Note: Descriptions are shown in the official language in which they were submitted.
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SURFACE ACOUSTIC WAVE PASSIVE
TRAM5PONDER HIVING ACOUSTIC REFLECTORS
The present invention rPlates to a "passive inter-
rogator label system" (PILS); that is a system com-
prising an interrogator for transmitting an interro-
gation signal, one or more "labels" or passive trans-
ponders which produce a reply signal containingcoded information in response to the interrogation
signal, and a receiver and decoding system for
receiving the reply signal and decoding the informa-
tion contained therein.
A passive interrogator label system of the type to
which the present invention relates is disclosed in
the U.S. Patent No. 3,273,146 to Horwitz, Jr.; U.S.
Patent No 3,7Q6,094 to Cole and Vaughan; U.S. Patent
No 3,755,803 to Cole and Vaughan; and Use Patent No.
4,058,217 to Vaughan and Cole. In its simplest form,
the systems disclosed in these patents include a radio
frequency transmitter capable of transmitting RF pulses
of electromagnetic energy. These pulses are received
at the antenna of a passive transponder and applied to
a piezoelectric "launch" transducer adapted to convert `
the electrical energy received from the antenna into
acoustic wave energy in the piezoelectric material.
Upon receipt of a upose, an acoustic wave is generated
within the piezoelectric material and transmitted along
a defined acoustic path. Further "tap" transducers
arranged at prescribed, spaced intervals along this path
convert the acoustic wave back into electric energy for
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reconversion into electrical energy by the launch transducer.
The presence or absence of tap transducers at the prescribed
locations along the acoustic wave path determines whether a reply
pulse will be transmitted with a particular time delay, in res-
ponse to an interrogation pulse. This determines the infor-
mational code contained in the transponder reply.
When an acoustic wave pulse is reconverted into an
electrical signal it is supplied to an antenna on the trans-
ponder and transmitted as RF electromagnetic energy. This energy
is received at a receiver and decoder, preferably at the same
location as the interrogating transmitter, and the information
contained in this response to an interrogation is decoded.
In systems of this general type, the energy contained
in the reply signal is substantially less than the energy
supplied to the transponder interrogating signal.
It is an object of the present invention to provide
a passive transponder adapted for use in an interrogation system
for transmitting a reply signal containing encoded information
in response to the receipt of an interrogating signal.
In accordance with a broad aspect of the invention,
there is provided, in a passive transponder adapted for use in
an interrogation system for transmitting a reply signal containing
coded information in response to the receipt of an interrogating
signal, said transponder comprising:
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(a3 an antenna or converting between electrical energy
and electromagnetic energy:
(b) a substrate having a substrate surface defining a
plurality of paths of travel for surface acoustic waves;
(c) transduce.r means, electrically connected to said
antenna and arranged on said substrate surface, for converting
between electrical energy and surface acoustic wave energy which
propagates along said paths of travel, said transducer means
comprising a plurality of transducer elements, electrically
connected together, for converting said interrogation signal
into surface acoustic wave energy, each one of said transducer
elements being responsive to produce surface acoustic waves within
a specified frequency band upon application of an interrogating
signal having a frequency within such band, the frequency bands
of at least two different transducer elements being exclusive
of each other so that an interrogating signal of a given
frequency will excite a specific one of said transducer elements;
(d) a plurality of acoustic wave reflectors arranged on
said surface along said paths of travel for reflecting said
surface acoustic wave energy back toward said transducer means,
said paths of travel extending in at least one direction from
said transducer means along at least one common line, wherein
said reflectors are arranged along said common line and wherein
at least one of said reflectors, closest to said transducer means,
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reflects only a portion of the acoustic wave energy received,
thereby to permit a portion of said acoustic wave energy to pass
beneath said one reflector to reach the next reflector along said
common line; and
(e) circuit means, connected to said transducer means, for
supplying said interrogating signal to said transducer means and
for receiving said reply signal from said transducer means.
In accordance with another broad aspect of the invention,
there is provided, in a passive transponder adapted for use in an
interrogation system for transmitting a reply signal containing
coded information in response to the receipt of an interrogating
signal, said transponder comprising:
(a) a substrate having a substate surface defining a
plurality of paths of travel for surface acoustic waves;
(b) transducer means arranged on said surface for converting
between electrical energy and surface acoustic wave energy which
propagates along said paths of travel;
(c) a plurality of acoustic wave reflectors arranged on
said surface along said paths of travel for reflecting said
surface acoustic wave energy back toward said transducer means; and
(d) circuit means, connected to said transducer means, for
supplying said interrogating signal to said transducer means and
for receiving said reply signal from said transducer means;
the improvement wherein said paths of travel extend in at
least one direction from said transducer means along at least one
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common line, wherein said reflectors are arranged along said
common line, and wherein at least one of said reflectors closest
to said transducer means, reflects only a portion of the acoustic
wave energy received,
thereby to permit a portion of said acoustic wave energy to
pass beneath said one reflector to reach the next reflector along
said common line.
