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Thread: RF cards: how do they work?
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face BY : Jim email (remove spam text)



A technology that has been around for at least 10
years (early 90's) that I can testify too - these
were used for secure access at a telecom carrier's
HQ building I worked at in '93.

I classify these as "coupled magnetic field" devices
as opposed to *true* RFID products (although the sales
dweebs will call them "RFID Products").

RFID products will work at distances of feet to many feet
(like the cards affixed to windshields that work to provide
automatic pay on the tollway systems around here) whereas
these cards won't work (ours didn't anyway) much more than
3 inches away form "the card reader".

Also - when TDMA RF technology cellular phones came into use
in our system we began encounter problems when entering the
parking garage (the garage required "card" access) if engaged
in an active callphone call ... these digital TDMA phones
interfered with either the "readers" or the cards to the
point that one had to end a call in order to get the card
"to read".

As to methodology - I can see where tuned circuits in the
frequency range of 100 KHz or so could be easily used to
couple to the card and the resulting 100 KHz energy rectified
to power a PIC or other low-power processor-based ID system.

RF Jim

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