Technical Insights > RF + Microwave

Complete your EMC Conducted Emissions Testing in Just 7 Steps

2018-09-26  |  9 min read 

There are two kinds of EMC Pre-Compliance tests you can perform – radiated and conducted.  Today, we will review conducted emissions testing – what it is and why it’s important.  For a similar discussion on radiated testing, check out EMC Basics:  What is Radiated Emissions & Immunity Testing?.

Conducted emissions tests focus on the unwanted signals that are on the AC mains generated by the device under test (DUT).  Conducted RF emissions are electromagnetic disturbances (noise voltages and currents) that are caused by electrical activity in a DUT and is conducted out of the DUT along its interconnecting cables – for instance, power, signal, or data cables.  Conducted disturbances, in particular, a conductor, can couple directly into another electronic device or component within the same device.  This will provide unwanted signals that could lead to issues, like inaccurate performance.  This type of testing is one of the first group of tests performed in the process for EMC Pre-Compliance, followed by Radiated Emissions testing, Radiated Immunity, and conducted immunity testing.  The general procedure is to connect the appropriate equipment, load the limit, and load the correction factors.

Before we go through the steps to complete the conducted emissions testing process, let’s gather the equipment required.  These are common items that a test bench should have – these include:

  • Spectrum Analyzer equipped with EMC pre-compliance measurement software
  • Line impedance stabilization network (LISN) - The LISN is important because it isolates power mains from the DUT, which must have as clean of a signal as possible
  • Limiter
  • DUT

Now let’s go through the conducted emissions testing process in seven steps:

1.  Set up your test

Connect the signal analyzer to the limiter, LISN, and DUT.  Make sure the cord between the DUT and LISN is as short as possible to avoid the power cord from becoming an antenna.  Measure the signals on the power line with the DUT off.  If you see a signal approaching the established limit lines, you’ll want to set up some additional shielding so that these signals do not interfere with your possible conducted emissions from your DUT.  Shielding isolates components from each other to avoid coupling and interference that unwanted.

2.  Select your frequency range

Be sure you are measuring within 150 kHz and 30 MHz, which is the correct bandwidth for this measurement.   This is the corresponding frequency span that meets the CISPR requirement, which is a standard that is used for compliance testing. We will talk more about CISPR in another blog.

3.  Load the limit lines and correction factors

The two limit lines used for conducted emissions are EN5502 Class A quasi-peak and EN55022 Class A EMI average.  To compensate for measurement errors, add a margin to each limit line.

Figure 1:  Scan table where you can select the frequency span needed for the corresponding measurement
Figure 2:  Conducted emissions display with limit lines and margin set

4.  Correct  for the LISN and the transient limiter

The transient limiter is used to protect the input mixer, basically acting as a filter or attenuator and is used with the LISN.  The correction factors for the LISN and the transient limiter are stored within the signal analyzer and can be easily recalled.  Correction factors adjust the reference plane for the DUT compensate for any loss through cables, space, etc.  Now you are able to view ambient emissions.  During this step, the DUT must be turned off.  If your emissions are above the limit, the cord between the LISN and DUT may need to be shortened.

Most radiated and conducted limits in EMC testing are based on quasi-peak detection mode.  Quasi-peak detectors weigh signals according to their repetition rate, which is done by having a charge rate faster than the discharge rate.  As the repetition rate increases, the quasi-peak detector does not have enough time to discharge completely, resulting in a higher voltage output.

The quasi-peak and average of the signals need to be measured and compared to their respective limits.  There are three detectors – Detector 1 will be set to peak, Detector 2 to Quasi-peak, and Detector 3 to EMI average.

Figure 3:  Loading correction factor files

5.  Locate signals above the limit lines

Switch on the DUT to find signals above the limit lines.  This is a good time to check to make sure the input of the signal analyzer is not overloaded by stepping the input attenuator up in value and seeing if they display levels do not change.

Figure 4:  Scan and search for signals above the limit lines

6.  Measure the Quasi-peak and average of the signals 

Most radiated and conducted limits in EMC testing are based on quasi-peak detection mode, which is available in the EMC X application.  Quasi-peak detectors weigh signals according to their repetition rate, which is done by having a charge rate faster than the discharge rate.  As the repetition rate increases, the quasi-peak detector does not have enough time to discharge completely, resulting in a higher voltage output. 

The quasi-peak and average of the signals need to be measured and compared to their respective limits.  There are three detectors – Detector 1 will be set to peak, Detector 2 to Quasi-peak, and Detector 3 to EMI average. 

Almost there!  We’ve got one more step to go!

7.  Review the measurement results 

The quasi-peak detector delta to Limit Line 1 & average detector delta to Limit Line 2 should all have negative values.  If there are some measurements that are positive, then there is a problem with conducted emissions from the DUT.  Before redesigning / troubleshooting the DUT with these results, check to ensure there is proper grounding if there are conducted emissions problems.

Figure 5:  Quasi-peak and average delta to limit - the measurement results

Check these tips out for any troubleshooting issues: 

  • If the signals you are looking at are in the lower frequency range of the conducted band (2MHz or lower), you can reduce the stop frequency to get a closer look
  • You can add more data points by changing the scan table
    • The default scan table is two data points per bandwidth, or 4.5 kHz per point

To get more data points, change the points per bandwidth to 2.25 or 1.125 to give four or eight points per bandwidth. 

For more details on conducted emissions testings, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share!  Stay tuned for the next one!