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Ask Slashdot: Wooden Chasis and EMF
Red Leader
asks: "Hi. I'm writing in the hope that some electrical
people will be able to help me out with the nitty gritty
aspects of shielding a computer case. You see, I'm building
myself a computer case out of wood. I have already built
one, but it was merely a wood case (replacing the plastic)
which overlayed a standard metal chassis. Well, that was
too heavy, and I also want to use my own layout for the
'guts' of the box. So aside from heat, noise and grounding
(which i've pretty much figured out) - I'm worried about
the electromagnetic interference aspect of this new
machine. It's most likely going to be a dual Celeron based
Ultra-Wide SCSI2 system - which I think (!?!) will
generate quite a large electromagnetic field. I've been
thinking of shielding the case by lining it with this with
copper
fabric or something similar - but don't know how it
sizes up to a solid metal case. Any help would be greatly
appreciated - and I'll be sure to post pics of the case
(well it's actually more than that) in progress and when
completed.
You can get conductive paint ( powdered Nickel ) at electronic supply stores. It's meant for just this kind of thing. You've probabaly seen it, it's the grey stuff inside high-quality plastic cases. :)
Cool thing about wood-- where you need a grounding strap, just nail it on
Cowardly as ever
We are Linux. Resistance is measured in Ohms.
Copper fabric would work fine, as would aluminum foil (assuming you could get a good electrical connection to it) or even conductive paint (this has been used on some plastic cased computers).
As stated elsewhere, the real trick is to make certain that all shields are properly grounded and that there are no large gaps (ventilation is of course okay).
The trick in EM shields is not the thickness of the shield, but rather its conductivity and the size of the openings (smaller openings cut off lower frequency radiation better).
Seems to me there's no reason that a wooden computer case couldn't be both well shielded and beautiful as well.
Good Luck!
do a search for "Electro Dag" this is a conductive
paint type substance used to do EMI shielding
on plastic enclosures.
Faraday Cages work both ways. It is this that allows Foreign Embassies (ours and others) to keep people from reading the EM from the electronics without having physical access.
Not quite the point. I'm not sure whether of not computer equipment falls under FCC part 15 (since it's not designed as a transmitter), but they state that along with accepting any interference from other devices, the device in question must not *cause* any interference to others. Hence the need for shielding.
just remember to make it with no gaps and grounded to your power supply. Also use copper if you can. If you want to make sure it is grounded to the metal, there is a rf gasket that radio's use that is like steel wool and rubber mixed together. It will make a good electic ground.
Whoa, that's heavy. :-)
I know one of the SoundBlaster AWE64 Gold cards has a SPDIF connector. That would eliminate a lot of the noise! Of course your equipment has to accept it. It would be great for converting mp3's to DAT!
I did get this cheap all-in-one pentium motherboard (PcChips I think) which has an SPDIF input from the CD-rom but the sound was somewhat distorted.. maybe it was in the wrong format or something. Not like I could hear any difference between pure digital and an analog cable with those cheap computer speakers.
But _do_ stay away from the arsenic, please.
It might matter if you have other computers around it. I think they might get upset about that. I think most of the interference is going to be the power supply, which is always enclosed in it's own case. I seem to remember some guy who claimed he could crash computers from some distance by using a high-power radio transmitter. Probably not very safe for the user at the time.
Use a sound card with S/PDIF digital output (coax or optical, optical removes electrical connection interference possibility) and purchase an outboard D/A converter. Run that to your Video 2 or whatever input, and you're there. No noise added by the PC, except in the digital processing, and fine sound due the the relatively higher quality of outboard D/A converters. Have fun, sounds like a cool project.
Chris
I can just see Steve spending his many hours at Apple and Pixar playing with the Toy Story interactive CD-ROM on a lime iMac.
So, is my PowerBook G3 a big antenna?
Do you somehow want some kind of ego validation from the rest of the slashdot readers by:
A)trashing the iMac
B)parading your linux.com status?
Painting the inside of the case with a metallic based aerosol sould work.
Back when I had an Acorn machine it used to interfer with the TV something horrendous (UK safety reqirements are somewhat more las than US).
Spray painting the inside of the plastic case with car paint and then earthing it worked a treat.
You're Worrying about the wrong piece of equipment. You could have you machine running in a cardboard box and the emf thrown out of your PC would be nothing compared to the interference caused by your monitor....
What you describe occurs even with "well" shielded monitors. As you know, CRTs use a magnetic field to control the scan pattern of the electron gun. Because of this, monitors are very sensitive to magnetic fields. For example, if you position your monitor close to where the AC mains come into your house and/or near the circuit breaker panel you will likely see some "jitter" in the display - this jitter will get much more visable when large appliances (like the range or washing machine) are running. This jitter goes away as you position the monitor further from the mains. Sometimes you might see this in an office setting if your work area is close to an electrical closet, etc.
Now, a monitor will also pick up magnetic fields from other near by equipment - other monitors in particular, especially if the display scan rates are different on each monitor! That is likely what occured - the iMac was probably at a different refresh rate than your display.
You are correct that the iMac is, perhaps, slightly less shielded than a typical PC but the iMac must meet the FCC regulations just like any other piece of electronic equipment. And, if you really take a look at the iMac, you will notice that what you see through the plastic is mostly a metal frame - you dont really see much of the bare electronics - especially not the CPU, etc. which operate in the higher RF range and must be fairly well shielded.
> Heat and airflow from the fans would probably cause quite some lead in the air coming out of the thing, doesn't sound too healthy..
I bet there is no case-elves using grinders on the lead nor the lead is in liquid state. Lead isn't exactly asbestos.
Another example of this kind of inteference with monitors is to put a mobile phone near to the screen and watch when it rings :)
If an electricaly conductive
shield surrounds the object completely
then all electrical fields are shielded,
without a need for earthing.
It is also true for magnetic fields, if the shield
is magnetically "conductive".
I believe you are correct. As re the freqs blocked, as best I can recall from P-Chem,
;) if the mesh was 6 mm (approx 0.25"), then the frequency blocked would be
wavelength = frequency * c
c= the speed of light= 3e5 kps =3e10cm/second
In this case
equal to 3e10cm/sec/6cm or.. 5e9/second or 5 GHz.
Please someone, check my math.
Jim Gwyn
This was a Flec-tron add, through and through.
I thought I would comment about how U.S. Defense Mapping Agency in Glen Echo, Maryland (you know, the agency whose satelites last flew over Yugoslavia six years ago) has a mil spec for wrapping their *entire buildings* in copper mesh, to protect their computers from incoming EMF in a nuclear war. (I forget the required db drop in EMF density, but they were up there and plain old screen door copper mesh with lots of mechanically strong grounding apparently does the trick.) I was going to suggest that (plain old screen door copper mesh would probably do the trick) until I saw the ad.
Bang. Ya got me. That was a flec-tron setup all the way.
I am about as far from being a physicist as it is possible to be, but even so it seems to me that perhaps a maker of electric guitars would be able to give you some handy pointers on the materials and methods employed in the process of managing electronic emissions within a wooden structure.
A faraday cage need not be "earthed", and a metal screen material purchased at the hardware store should provide good shielding. The problem will be to keep signal paths/shields and power paths/shield separate to avoid ground loops. Thay can be no end of joy. Remember, the shielding must have no gaps. The biggest problem will be, cables passing through the shielding which will allow EMI generated in the box to pass outside, and EMI generated outside the box to pass inside. Recommend careful shielding and use of filter circuits to provide the isolation(at a particular frequency). Good luck.
When I have my TV and my box, which are about a meter apart, My cable reception is terrible. I tryed turning off the monitor and putting it in to suspend mode. (Two of the four fans go off) All this doesn't make a diffrence. I can't move it further away since I have a really small room. The Bios option about emf emission is truned off but this doesn't make a diffrence
Topless or not doesn't matter either. And no I don't have a cheap plastic case i have a heavy metal chassis. The main's aren't grounded so I thought that would be the problem. But again no it wasn't. I took a cable and connected it on a part on the topless chassis. I then hooked it up the the gas pipe used for the heater. So That is surley grounded.
