Inductively Coupled RFID - Analogy to a transformer?

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hcbowman - 27 Jul 2008 16:59 GMT

Hi,

I'm trying to understand how RFID tags work. One large class of tags
is "inductively coupled," which I understand to mean that an
alternating current applied to a coil in the reader tends to induce a
current in the coil of the tag. The circuitry in the tag can change
the load impedance on the tag coil, and this causes a change in the
impedance of the reader coil. By monitoring the impedance of the
reader coil, the reader system can "read" the tag.

Is it correct to think of this type of tag as being similar to a
circuit that connects a load to an AC power source through a
transformer? If I switch part of the load on and off, the power
source would see the load vary on its side of the transformer. Even
if other parts of the load change over time, could I be really clever
with my switching to create a distinguishable signal on the source
side?

Are there conditions under which the transformer analogy breaks down?

Thanks,

--Cliff

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Androcles - 27 Jul 2008 18:30 GMT

| Hi,
|
[quoted text clipped - 19 lines]
|
| --Cliff

The RFID tag evolved out of the metal detector when it was realised
that part of the transmitted energy could be used by the device, much
as the old "crystal set" radios can be used to hear AM radio without any
amplification. One simply hooks a diode to the antenna and listens
on an earpiece; no need to tune to a station, you hear the strongest signal
as the loudest.
http://earlywireless.com/images/crystal_sets_images/crysta4.gif

Once you have energy in the device you can use it any way you want to,
including an identifying digital retransmission from a chip.
There is no hard rule as to what you or any manufacturer does
with it, he can modulate the transmitter with a digital signal and look
for the same signal being returned or he can send a pure sine wave
and look for a modulated return. Different manufacturers will have
different protocols.
Such tags are usually blasted with enough energy to destroy them
at the checkout so if you are determined to steal an item from the
electronics section you need your own blaster which you can build
out of a microwave oven.
http://margo.student.utwente.nl/el/microwave/
I once bought a pair of shoes with a tag in the heel that was not
destroyed... It got old setting off the alarms every time I went shopping.

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Natural Science Forum / Physics / General Physics / July 2008

Inductively Coupled RFID - Analogy to a transformer?