homemade EFIS system and EMI

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.

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

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