Data Flow in an Application
A software application that is designed to read data from a contactless data carrier (transponder) or write data to a contactless data carrier, requires a contactless reader as an interface. From the point of view of the application software, access to the data carrier should be as transparent as possible. In other words, the read and write operations should differ as little as possible from the process of accessing comparable data carriers (smart card with contacts, serial EEPROM).

Write and read operations involving a contactless data carrier are performed on the basis of the master–slave principle (Figure 11.1). This means that all reader and transponder activities are initiated by the application software. In a hierarchical system structure the application software represents the master, while the reader, as the slave, is only activated when write/read commands are received from the application software.

To execute a command from the application software, the reader first enters into communication with a transponder. The reader now plays the role of the master in relation to the transponder. The transponder therefore only responds to commands from the reader and is never active independently.

A simple read command from the application software to the reader can initiate a series of communication steps between the reader and a transponder. In the example in Table 11.1, a read command first leads to the activation of a transponder, followed by the execution of the authentication sequence and finally the transmission of the requested data.

The reader’s main functions are therefore to activate the data carrier (transponder), structure the communication sequence with the data carrier, and transfer data between the application software and a contactless data carrier. All features of the contactless communication, i.e. making the connection, and performing anticollision and authentication procedures, are handled entirely by the reader.

Components of a Reader
A number of contactless transmission procedures have already been described in the preceding chapters. Despite the fundamental differences in the type of coupling (inductive – electromagnetic), the communication sequence (FDX, HDX, SEQ), the data transmission procedure from the transponder to the reader (load modulation, backscatter, subharmonic) and, last but not least, the frequency range, all readers are similar in their basic operating principle and thus in their design.

Readers in all systems can be reduced to two fundamental functional blocks: the control system and the RF interface, consisting of a transmitter and receiver (Figure 11.2). Figure 11.3 shows a reader for an inductively coupled RFID system. On the right-hand side we can see the RF interface, which is shielded against undesired spurious emissions by a tinplate housing. The control system is located on the left-hand side of the reader and, in this case, it comprises an ASIC module and microcontroller. In order that it can be integrated into a software application, this reader has an RS232 interface to perform the data exchange between the reader (slave) and the external application software (master).

RF Interface
The reader’s RF interface performs the following functions:

generation of high-frequency transmission power to activate the transponder and supply it with power;
modulation of the transmission signal to send data to the transponder;
reception and demodulation of RF signals transmitted by a transponder.
The RF interface contains two separate signal paths to correspond with the two directions of data flow from and to the transponder (Figure 11.4). Data transmitted to the transponder travels through the transmitter arm. Conversely, data received from the transponder is processed in the receiver arm. We will now analyse the two signal channels in more detail, giving consideration to the differences between the different systems.

Inductively Coupled System, FDX/HDX
First, a signal of the required operating frequency, i.e. 135 kHz or 13.56 MHz, is generated in the transmitter arm by a stable (frequency) quartz oscillator. To avoid worsening the noise ratio in relation to the extremely weak received signal from the transponder, the oscillator is subject to high demands regarding phase stability and sideband noise.

The oscillator signal is fed into a modulation module controlled by the baseband signal of the signal coding system. This baseband signal is a keyed direct voltage signal (TTL level), in which the binary data is represented using a serial code (Manchester, Miller, NRZ). Depending upon the modulator type, ASK or PSK modulation is performed on the oscillator signal.

FSK modulation is also possible, in which case the baseband signal is fed directly into the frequency synthesiser.

The modulated signal is then brought to the required level by a power output module and can then be decoupled to the antenna box.

The receiver arm begins at the antenna box, with the first component being a steep edge bandpass filter or a notch filter. In FDX/HDX systems this filter has the task of largely blocking the strong signal from the transmission output module and filtering out just the response signal from the transponder. In subharmonic systems, this is a simple process, because transmission and reception frequencies are usually a whole octave apart. In systems with load modulation using a subcarrier the task of developing a suitable filter should not be underestimated because, in this case, the transmitted and received signals are only separated by the subcarrier frequency. Typical subcarrier frequencies in 13.56 MHz systems are 847 or 212 kHz.

Some LF systems with load modulation and no subcarrier use a notch filter to increase the modulation depth (duty factor) – the ratio of the level to the load modulation sidebands – and thus the duty factor by reducing their own carrier signal. A different procedure is the rectification and thus demodulation of the (load) amplitude modulated voltage directly at the reader antenna.

Microwave Systems – Half-Duplex
The main difference between microwave systems and low-frequency inductive systems is the frequency synthesising: the operating frequency, typically 2.45 GHz, cannot be generated directly by the quartz oscillator, but is created by the multiplication (excitation of harmonics) of a lower oscillator frequency. Because the modulation is retained during frequency multiplication, modulation is performed at the lower frequency.

Some microwave systems employ a directional coupler to separate the system’s own transmission signal from the weak backscatter signal of the transponder (Integrated Silicon Design, 1996).

