U.S. patent application number 09/891187 was filed with the patent office on 2002-01-17 for surface acoustic wave component which can be interrogated by radio and has an optimum code size.
Invention is credited to Reindl, Leonhard, Schmidt, Frank, Sczesny, Oliver, Vossiek, Martin.
Application Number | 20020005677 09/891187 |
Document ID | / |
Family ID | 7892669 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005677 |
Kind Code |
A1 |
Reindl, Leonhard ; et
al. |
January 17, 2002 |
Surface acoustic wave component which can be interrogated by radio
and has an optimum code size
Abstract
A coding scheme is specified which, compared to the prior art
and with the system having a structure resolution of the same
magnitude, allows an enlarged code size for the same number of code
elements, and/or by which fewer code elements are required per code
for a predetermined code size. The code elements are disposed with
basic values in a matrix defined in a novel manner, with the matrix
having a finer subdivision of the basic values than the subdivision
corresponding to the structure resolution. Further development with
mean-value formations are possible.
Inventors: |
Reindl, Leonhard;
(Clausthal-Zellerfeld, DE) ; Schmidt, Frank;
(Poering, DE) ; Sczesny, Oliver; (Aschheim,
DE) ; Vossiek, Martin; (Munchen, DE) |
Correspondence
Address: |
LERNER AND GREENBERG, P.A.
PATENT ATTORNEYS AND ATTORNEYS AT LAW
Post Office Box 2480
Hollywood
FL
33022-2480
US
|
Family ID: |
7892669 |
Appl. No.: |
09/891187 |
Filed: |
June 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09891187 |
Jun 25, 2001 |
|
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PCT/DE99/04079 |
Dec 22, 1999 |
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Current U.S.
Class: |
310/313D ;
310/313R; 375/E1.017 |
Current CPC
Class: |
H03H 9/6406 20130101;
H04B 1/70712 20130101 |
Class at
Publication: |
310/313.00D ;
310/313.00R |
International
Class: |
H01L 041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 1998 |
DE |
198 60 058.5 |
Claims
We claim:
1. A coded surface acoustic wave component for an ID tag radio
interrogation system, the coded surface acoustic wave component
comprising: a substrate wafer having a surface with a piezoelectric
material characteristic; at least one electroacoustic transducer
having an interdigital structure disposed on said surface of said
substrate wafer, said electroacoustic transducer producing a
surface acoustic wave in said surface with a main wave propagation
direction governed by said interdigital structure; and a reflector
structure having reflectors functioning as code elements and spaced
apart from one another in said main wave propagation direction on
said surface of said substrate wafer; said substrate wafer having a
position matrix for positioning said reflectors at correct
distances apart, said position matrix having basic values at equal
distances from one another aligned in said main wave propagation
direction, in said position matrix a size of a matrix spacing is
dimensioned on a basis of a movement distance which the surface
acoustic wave travels within a time period predetermined by a
time-dimensioned measurement inaccuracy of a system, and of the
basic values of said position matrix only the basic values occupied
by said reflectors as positions distributed corresponding to a
respective code are those for which distances between adjacent
reflectors are always at least of equal magnitude to a structure
resolution resulting from a frequency bandwidth of the system.
2. The component according to claim 1, wherein an equal number of
said code elements are in each case assigned to individual codes
with a predetermined code size.
3. The component according to claim 2, wherein said substrate wafer
has a minimum physical length in a direction of said position
matrix resulting from said interdigital structure of said
electroacoustic transducer and the predetermined code size with
regard to said position matrix.
4. The component according to claim 1, including at least one
sensor structure disposed on said surface of said substrate
wafer.
5. The component according to claim 1, wherein said code elements
can be additionally weighted for enlarging a code size.
6. The component according to claim 1, including reference elements
disposed on said substrate wafer.
7. A coded surface acoustic wave component for an ID tag radio
interrogation system, comprising: a substrate wafer having a
surface with a piezoelectric material characteristic;
electroacoustic transducers having interdigital structures disposed
on said surface of said substrate wafer, said elecrtoacoustic
transducers producing surface acoustic waves in said surface with
main wave propagation directions governed by said interdigital
structures; and resonator structures functioning as code elements
each having a resonant frequency, said resonator structures each
with respect to a respective one of said electroacoustic
transducers, being disposed in a respective main wave propagation
direction thereof, said resonator structures formed according to a
frequency matrix having frequencies at equal intervals from one
another as basic values of said frequency matrix for determining
individual resonant frequencies of said resonator structures, a
size of a frequency interval between said basic values within said
frequency matrix dimensioned on a basis of a measurement inaccuracy
with which a frequency can be measured in a system, and in said
frequency matrix said resonant frequency for each of said resonator
structures selected such that, within said frequency matrix, only
resonant frequencies which are used for said resonator structures
are those that a respective frequency separation between two of
said resonator structures provided for adjacent resonant
frequencies is at least of equal magnitude to a structure
resolution of a frequency measurement resulting from a resonance
duration of an individual resonator in the system.
8. The component according to claim 7, wherein an equal number of
said code elements are in each case assigned to individual codes
within a predetermined code size.
9. The component according to claim 7, including at least one
sensor structure disposed on said substrate wafer.
10. The component according to claim 7, wherein said code elements
can be additionally weighted for enlarging a code size.
11. The component according to claim 7, including reference
elements disposed on said substrate wafer.
12. A method for carrying out a determination of a respective code
of a coded surface acoustic wave component, which comprises the
steps of: detecting response signals of individual code elements of
the respective code a number of times successively; forming an
average value from the response signals for each code element
resulting in a plurality of average values; and determining a
measure representing a measurement inaccuracy from the response
signals of the individual code elements of the respective code.
13. The method according to claim 12, which comprises deriving the
measure representing the measurement inaccuracy as a mean deviation
between measured values of the response signals of each code
element and its mean value (=statistical standard deviation).
