is bad news, as it can act like an antenna and re-radiate internal noise currents.

Figure 11: The LCD module was bonded to the sub-chassis. This was later replaced with strips of finger-stock.

Finally, a small ferrite choke was added to the internal video cable and that, plus the extra bonding, was enough to get the emission level down so it would pass the
CISPR 11-A limit.

Figure 12: A flat ferrite choke was added to the video cable as a final solution.

#3 Digitizing Oscilloscope

One of the most common sources of radiated emissions is due to poorly bonded connectors mounted on shielded product enclosures. This occurs especially if
the connectors are circuit board mounted and penetrate loosely through the shielded enclosure. Poorly bonded connectors allow internally generated CM currents to leak out and flow on the outside of I/O, mouse or keyboard cables. This also allows ESD discharges inside the product – more bad news. If these currents are allowed out of the enclosure, the attached cables will act as radiating antennas, often resonating around 300 MHz due to the typical 1m length.

This was the case for a new digitizing oscilloscope prototype. The I/O connectors were all soldered onto the PC board and the board was fastened to the rear half of the enclosure. The connectors simply poked up through cutouts in the front metal shield.

Notice the gap around the bonding fingers of the connectors (Figure 13)? While measuring the CM current flowing on the outside of the USB cable under test, and simply jamming the screwdriver blade of my Swiss Army knife between the connector bonding fingers and metal chassis enclosure, I was able to drop the overall cable currents by 10 to 15 dB.

Figure 13: Cables should be tested individually. Here I have a current probe clamped around the cable under test and am monitoring the harmonics with a simple hand-held spectrum analyzer. As I ground the connector shell to the chassis with the Swiss Army screwdriver blade, the harmonics were reduced considerably!

The solution was to fabricate a custom shim with spring-fingers that would slip over all the connectors and bond firmly between the connector ground shell and enclosure. More and more low-cost products are relying on PC board mounted I/O connectors as a cost-cutting measure. Any time you see this, be prepared to carefully examine the bonding between the connector ground shell and the shielded enclosure.

Summary

In order to pass required EMC tests for radiated emissions, it is necessary to understand the basic concepts of current flow through loops, as well as differential- and common-mode currents and how they are generated. Troubleshooting an existing design is simply a process of identifying the likely sources, determining the coupling paths via probing, and applying temporary fixes. Once these fixes have been applied and the product passes emission limits, the electronic and mechanical engineers may determine the most cost-effective solutions. Obviously, troubleshooting or characterizing products early in the design cycle is preferred in order to reduce overall implementation costs.


Notes

• The handheld spectrum analyzer being used is made by Thurlby Thander Instruments (www.tti-test.com). It sells for approximately $1,995 (USD) and covers 1 MHz to 2.7 GHz.A complete review of the low-cost Thurlby Thander PSA2701T may be found on the author’s Web site, www.emc-seminars.com.
• Probe manufacturers include Fischer Custom Communications (www.fischercc.com), Beehive Electronics (www.beehive-electronics.com), Teseq (www.teseq.com) and many others.
• I made my own broadband preamp using a MiniCircuits model ZX60-3018G-S, which covers 20 to 3000 MHz at 18-23 dB gain and 2.7 dB noise figure. It sells for $50. Beehive Electronics also makes a low-cost ($525) broadband preamplifier that covers 150 kHz to 6 GHz at 30 dB gain and around 5-6 dB noise figure.

Kenneth Wyatt Kenneth Wyatt, Sr. EMC Engineer, Wyatt Technical Services LLC, holds degrees in biology and electronic engineering and has worked as a senior EMC engineer for Hewlett-Packard and Agilent Technologies for 21 years. He also worked as a product development engineer for 10 years at various aerospace firms on projects ranging from DC-DC power converters to RF and microwave systems for shipboard and space systems. A prolific author and presenter, he has written or presented topics including RF amplifier design, RF network analysis software, EMC design of products and use of harmonic comb generators for predicting shielding effectiveness. He has been published in magazines such as, RF Design, Test & Measurement World, Electronic Design, Microwave Journal, Interference Technology, HP Journal and several others. Kenneth is a senior member of the IEEE and a long time member of the EMC Society where he serves as their official photographer. He is also a member of the dB Society and is a licensed amateur radio operator. His comprehensive yet practical EMC design, measurement and troubleshooting seminars have been presented across the U.S., Europe and Asia. He currently resides in Colorado and may be contacted at ken@emc-seminars.com This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His Web site is: www.emc-seminars.com.