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[on:]

Hi!

Well, I've been wondenring how do Passive RFIDs work...

"Well, it's easy..." you can say.

I know the basics: the tag uses the magnetic field from the reader to power up the LC Circuit. The capacitor is then charged and then discharged and charged and then discharged (with opposite flow direction) ad infinitum (or until there's no energy)... and this is the phenomena that generates the Radio Wave.

Now, my question is, how does the chip encode the data in the wave?

I thought of PWM, but that would imply the circuit to be turned-on and off and I can't think on that...

So how is the modulation of the wave done by the chip?

To modulate the wave in amplitude, the chip could have a variable resistor that would be very high on a 0 and low on a 1, right? Still, I guess RFID do not use AM. :-?

Regards
Krappy

P.S.: This is not related directly to Arduino, but there are people here who have knowledge on electronics, so

[Reply #1 on:]

Not sure if this link will work but try anyway.
www.ieice.org/proceedings/ITC-CSCC2008/pdf/p1073_P1-17.pdf

I've just used an Arduino to decode rfid tag data. I had never done it before & it was interesting reading and learning about them.

[Reply #2 on:]

how does the chip encode the data in the wave?

The chip switches a resistor across the coil and loads it, thus taking more energy out of the field. Then it switches the resistor out so taking less energy out of the field.

The energising coil "feels" this changing load. It's a transformer action where the changing  load in the secondary is reflected by a changing load in the primary.

The reader then senses this changing load an turns it into zeros and ones.

This changing load is interpreted as a modulated signal in a number of different ways depending on the card type. The most popular type uses Manchester encoding to convey zeros and ones.