Category Archives: EMC Design

Interference from LED traffic lights and large “jumbotron”-type signs

I’m starting to receive more field reports from EMC (and other) engineers regarding the radiated emissions from LED-based traffic lights and especially from the large “jumbotron”-style LED-matrixed signs and billboards.

The LED traffic lights typically emit broadband interference, which covers the AM broadcast band within 100 feet, or so, and the larger LED signs emit harmonics well up into the UHF bands. One of my colleagues just helped resolve an issue with one of those giant signs that was interfering with an established 3G microcell in a nearby hotel. Then, just a couple days ago, I ran into a report out of Sweden regarding interference to aircraft communications from a large advertising billboard sign located near the Trollhättan-Vänersborg Airport. This was reported by the National Electrical Safety Board via their www.elsakerhetsverket.se web site.

The following is a translation of the Swedish text. While not a perfect translation, I think you’ll get the gist.

In December, the National Electrical Safety Board decision on prohibition of two billboards at Trollhättan-Vänersborg Airport, which posed a serious threat to flight safety. Measures have been taken and the interference is not currently a pressing problem.

On 16 December, the National Electrical Safety Board decision on prohibition of two hoardings sending out radio signals due to flaws in the design. The decision was made because the air traffic radio communications were disrupted during takeoff and landing. Disturbed radio communications can call from the airport or from another aircraft missed.

Troubleshooting underway

Using ban was lifted after the disturbance moved to another frequency that does not interfere with aviation radio communications. Safety Board has presented the company to correct the interference, and the provider is working to resolve the issue.

While the manufacturer claims to be troubleshooting the problem, all they did to initially resolve the interference to aircraft communications was to shift the sign’s clock frequency slightly, moving the interfering harmonics sufficiently out of the aircraft band. So, I can’t help but wonder what the harmonics are interfering with now?

Apparently, the current emission standard for lighting, IEC/EN 55015 excludes LEDs and is being revised to correct this. I guess it was thought LED lighting technology was more passive as far as interference goes. However, today’s industrial lighting designs use multiple switching power converters operating with very fast edge speeds (for efficiency) and in the 100’s of kHz, creating broadband emissions out to 200 MHz, or more. As LED lighting continues to take hold over other forms of illumination, interference reports like these are bound to proliferate. For those of you working in the lighting industry, this is a “heads up”!

Terminating shielded cables for low-cost unshielded products

Here is one of my “seed” questions I developed for the June 20th webinar in case not enough attendees asked questions. Fortunately, I didn’t have that problem, due to the tremendous response I received. In any case, while this wasn’t officially asked, the question does come up frequently.

Q: What if my product uses a plastic enclosure? Where do I connect the shield of the I/O cable?

A: Using shielded cables with unshielded enclosures can lead to design and performance issue for EMC. Many low cost consumer products can not afford a shielded enclosure, so how do we resolve this? The important thing to keep in mind is that there will inevitably be common-mode noise sources on the PC board. To keep these noise currents off our I/O and power cables, we can either block the currents from getting to the cables with a ferrite choke or divert the noise currents back to their source. Often, a combination of blocking and diversion is the best method. Many higher-end handheld consumer products use a diversion plate under the PC board. This method was described in another blog posting on The Connecting Edge and is merely a metallic plate or metalized film with one end bonded or clamped well to the I/O and power connector ground shells. This offers a low impedance path for the common-mode currents to flow back to the source through distributed capacitance. It also protects sensitive circuitry from external ESD currents injected at the I/O connectors. In addition, it serves as an image plane which helps reduce radiated emissions. To answer the question more directly, the cable shield must be bonded in some way to the digital ground (if a signal or I/O cable) and power ground (if a power cable). Ideally, all I/O connectors and power connectors should be grouped together on one side of the board. If they are spread all around the perimeter, that’s often bad news, as any noise sources on the PC board are now potentially driving the midpoint of a dipole antenna!

R&S Webinar, June 20, 2013

My first EMC webinar hosted by Rohde & Schwarz went off pretty well last Thursday, with 276 attendees from around the world checking in. Thanks to all for listening to some of the most common EMC issues I deal with during my consulting.

There were a number of questions asked, that I wasn’t able to get to due to lack of time, however, Rohde & Schwarz will be providing a list of the unanswered questions early next week and I intend to group like ones together and answer these periodically via this blog. Some of the more interesting questions will be answered on The EMC Blog, hosted by Test & Measurement World.

Thanks again for all who attended. I suspect we’ll do this again some time.

Design West Conference and Expo (April 22-25, 2013)

Test and measurement company, Rohde & Schwarz, invited me out to the Design West conference to present a couple of EMC design and troubleshooting talks. Little did I know these were to be held in their new “classroom in a truck”, which was driven right into the exhibit hall!

Design West 201304-154

Figure 1 – Here I am standing next to the truck. It’s difficult to get an idea of the size of the trailer from this point of view.

For Wyatt 1

Figure 2 – Here I am teaching a class of 24 engineers in the expandable classroom.

Figure 2 - Here's a better view of the trailer. There are two large "slide-outs" which expand the classroom to 24 seats, place room for equipment demos around the perimeter.

Figure 3 – Here’s a better view of the trailer. There are two large “slide-outs” which expand the classroom to 24 seats, plus room for equipment demos around the perimeter. They plan to use this mobile classroom for their “truck tour” of major cities this year.

The Design West show evolved from the Embedded Systems Conference (ESC), at which I’ve presented my EMC seminars in the past. This year, was more of the same – just 10X better (is that a 10 dB increase?) with a couple hundred vendors plying their embedded processors, I/O and sensor products. Thanks to Rohde and Schwarz for hosting my seminars there this year!

Review: The ARRL RFI Book (3rd Edition)

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…

Using MathCad to Simulate a Square Wave

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…

Enclosure Resonances & Easy Demo

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:

rectangular-box.jpg

rectangular-res-eq.png

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.

resonance2-600-100.jpg

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…


Use of Guard Traces?

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.