Readers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 310

Demodulator U2270B

R 1 D 1

C 1
1.5 n

R 2
470 k

C 2
680 p

R i

From reader
antenna

4.7 k

220 k

Bias

Figure 11.13 Rectification of the amplitude modulated voltage at the antenna coil of the reader
(reproduced by permission of TEMIC Semiconductor GmbH, Heilbronn)

The on-chip oscillator generates the operating frequency in the range 100-150 kHz.
The precise frequency is adjusted by an external resistor at pin R F . The downstream
driver generates the power required to control the antenna coil as push-pull output. If
necessary, a baseband modulation signal can be fed into pin CFE as a TTL signal and
this switches the HF signal on/off, generating an ASK modulation.
The load modulation procedure in the transponder generates a weak amplitude
modulation of the reader's antenna voltage. The modulation in the transponder occurs
in the baseband, i.e. without the use of a subcarrier. The transponder modulation signal
can be reclaimed simply by demodulating the antenna voltage at the reader using a
diode. The signal, which has been rectified by an external diode and smoothed using
an RC low-pass filter, is fed into the 'Input' pin of the U2270B (Figure 11.13). Using
a downstream Butterworth low-pass filter, an amplifier module and a Schmitt trigger,
the demodulated signal is converted into a TTL signal, which can be evaluated by the
downstream microprocessor. The time constants of the Butterworth filter are designed
so that a Manchester or bi-phase code can be processed up to a data rate of f osc / 25
(approximately 4800 bit/s) (TEMIC, 1977).
A complete application circuit for the U2270B can be found in the following chapter.

11.4 Connection of Antennas for Inductive

Systems

Reader antennas in inductively coupled RFID systems generate magnetic flux ,which
is used for the power supply of the transponder and for sending messages between the
reader and the transponder. This gives rise to three fundamental design requirements
for a reader antenna:

• maximum current i 1 in the antenna coil , for maximum magnetic flux ;

• power matching so that the maximum available energy can be used for the gener-

ation of the magnetic flux;

•

sufficient bandwidth for the undistorted transmission of a carrier signal modulated
with data.