How to Modulate on UHF-RFID Tag ?

The RFID tag does not have a local oscillator of the carrier, and reflects the signal sent out of the RFID reader with the information written to the memory in the RFID tag to the RFID reader. The principle of modulation is shown in the figure below.

The carrier (radio wave) input from the antenna does not cause reflection, if the antenna, the transmission line and the load attached to the termination are equal impedance.

* However, when the end of the transmission line is open, the carrier reflects totally (100%) at the end of the transmission line in-phase.

* When the end of the transmission line is terminated at the same impedance of the transmission line, the carrier does not reflect at the end of the transmission line.

* When the end of the transmission line is short, the carrier reflects totally (100%) at the end of the transmission line in the anti phase.

Using this impedance mismatch, the modulated carrier of the information from the RFID tag in the different reflection characteristics. We can receive the information from the RFID tag.

An RF switch in parallel with the terminated resistor at the antenna feed-point can be configured by providing "a short circuit" or "the terminated resistor circuit" switch depending on the information stored in the memory in the RFID tag. This circuit is modulating the carrier "Amplitude Shift Keying (ASK)" by the information.

An RF switch at the feed point of the antenna can be configured by providing "a short circuit" or "an open circuit" switch depending on the information stored in the memory in the RFID tag. This circuit is modulating the carrier "Phase Shift Keying (PSK)" by the information.

In the actual RFID tag, as the length of the transmission line on the circuit is very short, rather than the wavelength of the carrier, the modulation circuit is a very simple FET "RF switch" at the feed point of the antenna.