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I've been looking around for answers for a while now and I haven't been able to find anything. Does anyone know how EMI filters are rated? The only specs I have been seeing use an attenuation curve or insertion loss curve to describe their filter but not much else. If I am getting noise on a power line how would be able to decide which filter would be able to screen it out? One of the filters I am looking at uses a spec sheet like this: enter image description here

Am I to understand that anything under the attenuation curve is eliminated or is it something else? I'm sure that I am completely misunderstanding the situation, but I was hoping I could get some help.

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    \$\begingroup\$ The attenuation curve is in decibels, so for every 20dB "increase" in attenuation, the amplitude at that frequency is cut by an additional factor of 10. E.g.: -60dB = 0.001. The selection of the filter depends on what noise exists at the input and how much noise you can tolerate at the output. \$\endgroup\$
    – vir
    Feb 24, 2022 at 23:02
  • \$\begingroup\$ Exactly as @vir said. You need to know what the noise that's coming back on your power lines look like. Is that noise caused by a SMPS running at 400 KHz? Maybe you have system where the load is switched, or pulls current at a 10 KHz rate, like a radar system. \$\endgroup\$
    – SteveSh
    Feb 24, 2022 at 23:23

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More specific to your question. That chart shows the attenuation of the filter - how much it reduces the noise - as a function of frequency. That's the curvy line. The straight line is from a Mil standard and specifies minimum attenuation required over a range of frequencies.

So it looks like this filter was designed for an application that is required to meet Mil-Std 220. And this chart shows how the filter does against that requirement.

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The rating on filter is simply its transfer function: at x MHz it attenuates y dB and so on. The choice depends on what you are trying to pass.

In a EMC standard (like CISPR 12 for appliances) there are tables and/or curves saying, like: at 10 MHz you can't emit more than 60dBµV.

I measure my equipment and I see that, for example, at 10 MHz I'm outputting 100dBµV. So I'll chose a filter which, at 10 MHz, attenuates at least 40dB (with some safety margin).

A big issue is that usually filters are rated on a 50:50 ohm system (50 ohm impedance both on input and output). This isn't really representative of the actual impedences seen on the system. Good filter vendor also state the 1:100 and/or 100:1 filter response which are usually more representative.

At the end of the day it's mostly trial and error (how you mount the filter will influence it's response and noise often will bypass it someway)

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