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'[OT] RF interference problem in a palmtop's IR inp'
1999\12\06@060946 by Mark Willis

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OT, yeah, I know, at least it's a Tech problem;  I'm trying to interface
a PIC to here later, but this problem's not helping.

Some of the HP200LX folks are having a bad problem.  They're trying to
use the IrDA link from a 200LX to connect to a cell phone;  The RF from
the phone, is interrupting their IR link and causing havoc.  They're
getting a 1.6 kHz induced voltage, sort of a ramp, and it's not helping
them.

If someone good in RF can talk with me, after glancing at
http://www.user.tu-berlin.de/~dherbgba/hplxir/index.html, these guys
really could use some help, they've tried ferrite beads (though I don't
know what they used exactly), and a bunch of other things;  Placing a
copper 1/8" thick plate 6"x6" over the IR port, and another 6"x8" under
the palmtop, seem to work as a shield, not very portable though!

I think it's just a "1/R squared" problem, but if they can get this
solved, they'll be happy.  There's a nice ground ring around this IR
input circuit, I'm just not as good in Analog as I once was.  Myself,
I'll make an IR extender for this machine, that will definitely use a
*shielded* IR module...

The 200LX's IR phototransistor is set up so that it has nice, long leads
coming from the phototransistor, almost 1cm long between it and the PCB
through holes it solders to.  Nice RF antennas at close range to a cell
phone...

I do plan to look at his schematic, he finally found the 8DIL chip in
here's .PDF file and I notice that it just plain doesn't MATCH the
chip's .PDF file (not a good idea to short 2 totem-pole outputs
together, see pins 6 and 7!) - something's not right there, I'll go look
through the 200LX I have apart on the bench here to improve his drawing.

Any RF help would be great - Thanks in advance!

 Mark

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