Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 144

carrier signal (GTAG, 2001). In order to make a precise prediction regarding the
sensitivity of a reader, however, this value must be established for the individual case
by measurement.
For the transmission of data the signal reflected by the transponder is modulated. It
should be noted in this connection that as part of the process of modulation the reflected
power P S is broken down into a reflected 'carrier signal' and two sidebands. In pure
ASK modulation with a theoretical modulation index of 100% 6 the two sidebands
would each contain 25% of the total reflected power P S (i.e. P 3 —6dB), and at
a lower modulation index correspondingly less. Since the information is transmitted
exclusively in the sidebands , a lower wanted signal should be specified according to
the modulation index. The reflected carrier contains no information, but cannot be
received by the reader since it is completely masked by a transmission signal of the
same frequency, as Figure 4.89 shows.
Let us now consider the magnitude of the power P 3 arriving at the reader that is
reflected by the transponder.
As in equation (4.75) the received power P 3 at the receiver of the reader is:

P 3 = A e · Reader · S Back ( 4 . 109 )

20

0

Reader-transmitter
P 1 : 0 dBc

− 20

− 40

− 60

− 80

Transponder-signal
P 3 : abt. − 90 dBc

− 100

− 120

− 100 − 50 0 50 100

f

Figure 4.89 Example of the level relationships in a reader. The noise level at the receiver of
the reader lies around 100 dB below the signal of the carrier. The modulation sidebands of the
transponder can clearly be seen. The reflected carrier signal cannot be seen, since the level of
the carrier signal of the reader's transmitter, which is the same frequency, is higher by orders
of magnitude

6 In practice 100% ASK modulation of the reflected signal cannot be achieved, since Z T would have to
have an infinite value in the modulated state.