Readers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 303

Quarz Oscillator Modulator Output

module

Antenna box

Transmission
data

/

Received
data

/

Demodulator Amplifier Bandpass filter

Figure 11.4 Block diagram of an HF interface for an inductively coupled RFID system

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 differ-
ent systems.

11.2.1.1 Inductively coupled system, FDX/HDX

First, a signal of the required operating frequency, i.e. 135 kHz or 13.56MHz, is gener-
ated 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 (Manch-
ester, 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 13.56MHz systems are 847 kHz 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 mod-
ulation sidebands — and thus the duty factor by reducing their own carrier signal.