Since reflectors of acoustic waves may be made ex-
tremely efficient--providing nearly 100% reflection of the acoustic
wave energy--substantially all the acoustic wave energy which is
generated by a transducer is reflected back to that transducer for
reconversion into electrical energy. Theoretically, therefore,
the total loss in energy conversion will be approximately 3ab
upon launching an acoustic wave and about 3db in reconversion of
the acoustic wave into an electrical signal, or 6db. Various
configurations of transducers and reflectors arranged on a
piezoelectric substrate are described in detail below.
For a full understanding of the present invention,
reference should now be made to the following detailed
description of the preferred embodiment of the invention and the
accompanying drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of a system for transmit-
ting an interrogation signal, receiving a reply sig-
nal and decoding information encoded in the reply
signal.
Fig 2 is a block diagram of a passive transponder
adapted for use with the system of Fig. 1.
Fig. 3 is a timing diagram indicating the clock out-
put in the system of Fig. 1.
Fig. 4 is a frequency vs. time diagram illustrating
the transmitted signal in the system of Fig. 1.
Fig. 5 is a frequency vs. time diagram illustrating
both the transmitted and the received signal in the
system of Fig. 1.
Fig. 6 is a plan view, in enlarged scale, of a par-
ticular implementation of the transponder of Fig. 2.
Fig. 7 is a plan view, in greatly enlarged scale, of
a portion of the implementation shown in Fig. I.
Fig. 8 is a plan view, in greatly enlaxged scale, of
a transducer and two reflectors of the type employed
in the present invention.
Fig. 9 is a plan view, in greatly enlarged scale, of
a transducer/reflector pattern according to a pre-
ferred embodiment of the invention.
Fig. 10 is a plan view, in greatly enlarged scale, of
a transducer/reflector pattern according to a second
preferred embodiment of the invention.
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Fig. 11 is a plan view, in greatly enlarged scale, of
a transducer/reflector pattern according to a third
preferred embodiment of the present invention.
Fig. 12 is a plan view, in greatly enlarged scale,
of a transducer/reflector pattern according to a
fourth preferred embodiment of the present inven-
tion.
Fig. 13 is a frequency diagram showing the frequency
bands of the respective interrogationsignal pulses
in the configuration of Fig. 12.
Fig. 14 is a plan view, in greatly enlarged scale, of
a portion of a transducer/reflector pattern according
to a fifth preferred embodiment of the present inven-
tion.
The present invention will now be described with
reference to Figs. 1 - 14 of the drawings. Identical
elements in the various figures are designated by the
same reference numerals.
Figs. 1-7 illustrate an interrogator-transponder
system employing a surface acoustic wave transponder
which may form the environment of the present inven-
tion. The transmitter/receiver and decoder system
shown in Fig. 1 comprises a ramp generator 20 which
supplies a sawtooth waveform to a voltage controlled
oscillator (VCO) 22. The VCO produces an output
signal of frequency f which repeatedly ramps linearly
upward from a frequency of 905 MHz to a frequency ox
925 MH~. This signal is amplified by the RF ampli-
fiers 24 and supplied to a transmit/receive switch 26.
The switch 26 directs the signal either to a trans-
mitter power amplifier 28 or to a decoding mixer 30.
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The switch 26 is controlled by a 100 KHz square wave
signal produced by a clock 32. The output signal S
from the amplifier 28 is supplied to an external
circulator or transmit/receive (TR) switch 34 and is
transmitted as electromagnetic radiation by an
antenna 36.
block diagram of the transponder associated with the
system of Fig. 1 is shown in Fig. 2. The transponder
receives the signal Sl at an antenna 38 and passes it
to a series of delay elements 40 having the indicated
delay periods To and T. After passing each succes-
sive delay, a portion of the signal Ion Il, I2 ... IN
is tapped off and supplied to a summing element 111.
The resulting signal S2, which is the sum of the
intermediate signals Io ... It, is fed back to the
antenna 38 for transmission to the antenna 36 in the
system of Flg. 1.