The sparcs were pretty neat, when I connected the cable and forgot to turn it off.
I even hooked the coaxial cable shield from the TV to my "ground" No diffrence at all.
So if you got any help, suggestions, tips, hints or whatever...
And yes the Source is my box. And it doesn't matter what OS i am using.
oliver@mediaone.net
With metal cases, the usual problem is thermal expansion of the board relative to the case as it heats up. So, if you look at a good build, everything has freedom for relative expansion-contraction during warmup, without stressing the plastic layers and insert connections, memory cards included. On a metal case, relative expansions are probably on the order of 10e-3 inch, but even those small changes can produce tremoundous stress densities at critical points, such as at the processor socket, and cause a separation in the board.
With a wooden case, we add new orders of magnitude to this problem. Based on humidity changes from summer to winter in my basement, it would not surprise me that an oak case (coeff of expansion for moisture = ??) might expand a quarter of an inch or so. Not only will the case itself need the required degrees of freedom and degrees of strength, per laws of carpentry (ugh), but the entire mb-cards will need lots of float in the wood case, and all of the card inserts will need lots of differential float relative to the mb and the case, each of which are expanding in different directions. The back plane (holding pinouts) will tend to expand and contract in one direction, and the side plane (holding mb) in another.
The emf is no challenge. Copper mesh with soldered grounding. But the interesting challenge will be stress free mounting of the board while at the same time having a strong case. Would like to hear a follow up on how he sets the grains of the case, works out the aesthetics, and achieves the requried mechanical freedoms.
Also, it's not like the FCC will knock on your door...
;-)
Not to up the flamage, but if you have a ham/CB/etc... radio operator, small-craft (or big) airport, or other place that uses radios (next door with their portable phone perhaps), then yes, you are inviting the FCC over for supper... (If they get a complaint, they'll investigate; Watch for the white van outside!
If you're using the computer in a basement, or are at least about 100 ft. away from annoying anybody, go ahead. Leave the case open... I don't beleive the crap about the health effects of EMI myself...
Does lead chip easily? That might be a factor against it.
I wonder how much lead lead paint contained. It would be interesting to know if a room painted with lead paint reduced EMF...then again, I seem to be pretending to remember that the lead in the paint served a purpose that made painting or paint making easier.
Anyways, just thought it would be ironic that if the lead in lead paint was at high enough levels to reduce EMF from getting out of a room, that we phased it out in the late 70s, about the time electronics was booming and home computer use was increasing.
Who the hell worries about shielding???? I've mounted PC's on a piece of plywood.
This sounds like something I'm thinking about building as well but I don't have the audio requirements you do, so I can't help you wrt the cards (plenty of others have).
But regarding the control of your box, look into the VNC program. It's free, works on a variety of platforms, and allows your to control your headless computer remotely by replicating what would be your displayed desktop (is your desktop, if you had a monitor).
Oh, by the way, no real work gets done on the iMac--it's just there to verify Web sites and to, well, look cute and attract investors. Heh. No one deserves the torture of having to work on an iMac :)
So you bought a machine to attract investors and look cute for your company? A linux.com user, if true, selling out. Heh.
btw, not Apple's fault you couldn't figure out the thing. This is the "Gee, I needed a B&W but bought a 603. Why does it run slow?" syndrome. Other examples include, "I need to do high end graphics but stuck with the 64 megs. Why's my machine so slow?" Or the one we hear a lot, "I needed a high end server and bought NT".
Plenty of people do "real work" on iMacs. Maybe you didn't account for what you needed to work on when one of you folks ran out to buy it. That's your stupidity for wasting money. Shoulda just spent $60 on a 586 outfitted with 32 megs, 230 mb drive, slap a network card, and painted a mini tower case with some Rustoleum and Lexan model paint if you wanted a web terminal. Would cost you $100 max plus some break time and monitor.
The supply company Grainger (did i spell that right?) offers a liquid alminum that you can apply with a brush. They also sell a aluminum putty that works equally well. If I'm not mistaken I think they offer copper versions as well.
Expansion of wooden parts of the case may mean each part will need its own shielding and mechanical ground.
The wooded pieces will probably expand and contract differently than the emf shielding, of whatever form, and the emf screen and ground system may also need mechanical float. (If a single wrap of screeing is glued to two perpendicular pieces, it could become separated with repeated expansion and contraction from humitidy, and lose its ground.) That probably means an organized bundle of groundwires running around the case.
By adding some interior ducting for intake air on an atx positive pressure system, the sound deadening might be really cool. We're so used to the louder and louder whirr from all these new fans.
Is there a reliable, quantitative, and sensitive way to measure EMF for people looking to build their own cases? Cheap too. Or do I need an EMF proof room first?
Been thinking about building a plastic case lately since almost all non-custom PC cases (there are a couple of exceptions) absolutely suck in looks. On the other hand, maybe just buying a used case and painting it would be cheaper and just as neat looking.
Sorry if this was already covered; I'm sorta whizzing through the comments and haven't quite figure out this nesting stuff.
I think lead was replaced by other pigments due to other pigments (i.e. TiO2) having higher indexes (indices?) of refraction. The 'poison' aspect helped too. I would guess that the lead was only 5%-20% by volume. If the size of the lead pigments was the same size as current metal pigments (50-200 microns, I think?), and if the pigments were somehow grounded (not likely, suspended in non-conducting polymer), maybe the
inter-particle spacing would have been enough
to shield some higher wavelength radiation (acting like a really fine mesh?). There probably wouldn't be a straight path through anyway, anyone want to speculate on how the radiation reflecting/refracting amongst the pigments would affect it?
Yeah. Lead is pretty inert environmentally. That was why is made such good paint, flashing, solder joints, and so on. Having done a bit of modelling for restoration of old buildings where the goal was to go for the original look and feel, it has been tough to replace lead. Anyway, my point is that lead doesn't exactly sublimate into the air and your heat sink would have to be pretty damned hot to melt and vaporise it.
I would probably use copper and lacquer it, but then I AM an interior designing fairy after all (tongue only so slightly in cheek).
Seriously, that sounds like a nice idea. As much as I like SGI stuff, it costs an arm and a leg and IRIX is a real pain. Linux is better, the tools are slowing getting there for what I do (and I am seeing the good stuff come out little by little -- Linux is now a serious commercial contender and I would be surprised to see anything available for Macs and NT not available for Linux in 18 months). It would just be nice to have something other than (erk) cream cases. Although if you are going to get cases, I have always liked CalPC, becuase you can stand of them holding several of your closest friends, Walenda-like, and still open and shut the doors!
Lead foil is normally used as soundproofing in airplanes, also lead-loaded foam. The lead stays put for decades without poisoning anyone.
...
Take a pill, man. Or stop chewing on your electrical equipment
On the issue of lead vapors, do you have any citations? I don't recall that this has ever been an issue with medical cases that sheilded Cobalt 60 (that were also quite warm for other reasons). Wouldn't we have radiologists foaming at the mouth, turning beet red, and collapsing from heart failure if "vaporous lead and mercury" was leaking out from the heat? I don't recall that this was an issue with the equipment. I also have been handloading guns for years and I have never seen the generally quite safety-conscious handloading community be at all concerned with anything other than lead vapor when it is really, really hot. Perhaps I am wrong (and let me know with references if I am), but rednecks aren't dropping dead with any greater frequency than radiologists (although they do seem to get into more bar fights).
Anon former ME
It's not just Imac's that will degauss your monitor -- bonehead. I sit in a pod with three monitors on each side (2 ADI, 1 Applevision), every time someone starts up my monitor wonks out for a second.
