RFID coupling techniques - backscatter, capacitive, inductive

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RFID readers and writes communicate or couple to the RFID tags using a number of different techniques.

The main RFID coupling techniques include backscatter, inductive and capacitive coupling.

The type of coupling technique that is used will depend upon the particular application, and in turn this affect the choice of frequency band.

RFID coupling basics

There are several ways in which the RFID reader-writer can communicate with the RFID tag. The main RFID coupling techniques that are involved are:

• RFID backscatter coupling
• RFID inductive coupling
• RFID capacitive coupling

The type of coupling used affects several aspects of the RFID system including the range, frequencies needed and other elements of the RFID hardware.

The range of the RFID system can broadly be categorised into three:

• Close range - within 1 centimetre
• Remote - between 1 cm and 1 metre
• Long range - more than 1 metre

Of these types of RFID coupling, magnetic and capacitive types are normally used for close range links, inductive coupling for remote links and RFID backscatter coupling for long range links.

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 reader. 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.

RFID inductive coupling

RFID inductive coupling is used for what are termed "vicinity coupled" cards. RFID inductive coupling is defined in ISO 15693 standard, although not all RFID inductively coupled tags need conform to this standard.

In terms of operation, inductive coupling is the transfer of energy from one circuit to another via the mutual inductance between the two circuits. For RFID inductive coupling to be used, both the tag and the reader will have induction or "antenna" coils. When the tag is placed close enough to the reader the field from the reader coil will couple to the coil from the tag. A voltage will be induced in the tag that will be rectified and used to power the tag circuitry.

To enable data to be passed from the tag to the reader, the tag circuitry changes the load on its coil and this can be detected by the reader as a result of the mutual coupling.

RFID inductive coupling is a near field effect. Accordingly the distance between the coils must be kept within the range of the effect - normally this is taken to be about 0.15 wavelength of the frequency in use.

RFID inductive coupling is normally used on the lower RFID frequencies - often LF, i.e. below 135 kHz or at 13.56 MHz.

RFID capacitive coupling

RFID capacitive coupling is used for short ranges where a form of RFID close coupling is needed. As the name implies the system uses a capacitive effects to provide the coupling between the tag and the reader.

The system is often used for smart cards for which the standard ISO 10536 may be applied.

RFID capacitive coupling operates best when items like smart cards are inserted into a reader - in this way the card is in very close proximity to the reader. Rather than having coils or antennas, capacitive coupling uses electrodes - the plates of the capacitor to provide the required coupling.

The capacitance between the reader and card tag provide a capacitor through which a signal can be transmitted, although an earth return is required. The AC signal generated by the reader is picked up and rectified within the RFID tag and used to power the devices within the tag. Again the data is retuned to the RFID reader by modulating the load.

The three main types of RFID coupling each have their own characteristics. As a result, capacitive RFID coupling is used for very short ranges, inductive RFID coupling for slightly longer ranges and RFID backscatter coupling or RFID backscattering is normally used where longer distances are needed. Using the best form of coupling will enable the RFID system to operate to its best in any given application.

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