14. The method according to claim 12, which comprises detecting the
response signals of the individual code elements a large number of
times in such a manner that a determined mean measurement
inaccuracy of all the averaged values is reduced to a measure such
that the measure is less than a predetermined measure of the
measurement inaccuracy on a basis of which a matrix size of the
coded surface acoustic wave component is formed.
15. The method according to claim 13, which comprises detecting the
response signals of the individual code elements a large number of
times in such a manner that a determined mean measurement
inaccuracy of a representative mean value is reduced to a measure
such that the measure is less than a predetermined measure of the
measurement inaccuracy on a basis of which a matrix size of the
coded surface acoustic wave component is formed.
16. The method according to claim 12, which comprises: carrying out
the determination of the respective code of the coded surface
acoustic wave component having reference elements; interrogating
the reference elements a number of times successively and at least
one of averaged scaling values and offset values are determined
from the response signals obtained a number of times; and
correcting the response signals of the code elements using the
averaged scaling values and the offset values.
17. The method according to claim 14, which comprises detecting the
response signals of the individual code elements a large number of
times in such a manner that a determined mean measurement
inaccuracy of a representative mean value is reduced to a measure
such that the measure is less than a predetermined measure of the
measurement inaccuracy on a basis of which a matrix size of the
coded surface acoustic wave component is formed.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application No. PCT/DE99/04079, filed Dec. 22, 1999,
which designated the United States.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a coded surface acoustic
wave component which can be interrogated by a radio, as is known in
principle from the prior art (see U.S. Pat. Nos. 4,263,595, and
5,469,170, 1995 IEEE Ultrasonics Symp., pages 117-120, and
International Patent Disclosures WO 96/14589, WO 97/42519, and WO
97/26555).
[0004] In terms of its physical configuration, a surface acoustic
wave component contains a substrate wafer formed from a
piezoelectric material or a material with a piezoelectric coating.
At least one interdigital structure is disposed as a piezoelectric
transducer on or in its surface/coating having the piezoelectric
characteristic. When the structure elements of the transducer are
electrically excited appropriately, the transducer results in an
acoustic wave, which is generally referred to as a surface acoustic
wave, being produced in the surface of the substrate. The surface
acoustic wave has a movement direction/form there that is governed,
as is known, by the interdigital structure. Such a structure
defines a main wave propagation direction in the plane of the
surface.
[0005] In a manner corresponding to a surface acoustic wave
component which can be interrogated by radio, the surface acoustic
wave in the component can be excited by the transducer being
excited/fed by radio. To this end, the transducer is equipped with
an appropriate antenna for radio reception and, generally, also for
radio return transmission of a response signal from the transducer
to a receiver. A separate transducer with an antenna can also be
provided for the interrogation signal.
[0006] The interrogation signal is transmitted by a transmitter
which can transmit with a minimum bandwidth which can be
predetermined. The radio signal transmission can be carried out
using, for example, an apparatus that can use thermal and/or
mechanical energy to produce a radio-frequency pulse with the aid,
for example, of a nonlinear electronic component, like a radio
path. Details of this are known.
[0007] The receiver which is provided for the radio response signal
transmitted back from the component must be configured, as is
known, particularly in terms of its bandwidth to satisfy the
requirements of the system operating with the surface acoustic wave
component.
[0008] In the case of surface acoustic wave components which are
used for identification, it is necessary to ensure that a received
signal can be uniquely associated, as a response signal, with a
predetermined surface acoustic wave component which is
appropriately individually coded for this purpose, where a system
contains a number of such components which can be interrogated but
are coded differently from one another, and/or where other signals
are received which arrive in the system receiver in some other
way.
[0009] It is thus known and normal practice for such surface
acoustic wave components which can be interrogated by radio to be
provided with respective individual coding, which makes it possible
to distinguish the individual components from one another uniquely
in the respective received signal within a large number of such
surface acoustic wave components contained in the system.
[0010] First of all, two examples should be cited of the
application options for such coded surface acoustic wave components
that can be interrogated by radio. One of these examples is for
such a surface acoustic wave component with coding to be fitted,
for example, to an object that can be identified in an appropriate
manner by the component or its coding. Such components are also
known as ID tags. Another example is where the surface acoustic
wave component has the additional characteristic, or is equipped
with such an additional characteristic, as a sensor for, for
example, measuring a temperature, a force variable and/or other
physical, chemical or such like state variables. Such applications
and refinements of a surface acoustic wave component relating to
them are known.
[0011] Various principles are possible for producing a coded radio
response signal from an interrogation signal. One example is to
provide reflector elements for the coding, which are disposed such
that they are managed in a known manner to the configuration of the
already mentioned interdigital structure of the transducer. Such
reflector elements are generally strip elements, which are provided
on/in the surface of the substrate wafer in the path of the main
wave propagation direction of the surface acoustic wave. As a
further example for code elements and instead of such the reflector
elements, resonators can also be assigned to the transducer or
transducers, and they will also be described further below.
[0012] An individual reflector element produces a surface acoustic
wave component response signal that is shifted in time with respect
to the interrogation signal, that is to say with respect to the
transmitted pulse. A component which, instead of this, is provided
with resonators produces a response signal at an appropriate
specific (resonant) frequency. A respective large number of
reflectors disposed in different positions (with respect to one
another and with respect to the transducer) produce a corresponding
large number of pulse response signals shifted differently in time,
with the mutual time shifts being dependent on the positions of the
relevant reflectors with respect to one another. A corresponding
situation applies to the various response resonant frequencies for
a respective number of different resonators provided for different
frequencies.
