AVR Freaks

As I understand it... the card or fob is energised by the field of the reader when in range. In turn the card/fob switches its own coil in and out of a tuned circuit which loads the reader's own coil thereby modulating the reader's circuitry. A demodulator extracts the data.

What is discussed above is actually THREE separate things.

RFID tags, including the implantable ones used for pet id, are passive. These are rarely "fobs" but are attached  to things. They get their power from the interrogating signal. For the pet trackers, the interrogating signal is low frequency (some hundreds of KHz, max) and requires VERY close proximity (typically under 10cm) and coupling is magnetic (eg, the interrogator has one half of a transformer and the device forms the other half).  The tag returns a fairly long number. This is the first type and it is not called a "fob" (except when they are in the form that jgm shows). 

The second is also passive, but is used in warehouses, retail establishments, and such. The interrogator works on UHF (typically  860MHz or 915MHz). The tag still gets it power from the interrogator and it, similarly, returns a fairly long ID number. This, likewise, is NOT called a "fob".

The third is what we normally call a "fob". It is used to unlock or open car doors, garage doors, and such. The fob has an actual transmitter, typically in a special band around 300MHz (at least in the U.S.). A long pass-code is transmitted and the receiving device responds if the received code matches what was previously programmed into it.

There is a fourth device, called "near-field communicator". It is commonly used between equipped smart phones and vending machines, mostly in Europe. Range is typically a meter or so, and, as far as I understand, both partners in such transactions are active. The name "fob" is not used with this system.

Jim who worked in the UHF RFID industry.

My understanding on the swipe-less cards is the same. They appear to get their power from the interrogator, inductively, like the fobs shown above. Had never seen that type until this thread, the only ones I had previously seen were the pet-track ID capsules.

Jim

NFC is ISO18000, which is fundamentally compatible with ISO14443 (e.g. Mifare, Desfire, London Oyster card, etc). These have encryption and storage. As does the other 13.56MHz standard called ISO15693, which is used by iClass and others. The 125KHz systems are non secured and transmit in the raw (ask, fsk, or psk are all used in varying bit lengths). I think the livestock tags might be 134kHz - similar to the 125KHz stuff. If anyone wants to know more I design RFID systems but I only know the hardware/RF side of things - not strong on the software side at all. Just ask...
Murdo.

awneil,
Technically they do emit a signal - they modulate the carrier, with a back scatter, which leads to side bands at the relevant frequency. Its just that they (ignoring the uhf tags here!) are powered by the reader device.
Murdo.

Hmm, there are TWO kinds of wireless cards that I know of.

Some access-control systems (eg, for buildings) have an interrogator that you hold the card up to, usually quite close. I suspect that these are LOW frequency (125KHz, more or less).

The card like Cliff posted uses a 13-odd MHz system.

Both are near field (NFC). These basically work as the secondary of a transformer. The transformer primary is in the interrogator. The secondary is in the card. By modulating the impedance across the secondary (often a FET switch driven from a shift-register), there is a resulting change in the primary that can be detected. While there is a bit of RF radiation involved (especially the higher frequency system), the primary coupling mechanism is that of a VERY loosely coupled transformer.

The UHF "fobs" used for car doors, garage doors, and such, are straight out transmitters that run near 310MHz in the US. They are battery powered and you DO have to change the batteries periodically. I have a couple, right now, that are waiting for new batteries.

There is a wireless anti-theft system that is used for retail goods and often in libraries. The really cheap one (retail) is built into the packaging and can be deactivated with a magnetic field. The more expensive one (libraries) is, I have been told, more like the pet tags. Both of these appear to be passive low frequency and require walking through something equivalent to a big transformer primary. There are also some, which I do not know how they work, that are fairly large and used on apparel; the clerk has to put it into a machine that detaches it from the item. These also appear to be passive low-frequency devices.

The kind I worked with are the UHF passive tags. These work on the fact that an antenna will re-radiate a signal that impinges on it. The incoming field generates a current in the antenna and the current causes a new signal to be radiated. By modulating the feed-point impedance of the antenna, you can control the strength of the re-radiated signal (which, of course adds sidebands). The interrogator looks for the sidebands because all this has to happen precisely WHILE the transmitter is transmitting and the sidebands are the only part that is at a different frequency; the transmitter in the interrogator swamps anything else (eg, the tag) on the same frequency. 

Jim