Separate Common-mode and Differential-mode Signals

By Dr. Min Zhang, the EMC Consultant

Mach One Design Ltd

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There are two common ways of separating common-mode and differential-mode noise. One is the voltage method; the other is the current method. 

When using voltage method, a LISN-Mate is often used [1] [2] [3]. When using current method, an RF current probe can be configured to measure either common-mode or differential-mode noise depending on the wiring configuration. 

I have demonstrated how to set up the test using both methods, see https://youtu.be/0uCXs602_6M. The set-up can be found in Figure 1. This article is a follow-up, which discusses the test results and tries to make sense of it. 

Figure 1  Test set-up; (a) LISN-MATE common-mode measurement (b) current probe common-mode measurement (c) LISN-MATE differential-mode measurement (d) current probe differential-mode measurement (insulation form removed to show the configuration)

We used a buck converter as the DUT and set up the test. The LISN-MATE test result can be found in Figure2. 

Figure 2 LISN-MATE results

We then used the current probe method and compared the results with the LISN-mate result. Because current probe measures the current, we need to find a way of converting the results into voltage. This is done in the control software EMCView, where two settings are very important. 

Figure 3 The Tekbox EMCview can be set up to convert current reading to voltage

The first setting is the Lisn/Att Cor where I selected the transfer impedance file of the current probe I used. The current probe manufacturers should always provide you with the transfer impedance file of the probe you purchase. This file converts the voltage reading of a spectrum analyser to current reading (dBμV to dBμA, as dBμA=dBμV-dBΩ, where dBΩ is the transfer impedance). 

The second setting is that I added a 28 dB attenuation compensation. And the reason is:

Both LISNs used for the current probe set-up are terminated with 50Ω resistors. This is important. When I measured IDM using a current probe, I have measured 2×IDM due to the wiring configuration of the current probe on the cable, so I would need to divide the value by 2, or subtract 6dB on the output. The differential voltage is measured on a 50Ω in the LISN voltage set-up, this means I will then need to add 34dBΩ (50Ω) to give me the DM voltage. That means 28dB compensation file in EMC view. 

When I measured ICM using a current probe, I also measured 2×ICM according to the picture below, so I also need to subtract 6dB on the output, then add 34dBΩ (50Ω) to give me the CM voltage reading. That means, again, 28dB compensation file in the EMC view.

Figure 4 The DM and CM voltage relationship to current [4]

The comparison between voltage (LISN-MATE) and current measurement are shown in Figure 5 and Figure 6. Note that in order to see the difference more clearly, we turned off the attenuator in the current probe measurement, this results in a lower noise floor. 

On the differential-mode noise (Figure 5), from 150kHz to 30MHz both methods are amazingly close (error difference is within 2-3 dB), from 40-100 MHz, the LISN-MATE gives 4-6dB higher reading. But from 30MHz upwards, common-mode noise starts to dominate, there will be CM to DM conversion in the system set-up, so I expected measurement difference between the two in this frequency range.

On the common-mode noise (Figure 6), it is the low frequency range (150kHz to 30MHz) that shows 6-10dB measurement difference. From 30MHz onwards, the noise is predominantly common-mode, therefore the measurement difference in this frequency range is not big. 

Figure 5 Differential-mode noise
Figure 6 Common-mode noise

In summary, both the LISN-MATE and current probe methods can separate differential and common-mode noise well. The differential-mode noise is dominant in the lower frequency range (sub 30MHz), so I tend to trust the differential mode noise results in this region. From 30MHz, noise is predominantly common-mode, so in this frequency range, I tend to trust the common-mode noise results.  

Reference

[1] H. W. Ott, Electromagnetic Compatibility Engineering, New Jersey: Wiley, 2009. 
[2] Tekbox, “LISN-MATE,” [Online]. Available: https://www.tekbox.com/product/TBLM1_LISN_Mate_Manual.pdf.
[3] K. Wyatt, “Review: Tekbox LISN Mate is valuable for evaluating filter circuits,” [Online]. Available: https://www.edn.com/review-tekbox-lisn-mate-is-valuable-for-evaluating-filter-circuits/.
[4] T. Hegarty, “An Engineer’s Guide to Low EMI in DC/DC Regulators,” [Online]. Available: https://www.ti.com/lit/eb/slyy208/slyy208.pdf?ts=1629121242544&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FLM5155.
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