Best box to shield interference down to 100kHz - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: January 12, 2016, 05:56:20 am
content: I'm trying to source a shielded box for use as an enclosure for my crazy project to build a NQR (Nuclear Quadrupolar Resonance) spectrometer. I need the environment inside to be shielded from external interference. However, I'm confused about the utility of mu-metal (high permeability) vs simple aluminium (high conductivity).I'm concerned with frequencies ranging from 100kHz to 10MHz or so (so, right in the range of AM broadcast, amateur radio and many other interference sources.) I want the environment inside the box to be so interference-free that any detection that I do will be thermal noise limited (with an air-coil inductor being the sensing element here, in case that's relevant). So, the attenuation I'm after is (typical RF noise in a typical city in the 100kHz to 10MHz range) / (thermal noise). Not a fixed figure per se, but hopefully that's enough to define the problem!It seems natural to me that I'd use a simple aluminium box, but that leaves me wondering what mu-metal is for. I mean, clearly an aluminium box won't shield out the earth's ("DC") magnetic field, but it'll do great with 3 GHz radiation -- so where is the dividing line? What frequencies does an aluminium box start to fail? Would a thicker, more conductive box lower this cutoff frequency?Does an air coil operate differently to an antenna for the purposes of this problem?EDIT:I thought I should probably Google this first, and according tothis articleit seems that a steel box is the way to go for low frequencies. The skin depth for steel at 100kHz is0.06mm, whereas for copper it's0.2mm, which isn't very good given that the skin depth is a mere 9dB attenuation.But then again, every steel box I can find (example) is painted or powder coated for obvious reasons, and thus appears to be pretty poor from an RF point of view. They certainly don't claim to have good RF properties. In short, I still can't figure out a compelling solution.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 12, 2016, 06:14:38 am
content: I considered a mu-metal (both the actual 80% Ni stuff and the 49% Ni stuff) stamping... the application was for shunting the magnetic field of a (possibly moving) neodymium permanent magnet from some... magnetically sensitive parts. Lets say a DC to 20 Hz bandwidth.The quote for the mu-metal part was around 25 times the cost of an equivalent stamped steel part.When you cold-work mu-metal, the magnetic permeability is ruined in the areas where it is cold-worked. So the last step after stamping or welding your enclosure together must be the annealing. The annealing reverses the cold-working, and it restores the magnetic permeability to the level of the virgin mu-metal. The magnetic annealing is a fairly particular process...Ultimately, the mu-metal turned out to be too much trouble (and cost) for the gain. Finally, steel was passably good enough... so you just stamp and install, without worrying about annealing.

author: Theboel
author_rank: Frequent Contributor
timestamp: January 12, 2016, 06:21:16 am
content: I more concern about interference come from in/out connection compare to the box itself.for me double box steel + aluminum is "enough" thats my experience with high gain moving coil phono preamplifier but this is interesting topic if anybody else have more experience than me.

author: Marco
author_rank: Super Contributor
timestamp: January 12, 2016, 06:23:17 am
content: Quote from: rs20 on January 12, 2016, 05:56:20 amWhat about tin plated steel enclosures?http://nl.rs-online.com/web/p/pcb-mounting-enclosures/6276376/

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 12, 2016, 06:35:10 am
content: Paint isn't bad... you just need to paint the assembly after the seams have been welded/riveted/screwed together. Or use masking tape.Or grind off the paint.I would just use a two-piece die-cast aluminium enclosure. Due to not being sheetmetal, you can tighten the screws enough to get a good seal, without distorting anything.Hammond seems to have them.

author: tggzzz
author_rank: Super Contributor
timestamp: January 12, 2016, 10:41:24 am
content: A good source for your research would be the RF community; they have been dealing with these problems for a very long time. In particular, ham radio enthusiasts have developed and published many many articles on how to keep RF in/out of enclosures.Be aware that getting the right enclosure is only part of the solution. The other part is minimising the effects of imperfections in the enclosure such as gaps and joints and conductors that enter/exit the enclosure.