A directional coupler (Figure 11.6) consists of two continuously coupled homogeneous wires (Meinke and Gundlack, 1992). If all four ports are matched and power P1 is supplied to port 1, then the power is divided between ports 2 and 3, with no power occurring at the decoupled port 4. The same applies if power is supplied to port 3, in which case the power is divided between ports 1 and 2.

Directivity is the logarithmic magnitude of the ratio of undesired overcoupled power P4 to desired coupled power P2.

A directional coupler for a backscatter RFID reader should have the maximum possible directivity to minimise the decoupled signal of the transmitter arm at port 4. The coupling loss, on the other hand, should be low to decouple the maximum possible proportion of the reflected power P2 from the transponder to the receiver arm at port 4. When a reader employing decoupling based upon a directional coupler is commissioned, it is necessary to ensure that the transmitter antenna is well (anechoically) set up. Power reflected from the antenna due to poor adjustment is decoupled at port 4 as backward power. If the directional coupler has a good coupling loss, even a minimal mismatching of the transmitter antenna (e.g. by environmental influences) is sufficient to increase the backward-travelling power to the magnitude of the reflected transponder power. Nevertheless, the use of a directional coupler gives a significant improvement compared with the level ratios achieved with a direct connection of transmitter output module and receiver input.

Sequential Systems – SEQ
In a sequential RFID system the RF field of the reader is only ever transmitted briefly to supply the transponder with power and/or send commands to the transponder.

The transponder transmits its data to the reader while the reader is not transmitting. The transmitter and receiver in the reader are thus active sequentially, like a walkie-talkie, which also transmits and receives alternately.

The reader contains an instantaneous switching unit to switch between transmitter and receiver mode. This function is normally performed by PIN diodes in radio technology.

No special demands are made of the receiver in an SEQ system. Because the strong signal of the transmitter is not present to cause interference during reception, the SEQ receiver can be designed to maximise sensitivity. This means that the range of the system as a whole can be increased to correspond with the energy range, i.e. the distance between reader and transponder at which there is just enough energy for the operation of the transponder.

Microwave System for SAW Transponders
A short electromagnetic pulse transmitted by the reader’s antenna is received by the antenna of the surface wave transponder and converted into a surface wave in a piezoelectric crystal. A characteristic arrangement of partially reflective structures in the propagation path of the surface wave gives rise to numerous pulses, which are transmitted back from the transponder’s antenna as a response signal.

Due to the propagation delay times in the piezoelectric crystal the coded signal reflected by the transponder can easily be separated in the reader from all other electromagnetic reflections from the vicinity of the reader. The block diagram of a reader for surface wave transponders.

A stable frequency and phase oscillator with a surface wave resonator is used as the highfrequency source. Using a rapid RF switch, short RF pulses of around 80 ns duration are generated from the oscillator signal, which are amplified to around 36 dBm (4 W peak) by the connected power output stage, and transmitted by the reader’s antenna.

If a SAW transponder is located in the vicinity of the reader it reflects a sequence of individual pulses after a propagation delay time of a few microseconds. The pulses received by the reader’s antenna pass through a low-noise amplifier and are then demodulated in a quadrature demodulator. This yields two orthogonal components (I and Q), which facilitate the determination of the phase angle between the individual pulses and between the pulses and the oscillator (Bulst et al., 1998). The information obtained can be used to determine the distance or speed between SAW transponder and reader and for the measurement of physical quantities.

To be more precise, the reader circuit in Figure 11.8 corresponds with a pulse radar, like those used in flight navigation (although in this application the transmission power is much greater). In addition to the pulse radar shown here, other radar types (for example FM-CW radar) are also in development as readers for SAW transponders.

Control Unit
The reader’s control unit performs the following functions:

communication with the application software and the execution of commands from the application software;
control of the communication with a transponder (master–slave principle);
signal coding and decoding.

In more complex systems the following additional functions are available:

execution of an anticollision algorithm;
encryption and decryption of the data to be transferred between transponder and reader;
performance of authentication between transponder and reader.
The control unit is usually based upon a microprocessor to perform these complex functions. Cryptological procedures, such as stream ciphering between transponder and reader, and also signal coding, are often performed in an additional ASIC module to relieve the processor of calculation intensive processes. For performance reasons the ASIC is accessed via the microprocessor bus (register orientated).

Data exchange between application software and the reader’s control unit is performed by an RS232 or RS485 interface. As is normal in the PC world, NRZ coding (8-bit asynchronous) is used. The baud rate is normally a multiple of 1200 Bd (4800 Bd, 9600 Bd, etc.). Various, often selfdefined, protocols are used for the communication protocol. Please refer to the handbook provided by your system supplier.

The interface between the RF interface and the control unit represents the state of the RF interface as a binary number. In an ASK modulated system a logic ‘1’ at the modulation input of the RF interface represents the state ‘RF signal on’; a logic ‘0’ represents the state ‘RF signal off’.