Category Archives: General

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!

EMC Integrity (Longmont, CO)

Many of you may know, as an EMC consultant, I’ve been partnering with one of the best EMC test labs in Colorado, EMC Integrity, in Longmont, north of Denver. EMC Integrity was founded by Vince Greb in 1993 and now owns two of only three 10m semi-anechoic chambers in the state (the other is owned by Hewlett-Packard in Ft. Collins). They specialize in both commercial and military EMC testing. Check them out at www.emcintegrity.com. They were recently featured in an article in the Boulder County Business Report (March 15-23, 2013). Check it out here…

I’ll be presenting a comprehensive two-day EMC design and troubleshooting short course there April 2-3 (sold out), with a possible follow-up course later this year.

EMCI_Lockhart - CastnerTechnician, Casey Lockhart running a radiated emission test at one of the 10m chambers. Photo courtesy BCBR (© Jonathan Castner).

 

Milwaukee EMC Mini-Symposium – March 2013

I was honored to be the featured speaker at the 13th annual EMC Society Chapter of the IEEE Milwaukee Section “EMC Mini-Symposium” this last March 19th. Hosted and managed by EMC engineer Jim Blaha (GE Medical), this was actually no mini-symposium – but is the largest regional gathering of EMC engineers in the country. There were a record 180 engineers from around the area, as well as a record 42 vendors showing their wares. The event sold out within just a few weeks.

The title of my talk was a mouthful: “EMC Essentials and Pre-Compliance Testing with your own Affordable EMC Troubleshooting Tools Kit”. While I covered some of the major EMC theory for issues I generally end up addressing at various client companies, most of the day was spent on how to collect a set of useful tools, probes and measuring instruments to make up a portable EMC troubleshooting kit. I then went on to explain how I use these tools to perform pre-compliance and evaluation testing of prototype products.

I’d like to thank Jim Blaha for his superb organization and management of the event, all the vendors, who helped sponsor the event and Agilent Technologies for supplying the oscilloscope and spectrum analyzer for the demonstrations.

Here are a few pictures taken by my colleague and fellow EMC consultant, Jerry Meyerhoff. Thanks Jerry!

P3190643BlahaIntroJim Blaha from GE Medical manages the symposium and is providing introductory comments.

P3190408KenNcrowdDemoTableChatting during one of the breaks.

P3190645ClassAudienceA portion of the 180 attendees.

P3190675KenPigtailNspectrumHere I am explaining why pigtail shield connections cause common-mode currents resulting in radiated emissions.

P3190677RoomFrom BackA very full house!

EMC Troubleshooting Parts Storage

For those of you who regularly troubleshoot EMC issues, wouldn’t it be convenient to have all your “fix-it” parts all in one place? That’s what I thought as I was wandering around JoAnns (a fabric and hobby store) with my wife. I spotted this “photo keeper” storage box and thought it would be perfect to store all the little parts, gasketing, filters, etc., for quickly implementing fixes during the troubleshooting process.

IMG_1431

 

I had a chance to use this kit in a real-life consulting application as I helped a client in Chicago recently. I transported the kit inside a large Pelican case and used a quantity of bubble-wrap to protect it from shifting around. It survived just fine and added a sense of professionalism in the client’s eyes.

Hobby stores sell many similar storage boxes and you may find several to your liking. Read on for more details…