EMC Sleuth: Mysterious Interference at the Doctor's Office

As Membership Officer of the IEEE EMC Society’s Chicago Chapter and active member of the American Council of Independent Laboratories (ACIL) and various Smart Grid committees, he likes to stay involved in the industry and act as an “EMC Sleuth” if the occasion arises.

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November 20, 2013

Sometimes, I have the fortunate opportunity to step outside of the EMC lab and investigate real-world EMI problems.  Several years ago, I received a call from a desperate general contractor who had an EMI situation in a new state-of-the-art audio clinic.  The facilities included an MRI suite in the lower levels.  The doctors complained about issues of networking errors with their new computer system, electronic noise problems in their audio lab, and incorrect results from their blood analyzers.  They were threatening to withhold payments until the issues were resolved. 

The GC’s first thoughts were that the MRI equipment was the culprit.  He called us in the survey the site to determine if the MRI was indeed causing these interference problems.  We arrived at the site and set up a spectrum analyzer with various antennas and current probes in order to monitor the field intensity levels.  We started by taking measurements in the MRI suite and then moved on to the audio labs where the equipment problems had occurred.

In a short time, we were able to diagnose the problem.  The interference was due to a broadband source.  We could see significant broadband switching noise throughout the high-frequency (HF) spectrum.  These voltage spikes were both radiating from the power lines, as well as, conducting into the power lines.  We could not attribute any significant RF interference as generated by the MRI equipment.  We proceeded to walk into the waiting area of the office where the levels intensified.  This waiting room was state-of-the-art and included high-tech lighting that dimmed to create a soothing and relaxing atmosphere.

Can you deduce what else the light system created?

At an opportune time, we turned off the lights in the waiting room.  We immediately noticed a significant drop in the HF noise level and the spikes all but disappeared.  The contractor was beside himself to think that such a low-tech device as a dimmer switch could wreak such havoc upon the doctors’ offices.  We recommended that the dimmers be replaced with dimming technology that does not produce noise.  The problem was solved and everyone was happy.

Below are some questions for further thought and discussion on real-world interference applications:

  1. What lighting technologies are significant sources of EMI?
  2. What are some ways to reduce this interference?
  3. Will interference problems increase as more high-tech lighting is installed to reduce energy consumption?
  4. What regulations are in place to control the level of interference from these sources?

Do you have answers to the above or any questions about Interference IssuesEMC Testing, or other related topics? Please share your comments or questions below and this week’s expert, Ray Klouda, will get back to you as soon as possible.