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Experimental aircraft EFIS system using EPIA M

J

jcpearce

Jan 1, 1970
0
For my Long Ez aircraft 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 a custom aluminum case with very little
improvement.

I have seperated the entire unit from the airplane, turned off the data
aquistion unit and get the interference so it is coming solely from the
EPIA M motherboard. I get the same interference from my 2 home PC's
which are running different CPU's (AMD and Intel). I can test this with
a portable aircraft radio and walking towards the PC, around 6' or so
from them I get interference. So I would guess some supporting chip on
the motherboard is emmiting in this range (107-130 Mhz, right above
FM).

Any ideas on how to smother the EMI or some other small motherboard
which may not have as much an issue?

Thanks
 
J

Joerg

Jan 1, 1970
0
Hello jcpearce,

I believe the Long EZ is composite which would make it a bit more tough.
The case is already a good start. Ideally it should not be anodized
aluminum because of a sub-optimal surface conductivity. Maybe you could
find alodine or something more conductive. I prefer conductively
galvanized steel. But right now I wouldn't worry about it and instead
try these measures first:

More important that the enclosure are the wires going in and out. Since
you usually don't know all of the exact data rates or impedances,
capacitive filtering is not so easy. So buy yourself a large bag of 43
ferrite material. Toroids, all kind of sizes. Ideally you should also
buy some clip-ons and some of the Radio Shacks still have these. Then
while running the radios and the EPIA keep adding toroids, as close to
your enclosure as you can and see what happens. They have to go over the
whole cable, including (and most of all) the power cable. If you slid
one over +12V alone the DC current might saturate the core and then it
does no good.

Please mind that this may void the airworthiness of the Long EZ.
Especially if you use clip-on toroids these can fall off in flight and
land in places where they might cause a lot of grief. Also, ferrite
material is brittle and vibration against something else or a shock can
shatter that stuff.

Another product that can come to the rescue if you have D-Sub connectors
on there: These are also available as "EMI fortified" where there is a
hole-pattern ferrite integrated over the inside pin contacts. See if
they are protected against vibration though, for the same reason as
mentioned above.

Your situation is better then the EMI job I had to do on a new composite
design. The engine magnetos and a pump controller were messing up
NAV/COM. So I had to crawl around while the prop and other stuff was
running.

Regards, Joerg
 
T

Tim Wescott

Jan 1, 1970
0
Joerg said:
Hello jcpearce,

I believe the Long EZ is composite which would make it a bit more tough.
The case is already a good start. Ideally it should not be anodized
aluminum because of a sub-optimal surface conductivity. Maybe you could
find alodine or something more conductive. I prefer conductively
galvanized steel. But right now I wouldn't worry about it and instead
try these measures first:

More important that the enclosure are the wires going in and out. Since
you usually don't know all of the exact data rates or impedances,
capacitive filtering is not so easy. So buy yourself a large bag of 43
ferrite material. Toroids, all kind of sizes. Ideally you should also
buy some clip-ons and some of the Radio Shacks still have these. Then
while running the radios and the EPIA keep adding toroids, as close to
your enclosure as you can and see what happens. They have to go over the
whole cable, including (and most of all) the power cable. If you slid
one over +12V alone the DC current might saturate the core and then it
does no good.

Please mind that this may void the airworthiness of the Long EZ.
Especially if you use clip-on toroids these can fall off in flight and
land in places where they might cause a lot of grief. Also, ferrite
material is brittle and vibration against something else or a shock can
shatter that stuff.

Another product that can come to the rescue if you have D-Sub connectors
on there: These are also available as "EMI fortified" where there is a
hole-pattern ferrite integrated over the inside pin contacts. See if
they are protected against vibration though, for the same reason as
mentioned above.

Your situation is better then the EMI job I had to do on a new composite
design. The engine magnetos and a pump controller were messing up
NAV/COM. So I had to crawl around while the prop and other stuff was
running.

Regards, Joerg

Some other things to remember:

EMI is black magic. It's based on sound scientific principals but
there's so many variables that when you actually start attempting to fix
things it's black magic. So don't feel bad if you can't make it work,
or if the solution makes no sense.

If the EMI is happening at a specific frequency you can go looking for a
circuit that works right at that frequency. I once worked for a month
or so revamping software to work on different crystal frequency because
with the old frequency the customers favorite tactical comms frequency
was upset, even though the equipment squeaked by the EMI standards. For
example if you have significant radiation at 120MHz look for a 10 or
12MHz crystal and concentrate on wires from that part of the circuit.

The ideal EMI solution is to keep it from getting off the board in the
first place. The second-best is to get all the beads in place _inside_
the case, then if that isn't enough make the case EMI proof to keep
everything inside -- but this is much easier said than done. If you do
address the case pay attention to Joerg's comments about finishes and
use _lots_ of screws to attach the lid to the case.
 
R

Rene Tschaggelar

Jan 1, 1970
0
jcpearce said:
For my Long Ez aircraft 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 a custom aluminum case with very little
improvement.

