SURFACE ACOUSTIC WAVE PASSIVE TRANSPONDER HAVING AMPLITUDE AND PHASE-MODIFYING SURFACE PADS

REPETEUR PASSIF A ONDES ACOUSTIQUES DE SURFACE EQUIPE DE PASTILLES DE MODIFICATION D'AMPLITUDE ET DE PHASE

English Abstract

ABSTRACT
A passive transponder for use in an interrogation/trans-
ponder system comprises a substrate having a substrate
surface defining a path of travel for surface acoustic
waves; a launch transducer element arranged on the surface
for converting interrogating signals into surface acoustic
waves which propagate along the path of travel; a plurality
of tap transducer elements arranged on the surface at spaced
intervals along the path of travel for converting surface
acoustic waves into respective output signals; and a cir-
cuit, connected to the tap transducer elements, for combin-
ing the output signals of these transducer elements to form
reply signals. In order to control the delay time from
transducer element to transducer element, one or more "delay
pads" are provided on the substrate surface between these
transducer elements.

Claims

-14-
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED US FOLLOWS:
1. 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 substrate surface defining
a plurality of paths of travel for surface acoustic waves,
each path of travel having a different length from its
beginning to its end; and
(b) electric circuit means, disposed on said
substrate, for propagating surface acoustic waves along
said paths of travel, from said beginning of each path to
said end thereof;
the improvement comprising a plurality of selectively
removable surface acoustic wave delay pads of prescribed
width disposed on the surface of said substrate at
prescribed locations along a plurality of said paths of
travel to increase the surface acoustic wave propagation
time by prescribed increments from said beginning of each
of said plurality of paths to said end thereof, the number
of said pads in each travel path determining said increase
in acoustic wave propagation time along that path,
whereby the number of said pads in each travel path
may be selected to impart said coded information to said
reply signal.

-15-
2. The improvement defined in claim 1, wherein from zero
to three pads are provided along said at least one travel
path to provide one of four possible codes for that travel
path.
3. The improvement defined in claim 1, wherein said pads
are identical in size.
4. The improvement defined in claim 2, wherein said pad
is dimensioned to provide a propagation time phase delay
of 90° in the surface acoustic wave during propagation
of such wave from said beginning of its respective travel
path to said end thereof.
5. The improvement defined in claim 1, wherein said pad
is formed of a metal layer on said surface of said
substrate.
6. The improvement defined in claim 5, wherein said
metal is aluminum.
7. The improvement defined in claim 1, wherein the width
(L) of said pad in the direction of travel of said
acoustic wave and the thickness (t) of said pad are
selected in accordance with the following formula:
<IMG>

-15A-
wherein .DELTA. .PHI. is the phase delay provided by the pad, .lambda. is
the center wave length of the acoustic wave, and Ks and k
are constants.
8. The improvement defined in claim 7, wherein the phase
delay .DELTA. .PHI. is 90°.
9. The improvement defined in claim 1, wherein at least
one edge of said pad is serrated on a side thereof
perpendicular to said path of travel, thereby to control
attenuation of said acoustic wave.
10. The improvement defined in claim 9, wherein said
serrated edge forms first and second edge portions which
are perpendicular to said path of travel but displaced
with respect to each other in the direction of said path
of travel by a distance D such that the difference in
delay provided by said pad at said first and second edge
portions, respectively, is n times 180°, where n is an odd
integer.

Description

39~;2
SURFACE ACOUSTIC WAVE PASSIVE TRANSPONDER HAVING
AMPLITUDE AND PHASE-MODIFYING SURFACE PADS
The present invention relates to a "passive interrogator
label system" (PILS); that is a system comprising an
interrogator for transmitting an interrogation signal, one
or more "labels" or passive transponders which produce a
reply signal containing coded information in response to the
interrogation signal, and a receiver and decoding system for
receiving the reply signal and decoding the information
contained therein.
A passive interrogator label system of the type to which the
present invention relates is disclosed in the US Patent
No. 3,273,146 to Horwitz, Jr.; U.S. Patent No. 3,706,094 to
Cole and Vaughan; U.S. Patent No. 3,755,803 to Cole and
Vaughan; and U.S. 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 transmit-
ting 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 acous-
tic wave energy in the piezoelectric material. Upon receipt
of a pulse, 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 reconversion
into electrical energy by the launch transducer. The
presence or absence of tap transducers at the prescribed
:.