[0013] The response signal to be produced by the surface acoustic
wave component in response to a radio interrogation signal is thus,
in the case of reflectors, an additive superimposition of response
signal elements offset in time with respect to one another or, in
the case of resonators, an additive superimposition of a
correspondingly large number of sinusoidal, limited-time (generally
exponentially decaying) response signal elements at frequencies
which differ from one another. A respective surface acoustic wave
component is normally identified by determining the reception times
corresponding to the selected positions of the individual
reflectors in the relevant component. The resonator principle
results in amplitudes in the received spectrum at frequency support
points that correspond to the selected resonant frequencies of the
individual resonators. The coding or the impressed code of a
relevant reflector-coded surface acoustic wave component thus
physically/structurally contains coded positioning of the
individual reflectors that are provided, with respect to a
reference reflector element or with respect to the position of the
transducer on the surface of the substrate wafer. When resonators
are used for coding, the various resonant frequencies, which are
provided in a selective manner, of the individual resonators result
in the code impressed on the respective component.
[0014] One problem that is associated with this is that the
structure resolution of the associated measurement system is always
limited. In this case, structure resolution refers to the
capability of the system (in this case essentially containing the
transmitter, the surface acoustic wave component and the receiver)
to identify two reflection or resonant response signal elements
from two reflectors disposed immediately adjacent to one another on
the substrate wafer or from two resonators with immediately
adjacent resonant frequencies, as being two response signal
elements, which are separated from one another, in each case. In
systems with time measurement (reflectors), the time structure
resolution (.DELTA.t) is inversely proportional to the spectral
bandwidth B used for the system/the measurement, that is to say
.DELTA.t is proportional to 1/B.
[0015] In a system using frequency measurements (resonators), the
relationships are in principle analogous, that is to say, in this
case, the structure resolution, .DELTA.f is in this case based on
the quality of the system, that is to say it is inversely
proportional to the time duration t of the measurement signals
(.DELTA.f is proportional to 1/t). For normal measurement signals
with a Gaussian envelope, the proportionality factor is
approximately 0.5.
[0016] The fundamentally limited structure resolution results in
that all the code elements in the case of reflectors must be at a
minimum distance from one another and, in the case of resonators,
must have a corresponding minimum interval between mid-frequencies
since, otherwise, the signal components from elements (reflectors
or resonators) respectively adjacent in terms of position or
frequency would be superimposed in the response signal such that
reliable evaluation (identification) of a code of a relevant
component would no longer be possible.
[0017] Purely for the sake of completeness, it should be mentioned
that more far-reaching coding options can, additionally, also be
provided for the invention, which is still to be described in the
following text. For example, codes with a base higher than 2 can
also be used instead of a binary system (reflector
present/reflector not present). One possibility for achieving this
is to provide a number of amplitude thresholds/steps for a
respective code element. Another possibility is (additionally) to
evaluate (in steps) the phase difference between two signals from
two code elements.
[0018] Depending on the required code size, the known type of
coding is to dispose a greater or lesser number of reflector strips
distributed in terms of position along the main wave propagation
direction of the surface acoustic wave produced by the transducer,
on the surface acoustic wave component. For example, for a code
size of 32 bits, it is known as prior art for 32 spaces, located
one behind the other in the direction of the main wave propagation
direction, to be provided for up to 32 reflectors to be positioned
there. Thus, for a structure resolution (measured on the basis of
the delay time=path length s divided by the speed v of the acoustic
wave) of the system of 1 .mu.s, a delay time length of 31 .mu.s,
that is to say from the first bit to the 32nd bit, is thus required
for the dual coding for the configuration of the reflectors.
Therefore, the substrate wafer required for the component must have
a considerable length. This is associated with technical problems
that will also be discussed in the following text, in conjunction
with the invention. Reference should also be made to the detailed
description provided (further below) with regard to the use of
resonators as code elements.
SUMMARY OF THE INVENTION
[0019] It is accordingly an object of the invention to provide a
surface acoustic wave component which can be interrogated by radio
and has an optimum code size which overcomes the above-mentioned
disadvantages of the prior art devices and methods of this general
type. The object of the invention is, for a predetermined, in
particular large, code size, to manage with a shorter/smaller
(compared to the prior art) substrate wafer length/size and/or with
as few code elements per individual code as possible. In other
words, the aim is to find a coding scheme for a predetermined code
size which manages with an optimally small number of code elements
per code and which, furthermore, are disposed on an individually
selected basis, on a shorter/smaller substrate wafer
length/area.
[0020] With the foregoing and other objects in view there is
provided, in accordance with the invention, a coded surface
acoustic wave component for an ID tag radio interrogation system.
The coded surface acoustic wave component contains a substrate
wafer having a surface with a piezoelectric material
characteristic, and at least one electroacoustic transducer having
an interdigital structure disposed on the surface of the substrate
wafer. The electroacoustic transducer produces a surface acoustic
wave in the surface with a main wave propagation direction governed
by the interdigital structure. A reflector structure having
reflectors functioning as code elements are spaced apart from one
another in the main wave propagation direction on the surface of
the substrate wafer. The substrate wafer has a position matrix for
positioning the reflectors at correct distances apart. The position
matrix has basic values at equal distances from one another aligned
in the main wave propagation direction. In the position matrix, a
size of a matrix spacing is dimensioned on a basis of a movement
distance which the surface acoustic wave travels within a time
period predetermined by a time-dimensioned measurement inaccuracy
of a system. And of the basic values of the position matrix, only
the basic values occupied by the reflectors as positions
distributed corresponding to a respective code are those for which
distances between adjacent reflectors are always at least of equal
magnitude to a structure resolution resulting from a frequency
bandwidth of the system.
[0021] With the foregoing and other objects in view there is
further provided, in accordance with the invention, a coded surface
acoustic wave component for an ID tag radio interrogation system.
The coded surface acoustic wave component containing a substrate
wafer having a surface with a piezoelectric material characteristic
and electroacoustic transducers having interdigital structures
disposed on the surface of the substrate wafer. The elecrtoacoustic
transducers produce surface acoustic waves in the surface with main
wave propagation directions governed by the interdigital
structures. Resonator structures are provided and function as code
elements each having a resonant frequency. The resonator structures
each with respect to a respective one of the electroacoustic
transducers, are disposed in a respective main wave propagation
direction thereof. The resonator structures are formed according to
a frequency matrix having frequencies at equal intervals from one
another as basic values of the frequency matrix for determining
individual resonant frequencies of the resonator structures. A size
of a frequency interval between the basic values within the
frequency matrix is dimensioned on a basis of a measurement
inaccuracy with which a frequency can be measured in a system. And
in the frequency matrix the resonant frequency for each of the
resonator structures is selected such that, within the frequency
matrix, only resonant frequencies which are used for the resonator
structures are those that a respective frequency separation between
two of the resonator structures provided for adjacent resonant
frequencies is at least of equal magnitude to a structure
resolution of a frequency measurement resulting from a resonance
duration of an individual resonator in the system.
[0022] A novel position or frequency distribution, which forms the
respective codes, is provided for the novel coding principle
according to the invention and (in this case explained first of all
for the case of position distribution of reflectors, in the
following text) allows a greater number of different codes for a
given structure resolution, as defined above. In order to remain
with the abovementioned example, and in the case, for example, of
the surface acoustic wave component and its associated system (in
particular the receiver which evaluates the signals) having a
structure resolution of, for example, 1 .mu.s, and with the
acoustic wave having a delay time of 31 .mu.s of the abovementioned
length, the invention results in a code size (17 167 680 177 565
codings) increased by a factor of around 4000 times in comparison
to the 2.sup.32 different codings available in the prior art, which
corresponds approximately to a code size of 43 bits based on known
codings. In order, instead of this, to allow the above code size of
32 bits to be used with the measure according to the invention
specified in the following text, the invention in each case
requires only 23 reflectors (resonators) to be fitted for each
individual one of the codes, for which the considerably shorter
substrate wafer length corresponding to a delay time of 22.5 .mu.s
is then sufficient. The above numerical comparison is only one
example of the advantage that can be achieved by the invention. If
it is assumed that the delay time measurement in the case of
reflectors (or the frequency measurement in the case of resonators)
in the system is subject to even smaller measurement inaccuracy,
this can even be chosen to be many times greater still so that, for
example, the known 32-bit coding, in each case having only a
maximum of 20 reflectors fitted per code, can then be generated
even with a length of 19 .mu.s.
[0023] Further exemplary notes will now be provided primarily with
respect to and on the basis of the configuration of a component
according to the invention with reflectors (=coding in the time
domain), and this will be followed, further below, by additional
information relating to the embodiment with resonators (=coding in
the frequency domain).
[0024] Accordingly, the characteristic of the measurement
inaccuracy of the system for delay-time measurement or frequency
measurement was made use of, and was introduced here, for the known
structure resolution .DELTA. defined above. The measurement
inaccuracy denotes the random and systematic error with which the
delay time/frequency value measured using the system differs from
the actual delay time/frequency value of the physical structure.
The time position of a reflector or the frequency of a resonator
can be defined accurately only within an interval, which is
referred to as the measurement inaccuracy, owing to the inaccuracy
of the measurement carried out in or using the system. The
magnitude of the measurement inaccuracy .delta. in systems with a
surface acoustic wave component is generally considerably smaller
than the magnitude of the structure resolution. The measurement
inaccuracy can be reduced even further by averaging over a number
of measurements if the measurement errors are random, or by
calibration procedures in the case of systematic errors, and this
will also be described further below.
[0025] According to the invention, the magnitude .delta. is used to
form the respective position matrix with equidistant matrix
intervals .delta.t, or the frequency matrix with identical
frequency matrix intervals .delta.f for the coding, corresponding
to the teaching of the invention.
[0026] The principle of the invention is, despite the available
structure resolution .DELTA., for example, which is still constant
and wide as provided by the bandwidth, to dispose the reflectors
according to the invention in such a position matrix and, in order
to allow the position of a respective reflector to be determined
uniquely despite the limited structure resolution, to provide that
the matrix spaces in a position matrix are occupied only in such a
manner that spaces which are adjacent to one another in the
position matrix are occupied when there is no code, that is to say
when there is no code configuration. If, for example, the
measurement inaccuracy of the delay time measurement is dimensioned
to be half as great (for example .delta.t=0.5 .mu.s) on a time
scale as the structure resolution .DELTA. (for example 1 .mu.s)
measured on the same time scale, then the rule according to the
invention provides for at least one space in the matrix to remain
unoccupied between two occupied spaces in the position matrix. If,
for example, the variable .delta.t is in fact only 1/3 as great as
the structure resolution, this would result in the position matrix
being subdivided three times as finely as a matrix whose size
matches the structure resolution. Although, in accordance with the
rule according to the invention, at least two spaces in the
position matrix must then remain unoccupied between two adjacent
occupied reflectors of the code, namely once again due to the
limited structure resolution, the code size of the principle
according to the invention then nevertheless rises, however, to
5.times.10.sup.15 codings with, for example, 32 fitted reflector
positions for the individual code. The code size of the above
32-bit coding would in this case be capable of being generated with
a maximum of only 20 fitted reflectors per code on a chip length
which is now only 19 .mu.s.
[0027] For the invention, the above statements also apply in the
same sense when resonators are used as code elements instead of the
reflectors, as will be explained in more detail further below.
[0028] A development of the invention provides for an equal number
of code elements, that is to say reflectors or resonators, always
to be provided as standard in each individual one of the components
for the single individual codes/(coding options) for the single
individual components within the group of components of a
predetermined overall code size. For the surface acoustic wave
component, therefore as, seen from the transducer and with regard
to the propagation of the acoustic wave, the wave is always
attenuated to the same extent, and a faulty code is identified from
there being a different number of received signal elements.
According to this development of the invention with a constant
number of code elements, the code elements are just disposed
distributed differently in the position matrix for the respective
codes. The entire position matrix has a standard--compared to the
prior art--optimally short length, thus allowing a short component
dimension.
[0029] If the measurement inaccuracy is even lower (for example, as
mentioned above, 0.33 .mu.s), the code size and the number of
coding options with a predetermined number of code elements per
code for reflectors can be increased even further for a
predetermined length and for resonators on a predetermined surface
of the component chip. Conversely, if the measurement inaccuracy is
reduced even further for a given code size the number of code
elements required per code and/or the required length or area of
the substrate wafer of the component can be reduced.
[0030] In other words, the idea of the invention can also be
described as follows. A matrix according to the invention is formed
for the code elements of the individual codes of the code size. For
reflectors, this is a position matrix, and for resonators, as code
elements, is a frequency matrix. In the relevant matrix, the matrix
basic values are at equidistant (position or frequency) basic
intervals .delta.g from one another.
[0031] The intervals are dimensioned on the basis of the
measurement inaccuracy of the system, in which a delay time or a
frequency can be measured with an error .delta.(.delta.t for delay
time measurement; .delta.f for frequency measurement).
[0032] In a matrix according to the invention, the intervals are
optimally dimensioned to be of equal magnitude or else greater than
the magnitude .delta..
[0033] However, according to the invention, the only basic values
(positions/frequencies) which are in each case "occupied" by code
elements are those for which the intervals between code elements
positioned in such a manner are equal to or greater than the
structure resolution .DELTA. of the overall system. If the
measurement inaccuracy is .delta.=0.5 .mu.s or 0.33 .mu.s, the
intervals .DELTA., which are required according to the invention,
between positioned reflectors as code elements are each 1 .mu.s,
measured on a time scale for a structure resolution of 1 .mu.s. The
major difference from the prior art is that a position matrix,
which is several times finer in a corresponding manner, is
available for the fitting of reflectors as code elements by using
the smaller dimension than the structure resolution. If the
structure resolution .DELTA. remains unchanged, it is thus
possible, with a constant number of code elements per code, to
achieve a greater code size, or to achieve the previous code size
with fewer code elements per code. The division ratio between the
structure resolution and the selected matrix size may also be other
than an integer (greater than 1).
[0034] If this teaching relating to the technical craft is applied
to surface acoustic wave components coded using resonators, this
results in the following analogy. Instead of R reflectors, the
number R of resonators are provided and disposed on the substrate
wafer of the respective surface acoustic wave component (which
forms part of the predetermined code size). The R resonators have
resonant frequencies f.sub.i, where i=1 to R, which each differ
from one another. The matrix for selection of these frequencies
f.sub.i is the frequency matrix according to the invention with its
basic values f, for example 1 to 46. These have an equidistant
interval .delta.f. The basic interval of the resonant frequencies
available for coding (subject to a restriction which will be
mentioned in the following text) is dimensioned in such a manner
that it is greater than or, optimally equal to, the magnitude of
the measurement inaccuracy .delta.f with which it is possible to
measure an individual frequency in the system containing the
transmitter, the receiver and the component. In a comparable manner
to the situation with reflectors, in this case as well, the only
resonant frequencies f.sub.i which may be selected for resonators
to be used as code elements, within the matrix, from the (for
example 1 to 46) basic value frequencies in the matrix, according
to the invention, are those for which the intervals between
adjacent selected resonant frequencies (f.sub.j, f.sub.j+1) of two
resonators are in each case greater than or optimally equal to the
structure resolution, that is to say the frequency resolution
.DELTA.f of the overall system, including the natural bandwidth of
these resonators, which results from their quality factor.
[0035] The following calculation rule can be used to calculate the
code size. It is assumed that P is the number of basic values per
interval in the structure resolution
.DELTA.(p.multidot..delta.=.DELTA.). For example, P=2 for the
example with a measurement inaccuracy of 0.5 .mu.s and a structure
resolution of 1 .mu.s. P=3 for the example with a measurement
inaccuracy of 0.33 .mu.s and a structure resolution of 1 .mu.s once
again. It is assumed that the code size Cu for the number R of
respective code elements (that is to say Cu{R}) and for the number
of code elements R+P-1(Cu{R+P-1}) are given. The code size
(Cu{R+P}) is then obtained from the sum, that is to say
(Cu{R+P}=(Cu{R}+(Cu{R+P-1}). If, in consequence, the code size for
P successive numbers of code elements (Cu{R}), (Cu{R+1}), . . .
(Cu{R+P-1}) is known, then the code size can be calculated
successively for all the subsequent numbers of code elements.
[0036] The code size can also be increased even further when
resonators are used as code elements, provided the receiving unit
is additionally configured such that it allows determination of the
amplitude and/or phase or frequency position of the received signal
elements of the individual code element resonators. In this case,
the amplitude or the phase, or else both types of information, can
also additionally be used, in accordance with a principle which is
once again known per se, for additional expansion of the code
size.
[0037] If the structure of the code elements is also intended to be
used for sensory measurement purposes (as already mentioned above)
as well, then it may also be advantageous not to dispose the
possible positions of the reflectors or frequencies of the
resonators exactly in the equidistant matrix according to the
invention, but to introduce defined discrepancies in the code
element position (frequencies), so that the intervals/frequency
intervals between the code elements are not exactly equal to the
equidistant matrix. This avoids all the reflectors or resonators
supplying information which is redundant in a sensory manner. In
this type of embodiment, of course and as before, care must be
taken to ensure that all the position and frequency intervals
between code elements are, according to the invention, at least not
less than the structure resolution .DELTA. (1 .mu.s for example,
above) of the overall sensor system. To achieve this, it is then
possible either to dimension the basic interval .delta. to be
larger (as a minimum), or fewer codes can be provided.
