Inductive Coupling
Power Supply to Passive Transponders
An inductively coupled transponder comprises an electronic data-carrying device, usually a single microchip, and a large-area coil or conductor loop that functions as an antenna.

Inductively coupled transponders are almost always operated passively. This means that all the energy needed for the operation of the microchip has to be provided by the reader (Figure 3.13). For this purpose, the reader’s antenna coil generates a strong, high-frequency electromagnetic field, which penetrates the cross-section of the coil area and the area around the coil. Because the wavelength of the frequency range used (<135 kHz: 2400 m, 13.56 MHz: 22.1 m) is several times greater than the distance between the reader’s antenna and the transponder, the electromagnetic field may be treated as a simple magnetic alternating field with regard to the distance between transponder and antenna (see Section 4.2.1.1 for further details).

A small part of the emitted field penetrates the antenna coil of the transponder, which is some distance away from the coil of the reader. A voltage Ui is generated in the transponder’s antenna coil by inductance. This voltage is rectified and serves as the power supply for the data-carrying device (microchip).

A capacitor Cr is connected in parallel with the reader’s antenna coil, the capacitance of this capacitor being selected such that it works with the coil inductance of the antenna coil to form a parallel resonant circuit with a resonant frequency that corresponds with the transmission frequency of the reader. Very high currents can be generated in the antenna coil of the reader by resonance step-up in the parallel resonant circuit, which can be used to generate the required field strengths for the operation of the remote transponder.

The antenna coil of the transponder and the capacitor C1 form a resonant circuit tuned to the transmission frequency of the reader. The voltage U at the transponder coil reaches a maximum due to resonance step-up in the parallel resonant circuit.

The layout of the two coils can also be interpreted as a transformer (transformer coupling), in which case there is only a very weak coupling between the two windings. The efficiency of power transfer between the antenna coil of the reader and the transponder is proportional to the operating frequency f , the number of windings n, the area A enclosed by the transponder coil, the angle of the two coils relative to each other and the distance between the two coils.

As frequency f increases, the required coil inductance of the transponder coil, and thus the number of windings n decreases (135kHz: typical 100–1000 windings, 13.56MHz: typical 3–10 windings). Because the voltage induced in the transponder is still proportional to frequency f (see Chapter 4), the reduced number of windings barely affects the efficiency of power transfer at higher frequencies.

Table 3.6 Overview of the power consumption of various RFID-ASIC building blocks (Atmel, 1994). The minimum supply voltage required for the operation of the microchip is 1.8 V, the maximum permissible voltage is 10 V (Atmel, 1994)

Memory Write/read Power Frequency Application (bytes) distance(cm) consumption

ASIC#1 6 15 10 µA 120 kHz Animal ID
ASIC#2 32 13 600 µA 120 kHz Goods flow, access check
ASIC#3 256 2 6 µA 128 kHz Public transport
ASIC#4 256 0.5 <1mA 4MHz∗ Goods flow, public transport
ASIC#5 256 <2 ∼1 mA 4/13.56 MHz Goods flow
ASIC#6 256 100 500 µA 125 kHz Access check
ASIC#7 2048 0.3 <10 mA 4.91 MHz∗ Contactless chip cards
ASIC#8 1024 10 ∼1 mA 13.56 MHz Public transport
ASIC#9 8 100 <1 mA 125 kHz Goods flow
ASIC#10 128 100 <1 mA 125 kHz Access check

Close-coupling system.