Divide and Conquer

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.

Advertisements
Explore posts in the same categories: Troubleshooting

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


%d bloggers like this: