Communications: directive radio wave systems and devices (e.g. – Radar transponder system – Radar transponder only
Reexamination Certificate
2002-05-03
2003-10-07
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar transponder system
Radar transponder only
C342S042000, C342S043000, C342S044000, C342S050000, C342S175000, C342S195000, C342S196000
Reexamination Certificate
active
06630900
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for carrying out a non-contact remote interrogation in a system comprising a group of mobile transponders, wherein an interrogation station emits an interrogation signal, said interrogation signal is converted into an information-carrying response signal in a transponder comprising a SAW element and then returned to the interrogation station
1
.
The invention furthermore relates to an arrangement for carrying out the process, a transponder for such an arrangement, a SAW element suitable for a transponder and an algorithm for the reduction of disturbing influences on the propagation delay of the response signal.
BACKGROUND TO THE INVENTION
Processes of the above-mentioned kind which describe the identification of transponders are known e.g. from U.S. Pat. Nos. 4,737,790 (Skeie et al., X-Cyte Inc.), 4,734,698 (Nysen et al., X-Cyte Inc.) and 4,096,477 (Epstein et al., Northwestern University). In these processes passive SAW transponders (so-called SAW tags) are identified by means of an interrogation station. The transponders comprise a suitably packaged SAW element (SAW=surface acoustic wave) consisting of a piezoelectric material and suitable antennas for receiving and emitting electromagnetic waves in the range of 905-925 MHz. The SAW element modifies the received interrogation signal and generates a plurality of response signals each having a characteristic propagation. Two different processes are used for encoding the SAW element. The U.S. Pat. No. 4,096,477 uses a SAW element which does not comprise reflectors and on which a binary encoding is realized due to the use or omission of an output transducer. The response signal thus contains a different, code-specific number of signal components. This means, for example, for a code 1000 (binary) that exactly one response signal component is present and for the code 1111 (binary) exactly 4 response signal components. A short pulse is used as the interrogation signal.
In the processes described in the X-Cyte patents, the SAW element modifies additionally the phases of the response signals. The number of response signal components is thereby independent of the realized code; this is an advantage for decoding. The interrogation signal is a so-called chirp signal whose frequency varies in serrations in the range of 905-925 MHz. The SAW element comprises 16 different propagation paths (acoustic encoding channels). As a result, there are 16 different response signals whose signal periods are determined for all SAW elements of the system such that they each differ by one predetermined time interval &Dgr;T. The response signals expanding in the different paths thus have a time cascading which is constant (i.e. equal for all tags). When mixing the interrogation signal with the response signals, a predetermined number of known difference frequencies is generated in the interrogation station.
The difference frequency signals correspond to the beats between the interrogation signals and the time-delayed response signals. They are processed by correspondingly tuned filters. Since in each propagation path of the SAW element attenuation or phase shift elements are incorporated corresponding to the transponder-specific code, the transponder-specific code information can be obtained from the phases or amplitudes of the difference frequency signals.
Temperature changes and production tolerances cause disturbing propagation variations of the response signals. As a result, phase variations occur which distort a decoding or make it more difficult or even impossible. Thus, a calibration process as described in the above-mentioned U.S. Pat. No. 4,734,698 is used in practice. In this process two decoding channels in the transponder must have a uniform, transponder-unspecific code. The difference between the two corresponding response signals is used as the reference for the more exact phase information determination of the other response signals.
The process described above or similar processes have some of the following problems. At a frequency of 2.45 GHz, the phase encoding used is very susceptible to disturbances since already little temperature variations lead to great phase changes. If, for example, lithium niobate (0.7%/100° C.) is used as the substrate material for the SAW element, at a temperature change of 100° C. and a relative propagation difference of 100 ns, the relative phase change of the corresponding response signals is about 230° at 905 MHz and about 615° at 2.45 GHz. At 2.45 GHz ambiguity problems arise, or the reflectors must be arranged so close together that resolution problems arise and/or problems concerning the positioning of the phase elements used for encoding.
A further problem results from the described kind of calibration. The use of two response signals having a uniform, transponder-unspecific encoding reduces the number of the response signals usable for identification by 2. In the described embodiment of X-Cyte, the number of independent codes is thus reduced by the factor 4×4=16 as compared to the same system without calibration.
A further problem results from the necessity to optimally separate the response signals and disturbing signals. The disturbing signals are generated outside the transponder (e.g. by reflections of the response signal at metallic objects) as well as inside the transponder (e.g. multiple reflections between transducer (converter) and reflectors in the acoustic channels). It is known that the disturbing internal reflections can be reduced effectively if in the response signal the component having the longest propagation is at least twice as long as the shortest propagation. However, in the known realization of the SAW element and in case there is a large number (e.g. >6) of encoding channels, this prerequisite leads to SAW elements requiring large chip surfaces. A further problem arises from the demand for a cost-saving production of the SAW element. In this connection, the size of the SAW element is an important factor for the cost per article. The smaller this size, the cheaper is the tag. In this connection, the known SAW tags are not satisfactory.
Further problems arise in the process for encoding the SAW elements mentioned in U.S. Pat. No. 4,096,477 in that the number of the response signals to be processed is code-specific, the internal disturbing signals (multiple reflections between the transducers) are strong and numerous and a large number of output transducers is required (e.g. 2
16
codes require 16 output transducers).
A further problem is to convert the incident interrogation signals most efficiently in response signals, i.e. with minimum losses. The better the interrogation signals are reflected by the reflectors, the greater is e.g. the maximum reading distance that can be achieved. Suitable reflectors for this process are e.g. described in U.S. Pat. No. 4,737,790. They operate with a basic frequency of about 915 MHz. The manufacturing of reflectors with a basic frequency of about 2.45 GHz is very difficult with respect to production technology since the width of the electrode fingers is about 0.4 &mgr;m and thus leads to considerable cost disadvantages. The use of reflectors operating on the third harmonic and having a electrode finger width of about 0.6 &mgr;m is known from the prior art e.g. K. Yamanouchi, G. Shimuzu and K. Morishita, “2.5 GHz-range SAW propagation and reflection characteristics and application of passive electronic tag and matched filter”, Proc. IEEE Ultrasonics Symposium 1993, pp 1267-1271. The exact width of the electrode fingers particularly depends on the substrate material used. The above indications relate to 128°-LiNbO
3
.
A further problem results from maximizing the number of possible codes (cost reduction) and, at the same time, the strength of the response signals (long reading distance or safe reading-out in an environment with strong disturbance signals) on a given chip surface.
An effective increase in the response signal strength is achieved if as few acoustic cha
Küng Roland
Stierlin Roland
Gregory Bernarr E.
Hera Rotterdam B.V.
Ohlandt Greeley Ruggiero & Perle LLP
Rauchfuss, Jr. George W.
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