[aprssig] Re: IGPS RFID tags at 915 MHz

Jim Wagner wagnerj at proaxis.com
Mon Nov 26 18:15:17 EST 2007

I did a LOT of work with passive RIFD scanners for a large barcode 
reader company.

These things, if they are passive, are totally passive. They rely on 
modulated backscatter from the antenna. They are really pretty simple, 
in principle anyway. The reader sends clock "bits". The tag uses those 
to shift data out of a flash-based data shiftregister. The data opens 
and closes a switch in the antenna circuit that allows antenna current 
to flow, or not flow. When current flows, the antenna re-radiates some 
the incident energy.

The data capacity is pretty small, varying from 64 bits to (I think) 
256 bits and now, maybe up to 1024 or so.

These are totally different from the FastPass units that are used in 
toll road/bridge situations. Those have real receivers and transmitters 
though power is pretty low. They will typically only respond within 
100m or so of the query unit's antenna.

There are a whole bunch of newer tags coming onto the market. Some are 
intended for shipping containers. There is a whole group of them with 
encrypted data, etc, that are being used to track and validate certain 
drugs in pharmacies. These tend to work at low frequencies (under 
500KHz) and use magnetic coupling. NXP (ex-Philips) is also working on 
a family of "near-field" tags that are RF, but are intended for things 
like credit cards, vending machines, and such. One can argue the wisdom 
of such stuff, but it is being worked on.


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