Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 33

1 /n 2 . However, if nonlinear energy storage is used
for multiplication, then in the ideal case there are no losses (Fleckner, 1987).
Capacitance diodes are particularly suitable nonlinear energy stores for frequency
multiplication. The number and intensity of the harmonics that are generated depend
upon the capacitance diode's dopant profile and characteristic line gradient. The expo-
nent n (also γ ) is a measure for the gradient (

occurs with an efficiency of η =

capacitance-voltage characteristic).
For simple diffused diodes, this is 0.33 (e.g. BA110), for alloyed diodes it is 0.5 and
for tuner diodes with a hyper-abrupt P-N junction it is around 0.75 (e.g. BB 141)
(Intermetal Semiconductors ITT, 1996).
The capacitance-voltage characteristic of alloyed capacitance diodes has a quadratic
path and is therefore best suited for the doubling of frequencies. Simple diffused diodes
can be used to produce higher harmonics (Fleckner, 1987).
The layout of a 1-bit transponder for the generation of harmonics is extremely
simple: a capacitance diode is connected to the base of a dipole adjusted to the carrier
wave (Figure 3.5). Given a carrier wave frequency of 2.45 GHz the dipole has a total
length of 6 cm. The carrier wave frequencies used are 915 MHz (outside Europe),
2.45 GHz or 5.6 GHz. If the transponder is located within the transmitter's range, then
the flow of current within the diode generates and re-emits harmonics of the carrier
wave. Particularly distinctive signals are obtained at two or three times the carrier
wave, depending upon the type of diode used.
Transponders of this type cast in plastic (hard tags) are used mainly to protect
textiles. The tags are removed at the till when the goods are paid for and they are
subsequently reused.
Figure 3.6 shows a transponder being placed within the range of a microwave trans-
mitter operating at 2.45 GHz. The second harmonic of 4.90 GHz generated in the diode
characteristic of the transponder is re-transmitted and detected by a receiver, which is
adjusted to this precise frequency. The reception of a signal at the frequency of the
second harmonic can then trigger an alarm system.
If the amplitude or frequency of the carrier wave is modulated (ASK, FSK), then all
harmonics incorporate the same modulation. This can be used to distinguish between
'interference' and 'useful' signals, preventing false alarms caused by external signals.

=

Dipole

f A

Capacitance diode

Housing

f A

2 × f A

Basic circuit Mechanical design

Figure 3.5 Basic circuit and typical construction format of a microwave tag