Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 46

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EEPROM Mod.

CHIP

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Figure 3.19 Basic circuit of a transponder with subharmonic back frequency. The received
clocking signal is split into two, the data is modulated and fed into the transponder coil via a tap

3.2.2 Electromagnetic backscatter coupling

3.2.2.1 Power supply to the transponder

RFID systems in which the gap between reader and transponder is greater than 1 m
are called long-range systems . These systems are operated at the UHF frequencies of
868 MHz (Europe) and 915 MHz (USA), and at the microwave frequencies 2.5 GHz and
5.8 GHz. The short wavelengths of these frequency ranges facilitate the construction
of antennas with far smaller dimensions and greater efficiency than would be possible
using frequency ranges below 30 MHz.
In order to be able to assess the energy available for the operation of a transponder
we first calculate the free space path loss a F in relation to the distance r between the
transponder and the reader's antenna, the gain G T and G R of the transponder's and
reader's antenna, plus the transmission frequency f of the reader:

a F =− 147 . 6 + 20 log (r) + 20 log (f ) − 10 log (G T ) − 10 log (G R ) ( 3 . 1 )

The free space path loss is a measure of the relationship between the HF power
emitted by a reader into 'free space' and the HF power received by the transponder.
Using current low power semiconductor technology, transponder chips can be pro-
duced with a power consumption of no more than 5 W (Friedrich and Annala, 2001).

The efficiency of an integrated rectifier can be assumed to be 5 - 25% in the UHF and
microwave range (Tanneberger, 1995). Given an efficiency of 10%, we thus require
received power of P e = 50 µ W at the terminal of the transponder antenna for the oper-
ation of the transponder chip. This means that where the reader's transmission power
is P s = 0 . 5 W EIRP (effective isotropic radiated power) the free space path loss may
not exceed 40 dB (P s / P e = 10 000 / 1) if sufficiently high power is to be obtained at the
transponder antenna for the operation of the transponder. A glance at Table 3.7 shows
that at a transmission frequency of 868 MHz a range of a little over 3 m would be
realisable; at 2.45 GHz a little over 1 m could be achieved. If the transponder's chip
had a greater power consumption the achievable range would fall accordingly.
In order to achieve long ranges of up to 15 m or to be able to operate transponder
chips with a greater power consumption at an acceptable range, backscatter transpon-
ders often have a backup battery to supply power to the transponder chip (Figure 3.20).
To prevent this battery from being loaded unnecessarily, the microchips generally have

µ