From the above figure, we see that the differential mode insertion loss of the filter has a great relationship with the network impedance of the filter. 99% of filter manufacturers only provide curves A and B, but not C and D. The selection of filters based on manufacturer curves A and B alone does not guarantee that the test will pass.

When the network impedance of the input and output of the filter is very different, the insertion loss characteristics of the filter will be seriously reduced, and sometimes even the insertion gain phenomenon. Electronic engineers need to pay attention to this feature. Because at this time, the filter amplifies the disturbance of certain frequencies.

The insertion loss characteristics of the two filters shown in the figure above, the corresponding filter on the right is much better than the corresponding filter on the left. Because the insertion gain of the filter on the right is not only small, but also appears at lower frequencies. Since the lowest frequency of the conducted emission test of commercial equipment is 150kHz, the insertion gain below 100kHz will not cause the test to fail.

Of course, for the electromagnetic compatibility CE102 test, since the lower frequency limit is 10 kHz, even if the insertion loss characteristic on the right may not meet the requirements. In particular, when the frequency of the switching power supply is 40 kHz, the conducted disturbance may be amplified by the filter.