The connection from shield to ground must also be wide and flat to reduce the inductance of the connection. Preferably the shield should be connected at many points. For example, a solid grounded metal sheet could line the bottom and the shielded top could connect to that sheet all the way across its bottom edges.
Simply pulling a 'pig-tail' off the corner of the shield and connecting it to ground through a single wire will not be very effective. This would create a single path with high inductance that will block high frequency signals.
For reference, the SBLive DOES work in Linux
With a 400Mhz computer, I'd worry about EMF. (I'd also worry about heat dissipation, big time.)
But there's something to be said for convenience and ease of construction.
I'd use aluminum foil. The cheap stuff from the supermarket. And build a sandwich of two thin layers of wood with the aluminum foil pressed between them.
Make sure the aluminum foil on each side is electrically connected with the other sides. And connected to the ground receptacle on your outlet. (And that your outlet is properly grounded.)
Most grounded receptacles will be connected, in your basement, to the incoming cold water pipe. This gives you an awfully good ground. Unless you're dumping huge amounts of current to ground, which I've only seen happen in research labs, I wouldn't worry about getting a better ground connection.
Connecting wires to the aluminum foil can be accomplished through direct physical contact -- i.e.: pressure. Just insert the wires into the sandwich.
Alternatively, mix graphite (finely powdered carbon) with one part of a two part epoxy mixture. (I forget which part. You will have to experiment a bit. But you can't miss it. Graphite acts to set epoxy, and greatly speeds up the harding time. This is why you mix it with the other part.) Put in as much graphite as you can. Then mix the two parts together, forming a conductive glue, and glue the wire to the aluminum foil really really quickly.
Yee-haa! A real tech-geek masterpiece:
/.
1) Uses TeX for equation, rather than ambiguous ASCII math, and cites a result derivable from the equation.
2) Statements proven with a trivial and obviously true thought experiment.
This is what's great about
Eric Monsler
Ha HA HA HA HA! Too bad for you.
As an electrical engineer myself, I want to point out that unshielded electrical equipment is very dangerous. EM wave espcapes from an unshielded equipment will induced voltage on another remote circuit as far as a mile away. Most cars, home appliances, alarms use micro controllers in their circuit. The induced voltage can cause unwanted behaviour on the micro controller. A 60mph car can shift to reverse gear under the interference, because automatic gear is controlled by the mirco controller. It may not harm you, but it will harm other!!!
I hate to state the obvious here, but have you given any thought to the risk of fire with this wooden case? I mean, here we have a massively-electrical box known as the computer which generates copious amounts of heat requiring fans and such and you're seriously going to enclose it all in wood?
I think this is one for the Dilbert newsletter.
contrary to popular opinion, you don't need sheet metal for a shield. as we all know, c = f * lambda, where c is our old friend (speed of light), f is frequency, and lambda is the wavelength. the highest (therefore shortest wavelength) is the processor, say, 1 gigaHz (why not?), so lambda is around 30 cm. so, to shield wavelengths of this size, you need a spacing less than 30cm. so, a rather course mesh is just fine. what, you don't like my back-of-the-envelope calculation? deal with it- i'm a physicist and therefore allowed to simplify.
then follow the others and make sure you ground it not only with respect to the motherboard but also to real ground (i especially enjoyed the post of a large sledgehammer and grounding rod)
Copper mesh should be more than adequate. It would also probably look a lot less cheesy than aluminum foil...
For lids and other movable stuff, it is important that the shielding on the lid and body either have electrical contact, or overlap some. The idea is to keep you from having a slot in the shield when the lid is closed; perforations are OK, though.
Consider how a microwave oven door is built. The shield on the door doesn't make electrical contact with the edge of the oven wall (it's all covered with putty-colored paint), but it does rest flat on it for a bit on all sides. Since you're making the case out of wood, this might be more practical than trying to ensure electrical contact when the lid closes.
Besides, this is sort of what you're doing. At 400 MHz, the harmonics easily extend into the low microwave region.
I hate the "cream" colored ones as well. So I break out the Krylon paint and now I have Red, Green, and Blue cases.
VA_BOFH
> The ability to shield RF with a faraday cage is
> easily accomplished. The key element to keep in
> mind is what frequencies to I want to shield?
> In this case you are trying to keep frequencies
> of approximately 400mhz(or below from other
> chips) inside the box. Since the wavelength at
> this frequency is in the range of 70+ cm, you
> should if a wire mesh is used), have no gaps or
> openings bigger then about 10cm(probably
> overkill).
Not so. Considering a 400 MHz processor, you can expect a good chance of interference up to and including the 10th harmonic of that frequency -- 4 GHz. Why? Processor clocks are primarilly high-rise-and-fall time signals, and such signals, when mapped on a spectrum analyzer, are revealed to consist of an infinite number of ODD harmonics of the fundamental frequency. Fortunately for most consumer electronics manufacturers, this is typically only a problem up to the 10th harmonic.
I do regulatory-level testing for this very thing for a living. In 90% of the products I've looked at, the fundamental frequency is RARELY a problem, as most EE's design with that freq in mind. What they often overlook are the harmonics, and that's what kills us time and time again.
So: calculate for 2 GHz and be done with it -- Oh, some higher harmonics might leak through a portal or two, but at those frequencies they are highly directional so the odds of any real interference are greatly reduced.
If the shielding can't be done right, don't use it. Bad shielding is worse than no shielding at all. Instead of building a poorly designed Faraday cage, you can instead build a hugely efficient antenna.
Grimmtooth
Always the Coward
If you buy a better-brand moboard, the shielding of the case will be negligable. Take a look around you at countless of electronic devices in your house. Are they shielded? Rarely, and usually only localized shielding is used. That's because today's manufacturers design smarter and cheaper.
:-)
Cost of shielding is hidiously expensive when compared to the actual cost of building and populating a circuit board. Manufacturers these days use smart technology (e.g. spread spectrum clocks) and specific-purpose components (SMT ferrite beads and arrays, etc) to keep emissiond down to a minimum.
There are a few things you can do WITHOUT shielding that are usually MORE effective:
==> Ensure that grounding is contiguous throughout your system. Don't tie everything together with copper-core phone wire and expect it to be good. You need to ground the motherboard where it was DESIGNED to be grounded -- those little circles around the mounting holes, and anywhere that it would penetrate the case (parallel, serial, mouse, monitor type connectors). Don't skimp on the material used to create your grounding network -- I recommend 1/2" or wider wire braid, soldered together at any intersections.
==> The second most important aspect in grounding your system is the power supply. Through it, all voltage originates and all supply voltages MUST return. You MUST have a good, clear return path to your power supply, and I don't mean those dinky black wires on the PS connetor. I mean that wide braid stuff, again.
==> An alternative to the wire braid approach is to mount the motherboard and the power supply to a sheet of metal. This is actually better and offers fewer chances for ground loops to develop (a ground loop is a return path that has to take a circuitous path back to its supply, thus making a pseudo-antenna within the path). Along with these two components, it is a good idea to bolt your hard drive mounting brackets to it too.
==> Use spur washers everywhere.
==> Keep internal cabling short and clean. If you're using SCSI, make sure the mounting for the drive itself is firmly grounded and has a clean path back to the power supply and motherboard.
==> If anything cabled creates a problem, try a ferrite supressor around the cable. You can buy these several places. Good companies for this are Fair-rite, Stewart, and Tokin.
How do you know if your system is emitting? Well, if it causes interference with other electronic components, it's a good bet it is. If it still is after all of the above, you might need to shield the entire motherboard. You don't need to line the case (in fact, attempting to create a Faraday cage is worthless since a Faraday cage can have no portals). You can simply shield the motherboard, specifically the area around the processor, memory, and bus drivers. Be sure it is grounded tightly to the backplane in at least four points (cardinal points are best). I bet it won't be necessary.
This is what I do for a living -- trust me
Grimmtooth
(grimmtooth@elric.cts.com)
you'll do even better with an optical link rather than a wired one
I sure hope you are not serious! Since when have optical bits been better than electro-magnetic bits? Bits are bits!