;~.2~39~
--2--
locations along the acoustic wave path determines whether a reply
pulse will be transmitted with a particular time delay, in response
to an interrogation pulse. This determines the informational 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 transponder
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 information code
associated with and which identifies the passive transponder is
built into the transponder at the time that the tap transducers
are deposited onto the substrate of piezoelectric material. As
mentioned above, the presence or absence of tap transducers at
prescribed locations along the acoustic wave paths determines
whether a reply pulse will be transmitted with a particular time
delay in response to an interrogation pulse. With this type of
encoding, the number of possible codes is 2N where N is the number
of tap transducers. For a large number of codes, it is necessary
to provide a large number of tap transducers; however, increasing
the number of tap transducers reduces the efficiency of energy
conversion and introduces spurious signals into the reply signal.
In accordance with a broad aspect of the invention,
there is provided, in a passive transponder adapted for use in an
I.

3.~Z8~
--3 _
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 substrate surface defining a
plurality of paths of travel for surface acoustic waves, each
path of travel having a different length from its beginning to its
end; and
(b) electric circuit means, disposed on said substrate, for
propagating surface acoustic waves along said paths of travel,
from said beginning of each path to said end thereof;
the improvement comprising a plurality of selectively re-
movable surface acoustic wave delay pads of prescribed width
disposed on the surface of said substrate at prescribed locations
along a plurality of said paths of travel to increase the surface
acoustic wave propagation time by prescribed increments from said
beginning of each of sald plurality of paths to said end thereof,
the number of said pads in each travel path determining said
increase in acoustic wave propagation time along that path,
whereby the number of said pads in each travel path may be
selected to impart said coded information to said reply signal.
Advantageously, some delay pads are constructed in such
a way as to also control the attenuation of the acoustic waves
that pass beneath them. To effect this attenuation, at least
one edge of such pads is serrated on a side thereof perpendicular
I,:

~.~Z~3912
-3a-
to the path of travel of the acoustic wave. The width of the
serration in the direction of the path of travel is substantially
equal to n~/2 where n is an odd integer and is the center wave-
length of the acoustic waves Such a serration thus causes
portions of the wave which pass beneath the serrated edge to be
180 out of phase with respect to each other.
The delay pads according to the present invention are
utilized in a passive interrogator label system. In this system,
the interrogator transmits a first, interrogation
, I: "I

~.~28~
signal having a first frequency which successively assumes a
plurality of frequency values within a prescribed frequency
range. This first frequency may, or example, be in the
range of 905-925 MHz, a frequency band which is freely
available in many parts of the world for short-range trans-
mission.
The remote, passive transponder associated with this
interrogator receives the first signal as an input and
produces a second, reply signal as an output. Signal
transforming means within the transponder convert the first
signal in such a way as to impart a known informational code
in the second signal which is associated with and identifies
the particular passive transponder.
Associated with the interrogator of the system is a receiver
for receiving the second signal from the passive transponder
and a mixer, arranged to receive both the first signal and
the second signal, or signals derived therefrom, for mixing
together these two signals thereby to produce a further
signal. This further signal may, for example, contain the
sum and the difference frequencies of the first and second
signals, respectively.
Finally, the system includes a signal processor responsive
to the signal produced by the mixer for detecting the
frequencies contained in this signal thereby to determine
the informational code associated with the passive transpon-
der.
For a full understanding of the present invention, reference
should now be made to the following detailed description of
the preferred embodiments of the invention and to the
accompanying drawings.
Fig. 1 is a block diagram of a system for transmitting an
interrogation signal, receiviny a reply signal and decoding
information encoded in the reply signal.

~2;~8912
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 output 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 particular
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. 6.
Fix. 8 is a plan view, in greatly enlarged scale, of acous-
tic wave delay pads having serrated edges for controlling
the acoustic wave attenuation.
The present invention will now be described with reference
to Figs. 1-8 of the drawings. Identical elements in the
various figures are designated by the same reference number-
als.
Figs. 1-7 illustrate an interrogator-transponder system
employing a surface acoustic wave transponder which may form
the environment of the present invention. The transmit-
ter/receiver and decoder system shown in Fig. 1 comprises a
ramp generator 20 which supplies a sawtooth waveform to a
voltage controlled oscillator tVCO) 22. The VCO produces an
output signal of frequency f which repeatedly ramps linearly
upward from a frequency of 905 MHz to a frequency of 925
MHz. This signal is amplified by the RF amplifiers 24 and
supplied to a transmit/receive switch 26. The switch 26
directs the signal either to a transmitter power amplifier

