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'[PIC]:Components (PIC) in a 13.56MHz field'
2002\07\26@044415 by tony

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Hello all

What implications if any exist by having various components (PIC
included) within a 13.56MHz RF coil that has been etched on a
PCBoard.

Is there any difference to having those same components
along the outside of the same etched coil.

Is the only difference that of tuning or does the sinister black art of
RF introduce other more sinister unknowns??

TIA


TONY BAIA

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2002\07\26@093440 by M. Adam Davis

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This would also depend on the strength of the field generated by the
coil.  Chances are you'd have effects both ways, the pic would detune
the coil in a non-linear way (depending on what code it's running it has
a different magnetic field) and the coil will induce effects in the pic,
some may be undetectable, but a strong enough field is going to do odd
things with it.

I'd be most worried about the a/d converter if you're using one, pins
which are not driven high or low, and pins which are inputs but are not
being driven adequetely externally.  The oscillator may be affected as well.

Those are just my guesses.  Obviously if your field is very tiny then
you probably won't have to worry about it.

-Adam

Tony Baia wrote:

{Quote hidden}

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2002\07\26@095517 by Alan B. Pearce

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>I'd be most worried about the a/d converter if you're using
>one, pins which are not driven high or low, and pins which
>are inputs but are not being driven adequetely externally.
>The oscillator may be affected as well.

>Those are just my guesses.  Obviously if your field is very
>tiny then you probably won't have to worry about it.

Judging by the frequency I take it you are getting involved in security or
ID tags. :)

I do not know the field strength of a typical shop doorway detector, but I
doubt it is high enough to be a problem to the PIC, unless you are trying to
run the clock at a close frequency, in which case the oscillator may try and
lock to it, PLL style.

If you are trying to make another type of ID tag, along the lines of a
product ID tag where the sense device is quite close, then I believe it
should be possible to run that at a level which should still be safe for the
PIC.

Either way I suspect the worst you will see is some detuning of the coil due
to the nearby presence of the PIC and its hardware, wether mounted inside or
outside the coil. This will probably be most easily adjusted by using a grid
dip oscillator, but again I would suggest that the tuning will be broad
enough for this not to be a worry.

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2002\07\27@061726 by Peter L. Peres

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On Fri, 26 Jul 2002, Tony Baia wrote:

>Hello all
>
>What implications if any exist by having various components (PIC
>included) within a 13.56MHz RF coil that has been etched on a
>PCBoard.
>
>Is there any difference to having those same components
>along the outside of the same etched coil.
>
>Is the only difference that of tuning or does the sinister black art of
>RF introduce other more sinister unknowns??

The sinister black art of RF will ensure that the field is maximum inside
the coil, i.e. in your circuit, and that you are thus forbidden:

- Closed circuit patterns
- Patterns that may cause RF from the coil or RFID exciter to cause
misread digital or analog signals by rectification or glitching
- anything else that I may have forgotten

Otoh, all RFID 'credit card' type devices happily work inside a 13.56MHz
fields. So there is a way to do it.

Peter

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2002\07\29@131403 by Barry Gershenfeld

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>On Fri, 26 Jul 2002, Tony Baia wrote:
>>What implications if any exist by having various components (PIC
>>included) within a 13.56MHz RF coil that has been etched on a
>>PCBoard.
Peter sez:
>The sinister black art of RF will ensure that the field is maximum inside
>the coil, i.e. in your circuit, and that you are thus forbidden:
>- Closed circuit patterns
>- Patterns that may cause RF from the coil or RFID exciter to cause
>misread digital or analog signals by rectification or glitching
>- anything else that I may have forgotten
>
>Otoh, all RFID 'credit card' type devices happily work inside a 13.56MHz
>fields. So there is a way to do it.

I don't think the chip is bothered much, what you have to look out
for is the antennas (meaning the signal wires coming into the chip)!
14 Mhz has a 20-meter wavelength, so wire length isn't as much an
issue as capacitively coupled energy.  (aka, the "E" field).  But
that's a guess.

Barry

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2002\07\29@132859 by Peter L. Peres

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On Mon, 29 Jul 2002, Barry Gershenfeld wrote:

{Quote hidden}

I'd agree unless I'd know that plain LM358s make great microwave detectors
(direct rectifying). Which they make. Now guess how I found out ?

Peter

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