homemade EFIS system and EMI

RFI is definitely a problem, and fixing RFI problems can be a black
art.
You don't actually have to have _any_ chip running at the problem
frequency. Square waves have a lot of odd harmonics. So, something
switching regularly at 1/3 or 1/5 the problem frequency can cause
problems. That something can be a software routine in a chip running
much faster than the problem frequency.

You might want to reconsider your preferred form factor. PC
motherboards are amazingly inexpensive, but they're relatively large.
Have you considered the PDA form-factor? Smaller, less power-hungry,
built-in display, and most of the RF problems will be already handled.
Should still be able to handle the audio and display functions.

If you prefer Linux over PocketPC or Palm OS, I know the Zaurus PDA has
had Linux ported to it.

Glider pilots use PDA's as glide computers, so they're known to run
without problems in systems with aviation band radios. If you're
sending the data from the 8051 via the serial port, this is how the
glide computer PDA's interface with the GPS and/or vario systems.

Downside is, RS232 ports seem to be going away in favor of USB, and USB
OTG (where the PDA can act as a host instead of a peripheral) seems to
be very slow in coming in. OTOH, older PDAs with serial ports are
dirt cheap on EBay.

And, for 8051 chips, the Dallas 89C440 is a pretty cool little chip.
It can run internally at up to 4x the xtal frequency, 1 machine cycle/
clock instead of 12.
Flash, so you can reprogram it in-circuit. Built-in loader so all you
need to program it is a PC and a serial port. 32K bytes of program
space. (64K on the 89C450)
2K bytes of xdata memory on chip, and the usual 256 bytes of the 8052.
There are C compilers for the 8051 architecture, so you're really not
limited to assembly for development. About five bucks a pop, small
quantities.

Tim Ward

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.

jcpearce wrote:
Thanks for the input

Aluminum can act as an RF shield but not as a magnetic field shield,
given that the interference is in the RF band I figured aluminum
would
be OK (and to keep it light) but some iron based material would do
both
and perhaps is a better choice.

Using the PC architecture has so many advantages (and obviously at
the
moment a showstopper bug), I can easily store the history of a trip
on
a removable USB card, on this USB card is a directory for music that
can be played and piped into the stereo intercom, high tech displays
with graphs, charts, etc.. is easy, it is inexpensive, and easy to
program with high level languages. Doing the assembly on the 8051
data
controller was interesting but to do the same functions as described
above would be very very hard (for me). I have gone so far down this
road I am loath to junk it (even if that is the right thing to do)
and
tell myself if I bang my head against the wall enough times the
answer
will show up. I know people use their laptops in cockpits without
this
problem so there must be a way.

Hi, JC!
Actually, any highly conductive box with minimal gaps will shield
against magnetic field intrusion. The way it works is that the incident
magnetic field generates a current in the metal surface, and this
current then in itself produces a magnetic field which opposes the
original one. That is why it is very important to use highly conductive
material and to have highly conductive, well-sealed joints. By the way,
RF doesn't reflect from a metal surface; the field generates a current
in the surface, which in turn radiates the energy, This is why the
counterpoise (ground-plane) to be effective must be of high
conductivity for good 1/4 wave antenna radiation. Aluminum and copper
make very good enclosures, both for RFI and EMI. Soft aluminum, such as
1100 0, is the best and has an IACS relative resistance of 1.69 making
it more conductive than some of the harder alloys such as 2017 T4 and
2024 T4 which are 3.33, double that of 1100, or 5056 H18 at 3.70. Don't
use brass; it's not as good as aluminum. I had a small circuit that had
to have extreme isolation from incident fields to work properly. I made
a box out of double-sided circuit board, and soldered all the inner
surfaces together as well as all of the outer surfaces except the lid,
which had conductive fingers all the way around on the inside, then
soldered on the outside. Worked great! You might try this if it is just
to be a one-off.
Here're some references you might find informative: Interference
Handbook, Nelson, Radio Publications Inc., 1981; Grounding and
Shielding Techniques in Instrumentation, Morrison, Wiley, 1967; Good
grounding and shielding practices, Electronic Design, 1977 Jan. 04,
p.110; Sniffer probe locates sources of EMI, EDN, 1998 June 04, p.155,
as well as the previously mentioned catalogs, which are from Chomerics
and Metex. Amuneal Mfg. has mumetal shields. Paul

