Electrical Coupling
Power Supply of Passive Transponders
In electrically (i.e. capacitively) coupled systems the reader generates a strong, high-frequency electrical field. The reader’s antenna consists of a large, electrically conductive area (electrode), generally a metal foil or a metal plate. If a high-frequency voltage is applied to the electrode a high-frequency electric field forms between the electrode and the earth potential (ground). The voltages required for this, ranging between a few hundred volts and a few thousand volts, are generated in the reader by voltage rise in a resonant circuit made up of a coil L1 in the reader, plus the parallel connection of an internal capacitor C1 and the capacitance active between the electrode and the earth potential CR-GND. The resonant frequency of the resonant circuit corresponds with the transmission frequency of the reader.

The antenna of the transponder is made up of two conductive surfaces lying in a plane (electrodes). If the transponder is placed within the electrical field of the reader, then an electric voltage arises between the two transponder electrodes, which is used to supply power to the transponder chips.

Since a capacitor is active both between the transponder and the transmission antenna (CR-T)and between the transponder antenna and the earth potential (CT-GND)the equivalent circuit diagram for an electrical coupling can be considered in a simplified form as a voltage divider with the elements CR-T,RL (input resistance of the transponder) and CT-GND (see Figure 3.26). Touching one of the transponder’s electrodes results in the capacitance CT-GND, and thus also the read range, becoming significantly greater.

The currents that flow in the electrode surfaces of the transponder are very small. Therefore, no particular requirements are imposed upon the conductivity of the electrode material. In addition to the normal metal surfaces (metal foil) the electrodes can thus also be made of conductive colours (e.g. a silver conductive paste)or a graphite coating (Baddeley and Ruiz, 1998).

Data Transfer Transponder → Reader
If an electrically coupled transponder is placed within the interrogation zone of a reader, the input resistance RL of the transponder acts upon the resonant circuit of the reader via the coupling capacitance CR-T active between the reader and transponder electrodes, damping the resonant circuit slightly. This damping can be switched between two values by switching a modulation resistor Rmod in the transponder on and off. Switching the modulation resistor Rmod on and off thereby generates an amplitude modulation of the voltage present at L1 and C1 by the remote transponder. By switching the modulation resistor Rmod on and off in time with data, this data can be transmitted to the reader. This procedure is called load modulation.

Sequential Procedures
If the transmission of data and power from the reader to the data carrier alternates with data transfer from the transponder to the reader, then we speak of a sequential procedure (SEQ). The characteristics used to differentiate between SEQ and other systems have already been described in Section 3.2.

Inductive Coupling
Power Supply to the Transponder
Sequential systems using inductive coupling are operated exclusively at frequencies below 135 kHz. A transformer-type coupling is created between the reader’s coil and the transponder’s coil. The induced voltage generated in the transponder coil by the effect of an alternating field from the reader is rectified and can be used as a power supply.

In order to achieve higher efficiency of data transfer, the transponder frequency must be precisely matched to that of the reader, and the quality of the transponder coil must be carefully specified. For this reason the transponder contains an on-chip trimming capacitor to compensate for resonant frequency manufacturing tolerances.

However, unlike full-and half-duplex systems, in sequential systems the reader’s transmitter does not operate on a continuous basis. The energy transferred to the transmitter during the transmission operation charges up a charging capacitor to provide an energy store. The transponder chip is switched over to standby or power-saving mode during the charging operation, so that almost all of the energy received is used to charge up the charging capacitor. After a fixed charging period the reader’s transmitter is switched off again.

The energy stored in the transponder is used to send a reply to the reader. The minimum capacitance of the charging capacitor can be calculated from the necessary operating voltage and the chip’s power consumption.

A Comparison between FDX/HDX and SEQ Systems
Figure 3.27 illustrates the different conditions arising from full/half-duplex (FDX/HDX) and sequential (SEQ) systems.

Because the power supply from the reader to the transponder in full-duplex systems occurs at the same time as data transfer in both directions, the chip is permanently in operating mode. Power matching between the transponder antenna (current source) and the chip (current consumer) is desirable to utilise the transmitted energy optimally. However, if precise power matching is used only half of the source voltage (= open-circuit voltage of the coil) is available. The only option for increasing the available operating voltage is to increase the impedance (= load resistance) of the chip. However, this is the same as decreasing the power consumption.

Therefore the design of full-duplex systems is always a compromise between power matching (maximum power consumption Pchip at Uchip = 1/2U0) and voltage matching (minimum power consumption Pchip at maximum voltage Uchip = U0).

The situation is completely different in sequential systems: during the charging process the chip is in standby or power-saving mode, which means that almost no power is drawn through the chip. The charging capacitor is fully discharged at the beginning of the charging process and therefore represents a very low ohmic load for the voltage source. In this state, the maximum amount of current flows into the charging capacitor, whereas the voltage approaches zero (= current matching). As the charging capacitor is charged, the charging current starts to decrease according to an exponential function, and reaches zero when the capacitor is fully charged. The state of the charged capacitor corresponds with voltage matching at the transponder coil.

This achieves the following advantages for the chip power supply compared to a full/half-duplex system:

The full source voltage of the transponder coil is available for the operation of the chip. Thus the available operating voltage is up to twice that of a comparable full/half-duplex system.
The energy available to the chip is determined only by the capacitance of the charging capacitor and the charging period. Both values can (in theory!) be given any required magnitude. In full/half-duplex systems the maximum power consumption of the chip is fixed by the power matching point (i.e. by the coil geometry and field strength H ).

Data Transmission Transponder → Reader
In sequential systems a full read cycle consists of two phases, the charging phase and the reading phase.

The end of the charging phase is detected by an end of burst detector, which monitors the path of voltage at the transponder coil and thus recognises the moment when the reader field is switched off. At the end of the charging phase an on-chip oscillator, which uses the resonant circuit formed by the transponder coil as a frequency determining component, is activated. A weak magnetic alternating field is generated by the transponder coil, and this can be received by the reader. This gives an improved signal-interference distance of typically 20 dB compared with full/half-duplex systems, which has a positive effect upon the ranges that can be achieved using sequential systems.

The transmission frequency of the transponder corresponds with the resonant frequency of the transponder coil, which was adjusted to the transmission frequency of the reader when it was generated.

In order to be able to modulate the RF signal generated in the absence of a power supply, an additional modulation capacitor is connected in parallel with the resonant circuit in time with the data flow. The resulting frequency shift keying provides a 2 FSK modulation.

After all the data has been transmitted, the discharge mode is activated to fully discharge the charging capacitor. This guarantees a safe Power-On-Reset at the start of the next charging cycle.