Design of the RFID transponder
The heart of an RFID system is a data carrier, referred to as the transponder, or simply the Tag. The designs and modes of function of the transponders also differ depending on the frequency range, just as with the antennas.
In the LF and HF range, a unique, worldwide ID number is stored on the chip. This can be connected to information in a database. There are coil designs for these two frequencies in the transponders that are used in the magnetic near field of the antennas (inductive coupling).
In the UHF range, the transponder has an EPC (Electronic Product Code) storage area that can be programmed by the customer. Dipoles are used here within the antenna design.
Communication is by means of a backscatter method. Here, data transfer is not by means of inductance, but by changes to the impedance at the transponder antenna, resulting in backscatter. If you now switch this on and off in time with the data flow to be transferred then this results in an amplitude-modulated signal which the scanner or antenna can then receive and process.
Passive transponders are now available with a storage capacity of up to 10kbit, thereby allowing additional information to be stored.
The most common distinguishing feature among transponders is their power supply. Both active and passive models are available.
Passive transponders do not have their own power supply and gain their energy from establishing an inductive field from the radio signals of the scanner. The missing source of energy does result in lower ranges, but allows smaller and lighter designs. Additionally, passive transponders are maintenance-free and can be obtained much more cheaply. They are mainly used for product authentication and tracking, but also as data storage media for access control systems.
Active transponders draw their energy from a built-in battery and can therefore transmit signals themselves for data transfer. Due to their integrated power supply, they are more expensive than their passive counterparts but have a wider scanning range of up to 100 meters. They are mainly used to identify objects with a long lifetime and that can be used repeatedly.
Depending on their purpose, transponders are available in various sizes, designs and protection classes. The most common types are self-adhesive labels or chip cards. They are available as "read only" versions, which can only be read-out, and "read/write" versions that can both read and write the transponder.