Best of luck. Dual Celery's rule! I especially like those three minute Linux kernel compiles (with make MAKE='make -j6'). :-)
It's expensive as hell, but it works great and extremely lightweight. You can also solder to it, which may or may not be a plus. I use it to shield the electronics cavity on guitars, which I think would be a very similar situation.
Your PC is already full of lead.
Solder is a Lead/Tin alloy
The digital output on a AWE64 Gold is only from the Midi chip so it's no good for mp3's.
SB live has proper digital out's.
If the interference is not great enough to obscure the bits altogether, the bits can be dug out of the wire intact.
Lead does not chip; it's a soft metal which bends very easily - in fact you can quite easily engrave a solid block of lead with a No.2 pencil.
The chipping problem has to do with the lead carbonate crystals used in paint. Ordinary metallic lead will not chip, it will just dent and bend.
The success of lead as a paint pigment has nothing to do with the index of refraction versus TiO2.
The whole purpose of using lead in the paint was the fact that it was poisonous, therefore mold and mildew would not grow on the painted surface and ruin the color.
Don't worry about thermal expansion disrupting your shielding (it's all screwed/bolted together, right?) but make sure your PCI/ISA cards don't come loose or a pin gets misaligned and doesn't make contact. It's happened to me a bunch of times and, oh boy is it a bitch to figure out which one is loose and get them all back in snugly.
In my experience the CPU clock is not the main problem, since the leads are fairly short so it doesn't make a very good antenna. The biggest problem by far is the data buses (memory, video, hard disk, etc) because there are a lot more wires that are much longer, and may extend outside the case. Since data transactions on the buses are dependant on the software being run, and occur at no particular frequency, you get noise distributed across the entire spectrum. Just static across all bands, which can be difficult to diferentiate from natural background noise.
Well, I didn't have an Apple I, but I did have an Apple II.
1MHz is right in the middle of the AM broadcast band, and in fact the Apple II made a real mess of AM radio reception anywhere nearby.
About rf chokes, what would be the proper method to install a rf on on video cable. video cable is not very forgiveing? Any links would be much appricated.
Chad
The two things to remember are to make the shielding continuous (i.e. no gaps in the corners, etc.) and to ground the shielding inside the case.
If you fail to ground the shielding, you defeat the purpose of it, which is to keep the voltage potential constant.
The ability to shield RF with a faraday cage is easily accomplished. The key element to keep in mind is what frequencies to I want to shield? In this case you are trying to keep frequencies of approximately 400mhz(or below from other chips) inside the box. Since the wavelength at this frequency is in the range of 70+ cm, you should
if a wire mesh is used), have no gaps or openings
bigger then about 10cm(probably overkill). I forget the exact numbers but generally as long as any gaps you have are less then 1/4 wavelenght at the frequency in question, you should do a fine job of shielding.
In addition to wire mesh, any ferrous metal should do just fine, regardless of thinkess, since
the idea here is bouncing light off a mirror, rather then stopping a bullet with a wall, to use a metaphor.
One final note..if you're like most people, you are much more likely to encounter problems from RF getting into your computer then RF escaping (radios with a few watts of output power near unshielded hardrives can sometimes produce
unpleasant effects)
Jean B
General Radiotelephone Lisc., Advanced Amateur.
My days as a radar tech. tell me that all the other posters are correct.
A faraday shield is an EARTHED conductive material that COMPLETLY surrounds something, and electromagnetis radiation from either side will strike the shield and be drawn to earth.
If your equipments isnt earthed properly the shield does nothing, the electromagnetism will be re-radiated by the case.
As far as a mesh goes i beleive that a mech will block lower frequencies but not higher frequencies. If the wavelength of the elecrtomagnetic radiation is less than the diamater of each hole of the mesh then the shield will do nothing at that frequency and higher.
The above is how waveguides work (waveguides dimensions have to be a multiple of the wavelength)
This is probably way too much information, but i couldnt resits dredging up some old memmories
To do it right, you will need to cover every area possible (except for door openings) and you will need to make sure that _ALL_ of the fabric is _PROPERLY_ grounded to an _EARTH_ ground. This is not as easy as it seems.
I have some experience in this area, some years ago I repaired monitors that had very noisy switching power supplies and the FCC "busted" me because we blocked out the cop station next door with our interference. The solution was to encase our monitor racks in screen that we grounded. I put a little radio next to the racks and kept it on, if I got interference on that then I knew something was wrong and had to shut down. It worked like a champ!
One other thing to remember about figuring out what size mesh to use when shielding PC's...harmonics. The PC bus may be using a 100 Mhz clock or the CPU running off a higher clock but that is not a sine wave, it's a square wave & square waves have harmonics, the degree of which depends on the rise & fall times. But since even Ku band satellite dishes (12-14 Ghz) have a mesh around the size of a window screen, those should be fine (window screen, not satellite dish ;-) ).
GOESMan (awaiting password)
If I remember my EMC class correctly (which is about 2 yrs ago) even metal computer cases hardly provide any EMC shielding.
The problem lies in the fact that there are gaps in any computer case (disk drive, CD-ROM, anything that can be seen on the outside of the case). These gaps leak almost as much of your EM fields as having no case at all (at least at the clock frequencies used by your computer).
The only normal case I have seen that shields _something_ is my Alpha XL case, which is lined with EMC springs around every gap. This prevents the computer from disturbing my screen too much (and yes, it even makes the screen wiggle when the case is closed).
If you want EMC protection, you need a special EMC cabinet and knowledge about connecting cables to such a cabinet (you can't plug in cables just like into an ordinary computer).
Gtnx
Marcel de Boer, 3rd year student EE/IT
Ugh. iMac's monitor isn't shielded very well. At the place I used to work, someone turned on the monitor for the iMac next to where I was working, and my monitor got funky for a little while. It's like I degaussed it for the first time in years or something.
:)
Oh, by the way, no real work gets done on the iMac--it's just there to verify Web sites and to, well, look cute and attract investors. Heh. No one deserves the torture of having to work on an iMac
Anything I say is my opinion and mine only, it is not shared with anyone for whom I work with.
I do not work for Linux.com full-time yet. That should tell you what you need to know.
I'm sorry for any inconvenience or grievance that I may have caused.
I would like to say that I apologise for the negligent posting on Slashdot.org regarding the shielding of the iMac. The shielding of the iMac is obviously well within legal tolerances, and using the iMac will suit users in much the same way as Windows or Linux. I did not mean to badmouth the reputation of Macintosh systems. Most of what I say in Slashdot is in fact a joke. I will endeavour to make the distinction between truth and simple teasing painfully clear; in the event that I cannot, I will post through other means or not post at all.
I would like to let you know that I am not yet in the employ of Linux.com. I currently work for a separate company for which the iMac is located. And its uses are just that--to test Web sites for accuracy and nothing else.
Once again, I apologise for any inconvenience or grievance I may have caused. Thank you for bringing this to my attention. If you have any questions or suggestions, do not hesitate to send me an e-mail. Thank you.
--Jeff
I'm not certain myself (been a while since I did anything electrical, and I never fully groked FCs anyway), but wouldn't this sort of thing work in both directions? ie fields outside don't affect the inside and fields inside doeen't affect the outside. I suspect it depends very much on your grounding.
Bill - aka taniwha
--
Leave others their otherness. -- Aratak
Thought so. Thanks for the confirmation.
Bill - aka taniwha
--
Leave others their otherness. -- Aratak
Posted by viperx2:
The Diamond MX300 has a daughterboard, the MX25 I think, that is fully digital and I think I read that it is compatable with minidisk format, so I assume it is also DAT ready. They use the same cables and stuff. The MX300 was 50$ on pricewatch, and the MX25 is available from diamond for only 25$. I looked at this a while back, so my numbers are off. The MX300 works in linux, while the SBlive does not. The MX300 is truly a fraction of the price, and, from what I understand, a much better card.
