Power line EMI filter + feedthrough filter for a Faraday cage

  • #1
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Dear forumers,

I need a 50hz 250VAC power line filter for an EMI shielding faraday cage. The filter should work between 100khz and 20ghz, attenuation should be >=60dB between 1mhz - 5ghz.

Regular power line EMI filters like this
EMI-Filter-Power-Line-Filter-Mains-Filter-EMI-EMC.jpg

don't seem to go that high (only up to a few hundreds of mhz or so).

I see that there are professional feedthrough filters
HPR-Installation-Example-300x279.jpg

that do the filtering alone but they are quite expensive.

So I am thinking about using a cheaper feedthrough filter or capacitor (one that starts to filter above 10mhz) in combination with the regular one.

The regular one's spice model would look like the one in the attachment.


Has someone any experience with this combination or any counter-proposal against this?

Regards,
Akos
 

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  • #2
berkeman
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If you want 60dB up to 5GHz, you need to pay for the expensive filter. And what are you using for your Faraday Cage? How big is it? What else are you trying to feed through the walls?
 
  • #3
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The cage's dimensions will be ~2m x 1m x 1m (not a rectangular but an irregular one). The cage is for personal use.
I've been reading datasheets e.g. https://eu.mouser.com/datasheet/2/382/resin-sealed-bolt-in-filters-372229.pdf
and these show that high frequency insertion loss is independent of capacitance, so in general the bigger the capacitance, the greater the low frequency attenuation. However, small capacitance (1000pf for example) filters are much cheaper.
I would also like to lead an usb and a cat-5 cable through the metal walls, these aren't that important.

So the small capacitance feedthrough filter would work as a high frequency filter (>10mhz) while the regular one would go from 100khz-100mhz or so.
 
  • #4
berkeman
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Filtering at 100MHz is very different from filtering at 5GHz. Are you familiar with why this is true? :smile:
 
  • #5
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No I don't think I'm familiar with it (I'm googling it). EMI shielding papers tell that any power & data cable that goes thru the shielding must be filtered. All I know that there are parasitic properties of inductors, capacitors that makes these filters problematic to work at microwaves. So for microwaves, very small values make a big difference. Thanks for the eye opening but I don't get it. Why I couldn't insert a feedthrough filter to filter microwaves, then a regular power line filter to filter radio frequencies?
 
  • #6
berkeman
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All I know that there are parasitic properties of inductors, capacitors that makes these filters problematic to work at microwaves.
Correct.
Why I couldn't insert a feedthrough filter to filter microwaves, then a regular power line filter to filter radio frequencies?
You may be able to series-connect filters for different parts of the stop-band that you want, but there are lots of other issues to make the shielding work. Seams in your Faraday Cage welding/soldering can be a problem, for example. And if you are not careful about your filtering, you can impair the communication between the inside and outside of the cage.

Can you say more about your application? Have you looked at commercially-available shielding solutions? I use shielded enclosures in my EE R&D work weekly, and even though I've built some of them on my own, the best solution is almost always to buy one from the available vendors (who also offer options for filtered feedthroughs)...

http://www.vicommtech.com/data/imag..._equipments/302016/manual/shielding_box_6.jpg

shielding_box_6.jpg
 

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  • #7
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Thanks for the answers berkeman, I'll try the series connection then, I have no idea how much it can impair the communication.
I know about the other issues (seams, gaps and honeycombs for air ventilation). I've been reading EMI shielding papers like this http://www.dtic.mil/dtic/tr/fulltext/u2/a257412.pdf for a while.
I've been checking commercial products as well, but they're out of budget in general.

I could say more about the application only in private.
 
  • #10
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For access port seams, it depends on what it is. Power line filters can be screwed to the enclosure, but it'll likely be something custom in which case I will screw it as densely as possible.
I'm using screws with washers to join the sheets (galvanized metal sheets).

For doors, I'm not sure. There was a paper that showed EMI spring fingers are preferable over gaskets.
This is for windows but I'm planning to implement something like this (by using a thicker metal door):

990810r.gif


For usb and cat-5 ethernet cables the situation is more problematic. The cat-5 cable isn't really important, but
it seems that only usb data line filters with optical cable (thru a waveguide) should be employed:


7895-DVI-USB-Data-line-filter-main-image.png


Some webpages suggest that filtering the Vcc and GND wires improves the situation but the data lines simply cannot be filtered.
(I'm thinking on this one too.)

For the power line filter, cheaper feedthrough filters like this one https://eu.mouser.com/datasheet/2/415/4106-000-32394.pdf
go down to 100mhz. On the contrary, filters that go down to 1mhz are ~50 times more expensive.
There are many manufacturers, but it also depends on what I can buy here..
 

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  • #11
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It may sound funny, meat grinder plates seem to be a cheap alternative for ventilation instead of honeycomb panels.
A grinder plate with 3mm holes and with a thickness of 15mm has a cutoff frequency of ~60ghz and works well up to 20ghz, attenuation is "160dB - 20*lg(n)" which is still >= 80dB.

