Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 135

100

10

1

0.1

0.01

10 − 3 1 ×

1 × 10 − 4

1 × 10 − 5

10 − 6 1 ×

− 60 − 50 − 40 − 30 − 20 − 10 0 10 20

Input power P in (dBm)

Figure 4.81 Output voltage of a Schottky detector in a voltage doubler circuit. In the input

power range − 20 to − 10 dBm the transition from square law to linear law detection can be

clearly seen ( R L = 500 k , I s = 2 µ A, n = 1 . 12)

However, in the range that is of interest for RFID transponders at output voltages
u chip 0.8 - 3 V and the resulting junction resistances R j in the range < 250 (Hewlett
Packard, 1088) the influence of the junction capacitance can largely be disregarded
(Figure 4.82; see also Figure 4.81).
In order to utilise the received power P e as effectively as possible, the input
impedance Z rect of the Schottky detector would have to represent the complex conju-
gate of the antenna impedance Z A (voltage source), i.e. Z rect = Z A . If this condition
is not fulfilled, then only part of the power is available to the Schottky detector, as a
glance at Figure 4.65 makes unmistakably clear.
The HF equivalent circuit of a Schottky detector is shown in Figure 4.78. It is the
job of the capacitor C 2 to filter out all HF components of the generated direct voltage
and it is therefore dimensioned such that X C2 tends towards zero at the transmission
frequency of the reader. In this frequency range the diode (or the equivalent circuit
of the diode) thus appears to lie directly parallel to the input of the circuit. The load
resistor R L is short-circuited by the capacitor C 2 for the HF voltages and is thus not
present in the HF equivalent circuit. R L , however, determines the current I b through