Standardisation - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 256

Sense coil

Reference
coil

Field
coil

PICC
(DUT)

RF drive

Scope

d r d s

d d

Figure 9.37 Mechanical structure of the measurement bridge, consisting of the field generator
coil (field coil), the two sensor coils (sense and reference coil) and a smart card (PICC) as test
object (DUT) (reproduced by permission of Philips Semiconductors, Hamburg)

the measuring bridge is disrupted in time with the switching frequency (this corre-
sponds with the subcarrier frequency f s ) as a result of the modulation resistor in the
smart card being switched on and off. As a result, a subcarrier modulated HF voltage
can be measured at the measurement output of the measuring bridge. This signal is
sampled over several periods using a digital oscilloscope and then brought into the
frequency range by a discrete Fourier transformation. The amplitudes of the two mod-
ulation sidebands f c ± f s that can be seen in the frequency range now serve as the
quality criterion for the load modulator and should exceed the limit value defined in
ISO 14443.
The layout of the required coils, a circuit to adapt the field coil to a 50 transmit-
ter output stage, and the precise mechanical arrangement of the coils in the measuring
arrangement are specified in the Annex to the standard, in order to facilitate its dupli-
cation in the laboratory (see Section 14.4).

Reference card As a further aid, the standard defines two different reference cards
that can be used to test the power supply of a card in the field of the reader, the
transient response and transient characteristics of the transmitter in the event of ASK
modulation, and the demodulator in the reader's receiver.