Communications: directive radio wave systems and devices (e.g. – Radar transponder system
Reexamination Certificate
2002-05-14
2003-08-26
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar transponder system
C342S043000, C342S175000
Reexamination Certificate
active
06611224
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to new configuration of a surface acoustic wave transponder, and more particularly to a method and apparatus of improving the spatial and layout efficiency of surface acoustic wave encoded transponder having a folded acoustic path reducing a size of the otherwise required substrate.
BACKGROUND OF THE INVENTION
A known radio frequency passive acoustic transponder system produces individualized responses to an interrogation signal. The code space for these devices may be, for example, 2
16
codes, or more, allowing a large number of transponders to be produced without code reuse. These devices provide a piezoelectric substrate on which an aluminum pattern is formed, for example by a typical microphotolithography process, with a minimum feature size of, for example, one micron.
These codes are imposed upon a received signal by signal transforming elements formed on the substrate. Typically, for each encoded symbol, a separate transforming element is formed. In order to facilitate decoding, the transforming elements interact with the interrogation signal at different respective time delays, which are ensured by the acoustic propagation delay on the substrate. These time delays therefore dictate the minimum path length, and therefore size of the substrate. As in many microphotolithographic systems, the substrate size is related to system cost, and smaller substrates imply lower costs. Therefore, a tradeoff exists between substrate size and available encoding complexity.
The aforementioned transponder devices include a surface acoustic wave device, in which an identification code is provided as a characteristic time-domain reflection, attenuation, phase delay, and/or transducer interaction pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal, and can be analyzed in a plurality of states. This analysis reveals the various delay components within the device. In such a device, received RF energy, possibly with harmonic conversion, is emitted onto a piezoelectric substrate as an acoustic wave by means of an interdigital electrode system, from whence it travels through the substrate, interacting with reflecting, delay or resonant/frequency selective elements in the path of the wave. A portion of the acoustic wave is ultimately received by an interdigital electrode system, which may be the same or different than the launch transducer, and retransmitted. These devices do not require a semiconductor memory nor external electrical energy storage system, e.g., battery or capacitor, to operate. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, the time for transmission between the radio frequency interrogation system and the transponder is typically short as compared to the acoustic delay of the substrate. This allows the rate of the interrogation frequency change to be based primarily on the delay characteristics within the transponder, without requiring measurements of the distance between the transponder and the interrogation system antenna.
The interrogation frequency is controlled to change sufficiently from the return or “backscatter” signal from the transponder, so that a return signal having a minimum delay may be distinguished from the interrogation frequency, and so that all of the relevant delays are unambiguously received for analysis. The interrogation frequency thus should not return to the same frequency within a minimum time-period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3.273,146; 3,706,094; 3,755,803; and 4,058,217, expressly incorporated herein by reference.
Passive transponder tag interrogation systems (ire also known with separate receiving and transmitting antennas, which may be at the same frequency or harmonically related, and having the same or different polarization. Thus, in these systems, the transmitted and received signals may be distinguished other than by frequency. The acoustic wave is often a surface acoustic wave, although acoustic wave devices operating with various other wave types, such as bulk waves, are known.
The information code associated with and which identifies the passive transponder is built into the transponder at the time that a layer of metallization is finally defined on the substrate of piezoelectric material. This metallization also defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g., reflectors and delay elements. Thus, the information code in this case is non-volatile and permanent. The information representing these elements is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as a specific complex delay pattern and attenuation characteristics. In the case of a transponder tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is N×2
M
where N is the number of acoustic waves launched by the transducers and M is the number of reflective element positions for each transducer. Thus, with four launch transducers each emitting two acoustic waves, and a potential set of eight variable reflective elements in each acoustic path, the number of differently coded transducers is 2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, efficiency is lost with increasing acoustic path complexity (i.e., power splitting), and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each. Therefore, the transponder design is a tradeoff between device codespace complexity and efficiency.
The known passive acoustic transponder tag thus typically includes a multiplicity of “signal conditioning elements”, i.e., delay elements, reflectors, and/or amplitude modulators, which are coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the acoustic wave interacting with it. Even where the signal is split into multiple portions, it is advantageous to reradiate the signal through a single antenna. Therefore, a “signal combining element” coupled to the all of the acoustic waves, which have interacted with the signal conditioning elements, is provided for combining the intermediate signals to produce tile radiated transponder signal. The radiated signal is thus a complex composite of all of the signal modifications, which may occur within the transponder, modulated on the interrogation wave.
In known passive acoustic transponder systems, the transponder remains static over time, so that the encoded information is retrieved by a single interrogation cycle, representing the state of the tag, or more typically, obtained as an inherent signature of an emitted signal due to internal time delays. In order to determine a transfer function of a passive transponder device, the interrogation cycle may include measurements of excitation of the transponder at a number of different frequencies. This technique allows a frequency domain analysis, rather than a time domain analysis of an impulse response of
Nysen Paul A.
Skeie Halvor
Gregory Bernarr E.
Milde & Hoffberg LLP
X-Cyte Inc.
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