RFID transmission
Radio Frequency Identification(RFID) is a radio technology for identifying goods and people. The RFID system consists of an RFID transponder with active or passive tags and the RFID reader. There are several different methods for communication between the RFID transponder and reader, all of which are based on the resonance principle with inductive coupling and the resulting behavior of the resonant circuit of the reader.
One method is load modulation. In this method, the resonant frequency of the RFID transponder is tuned to the transmission frequency of the reader. When a transponder comes into the electromagnetic resonance field of the RFID reader, the transponder extracts energy from the electromagnetic field.
This energy extraction corresponds to digitalinformation. The energy extraction is modulated by switching on a resistor that is in parallel with the oscillating circuit of the transponder and damps it. The switching on and off of the parallel resistor corresponds to an amplitude shift keying( ASK).
Another modulation method, which corresponds to frequency shift ke ying( FSK), works by switching a parallel capacitor on and off, which changes the resonant frequency of the transponder. By switching the capacitance on and off, the resonant circuit has two different resonant frequencies that can be evaluated as digital information.
In addition to the duplex methods mentioned, there is also half-duplex operation, in which wireless energy transmission from the reader to the passive transponder and data transmission from the transponder to the reader take place alternately. As soon as a transponder enters the electromagnetic field of the reader, the oscillating circuit of the transponder resonates. The induction voltage is rectified and this charges a capacitor that supplies the transponder. The charging time for the capacitor takes about 60 ms and is thus considerably longer than the data transmission time.