ViperX2
-- The more you tighen your grip.... --
Posted by iam1:
I would agree with the person above who said to check out the electronics setup in guitars. I am not sure about the copper "fabric", but a thin sheet of copper shielding will do everything fine. As far as grounding or "earthing" it, your power supply would do that for you.
Posted by HeapDump:
The cases we sheild at work have nickel acrylic sprayed on the inside. They say that the trend is towards copper, but only because it is easier to landfill in Europe. It is a big thing for CE certification, or so I am told.
I'm pretty sure the MX300 does not work under Linux but the SBLive does.
Correct me if I'm wrong, because I would love to use an MX300 in this computer.
You can't overestimate the importance of beads. All the machines in my house have full metal cases, and kill TV reception, unless you put inductors on every single lead. (Especially the power cable, as I think RF goes down that and up into our TV amp.)
retrorocket.o not found, launch anyway?
You should see the ground systems some hams have. But this is a lot more than what someone needs to install just for an "adequate" ground.
:) The ARRL Handbook (Basically one of the bibles of amateur radio) has a LOT of information on this.
It also depends on the conductivity of your dirt. A swamp is a lot better than a desert.
retrorocket.o not found, launch anyway?
As far as if you can't see the problem, no one else will - I have to agree with you on that.
But if there is a problem, it can possibly occur house-wide. The only thing I can think of is that RF is leaking through my AC mains and into our TV preamp. Whenenver any machine in my house is running, Channel 11 gets funky. Putting ferrit on the PS cable and a few others helped a lot, but it's still pretty bad.
retrorocket.o not found, launch anyway?
So if you design something which is almost an equipotential you get an OK shield. Now, I'd think a fine enough mesh (something magnetic is probably best) would be suitable. But IANAEE and IANAP (an Electrical engineer or physicist).
Heh, when I was little, for Christmas, my brother got a radio controlled car, and I got a Star Trek walkie-talkie, with a morse-code key button. It didn't take me long, listening to the walkie-talkie, to realize that I could hear muted signals coming through from my brother's car, so I tried the morse-code key, and I made my brother's car turn. He tried returning it, but they wouldn't take it back because it worked just fine at the store. I drove him nuts for weeks until he caught me in the act, and beat the crap out of me.
"The number of suckers born each minute doubles every 18 months."
-jafac's law
These are my friends, See how they glisten. See this one shine, how he smiles in the light.
The Trident 4DWave NX based cards support this. They're still a little hard to find, but you can order them from Hoontech at http://www.hoontech.com. They offer quite a bang for the buck.
Trident actually wrote a GPL'd Linux driver for this thing, so it's a good card to buy.
Lead is a serious poison, especially to anyone under the age of twelve.
And it doesn't require the children to lick it to pick up a bad dose. Basically, the big danger of lead is that it let's off lead dust constantly. This dust is then inhaled directly into your blood stream.
I speak as a father whose son has had high lead levels... It's not worth it, don't use it.
-- Slashdot sucks.
Reminds me of my father almost 20 years ago, building a keyboard with a wodden case for the TI99/4A. It looked like the Flintstone's computer and was a long - running gag in my family...
Dolby Digital, DiscMan and DAT all use SPDIF for digital I/O. More and more soundcards support SPDIF, but beware: some only sample and output at one speed (SBLive=48K). If you're going for analog I/O, spend money; some cards cost more for a reason.
SPDIF runs over fiber or copper, converters are available. The SBLive! forums seem to have good discussion of SPDIF issues. There are also cheap SPDIF-only cards available. Some are supported under Linux, I believe.
'twould be nifty to have a thinclient with speakers in each room, feeding off a MP3/netradio server. Of course, decompression lag would lead to echoey audio between rooms. Better to just use USB speakers.
Use copper-clad steel sheet if you can get it. That way you get the benefits of magnetic as well as electric shielding (an RF emission has both aspects).
If you really want to cut emissions, get some tin-snips so you can cut sheet stock to shape. Look at current cases to see how they even have petaled shielding cupping the keyboard connector to prevent leaks there.
You'll also want to get some clamp-on ferrite beads (All Electronics in LA is my source) for the cables. The beads are lossy inductors at the intended frequencies, so they eat signals that would otherwise be emitted (or injected: you're protecting the system from arriving interference as well as preventing it from emitting interference, after all).
You'll have to be more careful to have good airflow throughout that box, compared to an equivalent metal box, because the wood is a better thermal insulator than metal, so the case itself won't throw off as much heat.
Besides shielding your box, it's nice to put ferrite RF chokes on the cables, especially the video cable but preferrably all cables running out of the computer. The cables work as antennas, potentially leading some of the RF noise out of your shielded box and emitting it. Some cables have the choke molded in, if yours doesn't, get the clip-on kind from the Radio Shack. The more turns of wire through it, the better it works.
I also use shielded CAT-5 wire on my 100-base-T network, and I use that same wire for my DSL connection, from the point where it enters the house.
Thanks
Bruce Perens K6BP
Bruce Perens.
You certainly won't last long aganist our attacks, my friend.
Go boilers!
Oh. Right. You mean "potato chip bags." Heh. I forgot they say things weird on the other side of the big pond.
Each part DOES need to be certified individually, but if you assemble them it changes the picture: how well do they work TOGETHER.
If you assemble a system from parts and want to sell it, it needs FCC testing. JUST because the parts passed muster does not mean they will assembled.
How do I know? We sell workstations with custom hardware and software, currently based off Intergraph GT1 TZ2000's. Even if we make no modifications we would need retesting, but once you add in custom PCI cards with additional processors there's some work to be done.
When someone turns something on, and your monitor goes flakey like that it's because your POWER system's stretched. Plug into another circuit, or MAYBE add a UPS, and tell me if it still happens.
I'll ignore the rest of your comments since you knew they were unprofessional... but you probably think Mac users are people not "smart enough" to use Linux shouldn't use computers, riiight?
A little tip - when you use the linux.com URL and reply-to address in stupid posts, it's no different than using company letterhead to for dumb faxes or snailmail. You'll figure it out soon enough...
Actually, the Apple 1 came *without* case - you were expected to supply your own.
Some of the computer history sites show the same wood photo over and over, but that's just chance and distribution of the same photo.
And at 1ish MHz, the thing didn't throw out much heat or (guessing here...) interference. Heck, my G3 300 MHz doesn't even need a CPU fan - just a tiny heat sink. By contrast my AMD K2/450 has a heat sink the size of a coffee mug, and a fan atop to boot...
... btw: IIRC the Apple I ($666.66 mailorder price) featured a wood case.
Though I doubt the grain was anything to write home about..
I don't know how serious you are about the sound quality, but you'll do even better with an optical link rather than a wired one. A wired link will still introduce noise into your DAC, though the better ones will do a good job of keeping that noise out of the analog.
:)
Out of curiosity, what DAC do you plan on using? I've seen some mighty cheap ones in Stereophile as of late... I even recall one that was less that $100!
I seem to remember that a 100 MHz signal has a wavelength of 3m (c/100Mhz). Likewise, a 500 MHz signal has a wavelength of 3/5m = 60cm. Even at 1GHz, the wavelength is 10cm. I'd say you're fairly safe using a mesh (at least for now).
Just stick aliminum foil to the inside of the
case, just do not forget to connect it to ground,
also connect the motherboard to ground. All the
individual cards need to be grounded too.
Just stick aliminum foil to the inside of the
case, just do not forget to connect it to ground,
also connect the motherboard to ground. All the
individual cards need to be grounded too.
First of all , there are three "S"s in CHASSIS.
And secondly, you are all thinking too
complicatedly (sic), just take some metal mesh
(screen door?) and staple it to the insides of
the wooden computer case. And yer it is a
faraday cage, and yes it keeps emf out and yes
it keeps emf in. I should know this crap, I have
a degree in physics.