~;~Z891Z
28 or to a decoding mixer 30. The switch 26 is controlled
by a 100 KHz square wave signal produced by a clock 32. The
output signal S1 from the amplifier 26 is supplied to an
external circulator or transmit/receive (TR) switch 34 and
is transmitted as electromagnetic radiation by an antenna
36.
A block diagram of the transponder associated with the
system of Fig. 1 is shown in Fig. 2. The transponder
receives the signal S1 at an antenna 38 and passes it to a
series of delay elements 40 having the indicated delay
periods To and QT. After passing each successive delay, a
portion of the signal Ion i5 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 Fig. 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 intermittent-
ly presented by the switch 26.
The output S5 of the mixer 30 contains the sum and the
difference frequencies of the signals S3 and So. This
output is supplied to a band pass filter 46 with a pass band
between 1 and 3 XHz. The output of this filter is passed
through an anti-aliasing filter 4~ 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 to a processor 54
that analyzes the frequencies contained in the signal by
means of a Fourier transform. The sample-and-hold device 50
and the A converter 52 are strobed by a sampling signal
which serves to compensate for the non-linearity, with

~'~Z~912
respect to time, in the monotonically increasing frequency f
of the VCO output signal.
To effect compensation the signal of frequency f produced by
the VCO 22 is passed via an isolating amplifier 56 to a
delay element 58 with a constant signal delay T8. Both the
delayed and the undelayed signals are supplied to a mixer 60
which produces a signal S7 containing both sum and differ-
ence 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
Nero crossing. These pulses are used to strobe the sam-
ple~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 produced by the VCO
22. Fig. 5 shows, in solid lines 66, the frequency of the
transmitted signal S1 and, in dashed lines 68, the frequency
2b of the signal S2 as received from the transponder. As may
be seen, the signal 68 is received during the interval
between transmissions of the signal 66. These intervals are
chosen to equal, approximately, the round trip delay time
between the transmission of a signal to the transponder and
the receipt of the transponder reply. As indicated by the
multiple dashed lines, the transponder reply will contain a
number of frequencies at any given instant of time as a
result of the combined (i.e., summed) intermediate signals
having different delay times (Tol To + IT, To + 2~T, ...
To + NUT).
Figs. 6 and 7 illustrate an embodiment of a passive trans-
ponder which implements the block diagram of Fig. 2. This
transponder operates to convert the received signal Sl an
acoustic wave and then to reconvert the acoustic energy back
into an electrical signal S2 for transmission via a dipole

-I ~2~ Z
antenna 70. More particularly, the signal transforming
element of the transponder includes a substrate 72 of
piezoelectric 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" transducers 80. The bars 74 and 76 thus define a
path of travel 82 for an acoustic wave which is generated by
the launch transducer and propagates substantially linearly,
reaching the tap transducers each in turn. The tap trans-
ducers convert the 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 acoustic wave path 82, as shown in Fig.
6, and an informational code associated with the transponder
is imparted by providing a 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. Each delay pad has a width sufficient
to delay the propagation of the 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
acoustic wave received by a tap transducer may be controlled
to provide four phase possibilities:
1. No pad between successive tap transducers = -90,
2. One pad between successive tap transducers = 0;
3. Two pads between successive tap transducers = +90;
4. Three pads between successive tap transducers = +180.

~.~Z~
Referring to Fig. 2 the phase information JO the phase of
the signal picked up by the first tap transducer 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 transmit-
ted as the signal S2 by the antenna 70, contains the infor-
mational code of the transponder.
As shown in Flg. 7, the three delay pads 84 between two tap
transducers 80 are each of such a width (L) as to provide a
phase delay of 90 in the propagation of an acoustic wave
from one tap transducer to the next as compared to the phase
in the absence of such a delay pad. This width (L) is
dependent upon the material of both the substrate and the
delay pad itself as well as upon the thickness of the delay
pad and the wavelength of the surface acoustic wave. As
noted above, the substrate material is preferably lithium
niobate (LiNbO3) and, the delay pad material is preferably
aluminum.
In the equations below, VO equals the propagation velocity
of an acoustic wave on a "free surface" without a delay pad
(VO = 348~ meters/second for a lithium niobate substrate);
Vs = the propagation velocity of an acoustic wave on a
surface which is shorted with an infinitely thin delay pad;
= the nominal phase delay in the transmission of an
acoustic wave from one tap transducer to the following tap
transducer when there is no delay pad; and Q~ = the addi-
tional phase delay imparted by one delay pad. Let us now
define:
V0 Vs 1 2
= - K ,
V0 2
where X it a "coupling constant" for a metalized
(aluminum) piezoelectric (lithium niobate) surface.

