Rfid tag - load modulation

HI,

I am not fully aware of load modulation.

The concept I know about load modulation is : Passive RFID tags communicates and transfers data to the reader by means of load modulation in the LF and HF range. Because of inductive coupling, magnetic field induces a small current in the tag. Since, the reader and tag are inductively coupled, change in the load at the tag results in amplitude variations at the reader (due to voltage drop at the reader). This sounds OK. But what I am unable to understand is how to change the load ? The same applies to backscatter modulation too. In back scatter modulation which is used at UHF and microwave range, the incident energy at the tag is reflected back to the reader. Here, when you switch the transistor (load) connected to the tag antenna, data is transferred from tag to reader.

I don't understand the concept of how to change/switch the load or detune the resonant circuit. Please help me to understand the concept.


Thanks,
shaikss.

Consider a resistor as a load of the tag resonant circuit. Changing the resistance, e.g. by switching another resistor on and off results in load modulation.

My question is how to change the resistance. I am unable to realize it practically. If I make a tag as a wrist band, then how do I switch the resistor ON and OFF? I need the mechanism. Theoritically speaking, it sounds good. But how to do it practically?

Did you consider a transistor switch?

I need to switch the resistor connected to the antenna coil.
One option is to use a FET. BUt at what scenarios do the FET has to switch?
At what scenarios do the FET switch?

I don't see a complicated problem. To give an idea, see the Reference PICC circuit from ISO-IEC 10373-6, the 13.56 MHz RFID test specification. There are many more options, of course.

Whenever there is a impedence change, tag transfers data to the reader. In order to see the impedence change, switching has to happen. But how to control the switching as the reader is tuned to 13.5MHz. Can you clarify this?

I'm not sure, if you're familiar with the concept of PICC to PCD subcarrier modulation. Generally, the tag logic will be clocked by the 13.56 MHz carrier, so all switching operations are synchronous to the carrier anyway. The 847 kHz subcarrier is generated by switching the load on and off for 8 carrier periods each.

After you suggested, I have read PICC to PCD subcarrier modulation. All is well except the data rate of manchester coding. How to select the data rate? Is it in accordance to any standard? why to divide-by-16 the resonant frequency to derive subcarrier of 847KHz? I meant to ask why the division factor 16 and why not 32 or 8 or some other pow of 2???

The test specification is based on ISO_IEC 14443, which is most popular for proximity cards. It specifies the said sub carrier and data rates of 106 kbit/s to 847 kbit/s. There are also other RFID standards, e.g. ISO_IEC 15693 for vicinity (larger distance) RFID.