Data Transfer Transponder → Reader
Load Modulation
As described above, inductively coupled systems are based upon a transformer-type coupling between the primary coil in the reader and the secondary coil in the transponder. This is true when the distance between the coils does not exceed (λ/2π) 0.16λ, so that the transponder is located in the near field of the transmitter antenna (for a more detailed definition of the near and far fields).

If a resonant transponder (i.e. a transponder with a self-resonant frequency corresponding with the transmission frequency of the reader) is placed within the magnetic alternating field of the reader’s antenna, the transponder draws energy from the magnetic field. The resulting feedback of the transponder on the reader’s antenna can be represented as transformed impedance ZT in the antenna coil of the reader. Switching a load resistor on and off at the transponder’s antenna therefore brings about a change in the impedance ZT, and thus voltage changes at the reader’s antenna. This has the effect of an amplitude modulation of the voltage UL at the reader’s antenna coil by the remote transponder. If the timing with which the load resistor is switched on and off is controlled by data, this data can be transferred from the transponder to the reader. This type of data transfer is called load modulation.

To reclaim the data at the reader, the voltage tapped at the reader’s antenna is rectified. This represents the demodulation of an amplitude modulated signal.

If the transponder leaves the near-field, i.e. the range <λ/2π(0.16 λ), the transformer coupling between reader antenna and the transponder antenna will be lost with the transition into the far-field. Therefore, load modulation is not possible any longer in the far-field. This does not mean, though, that data transmission from the transponder to the reader is, in principle, not possible. With the transition into the far-field, the mechanism of the backscatter coupling becomes effective. In practice, data transmission to the reader usually fails because of the low efficiency of the transponder antennas (i.e. the low antenna gain) in the far-field.

Load Modulation with Subcarrier
Due to the weak coupling between the reader antenna and the transponder antenna, the voltage fluctuations at the antenna of the reader that represent the useful signal are smaller by orders of magnitude than the output voltage of the reader. In practice, for a 13.56 MHz system, given an antenna voltage of approximately 100 V (voltage step-up by resonance) a useful signal of around 10 mV can be expected (= 80 dB signal/noise ratio). Because detecting this slight voltage change requires highly complicated circuitry, the modulation sidebands created by the amplitude modulation of the antenna voltage are utilised.

If the additional load resistor in the transponder is switched on and off at a very high elementary frequency fS, then two spectral lines are created at a distance of ±fS around the transmission frequency of the reader fREADER, and these can be easily detected (however fS must be less than fREADER). In the terminology of radio technology the new elementary frequency is called a subcarrier). Data transfer is by ASK, FSK or PSK modulation of the subcarrier in time with the data flow. This represents an amplitude modulation of the subcarrier.

Load modulation with a subcarrier creates two modulation sidebands at the reader’s antenna at the distance of the subcarrier frequency around the operating frequency fREADER. These modulation sidebands can be separated from the significantly stronger signal of the reader by bandpass (BP) filtering on one of the two frequencies fREADER ± fS. Once it has been amplified, the subcarrier signal is now very simple to demodulate.

Load modulation with subcarriers is mainly limited to the frequency range 13.56 MHz. Typical subcarrier frequencies are 212 kHz, 424 kHz (e.g. ISO/IEC 15 693) and 848 kHz (e.g. ISO/IEC 14443).

Example Circuit–Load Modulation with Subcarrier
Figure 3.18 shows an example circuit for a transponder using load modulation with a subcarrier. The circuit is designed for an operating frequency of 13.56 MHz and generates a subcarrier of 212 kHz.

The voltage induced at the antenna coil L1 by the magnetic alternating field of the reader is rectified using the bridge rectifier (D1–D4) and after additional smoothing (C1) is available to the circuit as supply voltage. The parallel regulator (ZD 5V6) prevents the supply voltage from being subject to an uncontrolled increase when the transponder approaches the reader antenna.

Part of the high frequency antenna voltage (13.56 MHz) travels to the frequency divider’s timing input (CLK) via the protective resistor (R1) and provides the transponder with the basis for the generation of an internal clocking signal. After division by 26(= 64) a subcarrier clocking signal of 212 kHz is available at output Q7. The subcarrier clocking signal, controlled by a serial data flow at the data input (DATA), is passed to the switch (T1). If there is a logical HIGH signal at the data input (DATA), then the subcarrier clocking signal is passed to the switch (T1). The load resistor (R2) is then switched on and off in time with the subcarrier frequency.

