Enjoyrfid Technical

RFID backscatter coupling
RFID backscatter coupling or RFID backscattering uses the RF power transmitter by the tag reader to energise the tag. Essentially they “reflect” back some of the power transmitted by the reader, but change some of the properties, and in this way send back information to the reader.

Using RFID backscatter or RFID backscattering, some tags achieve their data transmission by changing the properties of the tags themselves, while others switch a load resistor in and out of the antenna circuit.

RFID backscatter coupling operates outside the near field region, and the radio signal propagates away from the RFID readers. When the signal reaches the RFID tag, this interacts with the ingoing signal and some energy is reflected back towards the RFID reader. The way in which the signal is reflected back depends upon the properties of the tag (or any other object for that matter). Factors such as the cross sectional area, and the antenna properties etc within the tag all have an effect. In particular the antenna will pick-up and re-radiate energy, and the way this energy is re-radiated is dependent upon the antenna properties – by changing factors such as adding or subtracting a load resistor across the antenna, the re-radiated signal properties can be changed.

Over short ranges, the amount of power reaching the tag from the reader is sufficient to allow operation of small low current circuits within the tag. This can be used to drive an electronic switch, e.g. a FET that can switch an antenna load resistor in and out of circuit. This will effectively modulate the returned signal and allow data to be passed back to the reader.

In order to allow transmission and reception of a signal at the same time, a directional coupler is often used to allow the received signal to be separated from the transmitted on. Additionally the reader must be able to detect the modulation in the presence of a host of other reflections, although these will normally be stable and not modulated in any way.