Feed Thru T Filter I Don't Understand

All,

I understand how the common T filter functions. But I was asked to review a BOM for a board we are doing out of house and stumbled upon this.

https://search.murata.co.jp/Ceramy/image/img/PDF/ENG/L0112S0113NFE31P.pdf

So...it's a T filter but they call them the feed through type. I've checked online and no one really explains(from what I found) how this all works electrically. Pros? Cons? Also, just a basic understanding of how this filter works electrically would be great. It just doesn't make sense to me how you can have oune plate connected to GND and have the other floating. I must be missing something.

-mike

wuchy143 said:

All,

I understand how the common T filter functions. But I was asked to review a BOM for a board we are doing out of house and stumbled upon this.

https://search.murata.co.jp/Ceramy/image/img/PDF/ENG/L0112S0113NFE31P.pdf

So...it's a T filter but they call them the feed through type. I've checked online and no one really explains(from what I found) how this all works electrically. Pros? Cons? Also, just a basic understanding of how this filter works electrically would be great. It just doesn't make sense to me how you can have oune plate connected to GND and have the other floating. I must be missing something.

-mike

Click to expand...

hi Mike,
The cap is 'concentric' around the central conductor.

Often used for getting clean signals/voltages in/out of screened enclosures.

The centre cap is soldered to the metal screening case. OK.

This isn't a normal T filter. This one is used to supress EMI when a wire has to pass through an enclosure or some other type of EMI barracade. Thus, the "pass through" normenclature.

Thanks. You guys are good. Reminds me of how much I still don't know and still need to learn. That's the exact application here. Basically we have almost all of the components on this one PCB encompassed by a metal enclosure. Then right at the connector(outside of the metal enclosure) we have these filters.

So, it seems you both are hinting that the centroidal cap has one end hooked up to GND and the other soldered to the metal enclosure around the majority of the PCB? If this is true what does that centroidal cap do? Maybe if there is some RF induced on the metal enclosure the cap shorts it directly to GND so that it does not effect the signal coming out?

The T feedthrough filter has an inductor in series, a capacitor to ground, and another inductor in series. The two inductors are usually some type of ferrite bead around the central conductor. The capacitor is also centroid around the conductor. It provides capacitance between the conductor and the filter case ground/enclosure.

The purpose of all this so to keep any noise from going through the conductor, either from out-to-in or in-to-out, sort of a fire-wall for noise.

Thanks all. Much clearer now!!