magnetic shielding

Hello,

I hope this okay to ask here. I'm looking for something to shield
monitors from one another. I run dual-monitors and of course they tend to
get wavy after running for awhile.

I've seen whole covers, but I'm hoping for some panel just to go
inbetween. Cost is an issue to me so I'm looking for something
inexpensive ($40 or less).

Thanks,

Jon

 

Have you tried something simple, like a piece or two of sheet steel?

 

You don't want to hear this, but it's not really going work. I have
had the same problem, and there have been several threads about it
over the years.

The main interference in magnetic, this is hard to stop. Unless you
invest in some mu-metal sheets (*very* *very* expensive) there is
nothing you can do that you will be able to lift (2 inch thick iron
plate is a heavy alternative).

My solution was to change monitor orientation (a few degrees can make
a big difference) and play with the refresh rates (notch up/down a Hz
or so until the interference stops - if your display driver allows
this).

One of my monitors (a 22" Iiyama) plays hell with the TV 12 feet away
if run at standard 85Hz refresh. The interdependence stops when
tweaked to 86Hz.

Cheers,
FoxyRick.

 

Higher refresh rates mean that materials such as aluminum are more
effective. Try a combination of mu-metal (which can be salvaged from old
monitors or television sets in some cases) and fairly thick aluminum sheet,
like 1/8". That seems to work okay for the high frequency components.

Cheers!

Chip Shults

 

Bullshit. The reason a Faraday cage works is by redirecting or
diverting magnetic energy. NOTHING will "stop" it.

Since it can pass through the planet, I doubt seriously that anyone
will be "stopping" it.

All he needs is a partition for this low energy, but that will have
fringing emissions which may end up providing no effect.

So, what is really needed is to box one monitor with a box of steel
that effectively traps, and redirects the energy from one monitor,
while keeping influences from the other out.

Mu metal is not required at this level.

The other solution is to get and use two FPDs which typically have
little or no emission.

The problem is that monitors are nothing like they used to be.

I have an old 19" that is fully faraday caged inside the plastic
outer case. Guess what? No proximity emission effects...
at all.

Today's monitors are rarely shielded internally via full cages.

 

Really? I thought differently. Near field couple, far field coupling,
ahh heck, it don't matter.

So what is all of this "line of sight" stuff???

That is probably true. I have used cheap steel sheet metal in video
games, that had dual monitors with different sync sources, with decent
results. Nintendo's answer in their games was to use a single board
with a common sync chain so both monitors were putting out the same
emmission.

Aint that the truth.

Hence the need for sheilded "computer" speakers.

 

It can be diverted, which for all intents results in an attenuation
in the area where protection is desired.

Yeah. Those old monitors went for $1800.00 each too. The primary
target market was established businesses, and the military. That
market has widened, and cost of manufacture is an issue... for them.
For us, it only means more electromagnetic "noise".
The monitor in question was further modified, and caged a second
time with thermal provisions for operation at 70,000 ft.

Yeah. My speakers are flat panel electrostatic jobs. They are
definitely cool. "Monsoon" is the brand. They are powered by a 6
channel amp in the sub's cabinet, so they don't even tax the sound
card. Best buy I've made in some time. They take up less space too.

The difference between "sub-woofer on" and "foot in sub-woofer port"
is amazing! ;-] Hehehehe...

 

On Sat, 24 Jan 2004 18:45:09 GMT, James Beck

Actually, as far as I can tell, speakers are never actually
"shielded" in the sense most of us think. There in no big
layer of metal (mu- or otherwise) around anything. What
they actually have is a second magnet stuck onto the
back of the main magnet in such a way that it bucks the
external field of the main magnet.



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

 

Yer nuts. It is the field fringes that need to be shielded. Your
reversed magnet would do no such thing.

Speakers have closed magnetic loops in that there is a cap on the
rear of the magnet, and a post for insertion into the coil, then a cap
with a hole in it for the cone side. It is all but closed, but there
re fringe fields on them. For "shielding" it, the things need a steel
or Mu cap over the entire magnet.