[0038] The type of coding of the surface acoustic wave component
according to the invention offers, inter alia, a number of
advantages which are described in the following text, for example
also with respect to the technical implementation and configuration
on the surface acoustic wave component. The type of coding
according to the invention is, for example and in contrast to
multiphase coding, dependent, within limits, on changes to the
speed with which the surface acoustic wave propagates in the
component. For example, an ID tag with a mid-frequency of 434 MHz,
a structure resolution of 1 .mu.s and a number of P=4 reflectors
per interval .DELTA. will be used for comparison. In this case,
4-stage phase coding (4 PSK modulation) has a comparable code size
to the coding carried out according to the invention, which may be
referred to as pulse position modulation. With this mid-frequency,
the minimum structure resolution with 1 .mu.s is
434.times..lambda.. With the known 4 PSK modulation, two adjacent
states are separated by a phase angle of 90.degree., that is to say
1/4.lambda.. A variation in the time position of a reflector by
only 0.25 .lambda.:434 .lambda.=1 .mu.s: 1736, for example caused
by a position inaccuracy in the production of the component or due
to a discrepancy in the speed of the surface acoustic wave of the
component, thus on its own leads to intolerable corruption of the
response signal, and thus to faulty identification. In the case of
a component with coding in the same way as that according to the
invention, such an error would occur only if the position of a
reflector were incorrectly positioned by 0.25 .mu.s within the
matrix. Only then would comparable corruption of the signal occur
for a surface acoustic wave component coded according to the
invention. This shows that a surface acoustic wave component with
coding carried out according to the invention is thus less
sensitive by a factor of 400 to fluctuations in the surface
acoustic wave speed and/or positioning errors, comparable to the
the PSK modulation. Apart from this, this is also apparent from the
fact that the type of coding according to the invention is an
extremely robust multivalue coding for a surface acoustic wave
component. This is associated with major advantages, which also
affects the production of a respective component coded according to
the invention.
[0039] The above text is based on a respective magnitude .delta.
for the measurement inaccuracy, which is smaller to a greater or
lesser extent than the structure resolution .DELTA. of the system,
for the matrix intervals .delta.t and .delta.f of the position
matrix/frequency matrix according to the invention or used
according to the invention. In this case, the magnitude of the
value .delta. is chosen, for example, on the basis of experience or
measurements obtained when working with surface acoustic wave
elements. The object of a development of the invention is to
specify measures using which a (small) measurement inaccuracy
magnitude .delta. to be achieved can be achieved deliberately in a
manner which can be predetermined, specifically in order to make it
possible to use the invention described above as optimally as
possible.
[0040] In accordance with an added feature of the invention, the
substrate wafer has a minimum physical length in a direction of the
position matrix resulting from the interdigital structure of the
electroacoustic transducer and the predetermined code size with
regard to the position matrix.
[0041] In accordance with another feature of the invention, the
code elements can be additionally weighted for enlarging a code
size.
[0042] The object is achieved by an advantageous way of carrying
out the process of reading a respective code of a respective coded
surface acoustic wave component. The process of reading provides
for the respective code to be read a number of times successively,
that is to say for the respective individual code elements to be
measured a number of times in a corresponding manner. The time
dimension t for the position of the respective individual reflector
or the frequency of the respective individual resonator is thus
detected by measurement, that it is to say it is measured, by the
interrogation signal. The multiple reading of the individual code
elements of the respective code is carried out in an extremely
rapid sequence in the course of the correspondingly multiple
reading of the code. This results in data records that contain the
multiple measurement results of the respective individual code
element. A data record of the same type is obtained for each code
element of the code that is read. The data records are analyzed to
produce the magnitude of the standard deviation or some other
measure that describes the statistics or the inaccuracy of the
individual measurements within the respective data record. If there
are an appropriate number of measurements, a respective mean value
is obtained (for the reflector position or for the resonant
frequency of the resonator), or else some other representative
position/frequency mean value with a measurement inaccuracy which
may be used as the basis for the measurement inaccuracy .delta.
used and defined according to the invention.
[0043] If the above multiple reading of the code elements of a
respective code has resulted in a, for example predetermined,
measurement inaccuracy .delta., that is to say a predetermined
matrix size .delta.t or .delta.f, even being undershot by a certain
amount, then this increases the probability of the respective code
element, is to say the reflector position/resonant frequency,
having been measured correctly, that is to say the entire code
which has been read has been read correctly. This development,
which relates to the way in which the reading process is carried
out with multiple reading and averaging, results in an evaluation
in which all the random inaccuracies which are inherent in any
measurement are reduced to a (sufficiently) small level.
[0044] In the process of averaging the measured values, as
described above, it is also possible to carry out, in a manner
comparable to this, a calibration to overcome any systematic
errors. To do this, the surface acoustic wave component needs to
have at least two reference elements, for example comparable to the
code elements. These may be reference reflectors or reference
resonators, for example in the form of a start element and/or a
stop element in addition to the described code elements. The
reference elements are disposed independently of the matrix at
known positions, or as resonators with known resonant frequencies,
on the surface acoustic wave component. By comparison of the
measured and possibly also still averaged measured values of the
reference elements with their respectively predetermined known
values, for example comparison of measured time/frequency
difference between the start and the stop element with the
structurally predetermined known difference and/or comparison of
the measured values of the positions/frequencies of the start
element and stop element with their respective known actual
positions/frequencies, a scaling factor and/or an offset value can
be derived, using which (using both of which) all the
time/frequency measured values of the code elements can be
corrected. In this case, it is also advantageous to carry out the
calibration process a number of times and to average the result
over a number of measurements in each case, until assurance is
obtained that the (reduced) inaccuracy of the position/time or
frequency values achieved in this way is even less, by a specific
amount, than the/a predetermined level of the measurement
inaccuracy .delta..