I have seperated the entire unit from the airplane, turned off the data
aquistion unit and get the interference so it is coming solely from the
EPIA M motherboard. I get the same interference from my 2 home PC's
which are running different CPU's (AMD and Intel). I can test this with
a portable aircraft radio and walking towards the PC, around 6' or so
from them I get interference. So I would guess some supporting chip on
the motherboard is emmiting in this range (107-130 Mhz, right above
FM).

Any ideas on how to smother the EMI or some other small motherboard
which may not have as much an issue?

Apart what has been told already, there are clock generators that
sweep and thus distribute the noise over bands instaed of emitting
at selected frequencies. Some motherboards, such as my new
Athlon64bit let you select the clock spread option.

I you don't need the computing power an EPIA M provides, something
simpler, such as an 8051 or an AVR, would generate less interferences.


Rene
 
T

Tim Shoppa

Jan 1, 1970
0
jcpearce said:
For my Long Ez aircraft 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 a custom aluminum case with very little
improvement.

A good chunk of the interference is conducted out via the power supply
wires, ethernet, video wires, etc.

To some extent you can put chokes on these leads but seeing as how the
signal bandwidths over the Ethernet and video are in the tens of MHz
you can't filter out the actual signal!

If you can, for example, use optical interfaces to carry Ethernet and
other necessary signals from your box then you can come out way ahead.
And using not-el-cheapo power supplies helps a lot with conducted
interference over the power leads.

Are you using 400Hz 120VAC power, or 12VDC, or something else to power
the EPIA? If it's 12VDC via one of those little 12V<->ATX-like
converter boards, keep in mind that those converter boards use almost
completely unfiltered SEPIC and step-down converters and put out
incredible hash even without a computer hooked to them.

Tim.
 
A

Anthony Fremont

Jan 1, 1970
0
jcpearce said:
Any ideas on how to smother the EMI or some other small motherboard
which may not have as much an issue?

Others have covered the major factors to look at. Basically any wire
coming out of the case is suspect. You could use feed thru caps to pass
the power cables out of the case. I'm not sure how that would work on
the video signals though, it might seriously degrade them. And that
brings us to the point of my post, what are you using for a display? In
my experience, displays and keyboards are notorious for emitting rf
noise. Your EPIA itself may not be the real problem, I suspect it's
probably not.
 
M

Mark Jones

Jan 1, 1970
0
Quick thought: 100/120/133MHz front-side bus?

I am not familiar with your equipment (I'm a hang-gliding guy myself)
but perhaps you could try "wrapping" the suspected emitter in grounded
metal window screening (faraday cage) and running the test again. This
is just to pinpoint/determine if the EMI can be shielded without
building custom enclosures.

That said, any chance a notch filter could be installed in the
receiving equipment? Better to fix the source of the problem rather
than band-aid everything else though of course.
 
J

Joerg

Jan 1, 1970
0
Hi Tim,
Are you using 400Hz 120VAC power, or 12VDC, or something else to power
the EPIA? If it's 12VDC via one of those little 12V<->ATX-like
converter boards, keep in mind that those converter boards use almost
completely unfiltered SEPIC and step-down converters and put out
incredible hash even without a computer hooked to them.

That is an excellent hint. Especially when considering that aircraft
communications are still done in old-fashioned AM which is prone to
interference.

I wonder whether the task at hand could be performed using a more quiet
laptop. The Linux requirement might be a challenge here because all
those nice 'plug and play' USB data acquisition pods are geared towards
Win 2000/XP. Some even won't work under older NT variants. However, the
OP mentioned an 8051 add-on and those usually come with RS232. That
should help, except with some new laptops. The one I just bought is all
USB, no other comm ports.

Regards, Joerg
 
J

Joerg

Jan 1, 1970
0
Hello Anthony,
...... And that
brings us to the point of my post, what are you using for a display? In
my experience, displays and keyboards are notorious for emitting rf
noise. ....

Oh yes. I remember when I did an EMI case in a screen room and part of
the noise pattern just didn't make sense, didn't compute. Then it turned
out it came from the flat panel display of yours truly, the analyzer.

Sometimes holding a hand in front can pinpoint that. But in a composite
structure aircraft that might not mean much because touching anything
conductive will likely alter the noise pattern.

Regards, Joerg
 
K

Ken Smith

Jan 1, 1970
0
Tim Wescott said:
The ideal EMI solution is to keep it from getting off the board in the
first place.

Almost :>
The ideal is not to make it on the board in the first place. Second
is trapping it in only part of the PCB and keeping it from getting off the
board only comes in third.

Source termination of things like clock signals can help a lot. On
switcher supplies, softening up the switching will cost you in the losses
but perhaps save you on the EMI filter.

Putting a shield around the offending portion of the PCB can help a lot.
The smaller the volume you put inside the shield the more effective it
tends to be so don't enclose unrelated stuff if you are doing this.