51YOMqDmAUL._SY463_.jpg
 

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  • #12
sophiecentaur
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People usually have unreasonable expectations of their home made Faraday enclosures. RF is insidious stuff and will leak in all over the place. If you don’t have deep pockets then your performance is bound not to be stunning.
The first thing to do is to specify what level of interference you actually need - seriously. You may not in fact need the ultimate screened box.
You may actually need no in/out leads at all if you use batteries and measure and record everything in a well sealed box. That would be one extreme design philosophy.
You May get away with just power cables in and optical links for signals in and out.
Anything less will be a compromise but that may not matter.
 
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  • #13
CWatters
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RF is insidious stuff and will leak in all over the place.
+1

I've done time in a Faraday cage hundreds of meters down a salt mine. Some of the most frustrating days of my career. You can spend a fortune and follow all the good practice and theory as much as you like and still struggle to keep it in/out. Then someone comes along with a bit of kit thrown together in 10 mins that should leak all over the place and yet it doesn't. Then your boss wants to know why you need another £10K and two more weeks while they don't.
 
  • #14
sophiecentaur
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I don’t believe your visitor’s claims unless the performance of their kit relates to some luckily chosen frequencies.
Bosses are like that, anyway!
 
  • #15
anorlunda
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Educate me. I'm Faraday cage ignorant. Aren't all these problems frequency dependent? Is it correct to call the museum image below a Faraday Cage?

hqdefault.jpg
 

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  • #16
sophiecentaur
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Educate me. I'm Faraday cage ignorant. Aren't all these problems frequency dependent? Is it correct to call the museum image below a Faraday Cage?
If it does the job at the required frequency then it is a FC. But you couldn’t sell it for wideband general lab use.
 
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  • #18
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RF is insidious stuff and will leak in all over the place
Sounds reasonable, most documents emphasize this all the time. And it's something I've experienced myself (and I take it seriously).
Someone said in a yt video if joints allow water to flow in, that will allow EM waves to leak in.

Technically, it's an enclosed EMI shielding box not just a Faraday cage, I mean it's solid (a cage that is made of wires is still a FC AFAIU) but I don't know too much about polarization.

I've done time in a Faraday cage hundreds of meters down a salt mine.
That must have really been frustrating, but what could have been the problem in following theory?


I don't exactly understand the propagation of EM waves btw. In theory, a waveguide can keep out EM waves below a specific frequency. EM waves are not longitudinal waves but how is it possible that particles can't go thru such a waveguide (let's assume a circular waveguide)?
 
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  • #19
berkeman
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Someone said in a yt video if joints allow water to flow in, that will allow EM waves to leak in.
Not necessarily. A microwave oven is not waterproof, right?

And the opposite statement is also not necessarily true -- it's easy to make metal-to-metal waterproof seams that are not RF tight...
I don't exactly understand the propagation of EM waves btw
This may help: http://www.rfcafe.com/references/electrical/waveguide.htm

:smile:
 
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  • #21
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Not necessarily. A microwave oven is not waterproof, right?
OK, but that is just for a specific frequency, cell phones still work in an oven because the frequency is not the same.
The youtube video was about an EMP-proof shielding box (where the frequency is unknown).

There are optical usb cables for 100$. But what if the equipment still transmits adverse frequencies thru it?

The following product is claimed to remove all the interference by reconstructing the USB signal, so no adverse signal will be found on the output:

https://www.djmelectronics.com/usb-emi-rfi-filter.html
 
  • #22
sophiecentaur
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Is it correct to call the museum image below a Faraday Cage?
It works well enough to protect the occupant from what is basically a DC risk. Inside it will be full of relatively low level RF shash at all frequencies but that is irrelevant in this application. The Faraday suits that workers on live HV cables use need to be of closer weave than that cage because there is significant RF power when the flash-over occurs. See after 1m in this video (it goes on a bit and is very dramatic).
Edit: It could also be to do with the fact that the suit is very close to the body. I would bet that a 'bird cage' could be more wide spaced.
 
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  • #23
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Interesting video.

btw. When mounting a waveguide or a honeycomb vent onto the shielding enclosure, does it matter which side I mount it?
 
  • #24
Baluncore
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btw. When mounting a waveguide or a honeycomb vent onto the shielding enclosure, does it matter which side I mount it?
The wall of a shielded room will need to be a double wall. The walls are not electrically bonded except at the door, and the power filter feed-through. A honeycomb vent will be mounted inside the wall cavity, but attached to only the inner wall. The outer wall will also need a honeycomb or mesh that does not contact the inner honeycomb.

A guide will have some length and be bonded to one wall at each end. The power filter box should be built as a long box with partitions and removable covers. Half of those covers will be accessible from outside the outer wall, the other end from inside the inner wall. Each box section will have another filter stage, separated by a partition with feed-through capacitors. The filters should not all be simple LC or RF choke filters, there must also be resistive snubbers, AC capacitor coupled, to absorb the reflected noise, or you will end up with all sorts of resonances.
 
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  • #25
Baluncore
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I built my first Faraday cage while I was still at school. That taught me the futility of trying to reach perfection. I can honestly say I have not needed a screened room since then.

There is a screened room in the Engineering Department at the local University that I can use if I ever need one. The availability of a screened room makes it possible to realise that a screened room is very rarely actually needed. When it is needed, the required specifications will be known, and will certainly not require attenuation of the band from DC to Daylight to below the thermal noise floor in your analyser.

In my legitimate surveillance activities I monitored many targets who needed a screened room, but did not know it. I am beginning to think that only criminals and government cipher machines actually need screened rooms.
 
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