Readers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 311

Depending upon the frequency range, different procedures can be used to connect
the antenna coil to the transmitter output of the reader: direct connection of the antenna
coil to the power output module using power matching or the supply of the antenna
coil via coaxial cable.

11.4.1 Connection using current matching

In typical low cost readers in the frequency range below 135 kHz, the HF interface and
antenna coil are mounted close together (a few centimetres apart), often on a single
printed circuit board. Because the geometric dimensions of the antenna supply line
and antenna are smaller than the wavelength of the generated HF current (2200m) by
powers of ten, the signals may be treated as stationary for simplification. This means
that the wave characteristics of a high frequency current may be disregarded. The
connection of an antenna coil is thus comparable to the connection of a loudspeaker
to an NF output module from the point of view of circuitry.
The reader IC U2270B, which was described in the preceding section, can serve as
an example of such a low cost reader (Figures 11.14-11.16).
Figure 11.14 shows an example of an antenna circuit. The antenna is fed by the
push-pull bridge output of the reader IC. In order to maximise the current through
the antenna coil, a serial resonant circuit is created by the serial connection of the
antenna coil L S to a capacitor C S and a resistor R S . Coil and capacitor are dimen-
sioned such that the resonant frequency f 0 is as follows at the operating frequency of
the reader:

1
2 π √ L s · C s

f 0 = ( 11 . 3 )

The coil current is then determined exclusively by the series resistor R S .

OSC
enable

CFE RF

VS VBatt

95 9860

Driver

COIL1

Standby

Power
supply

&

Oscillator

COIL2

MS

= 1

m C

U2270B

Output

CODE

NF read channel

Input

Ouput
enable

GAIN OE

Figure 11.14 Block diagram for the reader IC U2270B with connected antenna coil at the
push-pull output (reproduced by permission of TEMIC Semiconductor GmbH, Heilbronn)