magnetic shielding

[DarkMatter]

On Mon, 26 Jan 2004 13:23:29 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

Magnetic fields ARE extremely hard to effectively shield against;

Standing fields, yes. Fields in motion, however can be blocked, and
such blocking depends on frequency.

RF IS a magnetic field.

Slow rate fields are hard to attenuate, yes. Extremely hard...
don't know if I'd go that far.

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.).

Don't need a primer.

Still, don't expect any such solution
to be more than partially effective -

No shit. I said before in this very thread that such fields pass
through the entire planet. We do NOT block them, we attenuate their
effects on our "shielded" circuits. That is all we do. We are
electronics industry folks. Are you?

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.

Best bets? Have you ever even heard of a dual monitor set-up?

If you have, your "relocate" suggestion is KAKA. It's KAKA even if
you haven't heard of them.

One will never "match" the rates without a shared clock.

Running both at the same rate is a given as dual output cards do not
allow dual rates IIRC. The monitor is internally clocked as well.
That is where the mismatch occurs.

That slight rate mismatch can actually increase the wave effect as
they oscillation carries through very slowly, like windshield wipers
matching up, then mismatching a similarly timed blinking light.

I hope that analogy wasn't too hard to grasp.

Shielding is quite possible, however.

The military runs rows of monitors in quite close proximity all the
time. Why no wave effects? Shielding!

 

[DarkMatter]

On Mon, 26 Jan 2004 13:23:29 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

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.

It's a big fat bubble faced 19" monitor, hardly "more high-end"
It is several years old, and when the cage is open... guess what?

Doh!

 

[DarkMatter]

On Mon, 26 Jan 2004 13:35:01 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

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..

Right... NOT!

They mostly still have single sided PCBs in them. Hardly a
high-ended EMI killing form factor.

 

[DarkMatter]

On Mon, 26 Jan 2004 13:37:10 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

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.

No. You are confused in thinking that you know what I refer to.

 

[DarkMatter]

On Mon, 26 Jan 2004 13:39:33 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

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.

Not likely the CIA boys could read your screen from the street 100
ft outside the office, however, which is what tempest specs were
about.

Again. LCD screens emit very little compared to CRTs.

LCD drive electronics is a different matter.

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:7dib101aqklhjh80krolk9g4cnelgniqeb@4ax.com...

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

I'm afraid you've completely missed the point. The original
question concerned MAGNETIC fields, not RF/EM radiation.

A perfect Faraday cage will in fact block ALL EM radiation from
the outside. In practice, of course, such a "perfect" cage rarely, if
ever, exists. But it still doesn't have a thing to do with magnetic
interference (i.e., pure B-fields).

Bob M.

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:tiib10dm21rqg8131mora4ahrra1oqvbep@4ax.com...

On Mon, 26 Jan 2004 13:23:29 -0700, "Bob Myers"
nospamplease@addressinvalid.com> Gave us:

Magnetic fields ARE extremely hard to effectively shield against;

Standing fields, yes. Fields in motion, however can be blocked, and
such blocking depends on frequency.

RF IS a magnetic field.

No. RF is an ELECTROMAGNETIC field, which is
a different sort of beast (specifically, an EM wave is a pair
of waves, one electric-field and one magnetic, sustaining each other
in a specific polarization and phase relationship). And the sort of
interference
we're talking about here (low-frequency magnetic fields, originating from
CRT deflection yokes, and in this case at the vertical sweep rate) isn't
RF by any stretch of the imagination.

Don't need a primer.

You may wish to get a second opinion on that.

Best bets? Have you ever even heard of a dual monitor set-up?

Yes; I've been doing monitor design and qualification work for
most of my 24 years in this business. In fact, you might want to
review an article to which I contributed some years ago on this
very subject; it's at:

http://www.vesa.org/emc.pdf

We admittedly wrote this one at a level targeted to the average
consumer; still, it's perhaps a useful reference in such situations.

One will never "match" the rates without a shared clock.

Not exactly, no - but an exact match isn't required. For the
interference situation in question, bringing the vertical rates to
within 0.1 Hz or better (which is quite simple to do, given current
video timing standards and cards designed with those clock rates
in mind) will very often reduce the visible effects of the interference
to acceptable levels. Not many people are worried if their image
is moving less than a millimeter over a ten second or longer cycle;
not nearly as much as the thing sitting there jumping around at
a few Hz or greater.