Look for a product called mumetal (sp).
It generally comes in sheets which are
almost like thin cardboard.
Miles Lott
go irish!
When I was 14, my dad brought home some Compaq portable XTs (suitcase models) that he found in a dumpster. I got a couple working and sold them, but I kept the rest to fiddle with. I built what may have been the first wooden XT tower. It was nice, but even with three cooling fans, it got HOT!!
What I did was remove the plastic casing from a machine and use wire cutters to split the chassis in half. With the Compaq, the 9" monitor & PS were on the left and the drives and MB and cards on the right, each in their own separate cages. The I attached a board to the top of the MB cage and attached the drives and power supply (the PS was a bare board!) to the top of that. I attached a cooling fan behind the PS, cut a hole in the board and had another fan ventilate the MB, and had a third fan ventilate the 20MB Miniscribe HD. The outer casing was all plywood except for the base, which was 3/4" pine. The front bezel and back cover were screwed directly into the cages, but the top and side covers wrapped around with hinges and attached to the base with latches.
As far as shielding, it was very necessary to have the entire cage. If I left the side panel off, the EMI was enough to make the TV fuzzy and to cause the cordless phone to lose sync with the base and not work until it was replaced on the base. For anyone making a case, don't forget what a great INSULATOR wood is and to shield well. In my case, the large amounts of heat were also a result of having old equipment (such as the completely stepper driven HD and the 8087 chip), but I'll bet things haven't improved much since then. The cages in the machine were not solid, but sheet metal with nickel-size holes cut at regular intervals. If you have an old machine, why not make the conversion?
I've used a a couple coats of carbon paint to shield against RFI in electric guitars and even good old heavy duty tin foil (what a bitch to tape down though). This might do the trick for you, you just ground it with a screw. Thin sheet metal would be best but if your case is solid hardwood the weight savings of paint would help.
I have used 4 to 5 five computers in the same room with the covers off with no problem. Sound was fine, moniters were fine, TV was fine, and radio tuner card was fine. Why would computers be bothered by the EMP created by computers, They are always inside the Faraday cage of the case.
The amount of interference from AC wall power is worse in my experiance then the interference then a computers with there covers off.
One day people will learn the folly of Winbloze, Linux Rules!
Heat and airflow from the fans would probably cause quite some lead in the air coming out of the thing, doesn't sound too healthy..
Okay, I've read everyone's comments, and I think that the copper mesh should provide sufficient shielding. I don't know how overboard on that I want to go, considering what else I'm trying to do...
As I said, I hope to put up pictures of it in progress, and when complete. I just need to get to a scanner...
There was one comment about heat, and one about the movement of wood. To those guys: I've got that pretty well covered.
As of my current design, insulating against sound is my primary concern. I'm going to line the entire case with a really dense (1 lb. per sq. foot) dB blocking fabric, as well as baffle any ventilation openings. The sound concern is directly related to my ventilation solution. I've long since decided that a filtered positive pressure system is the only way to go. The implementation on the first case wasn't so hot - but I've learned from my mistakes. Both cases are supplied with cool air from a large 90mm AC fan (80? don't remember) of all metal construction. It's not a normal case fan, but rather specifically a high power 'pushing against resistance' fan. The only problem is that it's pretty damn loud, hence the complicated sound protection.
As for the wood movement: Absolutely. I really enjoy joinery and fine woodworking, and realize that one can't make anything without keeping all the grain essentially parallel. For those who don't know, wood expands and contracts with the weather by varying amounts across its grain (perpendicular to the stripe pattern). If you were to make a joint in which the grains were not expanding and contracting in the same directions, then your joint will literally tear itself apart with time. Then what you made will crack, and or, fall apart. Case in point - on the old case I have a mahogany and maple board (grain running up/down; two strips of mahogany with a central 'racing stripe' of maple) working as the face of the box. In it is a slot for the CD tray. I took off the plastic tray's end cap, and mounted a pice of ebonized maple on it. That piece has grain running left/right (naturally), and hence doesn't significantly expand left/right. The face does move, though, and in the winter I was surprised to see that the extra room I had given it on either side wasn't enough, and that the CD tray wouldn't close unless I took the end cap off!
I can go more into depth, and talk about this for many kilobytes, but I'll save that for the web site where I'm going to put all this up for everyone.
Thanks to everyone for their help and comments (sorry, aluminum foil people, I'm going with a little more style than that... this *IS* a piece of furniture, after all). If you have any questions, feel free to talk to me. I love bouncing things off people, and I've given this project a LOT of thought and planning.
Thanks!
Many of us run caseless w/o any problems. It really depends on what else you have around it. Neither wood nor plastic shield EMF. Sometimes running without a case is nice if you're lazy like me or just like blowing the dust around.
For example, the keyboards are milled from one single piece of wood, so that the age-lines go through the entire keyboard and the keys.
That is one of the coolest things I've ever seen. It would be interesting to see one of the keyboards or mice after a couple of years of continuous use. I'm sure there would be all sorts of interesting wear and colouration.
Is it possible to be a wood geek? Mmmm, hardwoods.
Laugh while you can, monkey boy!
But wait, all a Faraday cage does is ensure there's no EMF inside the cage, which is sort of the opposite of what you want... someone with more electrical engineering knowledge bail me out here?
-- Some things are to be believed, though not susceptible to rational proof.
I would like a "music server" in my house. That is, connecting an old computer with no peripherals to a music system. I would control it by telnet or a web interface. Obviously, good sound quality is a high priority, and that might very well mean shielding.
Actually, though, what I would really like is an all-digital sound card, with any analog processing completely cut out of the loop. Ideally, it would output a digital signal that a good home audio system will decode, such as Dolby Digital, if that is possible. Or maybe the DAT format.
Anyone know how, with digital signals, shielding, or some combination of the two, you could make the best, cleanest audio signal possible? This wouldn't just be for MP3s, but also higher quality stuff. Any advice on this greatly appreciated.
Vidi, Vici, Veni
I work at a printing company, and some of the packaging we have to do is done with wooden crates lined with a kind of aluminum foil where one side is, likely enough, aluminum foil, and the other side looks more like utility paper, or packing paper. The machinists in the building actually use the same foil to prevent an EM field, which causes nausea when working with the two pieces of equipment we do have that are highly electric. They were given this hint by some other printing company in Britain who uses the same devices.
:)
I'm not sure where you can purchase it exactly but with a little poking around I'm sure you could find it. One side looks like normal aluminum foil (a little higher gauge however) and the other like a brown paper bag. If you try it, lemme know if it works, I might just comission a computer that doubles as an endtable.
"It's here, but no one wants it." - The Sugar Speaker
Copper fabric will corrode over time. This will make the size of the holes efectively larger as the individual wires won't be connected at each cross anymore. This is a problem with all woven wire shielding methods, even ones made of Stainless Steel wire. The other problem is you need to ground both ends of every wire or those wires you don't properly ground will become capacitive radiators.
The way around this is to instead use perferated or expanded metal sheets. Perferated metal sheets are ones that have many holes stamped into them. Expanded metal sheets are done by slicing many short parallel slits into a metal sheet, then streaching the sheet to expand the holes. It can then be optionally rolled flat again. Either way it weighs less than a solid sheet, but still supplies EMI/RFI blocking if made of the right materials.
Well, the only one I know of (that's not a professional card) is the SB Live, full edition. With it's digital I/O card and the extra optical digital card, you can input digital, process the signal digitally, and output digital. It can even mix digital audio, internal digital CD, digital MIDI, one analog source, digital SP/DIF, and optical digital all at the same time. I use my computer as a piece of my stereo system, with the internal digital CD, two digital in's (MiniDisc and an external digital out CD player,) and MP3; all being processed for better sound (if you're running [god forbid] Windows, try using the "Concert Hall" environment for classical music.)
