Sanna Harma

Inline SAW RFID tag using time position and phase encoding

Surface acoustic wave (SAW) radio-frequency identification (RFID) tags are encoded according to partial reflections of an interrogation signal by short metal reflectors. The standard encryption method involves time position encoding that uses time delays of response signals. However, the data capacity of a SAW RFID tag can be significantly enhanced by extracting additional phase information from the tag responses. In this work, we have designed, using FEM-BEM simulations, and fabricated, on 128deg-LiNbO3, inline 2.44-GHz SAW RFID tag samples that combine time position and phase encoding: each reflective echo has four possible time positions and, additionally, a phase of 0deg, -90deg, -180deg, or -270deg. This corresponds to 16 different states, i.e., 4 bits of data, per code reflector. In addition to the enhanced data capacity, our samples also exhibit a low loss level of -38 dB for code reflections.

Feasibility of ultra-wideband SAW RFID tags meeting FCC rules

We discuss the feasibility of surface acoustic wave (SAW) radio-frequency identification (RFID) tags that rely on ultra-wideband (UWB) technology. We propose a design of a UWB SAW tag, carry out numerical experiments on the device performance, and study signal processing in the system. We also present experimental results for the proposed device and estimate the potentially achievable reading distance. UWB SAW tags will have an extremely small chip size (<0.5 times 1 mm2) and a low cost. They also can provide a large number of different codes. The estimated read range for UWB SAW tags is about 2 m with a reader radiating as low as <0.1 mW power levels with an extremely low duty factor.

P0-14 Inline SAW RFID Tag Using Time Position and Phase Encoding

Surface acoustic wave (SAW) radio-frequency identification (RFID) tags are encoded according to partial reflections of an interrogation signal by short metal reflectors. The standard encryption method involves time position encoding that uses time delays of response signals. However, the data capacity of a SAW RFID tag can be significantly enhanced by extracting additional phase information from the tag responses. In this work, we have designed, using FEM-BEM simulations, and fabricated, on 128deg-LiNbO3, inline 2.44-GHz SAW RFID tag samples that combine time position and phase encoding. Each reflective echo has 4 possible time positions and a phase of 0deg, -90deg, -180deg, or -270deg. This corresponds to 16 different states, i.e., 4 bits of data, per code reflector. In addition to the enhanced data capacity, our samples also exhibit a low loss level of -38 dB for code reflections.

Extraction of frequency-dependent reflection, transmission, and scattering parameters for short metal reflectors from FEM-BEM simulations

Reflectors comprised of only a single or a few electrodes provide controllable, weak reflectivity essential for surface acoustic wave (SAW) radio-frequency identification (RFID) tags. The reflection, transmission, and scattering parameters of such reflectors must be known as a function of frequency in order to be able to control the amplitudes of tag responses and to use phase-based encoding reliably. In this work, we present a method of extracting the main reflection, transmission, and scattering parameters for short metal reflectors as a function of frequency. We use test device S parameters obtained through finite- and boundary-element method (FEM-BEM)-based simulations and, as an example, determine the reflection and transmission coefficients (their absolute values and phase angles) and the energy scattered into bulk for a few different single-electrode reflectors. We compare these parameter values to earlier results. Although only used for simulated data in this work, the same method can be applied to measured data as well. Assuming the S parameters available, this method is very fast and does not require any heavy calculation or special software.

Low-loss, multimode 5-IDT SAW filter

Longitudinally coupled resonator filters provide unbalanced-balanced operation with wide bandwidth, low loss, arid high suppression levels. However, reducing the insertion loss in the 1.8-2.2 GHz range remains a challenging problem because at high frequencies the resistive losses arising from the relatively wide aperture of the filter may degrade the performance. A 5-interdigital transducer (IDT) filter has six gaps at which the periodicity of the grating is broken, resulting in additional loss due to scattering into the bulk. In this paper, we show that replacing the gaps between the transducers with short transducer sections having their pitch different from that of the main transducers reduces the insertion loss of the device. We present devices with balun operation at 1842 MHz with wide bandwidth of 4.5% and -40 dB suppression, with a minimum insertion loss less than 1 dB in the best devices, and a maximum insertion loss of -1.2 dB in the passband. The passband is quite flat, with <1 dB ripple. We also discuss the layout of the contact pads and the connections, and its effect on the device performance and balance characteristics.