Bob M.

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:h7jb10l21n5aotte740n10tjjj9klimt4q@4ax.com...

It's a big fat bubble faced 19" monitor, hardly "more high-end"
It is several years old, and when the cage is open... guess what?

I suspect you're going to tell us, but I also suspect you're still
incorrect as to the root cause of the problem you're seeing.

Bob M.

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:3cjb10t3tc83dm14t57cnsn3n2kkji6d1e@4ax.com...

On Mon, 26 Jan 2004 13:35:01 -0700, "Bob Myers"
nospamplease@addressinvalid.com> Gave us:

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..


Right... NOT!

They mostly still have single sided PCBs in them. Hardly a
high-ended EMI killing form factor.

Did I say they were using something other than a single-sided
PCB? Nope....

Do you think the ONLY way through which the PCB can improve
or degrade EMI performance is the number of layers it has?
Apparently...

Exactly how many monitors did you say you'd qualified, by the
way?

Bob M.

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:dgjb1052cr60v5iqbndr0bdvt4vca2i3t3@4ax.com...

Again. LCD screens emit very little compared to CRTs.

In general, true - but you've apparently never run into a
nasty case of radiation coming from the panel's column drivers
and electrodes....

Bob M.

 

[DarkMatter]

On Tue, 27 Jan 2004 12:09:00 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

I'm afraid you've completely missed the point. The original
question concerned MAGNETIC fields, not RF/EM radiation.

From MONITORS, dingledorf. Back to EM. Doh!

 

[DarkMatter]

On Tue, 27 Jan 2004 12:21:44 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

we're talking about here (low-frequency magnetic fields, originating from
CRT deflection yokes, and in this case at the vertical sweep rate) isn't
RF by any stretch of the imagination.

I didn't say it was.

But it IS EM, since it is sine wave, and not a standing DC field.

By any stretch of the imagination.

By the way... there are some very slow radio systems around.

Slower than monitor refreshes to be sure.

At what frequency do you think the moniker "RF" begins to apply?

 

[DarkMatter]

On Tue, 27 Jan 2004 12:21:44 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

You may wish to get a second opinion on that.

How's this? Fuck you.

 

[DarkMatter]

On Tue, 27 Jan 2004 12:21:44 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

Yes; I've been doing monitor design and qualification work for
most of my 24 years in this business. In fact, you might want to
review an article to which I contributed some years ago on this
very subject; it's at:

http://www.vesa.org/emc.pdf

Oh boy! 1998! Wow.

A little late, eh?

Do you happen to be familiar with the effect the planet's field has
on upright game video screens when placed in a near horizontal fashion
in the game cabinet? Depending on the position of the game, the
convergence and colors all get screwed up by the Earth. Imagine
that... Standing fields pass through the planet with nothing to stop
them... just like I stated.

 

[DarkMatter]

On Tue, 27 Jan 2004 12:22:40 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

I suspect you're going to tell us, but I also suspect you're still
incorrect as to the root cause of the problem you're seeing.

The tube has no such features as those you described.

 

[DarkMatter]

On Tue, 27 Jan 2004 12:24:51 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:3cjb10t3tc83dm14t57cnsn3n2kkji6d1e@4ax.com...
On Mon, 26 Jan 2004 13:35:01 -0700, "Bob Myers"
nospamplease@addressinvalid.com> Gave us:

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..


Right... NOT!

They mostly still have single sided PCBs in them. Hardly a
high-ended EMI killing form factor.

Did I say they were using something other than a single-sided
PCB? Nope....

Do you think the ONLY way through which the PCB can improve
or degrade EMI performance is the number of layers it has?
Apparently...

No, dingledorf, you said that "better PC board design."

Sorry, but the form hasn't changed, nor has the emission of the form
very much to speak of. Hardly the level you seem to be touting.

Exactly how many monitors did you say you'd qualified, by the
way?

You're an idiot if you think that sets you apart, and a bigger one
if you think that that is a requisite to know what is going on.

