Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 134

10

Linear law detection

1

0.1

0.01

10 − 3 1 ×

Square law detection

10 − 4 1

×

10 − 5 1 ×

Noise limit

10 − 6
−

1 ×

60 − 50 − 40 − 30 − 20 − 10 0 10 20

Input power (dBm)

Figure 4.79 When operated at powers below − 20 dBm (10 µ W) the Schottky diode is in the
square law range

Impedance
matching

C 1

D 2 C 2

R L

U in U D1

U T D 1

U chip

Figure 4.80 Circuit of a Schottky detector in a voltage doubler circuit (villard-rectifier)

In order to further increase the output voltage, voltage doublers (Hewlett Packard,
956-4) are used. The circuit of a voltage doubler is shown in Figure 4.80. The output
voltage u chip at constant input power P in is almost doubled in comparison to the single
Schottky detector (Figure 4.81). The Bessel function (equation (4.102)) can also be
used for the calculation of the relationship of P in to u chip in voltage doublers. However,
the value used for R g should be doubled, the value R L should be halved, and the
calculated values for the output voltage u chip should also be doubled (Figure 4.81).
The influence of various operating frequencies on the output voltage is not taken into
account in equation (4.102). In practice, however, a frequency-dependent current flows
through the parasitic capacitor C j , which has a detrimental effect upon the efficiency of
the Schottky detector. The influence of the junction capacitance on the output voltage
can be expressed by a factor M (Hewlett Packard, 1088). The following holds:

1
1 + ω 2 C j R s R j

M = ( 4 . 103 )