Common mode and differential mode noise can theoretically be analyzed in conducted and radiated EMI. However, when it comes to the actual noise suppression technology, it is more meaningful to discuss the conducted noise. It is also possible to seek reference countermeasures through measurement. The following figure shows the common circuit for conducted EMI suppression in power supply. CY, LCM is a common mode noise filtering network, and CX is a differential mode noise filtering network.
If we can measure the common mode component and the differential mode component of the conducted noise separately, we know that we should adjust the common mode filter CY, LCM, or the differential mode filter CX to reduce the noise. The following is a brief description.
and
in the figure represent the currents on the two power lines. In theory, we can treat
and
as the sum of the common mode and differential mode noise currents, namely:
and
,
is common mode current,
is differential mode current.
Furthermore, we found that
and
can be obtained from
Comparing the two equations, as long as the signal of one of the power supplies can be added to the other power line signal in the forward and reverse directions, the sum and the difference of
and
can be obtained separately, and the
and
can be measured. Therefore, a
and
coupler/adder can be designed at the power supply terminal of the DUT to obtain
and
.
There is an approximation, which uses a current probe with a spectrum analyzer to measure the sum of the two power lines, as shown in the following figures.
In Figure (a), run two power cables through a current probe, connect the probe with a spectrum analyzer to obtain
. Run one of the power cables around the probe (b) to make the current direction opposite to the direction as shown in. (a). By so doing, the
can be measured from the spectrum analyzer, and the results are shown below. It can be seen that the common mode noise component is higher than that of the differential mode, and we know that the common mode filter
and
need to be adjusted to reduce the common mode noise component. Although this is not an accurate measurement, it is an approximation that is worthy of reference.

Common mode and differential mode noise 