The transponder reply signal S2 is received by the
antenna 36 and passed through the circulator or TR
switch 34 to a receiver amplifier 44. The output S4
of this amplifier 44 is heterodyned in the mixer with
the signal S3 intermittently presented by the switch
26.
The output S5 of the mixer 30 contains the sum and
the difference frequencies of the signals S3 and S4.
This output is supplied to a band pass filter 46 with
a pass band between l and 3 KHz. The output of this
filter is passed through an anti-aliasing filter 48
to a sample-and-hold circuit 50.
The sample-and-hold device supplies each sample to
an analog-to-digital converter 52. The A/D converter,
in turn, presents the digital value of this sample
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to a processor 54 that analyzes the frequencies con-
tained in the signal by means of a Fourier transform.
The sample-and-hold device 50 and the A/D converter
52 are strobed by a sampling signal which serves to
compensate or the non-linearity, with respect to
time, in the monotonically increasing frequency f of
the VCO output signal.
To effect compensation the signal of frequency f pro-
duced by the VCO 22 i5 passed via an isolating ampli-
fier 56 to a delay element 58 with a constant signal
delay Ts. Both the delayed and the undelayed signals
are supplied to a mixer 60 which produces a signal S7
containing both sum and difference frequencies. The
signal S7 is supplied to a low-pass filter 62 which
passes only the portion of this signal containing the
difference frequencies. The output of the low-pass
filter is supplied to a zero-crossing detector 64
which produces a pulse at each positive (or negative)
going zero crossing. These pulses are used to strobe
the sample-and-hold device 50 and the A/D converter 52.
Figs. 3-5 illustrate the operation of the circuit of
Fig. 1. Fig. 3 shows the 100 KHz output of the clock
32; Fig. 4 shows the frequency sweep of the signal pro-
duced by the VCO 22. Fig. 5 shows, in solid lines 66,
the frequency of the transmitted signal Sl and, in
dashed lines 68, the frequency of the signal S2 as re-
ceived from the transponder As may be seen, the sig-
nal 68 is received during the interval between trans-
missions of the signal 66. These intervals are chosen
to equal, approximately, the round trip delay time be-
tween the transmission of a signal to the transponder
and the receipt of the transponder reply As indi-
cated by the multiple dashed lines, the transponder
9~
reply will contain a number of frequencies at anygiven instant of time as a result of the combined
(i.e., summed) intermediate signals having different
delay times (To To IT, To 2 Q T, . . . To N IT).
Figs. 6 and 7 illustrate an embodiment of a passive
transponder which implements the block diagram of
Fig 2. This transponder operates to convert the
received signal Sl into an acoustic wave and then to
reconvert the acoustic energy back into an electrical
signal S2 or transmission via a dipole antenna 70.
More particularly, the signal transforming element
of the transponder includes a substrate 72 of piezo-
electric material such as a lithium niobate (LiNbO3)
crystal. On the surface of this substrate is
deposited a layer of metal, such as aluminum, forming
a pattern such as that shown in detail in Fig. 7. For
example, this pattern may consist of two bus bars
74 and 76 connected to the dipole antenna 70, a
"launch" transducer 78 and a plurality of "tap" trans-
ducers 80. The bars 7~ and 76 thus define a path of
travel 82 for a surface acoustic wave which is gener-
ated by the launch transducer and propogates sub-
stantially linearly, reaching the tap transducers each
in turn. The tap transducers convert the surface
acoustic wave back into electrical energy which is
collected and therefore summed by the bus bars 74 and
76. This electrical energy then activates the dipole
antenna 70 and is converted into electromagnetic
radiation for transmission as the signal S2.
The tap transducers 80 are provided at equally spaced
intervals along the surface acoustic wave path 82, as
shown in Fig. 6, and an informational code associated
with the transponder is imparted by providing a
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selected number of "delay pads" 84 between the tap
transducers. These delay pads, which are shown in
detail in Fig 7, are preferably made of the same
material as, and deposited with, the bus bars 74, 76
and the transducers 78, 80. Fach delay pad has a
width sufficient to delay the propagation of the sur-
face acoustic wave from one tap transducer 80 to the
next by one quarter cycle or 90 with respect to an
undelayed wave at the frequency of operation (circa
915 MHz). By providing locations for three delay
pads between successive tap transducers, the phase
of the surface acoustic wave received by a tap
transducer may be controlled to provide four phase
possibilities:
1. No pad between successive tap transducers =
- 9 0 ;
2. One pad between successive tap transducers
= 0;
3. Two pads between successive tap transducers
-~90;
4. Three pads between successive tap trans-
ducers = +180.