[0045] The averaging process described above or the above
calibration, or else both measures, can also advantageously be
carried out as a development of the invention.
[0046] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0047] Although the invention is illustrated and described herein
as embodied in a surface acoustic wave component which can be
interrogated by radio and has an optimum code size, it is
nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0048] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0049] FIG. 1 is a diagrammatic, plan view of a surface acoustic
wave component having reflectors according to the invention;
[0050] FIG. 1a is a block diagram of a component in a radio
interrogation system;
[0051] FIG. 2 is a plan view used to complete the explanation of
the definition of a position matrix;
[0052] FIG. 3 is a plan view of two surface acoustic wave
components with different codes for one code size, and each having
the same number of reflectors as the code elements;
[0053] FIG. 4 is a plan view of a physical configuration of the
surface acoustic wave component coded having resonators as the code
elements;
[0054] FIG. 5 is a frequency matrix, defined according to the
invention, with its individual resonant frequencies which are
available for (restricted) selection; and
[0055] FIG. 6 is an example of two different codes in the frequency
matrix.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] In all the figures of the drawing, sub-features and integral
parts that correspond to one another bear the same reference symbol
in each case. Referring now to the figures of the drawing in detail
and first, particularly, to FIG. 1 thereof, there is shown an
example of a surface acoustic wave (SAW) component 1 according to
the invention. The SAW component 1 has a substrate wafer 10
composed, for example, of lithium niobate, lithium tantalate or the
like, or else of quartz. These materials have the required
piezoelectric characteristic. First, an electroacoustic transducer
12 is disposed on a surface 11 of the substrate wafer 10 whose plan
view is illustrated. This is, for example, an interdigital
structure 12 having two comb-like structures and two electrical
connections 14. The connections 14 are positioned as pads on a
lower base layer 101 of the substrate wafer 10, and are
electrically connected to the respective comb-like structure 12. A
dipole antenna 114 that needs to be provided for a component that
can be interrogated by radio can be electrically connected to the
connections 14. 15 denotes a respective surface acoustic wave
(indicated schematically) to be produced piezoelectrically in the
surface 11 of the substrate wafer 10 by the transducer 12. A double
arrow 115 indicates an alignment of a main wave propagation
direction. 20 denotes a code element structure overall, which
contains code elements 21 positioned such that they are aligned to
correspond to the wave propagation direction 115. The numbers 1, 2,
3 . . . 46 numerically denote "basic values" 130 of a matrix, which
will be described in more detail further below. Of the basic
values, the positions 1, 3, 6, 8, 10, 13 . . . and 46 are each
occupied by one code element 21. Reflectors 21' are indicated for
this purpose in FIG. 1. The distribution of the code elements 21,
which is individually selected for a respective surface acoustic
wave component 1 of a group of such components, over the 46 basic
values, for example, corresponds to or forms an individual code
within the predetermined code size, which can be provided with this
group of components.
[0057] For the special way of carrying out the reading process,
described above, with calibration to overcome any possible
systematic errors, reflectors K.sub.1 and K.sub.2 are used as a
start reference code element and as a stop reference code element,
as reference elements.
[0058] For the sake of completeness, further structure elements
should also be mentioned, such as the reflectors which, in a manner
known per se, are part of a sensor structure 221 which is used, for
example, for temperature measurement, force measurement or the
like. 17 denotes conventional wave sumps for the surface acoustic
wave.
[0059] FIG. 1a shows an overview of a system, which contains the
surface acoustic wave component 1, a transmitter S and a receiver E
required for radio interrogation.
[0060] FIG. 2 shows, from the same view as that of the exemplary
embodiment in FIG. 1, only the substrate wafer 10 and the
interdigital structure 12 of the transducer (since this governs the
main wave propagation direction 115 in the surface 11 of the
substrate wafer 10). Instead of the code element structure 20
(which has not yet been described here) from FIG. 1, a position
matrix 30 according to the invention and defined for reflectors for
the invention is indicated with the basic position values 130
which, as in FIG. 1, are annotated 1, 2, 3 . . . up to 46. The
individual basic values 130 are each represented by a (center) line
thereof. The matrix 30 according to the invention is defined such
that, first, it is aligned in the main wave propagation direction
115 of the wave 15 produced by the transducer 12. Since the wave
propagation direction 115 is in this case linear, the position
matrix 30 is a linear matrix. A different configuration may also
occur in special cases, but the matrix always follows the wave
propagation in such a manner that reflectors as code elements at
the occupied positions of the basic values 130 can cause the
surface acoustic wave 15 to be reflected in a manner known per
se.
[0061] The linear matrix 30 has as many basic values 130 as
required for the predetermined code size taking account of a
further distribution condition, according to the invention, for the
individual code elements. In accordance with the definition
provided by the invention, the equidistant intervals a between the
basic values 130 are dimensioned such that the magnitude of the
respective distance between adjacent basic values (1 and 2, 2 and
3, . . . ) is equal to a movement distance .delta.t which the
surface acoustic wave 15 travels within a defined time period. For
delay time measurement with the reflectors 21' as the code elements
21, the time period is the measurement inaccuracy .delta. measured
in time as defined above or determined by timing details for the
system, which includes the surface acoustic wave component 1
together with the transmitter S and receiver E.
[0062] As stated with the teaching of the invention, the basic
values 130 of the position matrix 30 may each be occupied with a
code element 21 only at intervals .DELTA.t corresponding to the
structure resolution. If .delta.t.ltoreq.1/2.DELTA.t, one or more
basic values 130 are kept free between two basic values 130
occupied by the code elements 21.
