The App Note looks interesting.  Since I've never even wound a coil until last week
I'm sort of winging it.
Hadn't thought about the DC Resistance but I'm stuck with the 4 X 4 spacing to fit
the room on the board.

If it works I'll save $2-4 per assembly with the "free" antenna, so it's worth a
try.

Yes this is a 486 and in 3 or 4 years I've never had one piece of spam.  My other
computer that is part of the company's set-up get about 150 per day.  That's been
getting worse for years but last week they put in a filter and I'm down to 20 per
day.

Thank for everybody's input.  When I get some proto boards I'll let you know if it
worked.

John

Leo Potjewijd wrote:

> At 29-03-2004 12:56, [EMAIL PROTECTED] wrote:
> >In a message dated 2004-03-28 03:33:51 PM Eastern Standard Time,
> >[EMAIL PROTECTED] writes:
> >
> > >......The coil is about 3.25 X 4.25", 700uHy and takes 60 turns of mag
> > wire.  Can I do this with traces?........
>
> You can. EM does it in their proximity reader reference design, but use a
> lower inductance.
> They do 32 turns and swap layers every 4 turns.
>
> >I did something almost exactly like this a while back. Better do some
> >calculations before you do those 4/4 design rules. I was very disappointed
> >in the Q of the resulting coil; I needed a fairly high Q.
>
> But with prox readers, the Q has to be limited for bandwith reasons.
> Depending on the type of datarate and modulation used you can end up with a
> maximum Q of 1.5.......... Q's used with 2kb/s and ASK are in the range 10-25.
>
> >Should have done the calculations myself before layout, because it behaves
> >as theory would dictate. I used 10/10 rules. With 1 oz copper (1.4 mils
> >thickness), the cross-sectional area of  a 10 mil trace is only 14 sq mils
> >- roughly equivalent to #37 wire, which is about 2' per ohm. The coil you
> >describe is about 60' long, so you've got at least 30 ohms with 10/10
> >rules. If you go to 4/4, the cross section drops to 5.6 sq mils, which
> >puts you up around 90 to 100 ohms for 60' of coil. Now you need to bring
> >the rest of the circuit into the picture to compute Q, if that's relevant,
> >but you can at least say that you'll lose 1 volt of your drive for every
> >10 mA of current through that coil. If I were doing it over again, I'd
> >figure out a way to get some iron inside the coil and reduce the number of
> >turns, and if possible use wider traces and thicker copper.
>
> For proximity cards you will need to maximise the current, after all that's
> what powers the card. Some good computations can be found in Microchip's AN678.
>
> Leo Potjewijd
> hardware designer
> Integrated Engineering B.V.
>
> [EMAIL PROTECTED]
> +31 20 4620700



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