Fundamental Operating Principles - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 43

Table 3.6 Overview of the power consumption of various RFID-ASIC building blocks (Atmel,
1994). The minimum supply voltage required for the operation of the microchip is 1.8 V, the
maximum permissible voltage is 10 V

Memory
(Bytes)

Write/read
distance

Power
consumption

Frequency Application

ASIC#1 6 15 cm 10 µ A 120 kHz Animal ID

ASIC#2 32 13 cm 600 µ A 120 kHz Goods flow, access check

ASIC#3 256 2 cm 6 µ A 128 kHz Public transport

4MHz ∗ Goods flow, public transport

ASIC#4 256 0.5 cm < 1mA

ASIC#5 256 < 2cm ∼ 1 mA 4/13.56 MHz Goods flow

ASIC#6 256 100 cm 500 µ A 125 kHz Access check

4.91 MHz ∗ Contactless chip cards ASIC#7 2048 0.3 cm < 10 mA

ASIC#8 1024 10 cm ∼ 1 mA 13.56 MHz Public transport

ASIC#9 8 100 cm < 1 mA 125 kHz Goods flow

ASIC#10 128 100 cm < 1 mA 125 kHz Access check

∗ Close coupling system.

represented as transformed impedance Z T in the antenna coil of the reader. Switching
a load resistor on and off at the transponder's antenna therefore brings about a
change in the impedance Z T , and thus voltage changes at the reader's antenna (see
Section 4.1.10.3). This has the effect of an amplitude modulation of the voltage U L at
the reader's antenna coil by the remote transponder. If the timing with which the load
resistor is switched on and off is controlled by data, this data can be transferred from
the transponder to the reader. This type of data transfer is called load modulation .
To reclaim the data at the reader, the voltage tapped at the reader's antenna is recti-
fied. This represents the demodulation of an amplitude modulated signal. An example
circuit is shown in Section 11.3.

Load modulation with subcarrier Due to the weak coupling between the reader
antenna and the transponder antenna, the voltage fluctuations at the antenna of the
reader that represent the useful signal are smaller by orders of magnitude than the
output voltage of the reader.
In practice, for a 13.56 MHz system, given an antenna voltage of approximately
100 V (voltage step-up by resonance) a useful signal of around 10 mV can be expected
( = 80 dB signal/noise ratio). Because detecting this slight voltage change requires highly
complicated circuitry, the modulation sidebands created by the amplitude modulation
of the antenna voltage are utilised (Figure 3.16).
If the additional load resistor in the transponder is switched on and off at a very high
elementary frequency f S , then two spectral lines are created at a distance of ± f S around
the transmission frequency of the reader f READER , and these can be easily detected
(however f S must be less than f READER ). In the terminology of radio technology the
new elementary frequency is called a subcarrier ). Data transfer is by ASK, FSK or PSK
modulation of the subcarrier in time with the data flow. This represents an amplitude
modulation of the subcarrier.
Load modulation with a subcarrier creates two modulation sidebands at the
reader's antenna at the distance of the subcarrier frequency around the operating
frequency f READER (Figure 3.17). These modulation sidebands can be separated from