homemade EFIS system and EMI

RFI is best squelched at the souce, i.e. on the PC board. The biggest
source of RFI is the clock that drives the processor, etc. If it isn't
routed on the board very well or properly terminated, it will radiate like a
pig. Also, effective grounding of the PC board to the case is important.

Given that you have a noisy PC board, the only way to effectively shield it
is the fully enclose it in a conductive case, preferrable made out of steel
since that will also block the magnetic portion of the radiation. Any gaps
or slots in the case that are as large as the wavelengths of the radiated
signals will allow the signals to escape. In an airplane, aluminum cases
are preferred due to their light weight and lack of magnetic effects on the
compass.

Also, wires that run out of the enclosure will often carry the RFI signals
and re-radiate them by acting as antennas unless they are also well shielded
and properly terminated to the case ground. Ferrite cores can also be
effective in blocking such signals from traveling out through the wires.

Dean

These are all band-aids for the original problem of having an overly noisy
PC board.
"jcpearce" wrote in message
ups.com...
As a pet/learning project I made a data aquisition unit using an 8051
microprocessor and an EPIA M motherboard running a variant of Linux to
process and display the information. It all works but the EMI from the
EPIA M causes way too much noise to the aircraft radios. I have tried
shielding the whole device in an aluminum case with very little
improvement.

Any ideas on how to smother the EMI or some other small motherboard
which may not have as much an issue (as a test I took my portable
aviation radio and within 6' of any my home computers the same occurs
which gives me little hope)

Thanks

Thanks Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc & Tim, I
really appreciate the advice.

Your comments rekindle my drive, I had hit a low point after designing
the data capture board, OS, circuits etc.. and then... being killed
with RF noise.

An odd, but beneficial I suppose, confluence of circumstances
involving a broken window pane and a mischievous dog and a storm lead
to water getting on the DC to DC power supply (11~14 DC - 12V, 5V,
3.3V +-) for the EPIA M motherboard. Not surprisingly it decided not
to continue functioning. I then pulled out a traditional power supply
to use, and.. amazingly 80% of the noise went away! I would hate to
curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently little
RF and EMI protection. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.

I don't want to go through the space and elaborate hastle of going from
the 11-14V bus to 120V AC to a bulky PC power supply (space is
critical) to 12,5,3.3V so I need to find a suitable non noisy power
supply and then deal with the issues you knowledgeable fellows
enumerated in the posts.

Thanks again to Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc &
Tim.
Your comments rekindle my drive, I had hit a low point after designing
the data capture board, OS, circuits etc.. and then... being killed
with RF noise.

An odd, but beneficial I suppose, confluence of circucumstances
involving a broken window pane and a mischevious dog and a storm lead
to water getting on the DC to DC power supply (11~14 DC - 12V, 5V,
3.3V +-) for the EPIA M motherboard. Not surprisingly it decided not
to continue functioning. I then pulled out a traditional power supply
to use, and.. amazingly 90% of the noise went away! I would hate to
curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently all
the wrong harmonics. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.

I don't want to go through the space and elaborate hastle of going from
the 11-14V bus to 120V AC to a bulky PC power supply (space is
critical) to 12,5,3.3V so I need to find a suitable non noisy power
supply and then deal with the issues you knowledgable fellows
enumerated in the posts.

Thanks again to Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc &
Tim.

You're welcome! Keep us informed, please, of your trials and successes.



jcpearce wrote:
Thanks Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc & Tim, I
really appreciate the advice.

Your comments rekindle my drive, I had hit a low point after designing
the data capture board, OS, circuits etc.. and then... being killed
with RF noise.

An odd, but beneficial I suppose, confluence of circumstances
involving a broken window pane and a mischievous dog and a storm lead
to water getting on the DC to DC power supply (11~14 DC - 12V, 5V,
3.3V +-) for the EPIA M motherboard. Not surprisingly it decided not
to continue functioning. I then pulled out a traditional power supply
to use, and.. amazingly 80% of the noise went away! I would hate to
curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently little
RF and EMI protection. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.

I don't want to go through the space and elaborate hastle of going from
the 11-14V bus to 120V AC to a bulky PC power supply (space is
critical) to 12,5,3.3V so I need to find a suitable non noisy power
supply and then deal with the issues you knowledgeable fellows
enumerated in the posts.