I've done everything from 50 different upright and cocktail video
game displays to eyepiece sized viewers to high brightness jet fighter
ruggedized versions, to FPDs, and there is that big bubble faced job
for 70,000 ft I mentioned.

Are you qualified to use the word "qualify"? I mean you do seem to
think that you are some god compared to others, when you couldn't be
farther from the truth.

I've seen people like you in the industry many times. You're a
legend in your own mind. Sad that there are so many in the industry
that suffer from your disease.

 

[DarkMatter]

On Tue, 27 Jan 2004 12:26:28 -0700, "Bob Myers"
<nospamplease@addressinvalid.com> Gave us:

In general, true - but you've apparently never run into a
nasty case of radiation coming from the panel's column drivers
and electrodes....

Hahahah... and you qual'd it... That must have been some real POS
company. Ours was.... shielded.... yeah... that's the word....
Shielded. Not on the face, however. When it did get that treatment
(a $450.00 tempest compliant screen cover), the difference was not
that great. Big waste of money.

 

[Bob Masta]

On Sun, 25 Jan 2004 10:21:30 -0800, DarkMatter
<DarkMatter@thebarattheendoftheuniverse.org> wrote:

On Sun, 25 Jan 2004 13:16:08 GMT, NoSpam@daqarta.com (Bob Masta) Gave
us:

On Sat, 24 Jan 2004 18:45:09 GMT, James Beck
jim@reallykillersystems_INVALID_.com> wrote:

snip

Today's monitors are rarely shielded internally via full cages.
Hence the need for sheilded "computer" speakers.

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.


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.

Check out a few speaker component catalogs. <www.partsexpress.com>
is an example. You will find that drivers intended for A/V
applications say:
"A bucking magnet helps cut down on the stray magnetic field"
(Quoted from ViFA 4.5" Midbass #299-495)
The bucking magnet is conspicuous on the back of the regular
magnet structure.

What you won't find in these catalogs (at least, I never have) is
anyone selling mu-metal or boilerplate for shielding. Think about
all the cheap little desktop speakers that don't weigh much.

The problem with speakers is a static magnetic field that
affects nearby monitors; the AC field component (audio frequency)
is negligible. To block static fields with shielding in the sense
you are thinking would be formidable. For the speaker makers
to call their approach "shielding" may seem misleading to those
of us who know about iron and mu-metal, but it's a perfectly
reasonable way to describe things to the general public.
(I can hardly imagine a copy writer trying to explain the
advantages of "bucking magnets" to Joe Sixpack.)

Of course, if someone has actually opened up a speaker
and found it lined with boilerplate, I'll stand corrected.

Hey, maybe this is something we can sell to the "golden ears"
set! They love things that are totally ridiculous, so 800 pound
speakers might be a hot item!




Bob Masta
dqatechATdaqartaDOTcom

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

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:v9ce10d3omfk1u9ehbv473h25uauk1lp0p@4ax.com...

I'm afraid you've completely missed the point. The original
question concerned MAGNETIC fields, not RF/EM radiation.


From MONITORS, dingledorf. Back to EM. Doh!

I am certainly understanding the source of your reputation
in these discussions at this point.

Please explain how simply being about "monitors" somehow
magically takes this discussion from one concerning purely
magnetic fields to one concerning EMI. You DO understand
the distinction between the two, don't you?

Bob M.

 

[Bob Myers]

"DarkMatter" <DarkMatter@thebarattheendoftheuniverse.org> wrote in message
news:icce10hoqs1bu8udkd5tbu635st2v6ej31@4ax.com...

On Tue, 27 Jan 2004 12:21:44 -0700, "Bob Myers"
nospamplease@addressinvalid.com> Gave us:

we're talking about here (low-frequency magnetic fields, originating from
CRT deflection yokes, and in this case at the vertical sweep rate) isn't
RF by any stretch of the imagination.

I didn't say it was.

But it IS EM, since it is sine wave, and not a standing DC field.

No. There are "AC" magnetic fields, which are a completely
separate and distinct topic from electromagnetic (EM)
radiation, of ANY frequency.

By the way... there are some very slow radio systems around.

Agreed. And they operate by transmitting electromagnetic
waves, not simply time-varying magnetic fields. Again, you do
not appear to understand the distinction between these two
phenomena.

Bob M.