homemade EFIS system and EMI

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

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)

Physically separate the offender from the victim, especially the point of
entry of the signals into the victim. Don't forget about coax cables, comm
antenna and things like that, not just the radio box. You could try
reorienting the motherboard 90 degrees in various axis.

This is without the system connected to any of the sensors, just
turning it on and with an external battery so it is physically isolated
but within 6' or so it causes interference. Rotating the motherboard
does not help either. Thanks for the reply.

"jcpearce" wrote in message oups.com...
This is without the system connected to any of the sensors, just
turning it on and with an external battery so it is physically isolated
but within 6' or so it causes interference. Rotating the motherboard
does not help either. Thanks for the reply.


I have the same problems with my desktop machine and handheld radio...Wrap with aluminum foil and see what happens...

jcpearce wrote:

This is without the system connected to any of the sensors, just
turning it on and with an external battery so it is physically
isolatedseparateithin 6' or so it causes interference. Rotating the
motherboard
does not help either. Thanks for the reply.

Possibly try remotely mounting it back in the fusilage and running
fiberoptic to it. You can get inexpensive hobby FO and FO to rs232 modules
designed for experimenting.
Other than that try a non-aluminum case and use feed thru caps on all leads
and ferrite beads also. try some additional filtering on the PS and/or try
using separate PS for processor.
Just some thoughts.
John

ps those hobby aluminum cases do lousy for rfi prevention!

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 obviously are knowledgeable in the
subject, but have you tried the simple
stuff first, like ferrite doughnuts on
all cables to and from the boards and
grounding one side of cable shielding etc.?
It sounds like a very interesting
project, do you have a site for more info?
Kumaros
It's all Greek to me

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

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

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

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.