Look up info on "Tempest" PC's. Lots of good methods for RFI suppression.


jcpearce wrote:
Thanks for the suggestions, no I have not posted what I did anywhere
but I think I will as it took me a little while to learn rudimentary
assembly, come up with a variant of linux running off flash memory,
write the code to process the serial output and display etc..

I did not use a hobby aluminum case, I custom built one with no gaps
(motherboard has a temperature sensor so I could check this for
possible overheating) and this was grounded. Without hooking up the
data aquisition card and just powering up the EPIA M in the seperate
aluminum case with no connections of any kind to the airplane I get the
interference in the radio. There are no connections coming out of the
aluminum box except for power and this line has caps on it for
filtering.

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
home computer running at 2Ghz, another at 2.4 Ghz and the EPIA and 500
Mhz I would guess it is some supporting chip, but even if I knew I am
not sure that would do me much good. Perhaps changing the aluminum case
to a different size/shape would catch the offending frequency but I am
in the dark here and would be shooting in the dark.

There only seems two generic routes,
A) find a small computer which does not emit these frequencies (but I
do not know what is emmiting them so I would not know which computer
choice would alleviate this)
B) Some vastly better shielding approach for the motherboard.

Ideas?

Thanks



Bob wrote:

The "noise" could be coming through the connector and could be

emitting

from the wires.

Try using a connector with a metal housing and backshell and wires

with

shields, terminate the shields to the connector backshell/housing

which

will ground to the chassis.

The other end of the wires, try to terminate their shields where they
connect to.

Aircraft environment is not the same as home environment. Different
applications different techniques.

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

Some IMPRESSIVE experience in this group.

I have not heard "tempest" in years.

But again, in my experience, if you've got a metal box already. It's
most likely the connector and harness.

Carry those grounds (on the harness shields) through!



AINut wrote:
Look up info on "Tempest" PC's. Lots of good methods for RFI
suppression.


jcpearce wrote:
Thanks for the suggestions, no I have not posted what I did
anywhere
but I think I will as it took me a little while to learn
rudimentary
assembly, come up with a variant of linux running off flash memory,
write the code to process the serial output and display etc..

I did not use a hobby aluminum case, I custom built one with no
gaps
(motherboard has a temperature sensor so I could check this for
possible overheating) and this was grounded. Without hooking up the
data aquisition card and just powering up the EPIA M in the
seperate
aluminum case with no connections of any kind to the airplane I get
the
interference in the radio. There are no connections coming out of
the
aluminum box except for power and this line has caps on it for
filtering.

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
home computer running at 2Ghz, another at 2.4 Ghz and the EPIA and
500
Mhz I would guess it is some supporting chip, but even if I knew I
am
not sure that would do me much good. Perhaps changing the aluminum
case
to a different size/shape would catch the offending frequency but I
am
in the dark here and would be shooting in the dark.

There only seems two generic routes,
A) find a small computer which does not emit these frequencies (but
I
do not know what is emmiting them so I would not know which
computer
choice would alleviate this)
B) Some vastly better shielding approach for the motherboard.

Ideas?

Thanks



Bob wrote:

The "noise" could be coming through the connector and could be

emitting

from the wires.

Try using a connector with a metal housing and backshell and wires

with

shields, terminate the shields to the connector backshell/housing

which

will ground to the chassis.

The other end of the wires, try to terminate their shields where
they
connect to.

Aircraft environment is not the same as home environment.
Different
applications different techniques.

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

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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