Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 137

250

10 − 6 2 ×

200

150

100

50

0

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

Total current (I s + I b ) (A)

860 MHz
915 MHz
2.45 GHz

Figure 4.83 Voltage sensitivity γ 2 of a Schottky detector in relation to the total current
I T · C j = 0 . 25 pF, R s = 25 , R L = 100 k

at an operating current I b = 0. Additionally, the junction resistance R j = 40 k that
results at I T = 0 . 65 µ A can hardly be approximately transformed without loss using
real components at the low-ohmic source impedance of the antenna Z A = 73 +
j 0 . Finally, the influence of the parasitic junction capacitance C j at such high-ohmic
junction resistances is clearly visible, and shows itself in a further reduction in the
voltage sensitivity, particularly at 2.45 GHz.
In practice, Schottky detectors are operated at currents of 2.5 - 25 µ A, which leads
to a significantly lower junction resistance. In practice, values around 50 mV/ µ Wcan
be assumed for voltage sensitivity (Hewlett Packard, 1089).
Due to the influencing parameters described above it is a great challenge for the
designer designing a Schottky detector for an RFID transponder to select a suitable
Schottky diode for the operating case in question and to set all operating parameters
such that the voltage sensitivity of the Schottky detector is as high as possible.
Let us finally consider another example of the matching of a Schottky voltage
doubler to a dipole antenna. Based upon two Schottky diodes connected in parallel in
the HF equivalent circuit ( L p = 2nH, C p = 0 . 08 pF, R s = 20 , C j = 0 . 16 pF, I T =
3 µ A, R j = 8 . 6k ) we obtain an impedance Z rect = 37 − j374 ( | Z rect |= 375 ) .The
Smith diagram in Figure 4.84 shows a possible transformation route, plus the values
and sequence of the components used in this example that would be necessary to
perform a matching to 72 (dipole in resonance).
It is not always sensible or desirable to perform impedance matching between
transponder chip and antenna by means of discrete components. Particularly in the