Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 52

Transponder

Reader

C R-T

U

L 1 C 1

C R-GND

R L

R Mod

C T-GND

Figure 3.26 Equivalent circuit diagram of an electrically coupled RFID system

as a voltage divider with the elements C R-T , R L (input resistance of the transponder)
and C T-GND (see Figure 3.26). Touching one of the transponder's electrodes results in
the capacitance C T-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 elec-
trode 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 )ora graphite
coating (Motorola, Inc., 1999).

3.2.4.2 Data transfer transponder reader →

If an electrically coupled transponder is placed within the interrogation zone of a reader,
the input resistance R L of the transponder acts upon the resonant circuit of the reader via
the coupling capacitance C R-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 R mod in the transponder on and off. Switching
the modulation resistor R mod on and off thereby generates an amplitude modulation
of the voltage present at L 1 and C 1 by the remote transponder. By switching the
modulation resistor R mod on and off in time with data, this data can be transmitted to
the reader. This procedure is called load modulation .

3.2.5 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.