Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 48

3.2.2.2 Data transmission reader →

Modulatedreflectioncross-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 electromag-
netic waves is described by its reflection cross-section . Objects that are in resonance
with the wave front that hits them, as is the case for antennas at the appropriate
frequency, for example, have a particularly large reflection cross-section.
Power P 1 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 P 1 is
supplied to the antenna connections as HF voltage and after rectification by the diodes
D 1 and D 2 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 P 1 is reflected by the antenna and returned
as power P 2 .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 R L 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 P 2 reflected from the transponder can thus be modulated (

→

modulated backscatter).
The power P 2 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 back-
wards 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 transpon-
der ( P 1 /P 2 ) can be estimated using the radar equation (for an explanation, refer to
Chapter 4).

3.2.3 Close coupling

3.2.3.1 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 & 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 high frequency 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.