If "they" indeed do this they way you mention, then said speakers
are NOT shielded as they claim.

 

Yes, but the problem is that regular thin sheets of metal such as steel are
not a particularly good approximation of a 'perfect conductor' at 60Hz.
This is related to the skin depth (which I seem to recall is ~8mm in
copper -- a much better conductor than steel -- at 60Hz?) -- if the skin
depth is large compared to the condutors thickness, you can't build a very
good Faraday cage out of it. This is why people are always suggesting mu
metal -- its relative permittivity is on the order of thousands so the
requisite thickness of metal required is reduced by an order of magnitude or
so. It's unfortunate that the price is quite high and performance can
degrade remarkably due to mechanical stresses (bending, hammering, drilling,
etc.).

True, ut you can induce currents that set up their own fields out of phase
with the incident field and the superposition of the two can be (very close
to) zero. At that point, whether or not you've 'stopped' the field is more
of a philosophical question than physics.

Something I haven't seen anyone mentioning yet is that you (necessarily!)
have a nice big glass 'window' in the front of any monitor... it passes a
lot more than just THz (light wave) frequencies, you know.

Yes, but as one follow-up suggested, this is often not as effective as you'd
intuitively expect -- the frequencies involved are so low, you end up
needing a lot of metal for significant attenuation.

I'm not so sure this was done to reduce 60Hz interference as it was for the
low-MHz (RF) frequencies? To meet FCC requirements? I don't really know,
however.

---Joel Kolstad

 

Conduction skin depth is not the consideration. Magnetic properties
are. 8mm of steel is NOT ever needed. That is utterly ridiculous.

I made three shields last week. One was from a simple cookie sheet,
which I then laminated with transformer tape for isolation. It
reduced the field in question by a factor of 5. I then made one from
18 Ga steel, and it went to 6. We then had one fabricated from .032"
steel, and it ha hard tie points to mae the sides mate more intimately
with the sides of the chassis. That got us to seven.

That would be 15mV of ripple on a 1000 volt supply at .1 volt
regulation over 250 Watts. It will power 450 PMTs at one time, very
accurately. It started t over 100mV when we placed the circuitry in
the case. My shields made the difference, and the engineering
director (my boss) couldn't believe that I achieved that much with a
simple shield after we chased after this thing for two days.

I realized that the noise was much less out of the case, and the
only thing that changed was the proximity of the amp, and control
boards. I reached in, and picked up the Ctrl board, and raised it up
a quarter inch, and the noise cut in half. That is when I knew a
simple shield was all we needed. My first shield was a mere backplane
for the ctrl board. The two subsequent shields were full partitions
for the case. Big difference.

Sure, a Mu metal chassis and partition would be better, but one has
to shoot the engineers, and get on with production t some point. That
point would be the point at which cost of manufacture and meeting
customer spec are optimized. Mu metal cases and shields are not in
that equation. If they wanted a million pieces.. maybe. But for 500
a year... no thanks.

Faraday cages are typically for keeping rf noise inside a device
chassis, and for keeping external rf influences and lf influences from
"getting in".

We're also shielding magnetic field, not electrostatic field, so
conduction is not an issue. A complete magnetic circuit, however is.
That is why the shield I made with tie points worked better, despite
being thinner than the 18Ga sheet was. I had a complete magnetic
circuit.

Steel is fine for many purposes in this regard.

Nearly all rack mount chassis made for mil use are NOT Mu metal, and
meet all mil specs for EM shielding. Why would that be were it not
sufficient enough at attenuating EM fields?

It is also about magnetic fields, not conduction properties.

The order of magnitude more shielding achieved far outweighs any
"losses" in original spec incurred by deformation(s) of the original
sheet.

However... cost IS an issue, unless one is a huge conglom like Sony
or such, where incorporating the best is not a big impact on the
company's operational costs.

For us... we need NRE and development funding for such ventures.
Or a customer that actually pays for the quality we make, instead of
trying to think in chinese mass production pricing scheme numbers.

We do make the best, lowest noise HV & EHV supplies in the world,
though.

