SAW RFID Tag with Reduced Size

Surface acoustic wave (SAW) -based radio- frequency identification (RFID) tags are soon expected to be produced in very high volumes. The size and cost of a SAW RFID tag will be key parameters for many applications. Therefore, it is of primary importance to reduce the chip size. However, the number of distinct codes to be realized and the used frequency band impose limitations on the delays of coded responses and, consequently, on the tag size. The coded signal should arrive at the reader with a certain delay, that is, after the reception of the environmental echoes. An adequate initial delay is typically about 1 µs. If the tag uses a bidirectional interdigital transducer (IDT), the initial delay is needed on both sides of the IDT. In this work, we have replaced the bidirectional IDT by a unidirectional IDT. This allows to halve the space required by the initial delay, since all the reflectors must now be placed on the same side of the IDT. We have reduced the tag size even further by using a Z-path geometry with two strong inclined reflectors. In this configuration, the same space in the x-direction (the initial propagation direction) is used for both the initial delay and the code reflectors, which means that the chip length is finally determined only by the space required by the code reflectors. In this way, the tag length is reduced by about 2 mm compared to an equivalent single-track configuration using a unidirectional IDT with the code reflectors placed in line. The proposed configuration is especially advantageous for tags having a relatively long initial delay compared to the space required by the code reflectors. For such devices, a chip size of less than 2 mm by 1 mm (at 2.45 GHz) is realizable.