Monthly Archives: July 2013

Questions on Radiated Emissions Pre-Compliance Testing

Following the recent webinar sponsored by Rohde & Schwarz, I received way too many questions to answer during the live presentation, so I’m following up gradually and posting my answers on The EMC Blog, now hosted by

I just posted my third article:

3. Questions on EMC pre-compliance testing for radiated emissions

I’ll be posting one more on the questions I received on general EMC topics some time in August.

In the meantime, I’ll be attending Doug Smith’s ESD/EMC three-day seminar next week and may write up a description with pictures. The week after that, I’ll be attending the annual International Symposium on EMC right here in Denver. I’ll be posting at least two articles on new products and other activities.

Your questions answered regarding cables and PC boards

Following the recent webinar sponsored by Rohde & Schwarz, I received way too many questions to answer during the live presentation, so I’m following up gradually and posting my answers on The EMC Blog, now hosted by

I’ve posted two articles so far:

1. Questions on cables for EMC mitigation

2. Questions on PC boards for EMC mitigation

I’ll be posting more soon on general EMC topics and pre-compliance testing for radiated emissions.

Presentation during the IEEE EMC Symposium

I’ve been invited to be the keynote speaker at the annual NEC EMIStream evening seminar and reception during the IEEE International Symposium on EMC, Thursday, August 8th, from 5:30 to 7:00 PM. The title of my presentation is “EMC Troubleshooting and Pre-Compliance Testing on a Budget”. For more details and to register for this free event, check out this link.

Stitching Capacitors

Here’s a question from the last EMC webinar regarding “stitching capacitors” – what they are and how they’re used. While I attempted to answer the question during the Q&A session, things seemed a bit rushed, so here are some details.

A stitching capacitor provides a path for return current to flow back to it’s source. The capacitor would connect between the ground plane and power plane and must be located adjacent to where the signal trace changes reference planes. If the two planes are at the same potential, you only need to connect them together with a via at the place where the signal trace penetrates. Here’s a couple slides form my seminar that diagrams the issue. You can also run a ground trace (signal return trace through vias to do the same thing.


Figure 1 – Here’s one of the most common issues from a slide taken from my EMC design seminar. If a clock trace needs to penetrate through two planes, how does the return current get back to the source?

If the two planes are the same potential – for example, signal return planes – then you can merely connect the two planes together with (typically two, or more) vias on either side of the clock trace. However, if the two planes are different – for example, signal return and power – then obviously, vias would not work, so we replace them with high-frequency bypass capacitors – i.e., “stitching” capacitors – on either side of the clock trace. Unfortunately, connecting each side of the capacitor to buried layers will require multiple vias, so to keep lead/trace inductance as small as possible, the vias must be located so as to minimize trace length and should be located as close as possible to the source clock trace penetration.

Stitching capacitors may also be used to provide a high-frequency connection across isolated planes, but this is a more advanced technique we can discuss at a later time.

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 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”!