Referring to Fig. 2 the phase information ~0 (the
phase of the signal picked up by the first tap trans-
ducer in line), and 2 ... ON (the phases of the
signals picked up by the successive tap transducers)
is supplied to the combiner (summer) which in the
embodiment of Fig. 6 comprises the bus bars 74 and
76. This phase information, which is transmitted as
the signal S2 by the antenna 70, contains the
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informational code of the transponder.
While a system o the type described above operates
satisfactorily when the number of tap transducers
does not exceed eight, the signal to noise ratio in
the transponder reply signal is severely degraded as
the number of tap transducers increases. This is be-
cause the tap transducers additionally act as launch
transducers as well as partial reflectors of the
surface acoustic wave so that an increase in the
number of tap transducers results in a corresponding
increase in spurious signals in the transponder
replies. This limitation on the number of tap trans-
ducers places a limitation on the length of the
informational code imparted in the transponder
replies.
The present invention provides a means for reducing
spurious signals as well as insertion losses in a
passive transponder so that the informational code may
by increased in size to any desired length. Such
advantages are achieved by providing one or more
surface acoustic wave reflectors on the piezoelec-
tric substrate in the path of travel of the surface
acoustic wave to reflect the acoustic waves back
toward a transducer for reconversion into an electric
signal.
Fig. 8 illustrates the general concept of the inven-
tion whereby a transducer 86 is employed in conjunc-
tion with reflectors 88 and 90 in a unique configura-
tion which replaces the arrangement of Fig. 6 having
a launch transducer 78 and zap transducers 80. In
particular, the transducer 86 is constructed to con-
vert electrical energy received at the terminals 92
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and 94 into surface acoustic wave energy which propo-
gates outward in opposite directions indicated by the
arrows 96 and 98~ The launch transducer is con-
structed in a well known manner with an inter-digital
electrode assembly formed of individual electrode
fingers arranged between and connected to the two bus
bars 100 and 102. In the illustrated pattern, half
the fingers are connected to the bus bar 100 and the
other half are connected to the bus bar 102. Each
electrode is connected to one or the other bus bar
and extends toward a free end in the direction of
the other bus bar.
It will be appreciated that the size of the transducer
is expandable at will by merely adding electrode
fingers in the same pattern shown. The size of the
transducer is thus determined by the number of fingers
arranged in parallel.
Also in accordance with well known practice, the dis-
tance between successive fingers is equal to 3~/4
where is the center wavelength of the surface
acoustic wave. This distance 3~/4 is measured between
the centers of the individual electrodes. Further-
more, as may be seen, the length of the active regionbetween the ends of the electrodes connected to the
bus bar 100 and the ends of the electrodes connected
to the bus bar 102 is X where K is a proportion-
ality constant.
Surface acoustic waves which travel outward from the
transducer a6 in the directions 96 and 98 encounter
and are reflected back by the reflectors 88 and 90.
These reflectors comprise individual electrode fingers
which extend between the bus bars 104 and 106 on
31 2Z~
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opposite sides. As shown in Fig. 8, these electrodes
are spaced from center to center, a distance ~/2
apart.
The reflectors 88 and 90 serve to reflect nearly 100
of the surface acoustic wave energy back toward the
transducer 86; that is, in the directions 108 and 110,
respectively. Thus, after a pulse of surface acoustic
wave energy i5 generated by the transducer 86, it is
reflected back by the reflectors 88 and 90 and
reconverted into an electrical signal by the trans-
ducer 86.
The configuration of Fig. 8 may also include one or
more delay pads 112 which control the phase of the
surface acoustic wave received back by the transducer
86. For a 90~ phase delay (as compared to the phase
of the received surface acoustic wave without a delay
pad present) the delay pad should have a width equal
to 1/2 the width oE the delay pads in the transponder
configuration of Fig. 6 and 7 because the surface
acoustic wave will traverse the delay pads twice
(i.e., in both directions)O
Fig. 9 illustrates an entire transponder system
utilizing the concept shown in Fig. 8. In Fig. 9 a
plurality of transducers 114 are connected to common
bus bars 116 and 118 which, in tuxn, are connected
to the dipole antenna (not shown) of the transponder.
On opposite sides of this configuration are reflect-
oxs 120 and 122 which reflect surface acoustic waves
back toward the transducers which launched them.
