Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 45

R1 1K

1 2

4 ∗ BAT41

IC1

14

~

1 Q1

2 D3 D1

C1

12
11
9
6
5
4
3

Q2
Q3
Q4
Q5
Q6
Q7

2

2

− +

L1

CLK

1

4 2

1

C1

1

5V6

1 D4 D2

RES

3

~ 2

7

4024

7400

R2 560

T1

2
1

DATA

L1: 5 Wdg. Cul
0.7 mm, D = 80 mm

1 2 1

2 3

IC3a

3

Figure 3.18 Example circuit for the generation of load modulation with subcarrier in an induc-
tively coupled transponder

The voltage induced at the antenna coil L1 by the magnetic alternating field of the
reader is rectified using the bridge rectifier (D1 - D4) and after additional smoothing
(C1) is available to the circuit as supply voltage. The parallel regulator (ZD 5V6)
prevents the supply voltage from being subject to an uncontrolled increase when the
transponder approaches the reader antenna.
Part of the high frequency antenna voltage (13.56 MHz) travels to the frequency
divider's timing input (CLK) via the protective resistor (R1) and provides the transpon-
der with the basis for the generation of an internal clocking signal. After division by
2 6 ( = 64 ) a subcarrier clocking signal of 212 kHz is available at output Q7. The sub-
carrier clocking signal, controlled by a serial data flow at the data input (DATA), is
passed to the switch (T1). If there is a logical HIGH signal at the data input (DATA),
then the subcarrier clocking signal is passed to the switch (T1). The load resistor (R2)
is then switched on and off in time with the subcarrier frequency.
Optionally in the depicted circuit the transponder resonant circuit can be brought into
resonance with the capacitor C1 at 13.56 MHz. The range of this 'minimal transponder'
can be significantly increased in this manner.
Subharmonic procedure The subharmonic of a sinusoidal voltage A with a defined
frequency f A is a sinusoidal voltage B , whose frequency f B is derived from an integer
division of the frequency f A . The subharmonics of the frequency f A are therefore the
frequencies f A / 2 ,f A / 3 ,f A / 4 ... .
In the subharmonic transfer procedure, a second frequency f B , which is usually
lower by a factor of two, is derived by digital division by two of the reader's trans-
mission frequency f A . The output signal f B of a binary divider can now be modulated
with the data stream from the transponder. The modulated signal is then fed back into
the transponder's antenna via an output driver.
One popular operating frequency for subharmonic systems is 128 kHz. This gives
rise to a transponder response frequency of 64 kHz.
The transponder's antenna consists of a coil with a central tap, whereby the power
supply is taken from one end. The transponder's return signal is fed into the coil's
second connection (Figure 3.19).