homemade EFIS system and EMI

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.

jcpearce wrote:
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

Hi, JC!
As others have mentioned, it is important that the conductors carrying
signal and power in and out of the case be prevented from carrying the
RFI/EMI out of the box; ferrite beads and high-quality ceramic caps or
filter connectors can take care of that problem. Try not to use too
high a value of cap as the self-resonant frequency decreases with size.
Usually around 100pf is good at VHF. Your circuits have many frquency
dividers which reduce the initial oscillator frequency down to much
lower values. These square-waves are rich in harmonics. If you tune an
FM receiver or a VHF receiver across each band you can recognize the
various harmonics by seeing where the amplitude peaks. You can make a
list of these and see the periodicity of the responsible waveform. The
rf energy inside your box induces currents in the metallic structure of
the box. Any gaps, no matter how small, form a slot antenna which
radiates. Each fastener around the periphery of the box where the sides
and top and bottom attach when tightened cause a slight arch in the
surfaces which form the slots. Think of the pan and rocker-arm covers
opn a car engine! There are several ways to stop this. Use conductive
elastomer gaskets at each joint, make the box out of extremely thick
material with machined mating surfaces with joggles, or cover the box
with rf absorbent material with an overall metal cover such as aluminum
foil. Any rf energy leaking out of the box gaps will be absorbed
somewhat as they pass through the absorber, will reradite from the foil
back through the material, and then through many passes back and forth
be attenuated. One of the manufacturers of rf gaskets has an excellent
description of the radiation from a box in their product manual. Paul

I think your idea of putting a noisey mother board inside an aluminum
box for shielding is a reasonable one. The problem may be that the
aluminum box your're thinking of as a shield, looks like an antenna to
your VHF receiver. You need to make the potential of the shield box be
the same as the ground of the receiver. The lower he inductance this
connection is, the more alike the grounds will be and the receiver
won't be able to notice the potential of the case going up and down at
100MHz.

Some other things, make sure the ground of your MOBO is connected with
a thick stap to the case.

Solving EMI problems is like peeling an onion. Each layer makes you
wanna cry. You hit the low order effects first because they have the
largest effect and are easiest to fix. Eventually you get to a place
of diminishing returns (EMI gaskets, etc) were you can live with the
"birds" and squelch them out and live with the loss of sensitivity on
some channels of your receiver.

Regards

jcpearce wrote:
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

Do a Google search on mumetal. Some years ago I had a shielding
problem on a Gulfstream and was able to kill it with a mumetal shield.

You are experiencing a problem that can be solved easily, using techniques
known by radio amateurs. In particular, The Radio Handbook, bu Bill Orr,
has a chart showing the attenuation of signal levels by various bypassing
methods used at the case of a shielded piece of equipment.

At 100 MHz, the attenuation of a standard bypass capacitor is not very high.
Additional filtering will reduce the level to where you cannot hear the
noise coming from the microprocessor. There are special capacitors, called
feedthrough capacitors, which do a great job of attenuating the signal
level, and that combined with a pi-network will result in no signal coming.

I would suggest that you borrow a high bandwidth scope (200 to 400 MHz) to
look at the levels coming out.

Where ultimate attenuation is required, double shielding is often used,
however I do not think you need that level of attenuation.

Colin N12HS


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jcpearce wrote:
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

Hi, JC
Here's a couple more ideas on containing signals in the box. First,
have plenty of closely spaced fasteners along the joints; not just in
the corners. Second, the thin copper desoldering braid makes a fairly
good RF gasket. 'Best with your project! Paul

"Here's a couple more ideas on containing signals in the box. First, have
plenty of closely spaced fasteners along the joints; not just in the
corners. Second, the thin copper desoldering braid makes a fairly good RF
gasket. 'Best with your project!"

I would not use desoldering braid. Copper reacts with aluminum. The
electrolysis will form aluminum oxide, which is an excellent insulator, and
the rf tight enclosure will no longer be secure. It could also cause
rectification, and allow external signals to mix, resulting in many spurious
signals.

Aluminum likes stainless steel, since the electric qualities are similar.
Aluminum and copper plus moisture will form a battery.

Colin N12HS


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COLIN LAMB wrote:
"Here's a couple more ideas on containing signals in the box. First,
have
plenty of closely spaced fasteners along the joints; not just in the
corners. Second, the thin copper desoldering braid makes a fairly
good RF
gasket. 'Best with your project!"

I would not use desoldering braid. Copper reacts with aluminum. The
electrolysis will form aluminum oxide, which is an excellent
insulator, and
the rf tight enclosure will no longer be secure. It could also cause
rectification, and allow external signals to mix, resulting in many
spurious
signals.

Aluminum likes stainless steel, since the electric qualities are
similar.
Aluminum and copper plus moisture will form a battery.

Colin N12HS
Hi, Colin!
Excellent consideration! Thanks for the heads-up! Paul

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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