[0063] As an illustrative example FIG. 3 shows, alongside one
another, two position matrices 30' and 30" which each have (for the
sake of simplicity only) 13 of the matrix basic values 130. Of
these, there are preferably an equal number of positions in the
respective matrix which are occupied with the code elements 21,
that is to say with the reflectors 21', in each of the two
matrices, namely in each case six basic values 130. However, the
occupancy distribution differs depending on the different code in
the two matrices 30', 30".
[0064] As an example, FIG. 4 shows an embodiment with resonators
instead of the reflectors 21' as shown in the previous figures.
[0065] FIG. 4 shows a plan view of a surface acoustic wave
component 1' having resonators 220. 10' denotes a substrate wafer
on whose surface 11 transducers 212, the resonators 220, the
connections 14 for the dipole antenna 114 and the wave sumps 17 for
wave attenuation are disposed. A transducer 212.sub.1 and the two
resonator elements 220.sub.1' and 220.sub.1" which form a resonator
220.sub.1 are shown in the second line of FIG. 4. 115 indicates the
main wave propagation direction, and 15 the associated surface
acoustic wave. The two elements of the resonator 220.sub.1 contain
reflector strips, which are normal for such a component and are
spaced apart from one another, and the resonator is tuned, for
example to a selected frequency f.sub.1, by selection of a distance
between the strips. The resonator 220.sub.1 is a first code element
of the coded component 1' shown in FIG. 4. A j-th code element is
shown in the line underneath, with the resonator 220.sub.j, which
once again is composed of two elements, and its transducer
212.sub.j that is required to produce the wave 15. The frequency
f.sub.j is also selected from the frequency matrix according to the
invention. The R-th code element of the component 1' is shown in
the fourth line. Once again, the resonator contains two elements
220.sub.R. The resonator 220.sub.R is tuned to the frequency
f.sub.R, which is likewise selected in the predetermined frequency
matrix according to the invention. The resonant frequencies of
these resonators are thus surface acoustic wave structures tuned in
a manner known per se and having frequencies f.sub.1 to f.sub.R
which differ from one another. The selected frequencies produce the
overall code of the individual surface acoustic wave component. The
transducers 212.sub.1 to 212.sub.R can be connected in series or
else in parallel in the manner shown. A physically single
transducer construction can also be provided, but this covers the
illustrated main wave propagation directions 115.sub.1 to
115.sub.R. Normally, the bandwidth of such a transducer 212 is so
large that even identically configured transducers can form the
transducer chain.
[0066] A frequency matrix 230 according to the invention in FIG. 5
and which is relevant for the embodiment with resonators is
analogous to the position matrix 30 in FIG. 2. The matrix interval
.delta.f in a frequency domain, which is relevant to the invention,
is obtained from the measurement inaccuracy of the system,
containing the transmitter, the receiver and the component, or from
the measurement inaccuracy which can be achieved by multiple
measurements or by averaging, comparable to the interval between
the basic values 130 resulting from the time measurement
uncertainty, in FIG. 2. Based on the embodiments in FIGS. 1 and 2,
46 frequencies f.sub.i are also indicated, by way of example, in
FIG. 5. The total number of such frequencies f.sub.i which are
required for the component 1' in order to select the total number R
of resonant frequencies for its resonators 220.sub.1 to 220.sub.R
is once again governed by the predetermined code size in this case.
In order to allow the component 1 or 1' as shown in FIG. 1 or FIG.
4, respectively, to be kept as geometrically small as possible, the
total number R is also selected to be as low as possible in this
case and, by the invention, this can be achieved with reduced
measurement inaccuracy .delta. with, for example, an unchanged
coarse structure resolution .DELTA..
[0067] Accordingly, the "occupancy" of the possible basic values in
the frequency matrix shown in FIG. 5 is subject to the limitation
that the frequency interval between two adjacent frequencies
f.sub.j and f.sub.j+1 (j=from 1 to R) used for resonators must be
.DELTA.f, where .DELTA.f is at least of equal magnitude to the
structure resolution resulting from the quality factor of the
system. The structure resolution is the frequency interval .DELTA.f
which is required in order to make it possible to distinguish
between two resonant frequencies, which differ from one another, in
the system. For example, compared with the component 1 described
above and having the reflectors 21' with a minimum permissible
position interval corresponding to the time duration .DELTA.t, an
occupancy of the frequency positions f.sub.1 to f.sub.46 in the
matrix as shown in FIG. 5 can be used with a minimum frequency
interval .DELTA.f=2.times..delta.f for an embodiment with
resonators, if the frequency measurement inaccuracy of the system
is half the magnitude of the frequency structure resolution
.DELTA.f.
[0068] In the case of a component with resonators as reference
elements, resonators K.sub.11 and K.sub.12 are used as start and
stop elements, respectively, for carrying out the reading process
with calibration.
[0069] In a comparable manner to FIG. 3, FIG. 6 shows the frequency
scheme for two different codes from a predetermined code size with
six predetermined resonators as code elements of the code. For
example, these are the various codes of the components 1(n) and
1(n+1) of a total number N of coded surface acoustic wave
components 1' for a predetermined code size.
[0070] The use of the invention also results in advantages relating
to the configuration of the production process. For example, when
producing a component coded according to the invention, the
exposure time for production of the code elements
(reflectors/resonators) is reduced, for example being halved. This
is achieved due to the fact that, for example, two reflectors 21'
are always placed jointly on the exposure mask, and are exposed
jointly. For this purpose, these two reflectors 21' must be
provided with intervals that differ from one another, to be precise
with minimum intervals corresponding to the rule according to the
invention on the exposure mask. If, for example, the structure
resolution is 1 .mu.s and the number of code elements is P=4 per
interval .DELTA. of the structure resolution, then exposure masks
must be provided which each have two reflectors, which can be
exposed at the same time, for the reflector intervals of 1.00,
1.25, 1.50, 1.75 and, possibly, also 2.00 .mu.s.
* * * * *