The second-best is to get all the beads in place _inside_
the case,

This works fairly well. Using inductors of higher values is usually less
effective. You want the lossiness of the beads to suck up the RF power.
If you don't, it will just find a new way to leak out. This is usually
capacitively right past the inductors.

[...]
address the case pay attention to Joerg's comments about finishes and
use _lots_ of screws to attach the lid to the case.

Or (1) make the box from brass and solder it (2) Use those RF fingers to
connect all of the joint (3) buy some sticky backed copper tape and tape
over the joints.
 
J

Joerg

Jan 1, 1970
0
Hi Ken,
Or (1) make the box from brass and solder it (2) Use those RF fingers to
connect all of the joint (3) buy some sticky backed copper tape and tape
over the joints.

(1) Brass works but gets ugly over time. Fingerprints, oils, gunk all
leave pretty permanent 'souvenirs'.

(2) I prefer not to use any copper-beryllium. I believe its oxide can be
toxic.

(3) Copper tape works great but unless you spot solder it tends to come
off during a hot summer. Then it sails into some electrical area and,
bzzzzt - shazam, you have a whole other issue at hand.

Regards, Joerg
 
T

Tim Wescott

Jan 1, 1970
0
Joerg said:
Hi Ken,


(1) Brass works but gets ugly over time. Fingerprints, oils, gunk all
leave pretty permanent 'souvenirs'.

(2) I prefer not to use any copper-beryllium. I believe its oxide can be
toxic.

(3) Copper tape works great but unless you spot solder it tends to come
off during a hot summer. Then it sails into some electrical area and,
bzzzzt - shazam, you have a whole other issue at hand.

Regards, Joerg

I have a couple of home-built instruments who's cases are double-sided
PC boards soldered together. I covered them all over with clear 2" wide
shipping tape. The copper has turned green underneath the tape over the
years, but it's a consistent, even green -- the only fingerprints are on
the back panels which are (a) removable and (b) not taped.

Clear paint would probably also work, assuming you're careful about not
painting the parts that are supposed to connect.
 
D

Dave VanHorn

Jan 1, 1970
0
Something I have done to quiet down noisy PCs:

Obtain a large square (as large as will fit) of MosFoam. Digikey sells it,
this is the stuff that you put chips in to protect them from ESD in storage.

Cut to fit, and place inside a large freezer bag, and glue the freezer bag
with mosfoam in place in the case. The bag prevents any "crumbs" from
getting into the other goodies, and keeps the square itself out of anything
else.

RF inside a metal box, is like light in a box made of mirrors, it's gonig to
bounce around till it dissipates, or finds a way out. The mosfoam is a nice
material for it to dissipate in.
 
K

Ken Smith

Jan 1, 1970
0
years, but it's a consistent, even green -- the only fingerprints are on
the back panels which are (a) removable and (b) not taped.

Clear paint would probably also work, assuming you're careful about not
painting the parts that are supposed to connect.

How about getting it gold plated?
 
T

Tim Wescott

Jan 1, 1970
0
Ken said:
How about getting it gold plated?
Must resist temptation to -- oh to hell with it:

Why would you want to gold plate paint?
 
K

Ken Smith

Jan 1, 1970
0
Must resist temptation to -- oh to hell with it:

Why would you want to gold plate paint?

'Cause gold plating things is good!


But seriously, plating an aluminum case is a fair option. Nickel plating
looks quite good and resists finger prints and scuff marks well.
 
J

jcpearce

Jan 1, 1970
0
Thanks

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 #[email protected]! (
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 knowledgeable fellows
enumerated in the posts.

Thanks again
 
T

Tim Wescott

Jan 1, 1970
0
jcpearce said:
Thanks

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 #[email protected]! (
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 knowledgeable fellows
enumerated in the posts.

Thanks again
Anything that's called "DC/DC" by an honest person is going to be
switching -- if it's a linear supply it'll be called "linear".

You could always build a linear supply for the thing, if you think you
can stand your 12V not being quite 12V sometimes. Plan on _big_ heat
sinks, though.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that jcpearce <[email protected]>
I am trying to find another DC-
power supplies, I would rather deal with energy/heat loss than RF noise.

It's really difficult to convert DC voltages without switching! But in
your case, you can tolerate the lower efficiency of a sine-wave supply.
You need a sine-wave oscillator (not low distortion), an audio amplifier
chip and a suitable transformer at its output, which you will need to
design or have designed for you. I suggest you use a 400 Hz oscillator,
not 50 or 60 Hz. This should not present a problem with core loss if you
use a toroidal core, and a core for 400 Hz is much smaller and lighter
than one for 50 or 60 Hz. Since the voltages are low, hand-winding is
practicable.

You might be able to use just one rectifier to get 12 V and then
regulate down to 5 V and 3.3 V, depending on what currents you need. But
it isn't that difficult to provide more than one secondary winding to
get different DC voltages with less heat loss.
 
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