author: penfold
author_rank: Frequent Contributor
timestamp: January 12, 2016, 12:36:56 pm
content: The cutoff point is the frequency where the skin depth is in the region of the thickness of the material for non-magnetic materials like aluminum.

author: rs20
author_rank: Super Contributor
timestamp: January 12, 2016, 12:44:36 pm
content: Thanks for the advice everyone!Quote from: Marco on January 12, 2016, 06:23:17 amVery interesting, I didn't realise those came in the form of boxes (as opposed to PCB-mount shields). I need to open this box regularly to change the sample being tested; will the box have adequate attenuation if it's not soldered shut? (I'm not sure if soldering it shut is even considered normal practice).Quote from: pmbrunelle on January 12, 2016, 06:35:10 amAll good points, thanks!Quote from: tggzzz on January 12, 2016, 10:41:24 amYes, I'll be using BNCs which should maintain good shielding through to the oscilloscope that will ultimately be taking measurements; but I haven't figured out power and control yet. Indeed I should check those articles you mentioned.Quote from: penfold on January 12, 2016, 12:36:56 pmIndeed, althoughthis articlepurports to provide a skin depth formula that factors in permeability as well?

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 12, 2016, 12:55:58 pm
content: Generally speaking, above 100kHz you only need a conductive material to shield both electric and magnetic fields (E and H, respectively). Mu metal is only effective for low frequency, weak magnetic fields (like the Earth's, or from 50/60Hz power lines, etc.); a better general purpose material for keeping out magnetic fields is steel, believe it or not.Otherwise, at the circuit board level the best defense against both E and H field interference is a ground plane: the conductive sheet blocks E-fields, of course, but also minimizes the loop area of any signal on the board, and that inhibits induction.

author: beaker353
author_rank: Unknown
timestamp: January 12, 2016, 04:15:06 pm
content: I'm not sure if it would apply or not to your application but I've seen some highly isolated RF circuits actually have multiple metal enclosures inside one another. They are always thoroughly grounded to each other and the physical spacing between the cans are specifically designed to some end. Not sure on the physics at play or math.-EM

author: Marco
author_rank: Super Contributor
timestamp: January 12, 2016, 06:14:22 pm
content: Quote from: beaker353 on January 12, 2016, 04:15:06 pmYou could also fill the space in between withiron powder. That's getting a bit ridiculous though.

author: rfeecs
author_rank: Frequent Contributor
timestamp: January 12, 2016, 06:43:28 pm
content: There are companies dedicated to making boxes like this. Of course it wouldn't be do-it-yourself:http://ramseytest.com/product_list.php?category=1http://www.rfshieldbox.com/products.htmlRoom sized:http://www.ets-lindgren.com/RFShieldedEnclosuresI'm sure you can find plenty with google.My understanding is that a Faraday cage will only shield static electric fields and higher frequency EM waves. For a low frequency magnetic field, like you might get sitting right next to a large electric motor, it won't help you. That's where the mu-metal comes in.There are also various RF absorbing materials and ferrite loaded materials. Eccosorb comes to mind:http://www.eccosorb.com/products-eccosorb-high-loss-absorbers.htm

author: rs20
author_rank: Super Contributor
timestamp: January 13, 2016, 02:02:39 am
content: Rfeecs,Very interesting products, including the ones that provide EMI filtered data ports as well (and no prices listed on the site!). Very interesting links for sure, thank you. However, I must point out that the shielding on those is only specified down to 100 MHz, which is a few orders of magnitude short of the 100kHz I was after. I'm sure it's pretty good down there as well, although it'd be nicer to have it specified for sure, and the filtering afforded by those special ports would obviously be much more of an issue as well. I might email them asking about my requirements; I'm sure they'll have something interesting to say (if they respond).