The Road to Cost-effective Filter Design

There is no use designing a filter if we don t know how far we really need to go to achieve compliance. So it is a good idea to take stock of where we are at this point. 

The LISN sensitivity increases (at the rate of 10 dB/decade). So we would start getting higher and higher noise readings. 

But the EMI filter starts becoming more and more effective, attenuating the signal at a typical rate of 40 dB/decade. 

This swamps out the increasing LISN sensitivity, and so our measured noise actually falls at the rate of 40 — 10 = 30 dB/decade. (assuming a flat emission spectrum). 

But the limit lines are asking for us to decrease the noise level at the rate of only 20 dB/decade. 

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