Feasibility of ultra-wideband SAW tags

We discuss the feasibility of surface acoustic wave (SAW) radio-frequency identification (RFID) tags that rely on ultra-wideband (UWB) technology. We propose a design of a UWB SAW tag, carry out numerical experiments on the device performance, and study signal processing in the system. We also present experimental results for the proposed device and estimate the potentially achievable reading distance.

Surface Acoustic Wave RFID Tags

The first radio identification systems appeared already during World War II for military applications, namely, for identification of planes. However, it is only now that the technical conditions are right for a wide use of radio frequency identification (RFID). The two key issues for the RFID technology are the number of different codes that can be ‘written’ on a tag and the possibility to store, transfer, and communicate information. Due to the ongoing miniaturization of semiconductor integrated devices, mass production of such devices at a low cost has become possible. More specifically, microand nanometer lithographic technology allows for the fabrication of very small tags (with a chip size on the order of 1 mm and smaller) operating at the GHz-range where sufficiently wide frequency bands are available. These industrial, scientific, and medical (ISM) frequency bands can be used without licensing with a limited radiated power. The wide frequency bands finally allow for a practically infinite number of different codes to be written and read at microsecond time intervals. As to the second key issue, the omnipresent internet, intranet, and similar communication networks enable the processing of databases and developing of smart systems that use the information automatically read from RFID tags. It is interesting to point out that the dramatic development of mobile phones (that only combine a transmitter and a receiver, both used in radio communications for a century by now) was based exactly on the same two reasons: first, the development of technology allowing the use of high frequencies, wide frequency bands and, finally a large number of subscribers and, second, computer databases with high-speed data links enabling fast communication. The type of RFID tag introduced in this chapter, the surface acoustic wave (SAW) tag, is in many aspects similar to RF SAW filters, that are widely used in mobile phones. SAW tags and SAW filters use the same technology.

Properties of Narrow Metal Reflectors Used in Reflective Array Compressors and Surface Acoustic Wave Tags

Narrow open-circuited aluminum electrodes with metal ratio on the order of 0.2 can provide controllable weak reflectivity necessary for many applications such as reflective array compressors (RAC) and surface acoustic wave (SAW) tags. Etched grooves used in RACs can thus be replaced with metal strips. We determine the main parameters of such narrow electrodes using FEM-BEM simulations and experiments. We show that a weak reflectivity of 0.3% per wavelength can easily be achieved and controlled by varying the electrode width. We also demonstrate the use of narrow electrodes in an inline dispersive delay line (DDL).

PS-1 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 mus. 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

Experimental results for a longitudinally coupled 5-IDT resonator filter with distributed gaps

Longitudinally coupled resonator filters (CRFs) provide unbalanced-balanced operation with wide bandwidth, low loss and high suppression levels. One-track filters with five transducers show good performance at 2-GHz frequencies. At high frequencies, however, the resistive losses arising from the relatively wide aperture of the CRFs can degrade the performance. Reducing the insertion loss in the 1.8-2.2 GHz frequency range remains a challenge. A 5-IDT filter has 4 gaps between IDTs where the periodicity of the grating is broken, resulting in additional loss. We show that replacing these gaps with short transducer sections reduces the insertion loss of the device. We present devices on 42/spl deg/-LiTaO/sub 3/ at 1842 MHz with wide bandwidth of 4.5% and 40 dB of suppression, with a minimum insertion loss less than 1 dB in the best devices, and a maximum insertion loss of 1.2 dB in the passband. We also discuss the layout of the contact pads and the connections and its effect on the device performance and balance characteristics.