Divide and Conquer

Posted September 4, 2011 by Henry Ott
Categories: Troubleshooting

In my 8/28/11 blog I discussed the four basic tenets of EMC troubleshooting, the first of which was Divide and Conquer. The objective of this approach is to eliminate some of the components, parts of the system, etc. from being associated with the problem, and then concentrate our troubleshooting efforts on the remaining components, parts of the system, etc. In this blog I will discuss this approach applied to the case of a radiated emission problem, and the case of a conducted emission problem.

Radiated Emission Problem. In the case of a radiated emission problem it is advantages to determine if the emission is coming from the cables, or from the enclosure/PCB. Often the simplest way to do this is to remove the cables and see if the product passes without cables. If it does, the emission is coming from the cables. If not, fix the enclosure/PCB first, then worry about the cables. If the product does not pass without cables, it will never pass with cables.

Of course, when removing the cables, you cannot remove the power cable. Therefore, wrap the power cable around a big ferrite core to suppress the radiation from this cable.

Another approach, not requiring the removal of the cables, would be to measure the common-mode currents on the cables using a common-mode current clamp such as the Fischer Custom Communications Model F-33-1, or if the emissions are above 250 MHz, the F-61 current probe. See Measuring Common-Mode Currents on Cables.

Conducted Emission Problem. The first thing to do when facing a conducted emission problem, on the power line, is to determine the frequency of the failure(s). If it is at the low end of the conducted emission spectrum, the source is most likely the switching power supply or an ineffective  power line filter. If the failure is at the high end of the conducted emission spectrum, the source is most likely the digital logic.

If the emission is from the switching power supply, it is desirable to next determine if the emission is from a common-mode or a differential-mode noise current – since different components in the power supply, and the power-line filter, affect differential-mode emissions and common-mode emissions. Performing the conducted emission test as specified in the various EMC regulations, will not determine this.

We can, however, determine this by adding a differential-mode rejection network between the LISN and the measuring instrument. This will allow us to determine the magnitude of just the common-mode noise, see Distinguishing Between CM and DM Conducted Emissions.

If you make a habit of dividing up your EMC problems as discussed in this blog, your EMC troubleshooting will be easier and will go much faster.

In my next blog I will discuss the Predominate Effect principle.

The Four Basic Tenets of EMC Troubleshooting

Posted August 28, 2011 by Henry Ott
Categories: Troubleshooting

The lull in the middle of the storm! As I write this I am sitting at home in the eye of hurricane Irene, the rain has stopped, the wind has stopped, and the sun is almost out. In the last 10 hours we have received over 6″ of rain. I know, however, that we have at least another 8 hours of torrential rain and hurricane force winds left before this thing is actually over.

At my presentation as part of the Consultant’s Workshop at the 2011 IEEE EMC Symposium in Long Beach, CA on Friday August 19, I talked about the four basis tenets of  EMC troubleshooting. If you understand these four basic principles, your EMC troubleshooting will be a lot easier and will go a lot faster. The four tenets are:

  • Divide and Conquer
  • Predominate Effect
  • The “Kill it Dead” Strategy
  • Fix Implementation

My next few blogs  will cover each of these basic principles individually.

Portable Spectrum Analyzer

Posted August 21, 2011 by Henry Ott
Categories: Test Equipment

My new PSA-T Series portable spectrum analyzer manufactured by Thurlby Thandar Instruments in the UK. It is available from Newark Electronics, as well as Saelig Electronics (www.saelig.com), the importer, in the US for under $2,000. There are two models, Model PSA1301T covers the frequency range of 150 kHz to 1.3 GHz and Model PSA2701T covers the frequency range of 1 MHz to 2.7 GHz. While it does not have all the bells and whistles of a large, heavy, expensive analyzer it has more than enough for doing precompliance EMC measurements. I highly recommend it.
Ken Wyatt has posted a nice review of the spectrum analyzer on his web site, you can see it at:    http://www.emcseminars.com/Technical_Articles/files/TTi_PSA2701T_Spectrum_Analyzer_Wyatt.pdf