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”!

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

  1. Are you sure the issues are with the (switching) powersupplies?

    I would suspect the led’s themselve being the transmitters when switched off. This also was an issue in the past where you would multiplex led’s even with linear powersupply. You could see (or hear) them into hundreds of MHz, depending on mux frequency.

  2. Hi Benno, good question. My experience has been that the dominant radiator was the SMPS modules, but you may also be correct if the LEDs are being multiplexed with fast switching. In addition, the wiring to the LEDs could be serving as the antenna structure, which allows the emissions to propagate. I checked the LED stop light signals in the small town where I live and they seemed relatively quiet as I listened to the AM radio in my vehicle, however, others are reporting dramatic increases in broadband emissions from the AM band all the way up (and past) the FM band. So it likely depends on the manufacturer of the lighting.

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