Since the transducers 114 are connected in parallel,
an interrogation pulse at radio frequency is received
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by all the transducers simultaneously. Consequently,
these transducers simultaneously generate surface
acoustic waves which are transmitted outward in both
directions. Due to the particular configuration
shown, the reflected surface acoustic waves are
received at staggered intervals so that a single
interrogation pulse produces a series of reply pulses
after respective periods of delay. Fig. 9 illus-
trates the time order of the reflected signals as 1,
2, 3, ...18, 19 and 20.
Fig. 10 shows another embodiment of a passive trans-
ponder having transducers and reflectors according to
another preferred embodiment of the present invention.
In this case, four transducers 124 are connected
electrically in series between bus bars 126. These
transducers are interconnected by means of inter-
mediate electrodes 128, the electrical circuit through
each transducer being effected by capacitive coupling.
When energized by an RF electrical signal, the trans-
ducers simultaneously produce surface acoustic waves
which travel in four parallel paths 130.
To the right of the transducers 124 in Fig. 10 are
four sets 132, 134, 136 and 138 of reflectors 140
arranged in the paths of travel 130 of the surface
acoustic waves. In the example shown, three
reflectors 140 are arranged in each set; however, the
number ox reflectors may be varied. If only a single
reflector is provided in each of the positions 132,
134, 136 and 138, this reflector should be designed
to reflect nearly 100% of the surface acoustic waves
at the wavelength of these waves. If more than one
reflector is provided, these reflectors should be
designed to reflect only a portion of the acoustic
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wave energy.
In the embodiment shown in Fig. 10, for example,
where three reflectors are provided in each set, the
first and second reflectors should allow some of the
acoustic wave energy to pass beneath them to the
third and last reflector in line. In this way, if a
pulse of surface acoustic wave energy i5 generated
by a transducer 124, some of it will be reflected
by the first transducer, some by the second and some
by the third reflector in line.
Fig. 11 shows another preferred embodiment wherein
the transducers are arranged between common bus bars
140 and 142. These transducers 144 (designated "T" in
E'ig. 11) generate surface acoustic waves in opposite
directions as indicated by the arrows 146. These
acoustic waves are reflected by the reflectors 148
(designated with an "R" in Fig. 11~ and returned
toward the transducers in the direction indicated by
the arrows 150. As is illustrated in Fig. 11, the
distances between the transducers 144 and reflectors
148 are staggered so that a single interrogator pulse
results in a succession of reply pulses.
Fig. 12 shows another preferred embodiment of the in-
vention comprising a number of transducers 152 con-
nected electrically in series and a number of reflec-
tors 154 connected electrically in series. Both the
transducers and the reflectors are "tuned" to operate
at different surface acoustic wavelengths so that,
depending upon the particular frequency applied to
the terminal electrodes 156 and 160, a particular one
of the transducers will generate a surface acoustic
wave. This surface acoustic wave will travel toward
the right (in the sense of Fig. 12) and be reflected
back by the respective reflector 154 which is also
tuned to the same wavelength as its corresponding
transducer.
Fig. 13 illustrates the different frequency bands of
the interrogation signals required for the transponder
embodiment illustrated in Fig. 12. As is shown, there
are five frequency bands 162, one for each of the
five transducers 152 and corresponding reflectors
154.
In the embodiment of Fig. 12, the information code of
the transponder is imparted by providing a selected
number of delay pads 164 between the transducers 152
and reflectors 15~. These delay pads modify the
phase of the surface acoustic waves which propogate
toward the reflectors 154 and then return to the
transducers 152.
Fig. 14 illustrates still another embodiment of a
transponder according to the present invention which
comprises separate launch and receiving transducers.
As may be seen, surface acoustic waves are generated
by a launch transducer 166 and propogated in the
direction indicated by the arrow 168. These surface
acoustic waves pass beneath the receiving transducer
17Q and continue on toward one or more reflectors
172 in the direction indicated by the arrow 174. This
acoustic wave energy is reflected by the reflectors
172 and directed back toward the receiving transducer
170 in the direction indicated by the arrow 176.
In the embodiment shown in Fig 14, the launch and
receiving transducers may be connected to separate
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dipole antennas. This may be advantageous in certain
applications since the different antennas may receive
and radiate energy in different directions.
S There has thus been shown and described a novel sur-
face acoustic wave passive transponder, having acous-
tic reflectors, which fulfills all the objects and
advantages sought therefor. Many changes, modifica-
tions, variations and other uses and applications of
the subject invention will, however, become apparent
to those skilled in the art after considering this
specification and the accompanying drawings which
disclose preferred embodiments thereof. All such
changes modifications, variations and other uses and
applications which do not depart from the spirit and
scope of the invention are deemed to be covered by
the invention which is limited only by the claims
which follow.