~.~289~
--10--
Since,
V0 vs kt
- = + - , and
VO
- 2~L,
where kt/~ is an approximation term due to mass loading by
the pad;
k is a proportionality constant dependent on the
substrate and pad materials; and
t is the thickness of the pad,
therefore we have,
1 kt
= - K + - , and
2~L 2
L
2~[~ K -I (kt/~)].
The preferred thickness of the delay pad film is approxi-
mately 0.1 micrometers. The manufacture of the transponder
is facilitated if three delay pads are initially deposited
between all the tap transducers and, thereafter, delay pads
are selectively removed to impart the code to the transpon-
der.
With pads providing a 90 delay, there are four code pos-
sibilities for each set of three delay pads. Consequently,
for the transponder illustrated in Fig. 6 having seven sets
of delay pads, 4 code possibilities are provided.

:3 ZZ~39~Z
Fig. 8 illustrates delay pads 86 which make it possible to
control the amplitude as well as the phase of the acoustic
wave. Such amplitude modification may be detected by the
receiver and decoder system so that additional codes may be
imparted in the transponder without requiring additional tap
transducers and delay pads.
In this case, the amplitude modification of the surface
acoustic wave takes the form of a prescribed attenuation.
This attenuation is effected by wave cancellation at the
edge of the delay pad.
As is shown in Fig. 8, the serrated edge of the delay pads
86 have a first edge portion 88 and a second edge portion 90
which are perpendicular to the path of travel 92 of the
surface acoustic wave but are displaced with respect to each
other in the direction of the path of travel by a distance
D. The first edge portion 88 has a total length a whereas
the second edge portion has a total length b. It will be
understood that, whereas the serrated edge is shown to have
only two segments in Fig. 8, the edge may be divided into
several segments for each edge portion. It is necessary
only that the segments of the first edge portion all have a
delay pad width L1, whereas the segments of the second edge
portion have a delay pad width L2.
Thus, the distance a is the sum of all the first edge
portion segments, whereas the distance b is the sum of all
the second edge portion segments.
The maximum wave cancellation at the serrated edge is
provided when the distance D is selected such that the
difference in delay provided by the pad at the first and the
second edge portion, respectively, is n 180, where n is an
odd integer. MorP specifically, D is preferably selected
such that 2 = no radians, where l is the additional
delay provided by the delay pad over the distance Ll, and ~2

~l2Z89~
-12-
is the additional delay provided by the delay pad over the
distance L2.
From the formula derived above for the width L of a delay
pad, we have:
Q~l~
L1
2~X
where, X = Ks + -
~2
L2
2~X
D= L1 - L2 = l 2 [2~X]
[ ]
2~X
no
D
Ks + 2kt/~
The amount of attenuation provided by the serrated edge may
be controlled either by varying the distance D to provide
more or less than the optimum delay as defined by the
formula above, or by varying the relative total lengths a
and b of the first and second edge portions. If we let W be
the attenuation weighting factor, then
W = (a - b)/(a + b), under optimum delay conditions.

~2Z89~;~
-13-
As may be seen from this formula, the maximum attenuation
occurs when a equals b (W=O). The minimum attenuation
occurs when either a or b equals zero (W=1).
It is recommended that the amplitude modifying delay pads 88
be formed in mirror image before and after a tap transducer
94 as illustrated in Fig. 8. This arrangement compensates
for irregularities in the acoustic wave front caused bv
these delay pads.
There has thus been shown and described a novel surface
acoustic wave passive transponder, having amplitude and
phase-modifying delay pads, which fulfills all the objects
and advantages sought therefor. Man~v changes, modifica-
tions, variatior-s 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 embodi-
ments thereof. All such changes, modifications, variations
and other uses and applications which do not depart from the
spirit and scope of the inventiGn are deemed to be covered
by the invention which is limited only by the claims which
follow.