And, of course, it's 48Khz digital SP/DIF out is great for recording to MD or DAT. (Or going to a high-end reciever)
Another non-functioning site was "uncertainty.microsoft.com."
The purpose of that site was not known.
I don't know if copper cloth with cut it, but reatively thin sheet metal should do it. This is what they use for a lot of plastic cased PCs Make sure that the sheets are well sealed and are electically contiguous.
Your computer will generate RFI, and very little EMI. Copper mesh will work well, and is far easier to connect to than aluminum foil. If you emit significant RF energy, you will run the risk of upsetting neighbors who are ham radio operators, or perhaps will interfere with TV reception.
If you know of any nearby hams, they can help you with practical tips on how best to shield your case. A ham can also scan some of the radio bands looking for your noise, if willing.
To check for TV interference, use a TV with rabbit ears (not cable) and check the low VHF channels (2-6). Those are the most likely to be affected. If interference is present, shut down your PC to be sure whether it is the source.
Also be aware that the noise signature of a machine will vary as a function of what it is doing. Disk access operations will change the RF compared to purely CPU based operation, so do a variety of things with the machine while checking.
Also, RF, as with any field energy, decreases as the square of the distance, so move the TV within 6 feet or so for initial checking. If you can't see a problem there, chances are no one else will, either.
--- Bill
What's the big deal with EM worries? I run an O/C Celeron 400 with all kinds of sensitive parts (SBLive, DVD Decoder, etc.) just thrown caseless into a tiny little wood cabinet and I don't get the slightest bit of noise or interference in anything. I always figured the shielding bit was one of those stupid FCC regs that could be safely ignored.
Nice try but lead is not a ferro magnetic media. Steel wire cloth (window screen) would work better, be lighter, less toxic. You can cut window screen with kitchen shears and staple it to the inside of the case.
(Yes lead is a conductor, a poor one. Faraday cages are great for weak electric fields but leak magnetic fields unless a magnetic conductor is used to trap that nasty B field)
While your children are not licking the inside of your case the heat from the components in the case will cause vaporous lead and the trace mercury found in all lead to out gas. In and enclosed area like the typical bedroom you could get heavy metal poisoning in a few months of exposure.
Yup, some people use lead as a shield against radioactive particles. So it blocks Superman's x-ray vision. Funny thing is, things like cement, paraffin and water are better shielding per unit weight. It must be some 'other' property besides particle capture cross section that blocks Superman's sight. Otherwise he should not be able to see through a concrete wall.
3-4 feet isn't enough in a lot of areas, I have a friend who just put a new ground rod in, and code required something like an 18' rod in his area, he had one heck of a time getting that all the way down.
-Ted
I'd like to see how a finished one looks and how hard it is to make one. I'm getting ideas of a tower case that doubles as a couch endtable....
The emf-l mailing list comprises laymen, experts, and other concerned citizens. The topics are just about everything you could think of in the field (pun) of the ever-increasingly "blue" world that we're all living in. Unfortunately the list moderator and maintainer is (as of today) on a two week vacation which puts the list on hold as well. But, you may still join now (OH, the work load of the /. effect. Poor Roy...). Just send an email with your concerns to rbeavers@llion.org and surely there will be someone who is able to help you.
/Adam
PS. BTW, don't mail me, as my mail server is currently broken. DS
I work in television engineering and we deal with EMF issues on occasion. There are actually several issues to consider.
First, there is low frequency EMF, this is basically magnetic fields. The funny colors on your monitor that clear when you degauss are the result of low freq EMF. Put a speaker or magnet near your monitor and you can see the effect.
Second, there hum EMF, typically seen when a monitor is near a source of EMF at a nearby frequency. You may have seen this if you place a 60Hz monitor next to a 75Hz monitor. You will see hum bar rolling through the monitor.
Thirdly, there is high frequency EMF, not typically seen unless dealing with sensitive electronics with low signal to noise ratios.
For the absolute best shielding use mu-metal. It is specifically designed to block EMF. There isn't much better than mu-metal. Be prepared, mu-metal is expensive. See www.mushield.com (best) or Cutting Edge (okay).
If you can't afford that, the copper will probably work OK. Try to leave as few gaps as possible. Make absolutely sure you tie any shielding to ground. Other simple tips are making sure the power cords are plugged into grounded sockets (get a $5 circuit tester), use a good surge protector, and put those little ferrite clamp-ons on your I/O cables. Check out the EMF Do's and Don'ts here.
I think the thing to worry most about shield ing is your monitor. It is the most EMF susceptible thing you are likely to have. If the built in shielding is good, you will probably be OK.
But in all reality, you might try running with out the shield at first and see how it goes. If you don't see hum in your monitors, your speakers don't buzz, and the dog doesn't have his paws over his ears, you might be OK without it.
Good luck.
nuclear iraq bioweapon encryption cocaine korea terrorist
And don't forget your tin foil hat.... I never take mine off. Those aliens... err electomagnetic fields, aren't gonna get me!!!!
Opinionated Law Student Strikes Again!
No, it's quite a few moderately sized antennas and a couple thousand minutely sized ones.
Several places sell sheet lead about a 16th or 32nd of an inch thick for use in soundproofing. I've worked with it, and it is really easy to work with, incredibly flexible and easy to cut. The only problem with this is that it is *really* heavy. But for shielding, there is probably nothing better. Hell, superman can't even see through it.
Before anybody says it is dangerous for children/animals, do your children lick the inside of your computer case? I thought not.
Reading the articles in this thread is certainly interesting me now as I am doing a similar project on and off.
:-)*
I have a server in the rec-room that does masquerading for my local network (I can even hook up my laptop using PLIP and use it whilst lying down on the couch. Man, am I lazy!). Though the main reason I put it in was for an MP3 player to hook into the receiver. Here's the specs for those interested:
- K6-2/300
- 64MB RAM
- 2.5GB HD
- Good ol' GUS (though you guys have gotten me interested in using one of my TB Montegos (they've have gone waaay down in price recently, I believe they're cheaper than the SBLive), which has SPDIF and 18-bit AD/DA converters, so that might get interesting, especially since I now have a DVD and am thinking of getting a digital receiver. I can record sound from DVD now, though)
ATI All-In-Wonder Pro 8MB AGP (Waiting for gatos to get TV-Out... Then life will get more fun)
- Generic (Packard Bell) PC remote. With LIRC, I can use any remote to control MP3 playing.
- SuSE 6.1
It also does SETI@Home for Team Slashdot as does the other computers on the network (except the laptop of course)
So, if you want absolutely, positively, best playback on a system, get a digital receiver and either a TB Montego or a SBLive and you should be sittin' pretty without having to worry about shielding.
What you want is Tempest-grade sheilding. Do some research on tempest.
All computers emmit a certain amount of electomagnetic radiation. Now, let's take your monitor as an example. Obviously, it emmits visible radiation (light). But that doesn't travel very far. But your monitor ALSO emmits other kinds of radiation. These waves almost resemble the image on your screen, if you analyize them enough. These waves are usually much faster than visible light waves. That means some feds can park a black van accross your street, intercept the highspeed radiation ebing emmited from your monitor and regenerate the exact display of your monitor in the van.
The United States Government started research on this phenomenon many years ago. They code named the project 'Tempest'. Today the Tempest project has been mostly declassified and information on Tempest can be found all over the Internet. I can find the URL for a really informative site on Tempest, unfortunatly. You could try certain channels on IRC. I beleive someone from #2600 (EFNet) has a huge site on tempest.
You can buy snap-on chokes (basically coils) at Radio Shack to snap over all the cables leaving the case. These cut EM WAY back. You can test this with a cordles phone (old 49mhz type). Hold it (turned on) next to the cables at the back of your PC. You can hear the signals (sounds like new age music).
Last night I was working on my sons remote control truck. I set it next to my keyboard and it took off! It was picking up signals radiating from the keyboard, or more likely the keyboard cable. It acts like an antenna.