In radiology, for instance A lower noise supply means a higher
contrast ratio in the imagery. I have a 4kV supply that is at 2mV
ripple through its entire range of operation. That is like 0.00005%
A shielded multiplier (a mere partition), and a HV coaxial output
got us there, down from 11mV previously.

I AM the noise abatement crew! Heheheh...

 

Sure it is, although I'd grant you that at 60Hz the problem might be more
intuitively understood by thinking about magnetostatic concepts rather than,
e.g., TEM waves.

Well, from your examples... a reduction of seven (and assuming you mean
field strength or an induced voltage or current) is -16.9dB. That's a
noticeable amount, but by no means what I'd consider 'huge.' (And not that
I'd know, but I imagine the TEMPEST specs were shooting for at least 40 if
not 60+dB field strength reduction?)

I do appreciate your concrete examples -- they're worth a lot more than my
idle speculation about how effectively shielding between monitors might be.
I was primarily suggesting, though, that if you have really bad monitor to
monitor interference problems, you might find cookies sheets not 100%
effective in eliminating the problem. The original poster should certainly
try, though, and report back his results!

I agree that mu metal shields are not practical for most engineering budgets
for small production runs. It is handy to have a stash of them around just
to try out various shielding experiments -- to determine where the
interference is coming from, for instance.

Clearly it is effective, but keep in mind that the board layout and circuit
designs were/are done with an eye bent towards minimizing EMI as well. I
also couldn't begin to tell you what the frequency range of interest for
mil-spec shielding is, but imagine that 60Hz is slow enough that it bends a
CRT's deflection and messes up the picture long before it induces voltages
typically large enough to cause errant operation of a circuit and therefore
might not be covered (or have a really, really lax standard).

I believe the aviation equipment often uses a metal box within another metal
box for even more complete shielding?

But is that the limiting factor in the system's overall performance? Does,
e.g., the ADC in an image digitization system typically have enough bits to
recognize the advantage of 2mV ripple vs. 11mV? I mean, to resolve 2mV out
of 4kV you'd need a 21 bit ADC -- available (although not at live video
rates!), but are these actually used?

---Joel Kolstad

 

It took us back down to within the customer spec, nd was surely much
more as measured externally. I was talking about injected ripple.
For that, it was quite significant. The main ripple at 17kHz was at
5mV.

I made no mention of tempest specs. Military EM shielding specs
were here before tempest specs. Said specs, however, are also met
with mere steel in many cases. (hahaha I said "cases") hehehe...

We have had past contracts with FPDs that had covers on them that
were $450.00 each. It was a Tempest spec'd device. A ruggedized field
PC with IBM RISC Guts. All but immersible. The screens were
optically coated screens of glass, about 5/32" thick with foiled
edges. This machine had air pressure relief vents that were $30.00
each!

Why those dumbos thought that an LCD based FPD would emit, I'll
never know, because they don't, when compared to a crt, which can be
monitored from the roadside!

 

A good eng lab would. Ours got put on low or no budget back when
911 hit. Sad. Though more than a few PS companies folded in the last
3 years.... We eeked by. I eat less now as my last friggin raise was
a mere 2%. I am pissed too!

 

Well... in our shop, even that takes a backseat, when the customer
deadline was made well inside the actual window that we should have
gotten for the development time. Our future iteration of the same
device will assuredly be an even better product.

Slow waves pretty much pass through anything... 60 Hz rises and
falls fast enough for steel to do pretty good job. I am sure that
their equipment has sectioned areas where small Mu boxes encapsulate
certain circuit segments from others. Our entry module has a Mu
shield if I am not mistaken. I used shielded Thermax 18/3 to power
the unit, and 24/2 for the fans. That killed a lot too, as the simple
twisted PVC previously used surprisingly generated a lot of the
"noise".
Teflon twists up tighter, so I think it my have worked better, but
when I saw that Thermax on the wire stand, I knew that was our puppy.
It is teflon too, which means nothing except that insulation
thickness, and subsequently closeness of the twisted wires gets
affected.

The 19" rack mount display we made for them years ago was as it had
to fill a rack space, nd have rail mounting capability. We piped the
I/O and power to the back of the outer box, and made provisions for
conduction cooling on some components for use at 70,000 ft.