Thanks again to Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc &
Tim.
Your comments rekindle my drive, I had hit a low point after designing
the data capture board, OS, circuits etc.. and then... being killed
with RF noise.

An odd, but beneficial I suppose, confluence of circucumstances
involving a broken window pane and a mischevious dog and a storm lead
to water getting on the DC to DC power supply (11~14 DC - 12V, 5V,
3.3V +-) for the EPIA M motherboard. Not surprisingly it decided not
to continue functioning. I then pulled out a traditional power supply
to use, and.. amazingly 90% of the noise went away! I would hate to
curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently all
the wrong harmonics. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.

I don't want to go through the space and elaborate hastle of going from
the 11-14V bus to 120V AC to a bulky PC power supply (space is
critical) to 12,5,3.3V so I need to find a suitable non noisy power
supply and then deal with the issues you knowledgable fellows
enumerated in the posts.

Thanks again to Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc &
Tim.

curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently all
the wrong harmonics. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.



The 110v power supplies used in desktop computers are also switching
supplies. So the problem is the cheapness of the 12v unit you selected.
I'd try another make unit and see if that works if not try putting the power
supply in its own shielded case and shielding and ferrite beading the
input/output leads from it.
Good luck and let us know what you workout!
John

"jcpearce" wrote in message
ups.com...
Thanks Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc & Tim, I
really appreciate the advice.

Your comments rekindle my drive, I had hit a low point after designing
the data capture board, OS, circuits etc.. and then... being killed
with RF noise.

An odd, but beneficial I suppose, confluence of circumstances
involving a broken window pane and a mischievous dog and a storm lead
to water getting on the DC to DC power supply (11~14 DC - 12V, 5V,
3.3V +-) for the EPIA M motherboard. Not surprisingly it decided not
to continue functioning. I then pulled out a traditional power supply
to use, and.. amazingly 80% of the noise went away! I would hate to
curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently little
RF and EMI protection. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.

I don't want to go through the space and elaborate hastle of going from
the 11-14V bus to 120V AC to a bulky PC power supply (space is
critical) to 12,5,3.3V so I need to find a suitable non noisy power
supply and then deal with the issues you knowledgeable fellows
enumerated in the posts.

Thanks again to Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc &
Tim.
Your comments rekindle my drive, I had hit a low point after designing
the data capture board, OS, circuits etc.. and then... being killed
with RF noise.

An odd, but beneficial I suppose, confluence of circucumstances
involving a broken window pane and a mischevious dog and a storm lead
to water getting on the DC to DC power supply (11~14 DC - 12V, 5V,
3.3V +-) for the EPIA M motherboard. Not surprisingly it decided not
to continue functioning. I then pulled out a traditional power supply
to use, and.. amazingly 90% of the noise went away! I would hate to
curse but the blasted PW-60 #!@! (
http://store.ituner.com/ituner/pw60caatxpos.html ) power supply was the
primary culprit. It is a switching power supply with evidently all
the wrong harmonics. I am trying to find another DC-DC power supply
without these issues, they all appear to be switching power supplies, I
would rather deal with energy/heat loss than RF noise.

I don't want to go through the space and elaborate hastle of going from
the 11-14V bus to 120V AC to a bulky PC power supply (space is
critical) to 12,5,3.3V so I need to find a suitable non noisy power
supply and then deal with the issues you knowledgable fellows
enumerated in the posts.

Thanks again to Dean, Elippse, Colin Lamb, Bob, AINut Don, Kayroc &
Tim.


Ahhh.... yes. I should have thought to mention the importance of a quiet
power supply. A PI filter between the supply and the PC is a good idea to
help ensure that switching noise from your supply doesn't get into your PC
board.

I'm used to designing my own power supplies for systems like this, so I
didn't stop to think that you might be using an off-the-shelf supply.

