RE: [oc] Project suggestion: RFID tag reader

["Harvey, Wilbur" <Wilbur.Harvey@spirentcom.com>]

RFID tags use the model of a loosely coupled transformer.
The "reader" is, in reality the primary, and the tag is the secondary.
At 13Mhz, you can to out to about 1 to 1.6 meters using the standard credit
card sized tags and 1.2 meter dia reader "antenna". It would not ever be
possible to read them over long distances.

The tag is energized by the field from the primary winding (reader antenna)
and powered up, it then has a variable load (open collector/drain usually)
which in effect modulates the power being drawn by secondary winding. The
reader detects this variable amount of load from the secondary as the
"transmitted" signal from the tag.

Since it is really a transformer, the coupling is determined by the relative
angle between the two windings. If they are perpendicular to each other, a 1
meter range turns into 0 meters.

Typically to get long ranges, you try to make the Q of the windings high,
which makes them very easy to detune from things like the item which the
tags are applied to, or your body near the reader "antenna". Of course if
you place the tag on a conductive surface, it short circuits the signal and
you get extremely short reading ranges. Ferrite materials can be applied
under the tags when placed on conductive surfaces to get normal reading
ranges, but of course, this affects the tuning of the tags coil.

Typically the tags each have a unique ID code, and most have the ability to
be programmed with some data, that can be whatever you like, like the TI and
Philips tags. That is why they are used for disposable airline baggage
tracking tags.

Wilbur Harvey - Engineer
Spirent Communications Inc., www.adtech-inc.com
First Hawaiian Center, 999 Bishop St.. Suite 2700
Honolulu, HI 96813
Tel: +1 (808) 440 3494, +1 (808) 440 3363 (Direct)
email: wilbur.harvey@spirentcom.com


-----Original Message-----
From: Simon Gornall [mailto:simon@gornall.net] 
Sent: Sunday, November 17, 2002 9:31 PM
To: cores@opencores.org
Subject: Re: [oc] Project suggestion: RFID tag reader

On Mon, 2002-11-18 at 03:49, Rudolf Usselmann wrote:
> On Sunday 17 November 2002 19:54, John Dalton wrote:
> > Here is a suggestion for a project:
> >
> > A reader for Auto-ID RF identification tags:
> >
> > http://www.autoidcenter.org/
> >
> > As I understand it, auto-ID will be replacing the
> > barcodes you see on practically everything you
> > purchase.   The main differences to a barcode are:
> >
> > 1) You can't read the number without a reader.
> > 2) Individual items will be coded with unique numbers.
> >    With barcodes, all packets of "brand X" washing
> >    powder are marked with the same number.  With RFID
> >    tags, each packet carries a unique number.  If you
> >    are carrying that packet of washing powder (or wearing
> >    clothes), it will be possible to track your movements.
> > 3) RFID codes can potentially be read over longer distances
> >    than barcodes can (ultimately by satellite???).
> 
> Holly sh... !!!  I think what we really need is a RFID Jammer ! I'm
> not exactly thrilled by haveing any government agency (specially
> a foreign one) being able to track my movements.
> But than, aren't these things powered by the reader ? I guess
> they would have to Emmit some sort of energy beam from low
> orbit to activate a particular (or all) RFIDs in an area ...
> Scary though ...

The above is a bit optimistic as regards current technology, and a ways
into the future as well, IMHO. RFID runs on (typically) 125KHz or
13.46MHz (which interferes quite nicely with the PAL colour burst
frequency ...) Read range for the "normal" 2cm-wide tags is about 10cm
plus or minus 2cm depending on aerial and conditions (it's a lot less in
wet conditions, for example). It can be larger as a function of aerial
radiative power, but you're stuck with the inverse square law... It's
also larger with respect to aerial area, although there are limits due
to tuning requirements.

RFID is powered by the transmitter, and I wouldn't like to be in the
beam-width of any satellite which could power the chip from orbit! 

Oh yeah, they're also dependent on the phase of the moon. At least
that's the way it seemed when I was using them a year or so ago. They
may have got over that :-)

Simon.


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