Optionally in the circuit depicted, the transponder resonant circuit can be brought into resonance with the capacitor C1 at 13.56 MHz. The range of this ‘minimal transponder’ can be significantly increased in this manner.

Subharmonic Procedure
The subharmonic of a sinusoidal voltage A with a defined frequency fA is a sinusoidal voltage B, whose frequency fB is derived from an integer division of the frequency fA. The subharmonics of the frequency fA are therefore the frequencies fA/2,fA/3,fA/4 ….

In the subharmonic transfer procedure, a second frequency fB, which is usually lower by a factor of two, is derived by digital division by two of the reader’s transmission frequency fA. The output signal fB of a binary divider can now be modulated with the data stream from the transponder. The modulated signal is then fed back into the transponder’s antenna via an output driver.

One popular operating frequency for subharmonic systems is 128 kHz. This gives rise to a transponder response frequency of 64 kHz.

The transponder’s antenna consists of a coil with a central tap, whereby the power supply is taken from one end. The transponder’s return signal is fed into the coil’s second connection (Figure 3.19).

Electromagnetic Backscatter Coupling
Power Supply to the Transponder
RFID systems in which the gap between reader and transponder is greater than 1 m are called long-range systems. These systems are operated at the UHF frequencies of 868 MHz (Europe) and 915 MHz (USA), and at the microwave frequencies 2.5 and 5.8 GHz. The short wavelengths of these frequency ranges facilitate the construction of antennas with far smaller dimensions and greater efficiency than would be possible using frequency ranges below 30 MHz.

In order to be able to assess the energy available for the operation of a transponder we first calculate the free space path loss aF in relation to the distance r between the transponder and the reader’s antenna, the gain GT and GR of the transponder’s and reader’s antenna, plus the transmission frequency f of the reader:

aF =−147.6 +20 log(r) +20 log(f ) −10 log(GT) −10 log(GR) (3.1)

The free space path loss is a measure of the relationship between the RF power emitted by a reader into ‘free space’ and the RF power received by the transponder.

Using current low-power semiconductor technology, transponder chips can be produced with a power consumption of no more than 5 µW (Friedrich and Annala, 2001). The efficiency of an integrated rectifier can be assumed to be 5–25% in the UHF and microwave range (Tanneberger, 1995). Given an efficiency of 10%, we thus require received power of Pe =50 µW at the terminal of the transponder antenna for the operation of the transponder chip. This means that where the reader’s transmission power is Ps =0.5 W EIRP (effective isotropic radiated power) the free space path loss may not exceed 40 dB (Ps/Pe =10 000/1) if sufficiently high power is to be obtained at the transponder antenna for the operation of the transponder. A glance at Table 3.7 shows that at a transmission frequency of 868 MHz a range of a little over 3 m would be realisable; at 2.45 GHz a little over 1 m could be achieved. If the transponder’s chip had a greater power consumption the achievable range would fall accordingly.

In order to achieve long ranges of up to 15 m or to be able to operate transponder chips with a greater power consumption at an acceptable range, backscatter transponders often have a backup battery to supply power to the transponder chip (Figure 3.20). To prevent this battery from being loaded unnecessarily, the microchips generally have a power saving ‘power down’ or ‘standby’ mode.

If the transponder moves out of range of a reader, then the chip automatically switches over to the power-saving ‘power down’ mode. In this state the power consumption is a few µAat most. The chip is not reactivated until a sufficiently strong signal is received in the read range of a reader, whereupon it switches back to normal operation. However, the battery of an active transponder never provides power for the transmission of data between transponder and reader, but serves exclusively for the supply of the microchip. Data transmission between transponder and reader relies exclusively upon the power of the electromagnetic field emitted by the reader.

Modulated Reflection Cross-Section
We know from the field of radar technology that electromagnetic waves are reflected by objects with dimensions greater than around half the wavelength of the wave. The efficiency with which an object reflects electromagnetic waves is described by its reflection cross-section. Objects that are in resonance with the wavefront that hits them, as is the case for antennas at the appropriate frequency, for example, have a particularly large reflection cross-section.

