Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 95

('short-circuited' transponder coil)

ω 2 k 2 · L 1 · L 2

Z T (R L →∞ ) =

( 4 . 54 )

1
jωC 2

jωL 2 + R 2 +

(unloaded transponder resonant circuit).
Transponder inductance L 2 Let us now investigate the influence of inductance L 2
on the transformed transponder impedance, whereby the resonant frequency of the
transponder is again held constant, so that C 2 = 1 /ω TX L 2 .

Transformed transponder impedance reaches a clear peak at a given inductance
value, as a glance at the line diagram shows (Figure 4.36). This behaviour is remi-
niscent of the graph of voltage u 2 = f(L 2 ) (see also Figure 4.15). Here too the peak
transformed transponder impedance occurs where the Q factor, and thus the current
i 2 in the transponder, is at a maximum ( Z T ∼ jωM · i 2 ). Please refer to Section 4.1.7
for an explanation of the mathematical relationship between load resistance and the
Q factor.

4.1.10.3 Loadmodulation

Apart from a few other methods (see Chapter 3), so-called load modulation is the most
common procedure for data transmission from transponder to reader by some margin.

35

30

25

20

15

10

5

0

1 × 10 − 7 1 × 10 − 6 1 × 10 − 5 1 × 10 − 4

L 2 (H)

f RES f TX

f RES = f TX + 3%
f RES = f TX − 0.5%

=

Figure 4.36 The value of Z T as a function of the transponder inductance L 2 at a constant
resonant frequency f RES of the transponder. The maximum value of Z T coincides with the
maximum value of the Q factor in the transponder