Actually it is the opposite. Office/Industrial
equipment with oscilators over 10 KHz must be FCC Class A Aproved. Residential equipment must be Class B Aproved, which is generally 10dB more strigent for most frequencies.
The theory being, residential environments have more radio and TV receivers than industial/office environments, and residences are typically closer together than factories and office buildings.
------- CAIMLAS
~/ssh slashdot.org ssh: connect to host slashdot.org port 22: too many beers
The whole idea sounds pretty cool - I remember a company back in the 80's that was making cases like this for XTs and monitors - I guess a company still is (www.woodcomputers.com).
If you are going with an all wood case, I would line it with thin copper or steel sheet (heck, aluminium foil would prob. be ok), and solder at the edges, or use metallic tape. I would forget the fabric stuff. Build the case so that the back side/bottom is a normal metal case (with the front, top and sides being wood), and use "L" angle aluminium to screw the wood down to the PC chassis to complete the shielding.
Another thing to consider is to just "fake" it - bond/screw the wood to the metal case - in other words, build a facade case around a normal metal/plastic one - shielding would be taken care of by the regular case. Unless this is all to impress any woodworking buddies or something...
Reason is the Path to God - Anon
If you need better grounding, or think you do, and don't understand it well, then get a good electrician.
With that said:
You would want a steel-clad copper rod, not the other way 'round. Exposed copper will quickly oxidize into an amorphous, non-conducting blob. The steel should be galvanized, of course. Aluminum or a similar "rustless" covering is good, too.
Then you need to remove the inferior ground from your house first. This is a must-do. If you have >1 ground point, imagine what happens when lightning hits that shiny new ground rod you installed...
There's suddenly a huge positive potential at the rod, which will dissipate as quickly as possible wherever it can. It takes a while to do that, though, because the earth doesn't have a high density of delocalized electrons. It takes lots of earth, and some time, to sort out the potential difference introduced by a strike.
With just one ground point, you're pretty much OK -- there aren't enough idle electrons in your equipment (compared with how many there are in the earth surrounding the rod) to do much damage.
With a second ground point, though, there are *zillions* of handy electrons near the earth around the other ground rod.
They will flow from the old ground, up to your house, through your equipment (depending on exactly how things are wired), and out to the new ground. Before it burns out your wiring and sets your home on fire, this current will be very large.
Incidentally, this is also why it's not good to be near a lightning strike and standing on open ground. If your feet are planted, you have two ground points. There can be ~1e5V potential difference between one foot and the other as the charge dissipates. Without heavy rubber soles, that difference will resolve itself by sending current through your low-resistance legs and crotch. If you're holding a tree or metal rail or other somewhat-grounded thing, then add "heart, lungs, and spine" to "legs and crotch" above. Not fun.
cheers,
mike
The key in shielding is to surround whatever you wand shielded with a conductor. EM waves don't penatrate conductors very well. I'm not even sure you need to ground it, but it is a good idea for other reasons if not this one (unless you like getting electicuted). As a previous post mentioned, the smallest opening should be smaller than the wavelength of the EM radiation, in this case a few cm should do it.
Take a look at your mocrowave door sometime. It uses the same idea. THere is nothing special about the glass, it just has a piece of metal with small holes in it. The EM (microwave) frequency is about 2.5 GHz IIRC. Those little holes are smaller than that wavelength, so nothing gets through.
I haven't had a lid on my box for a long time now.
The only noticeable effect is that not one AM radio in the house works when my computer is on. But that also happens when my Win box is swtiched on (and I rarely have the lid off that).
xix.
"Everything is adjustable, provided you have the right tools"
Simple, effective, and cheap...
Use contact cement to put aluminum foil on the inside of the wooden case. Attach a grounding strap such as a screw and eye conntctor to attach the aluminum foil to ground. Attach the other end of the connector to chassis ground. Voila, a shielded case.
I originally saw this in a HAM radio manual and used it on a homebrew computer a few years ago.
Steven
NRRPT/RCT
Don't do that when you dont have a degaussing monitor... BAD BAD BAD idea.
I wonder what kind of noise protection wooden peripherals produce? How did this company provide adequate shielding?
I have seen a lot of posts here with people saying you only need to worry about 400 MHz or so based on the CPU speed. If we were dealing with sine wave signals for the CPU clock, that would be true, but we are dealing with gross attempts at square waves, which contain a very wide range of frequencies, as pointed out by Ping1400 in a reply burried down there somewhere, this means that we actually need to worry about frequencies at and over 3 GHz. Of course, we are lucky enough that the core voltage is only 2.2 volts (if that much) so you don't have a huge signal to kill....
The way I understand it, each device in a personal computer (i.e. CPU, motherboard, RAM, HDD, graphics card...) has to comply with FCC-regulations concerning electromagnetic radiation.
So, there is IMHO no need for any additional shielding, since even a traditional (metal) case won't shield any radiation, because it has slots for CD-ROMs, Floppies and on the back side for cards etc.
-- DJ Kat is where it's at
I'm not sure there's all that much EMF created by the PC...But don't quote me on that. First, is there any EMF shielding on the *shudder* iMac? And second, I've seen a clear plastic case on a celeron game machine...Although this might not apply since both of my data points are extremely weak systems, as opposed to your dual-celeron with u2 scsi...
Wire mesh is just fine for basic emf and rf shielding...
It should be roughly contiguous, although, that is hardly worth pulling out a micrometer of magnifying glass.
Just make sure each seperate piece of mesh when assembled makes good electrical contact with the frame? (if you have some sort of framework on this) or other conductive bus. To test if this is roughly adequate,
1. get a multimeter (measures voltage/resistance and usually current, sometimes mislabeled a voltmeter)
2. set it to measure resistance or continuity.
3. touch 1 probe to one portion of mesh, the second to a point opposite...as in as far as you can get inside the case from point 1
4. look at the resistance readout.
5. It should read 0 ohms (if yours has a floating needle, it may be a bit off, not to worry) If it reads anything > 1 ohm, you don't have a significantly conductive material to be useful as a shield/ground...
(For those who will argue here...I just finished measuring the foil from a pack of marlboro light 100's that came out to 200 mOhms , or 0.2 Ohms, in case you cared.)
Now for the final key point. Ensure that the power supply is properly grounded, your shield is properly grounded, and that your new case is plugged into an adequately grounded 3 prong receptacle.
For clarification...if you don't plug it into a grounded receptacle, what you've just configured is essentially a floating common attached nicely to a power cord which sould make a passable antenna for rebroadcasting all the stuff you just collected.
If you need to add a better ground to your house...i.e. you get lines in the tv when you run the vacuum...talk to an electrician unless you are comfortable in a circuit breaker box.
You can get a 3-4 foot copper clad steel rod, with copper wire, to act as a grounding rod. have the electrician hook up the copper wire to the ground bar in your circuit breaker box, then you attach it with a clamp to the grounding rod.
Note..you're going to need a sledge hammer...preferably a big one...
Apply the sledge hammer to thetop of the grounding rod until only about 3-4 inches sticks out of the ground....
now attach that wire, and your house ground should be much better.
Whew...wrote a lot more than I planned...but you get the idea.
That copper fiber does just fine if you just remember to connect it to ground potential. Power case will do just fine and some times there is screw for that.
Another thing is that copper layer must be as continuous as possible. Electro Magnetic Interferences goes from any hole big enough for the wave lenght.
If a wooden computer isn't enough, it looks like you can accessorize with wooden mice, keyboards and LCDs.
http://www.woodcomputers.com/
This is really cool... I never knew you could buy this stuff!
--
"A stable operating system is a no-Win situation"
--
Scott Brady
To address the issues of wood expandsion/contraction and also noise, would an inch-and-a-half to two-inch corkboard make sense to attach the motherboard to, on top of a metal sheet or some foil? Don't know if this would be a problem as far as EMI/RFI--just a thought, though.