Harmonic RFID System
Functioning of the RFID system assumes a sending of a probing pulse with the help of transceiver of basic unit which activates a transponder, established on the object. Transponder replies by the reciprocal radio-frequency pulse modulated by a unique identification code. The use of passive transponder represents the greater interest for consumers, as its power supplying is carried out due to the energy of a signal of basic unit within the communication time. Under clear reasons the link of such system should be duplex. Due to the increased power of such communication link, there is an opportunity in this case to send several commands to transponder and to organize, thus, any necessary exchange protocol.
The organization of duplex channel involves the engineers in complicated design of transponder and increases its cost. In practice there are widely used so-called harmonic RFID systems (Colpitts et al, 2004) and (Vanjari et al, 2007) in which the transceiver of the interrogator sends initial signal on one radio frequency, and transponder responds on frequency, which is multiple to the initial one. As a rule, frequency rate is equal to two. Such approach satisfies the main principle of duplex communication. On the other hand the doubling of frequency allows us to use the same known antenna for reception and radiation. Multiplication of frequency of the transceiver can be achieved by the elementary technical solution, for example by connection to the terminal of the transponder’s antenna of Shottki diodes, as it is shown on Figure1 a.
Figure 1. Transponder circuit scheme a), Spectrogram of combinatorial RFID system b)
Besides the multiplication of interrogator signal frequency, they make its rectifying for transponder modulator feeding. Obviously, the energy of signal, which is received from interrogator and rectified, is weak, and this signal must be presented during entire communication session. The Shottki diodes let us minimize the losses and the real value of rectified voltage can reach few volts. That is enough for the feeding of modulator.