Power P1 is emitted from the reader’s antenna, a small proportion of which (free space attenuation) reaches the transponder’s antenna (Figure 3.21). The power P1   is supplied to the antenna connections as RF voltage and after rectification by the diodes D1 and D2 this can be used as turn-on voltage for the deactivation or activation of the power saving ‘power down’ mode. The diodes used here are low-barrier Schottky diodes, which have a particularly low threshold voltage. The voltage obtained may also be sufficient to serve as a power supply for short ranges.

A proportion of the incoming power P1   is reflected by the antenna and returned as power P2.The reflection characteristics (= reflection cross-section) of the antenna can be influenced by altering the load connected to the antenna. In order to transmit data from the transponder to the reader, a load resistor RL connected in parallel with the antenna is switched on and off in time with the data stream to be transmitted. The amplitude of the power P2 reflected from the transponder can thus be modulated (modulated backscatter).

The power P2 reflected from the transponder is radiated into free space. A small proportion of this (free space attenuation) is picked up by the reader’s antenna. The reflected signal therefore travels into the antenna connection of the reader in the backwards direction and can be decoupled using a directional coupler and transferred to the receiver input of a reader. The forward signal of the transmitter, which is stronger by powers of ten, is to a large degree suppressed by the directional coupler.

The ratio of power transmitted by the reader and power returning from the transponder (P1/P2) can be estimated using the radar equation (for an explanation, refer to Chapter 4).

Close-Coupling
Power Supply to the Transponder
Close coupling systems are designed for ranges between 0.1 cm and a maximum of 1 cm. The transponder is therefore inserted into the reader or placed onto a marked surface (‘touch and go’) for operation.

Inserting the transponder into the reader, or placing it on the reader, allows the transponder coil to be precisely positioned in the air gap of a ring-shaped or U-shaped core. The functional layout of the transponder coil and reader coil corresponds with that of a transformer (Figure 3.22). The reader represents the primary winding and the transponder coil represents the secondary winding of a transformer. A high-frequency alternating current in the primary winding generates a highfrequency magnetic field in the core and air gap of the arrangement, which also flows through the transponder coil. This power is rectified to provide a power supply to the chip.

Because the voltage U induced in the transponder coil is proportional to the frequency f of the exciting current, the frequency selected for power transfer should be as high as possible. In practice, frequencies in the range 1–10MHz are used. In order to keep the losses in the transformer core low, a ferrite material that is suitable for this frequency must be selected as the core material.

Because, in contrast to inductively coupled or microwave systems, the efficiency of power transfer from reader to transponder is very good, close-coupling systems are excellently suited for the operation of chips with a high power consumption. This includes microprocessors, which still require some 10 mW power for operation (Sickert, 1994). For this reason, the close-coupling chip card systems on the market all contain microprocessors.

The mechanical and electrical parameters of contactless close-coupling chip cards are defined in their own standard, ISO 10536. For other designs the operating parameters can be freely defined.

Data Transfer Transponder → Reader
Magnetic Coupling
Load modulation with subcarrier is also used for magnetically coupled data transfer from the transponder to the reader in close-coupling systems. Subcarrier frequency and modulation is specified in ISO 10536 for close-coupling chip cards.

Capacitive Coupling
Due to the short distance between the reader and transponder, close-coupling systems may also employ capacitive coupling for data transmission. Plate capacitors are constructed from coupling surfaces isolated from one another, and these are arranged in the transponder and reader such that when a transponder is inserted they are exactly parallel to one another.

This procedure is also used in close-coupling smart cards. The mechanical and electrical characteristics of these cards are defined in ISO/IEC 10536.

Data Transfer Reader → Transponder
All known digital modulation procedures are used in data transfer from the reader to the transponder in full-and half-duplex systems, irrespective of the operating frequency or the coupling procedure. There are three basic procedures:

ASK: amplitude shift keying
FSK: frequency shift keying
PSK: phase shift keying
Because of the simplicity of demodulation, the majority of systems use ASK modulation.