Dean

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Hash: SHA1

JC,

Having tackled this issue myself for other apps, I can point you to an
excellent resource, "The Art Of Electronics" by Horowitz & Hill. In
there, they outline some techniques witch you may consider paranoid, but
I have found to work. For instance, building an enclosure within an
enclosure. The outer enclosure is little more than a way to terminate
your outside wire connections (including power), allowing you to bring
them into a controlled environment where you can then apply such gross
tricks as caps & ferrite filters on your power lines and data lines
(don't forget fuses!). After you have performed whatever magic you can
on the in/outbound leads, you then bring the signal & power into the
inner enclosure.

Also remember that for homemade enclosures of bent & riveted aluminum,
small gaps where one flap is riveted to another can have gaps in then
which WILL leak RF. To fix this, your local electronics parts supplier
(or Digikey) will sell conductive tape. Welding also works. When
attaching the top of the enclosure, you could employ either one of those
fine mesh springy wire gaskets, or ... more tape.

Good luck,
Evan Carew

P.S.
IF you want me to forward some of the specific steps outlined in TAOE,
let me know.
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jcpearce wrote:
So it is coming solely from the motherboard, some chip on the board is
oscillating in the 107~130 Mhz range, given this occurs against one


That frequency is awful close to the bus frequency of modern systems
(133Mhz). If you're using DIMM memory, do you have a card in every
slot, or do you have some of those little radiators hanging free?

Thanks Evan, I have eyed that book at the local bookstore and given
that you mention there is a section (and paranoid no less, a good
thing) on this I'll purchase a copy. Books like this can save a lot of
hair being pulled out.

The enclosure I made is riveted and there are very small gaps in
addition to where the mandrel pulls out leaving a small hole. Would
encompassing the whole thing in aluminum foil (as a test, not for
deployment) accurately simulate these sophisticated seals? Before going
through that effort I would want to know what, if any, gain I may
derive.

Ernest the MB only has one DDR slot, where a single 512 Mb card is
plugged in.

I am encouraged that if the power supply issue can be solved via
shielding I will be set. The people at ituner admitted the PW-60 DC-DC
power supply was a noise daemon but there new super improved model the
M1-ATX ( http://www.mini-box.com/s.nl/sc.8/ca...it.A/id.356/.f )
model would solve this and they are sending me a unit, they are willing
to work with me which is quite good customer service.

If it is still too noisy I'll get paranoid with cases within cases,
welding, prayer, etc... If still too noisy (please no) I'll do my own
power supply,

Thanks for the replies.

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JC,

[Comments follow your text]

jcpearce wrote:
Thanks Evan, I have eyed that book at the local bookstore and given
that you mention there is a section (and paranoid no less, a good
thing) on this I'll purchase a copy. Books like this can save a lot of
hair being pulled out.

The enclosure I made is riveted and there are very small gaps in
addition to where the mandrel pulls out leaving a small hole. Would
encompassing the whole thing in aluminum foil (as a test, not for
deployment) accurately simulate these sophisticated seals? Before going
through that effort I would want to know what, if any, gain I may
derive.


Well, when last I did this, I remember the RF "bonding" tape was nickel
plated coper foil with a collodial silver stickey backing. As I
remember, it worked perfectly, if somewhat expensively. Since this was a
little more than 10 years ago, I imagine that there have been some
cheaper alternatives developed since then.

Your problem when using tin foil will be getting it at the precise
potential of the rivited box underneath it. To accomplish this, it needs
to be "bonded", ie electrically grounded to the main box at regular
intervals. What these intervals is, has either to be determined
experimentally, or solved via the paranoid method (nickel plated coper
tape with $ilver......).

Ultimately, I think the better solution would be to get some cheap
double or single sided FR11 PC board and solder it together with coper
foil (cheap at your local electrical supply). To seal the top, you are
going to have to order some king of conductive compression tape (cheap
from digikey.

If you are interested, email me off group & I'll see if I can find you
some hardware ideas from Digikey.

Evan
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"Evan Carew" wrote

Well, when last I did this, I remember the RF "bonding" tape was nickel
plated coper foil with a collodial silver stickey backing. As I
remember, it worked perfectly, if somewhat expensively.

Sounds similar to something you can get in the heating and cooling sections
of the big box hardware stores. It is thin aluminum tape with sticky on it.
Seems like reversing it once in a while, or at the ends would get the al to
al contact, to bypass the non conductive sticky.

I have not tried it, but advice is worth what you pay for it! g
--
Jim in NC