As far as knowing what they typically do now... I do not. Or why.

I still think forcing passengers to turn off electronic equipment
for take off and landing is big time overkill.

According to the customer... I think so. They were in love with the
results they received from our lower noise supply of the same model.
We don't even make the other, non-shielded version anymore.

What actually happens is this..

When an e-beam for an x-ray tube is generated, the noise content in
the e-beam translates into a less pure x-ray flux. That translates
directly into a less even distribution of the focussed rays on the
target, and subsequently, the imaging digitizer, or film. Therefore,
the more pure the DC supply is that feeds the tube, the more pure the
flux emitted is, and the better the contrast ratio in the imagery is
as the passage of the beam through the target medium is more
homogeneous. At least, that was how I understood it when it was told
to me. I am not a radiologist, however. :] I am apparently a noise
abatement specialist wanna be! :] One that has a screwed up "A" key
on his PC! DANGIT! You may note that many or most of my spelling
errors are omissions of the letter "A"! Time for a new Kbd!

 

Sorry, but to use your own term, "bullshit." A Faraday cage
has to do with blocking ELECTRIC fields; shielding from
magnetic interference - purely magnetic - is another thing
entirely. For a simple, jr.-high-school-level discussion of a Faraday
cage, see:

http://www.physics.gla.ac.uk/~kskeldon/PubSci/exhibits/E3/

Magnetic fields ARE extremely hard to effectively shield against;
mu-metal shields (or any other material which a sufficiently high
permeability) are best, but you can try just about any ferromagnetic
material (iron, most steels, etc.). Still, don't expect any such solution
to be more than partially effective - if it truly is a case of magnetic
interference, your best bets have already been mentioned - either
physically relocate/re-orient one of the monitors, or try to match the
refresh rates as closely as possible to reduce the "beat" effect to
something tolerable.

Guess what? The "Faraday cage" (internal shielding) has nothing
to do with the fact that it has no "proximity" (magnetic) effects. Older,
particularly larger (which tended to be more high-end) CRT monitors
often included internal (to the CRT) magnetic shielding, and/or field
cancellation coils, which greatly improved their performance with
respect to external fields.

Bob M.

 

Sorry, Joel, but there's still a lot of confusion of electric (E-field),
magnetic
(B-field), and electromagnetic waves in the above.

"Permittivity" is the parameter concerned with ELECTRIC field behavior;
it also, in its "relative to free space" form, is known as "dielectric
constant."
The parameter which measures the ease with which MAGNETIC fields
are established in a given material is PERMEABILITY. Skin depth, at
least for non-ferromagnetic metals (i.e., those whose relative
permeabilities
are basically 1.0), is dependent on frequency and conductivity. But purely
MAGNETIC shielding (i.e., trying to block B fields) is a question of
permeability, not permittivity or "skin depth."

Exactly correct. The internal metal shielding of older monitors was
there ONLY as a counter-EMI measure; it has essentially nothing to
do with their low-frequency magnetic field performance, which is addressed
through other means.

Newer monitors pass the EM requirements just as well as the older ones
did, and without all the bulky (and expensive) shielding, primarily through
better design in the PC boards, grounding, etc..

Bob M.

 

Quite simply because EM radiation (i.e., high-frequency RF) is
a completely different beast than low-frequency magnetic interference.
You are confusing two very different areas of regulatory (and
user!) concern.


Bob M.

 

Possibly because LCD monitors DO emit EM; it doesn't
come from the same sources as in a CRT, and so the counter-
measures may appear a little different - but simply having an LCD
is by no means an assurance of getting a pass in your EMI testing.

Again, please note the difference between "EM" and "magnetic."
An LCD does NOT have problems with low-frequency magnetic
fields (as CRTs obviously did), but that has absolutely nothing to
do with their EMI performance.

Bob M.

 

Yeah, except that you are wrong.

It must be too simple then, 'cause you missed it.

Such cages ATTENUATE, not block completely, both EM AND ES fields,
AND RF IS an EM field.