Identification system using coded passive transponders

An identification system including a transmitter, a receiver, a decoding subsystem, and a passive transponder identifier. One passive transponder identifier of the invention is a surface acoustic wave device provided with pad means for applying and removing pressure on a substrate of the device at preselected locations. A second identifier of the invention is a microacoustic shear bulk wave device. The passive transponder identifiers are programmed to produce a characteristic coded electronic reply in response to an electromagnetic signal interrogation.

BACKGROUND OF THE INVENTION 
Passive identifiers capable of producing coded replies are known in the 
prior art. For example, in Cole et al. U.S. Pat. No. 3,706,094 issued Dec. 
12, 1972 there is shown an electronic surveillance system including a 
label in the form of a surface acoustic wave device. Labels are coded by 
maufacturing cards with a full array of elements and by later severing the 
connections of an appropriate number of array elements from the main 
transmission line. Alternatively, labels manufactured in accordance with 
the Cole et al. patent may be coded during manufacture by omitting one or 
more array elements or by not connecting them to the transmission line. 
Surveillance systems manufactured in accordance with the Cole et al. patent 
suffer from the disadvantage that severance of the connections of array 
elements from a transmission line is difficult to perform manually because 
of the small size of such elements. Labels coded during manufacture by 
omission or disconnection of elements lack the flexibility of labels coded 
by an ultimate use because severance of elements renders these labels 
incapable of replying with certain codes utilizing the severed elements. 
The Cole et al. labels suffer from the additional disadvantage that they 
are based upon surface wave devices, in which losses are extremely high 
compared with shear bulk wave devices. 
It is a principal object of the present invention to provide an 
identification system having an identifier that may be coded manually by 
the ultimate user by imposing pressure upon a pad means intermediate an 
input transducer and an output transducer to interrupt communication 
between the transducers, and by removing pressure from the pad means to 
restore such communication. Because output transducers may be reconnected 
after they have been disconnected, the identification system of the 
invention is more flexible in varying codes than prior art systems in 
which output transducers are permanently disconnected from a delay line. 
Another object of the invention is to produce an identification system 
having an identifier which is a microacoustic shear bulk wave device, 
thereby minimizing losses occurring during processing and increasing the 
maximum number of useful output transducers so that a larger number of 
different coded electronic replies may be produced in response to an 
electromagnetic signal. 
Additional objects and advantages of the present invention will become 
apparent to persons skilled in the art from the following specification, 
taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE INVENTION 
The present invention is practiced by transmitting a radio frequency pulse 
through an antenna to a coded passive transponder identifier. The radio 
pulse is then processed acoustically in the identifier, and a coded reply 
signal is retransmitted by the identifier. The reply signal is received in 
a decoding subsystem, processed by conversion to a string of digital 
pulses, and compared with stored information to determine whether a bearer 
of the identifier should be allowed access or other action taken. 
A schematic block diagram of an identification system of the invention is 
shown in FIG. 1. The pulse generator, radio frequency generator, and mixer 
are components of a transmitter. The radio frequency generator includes a 
standard radio frequency (e.g. 200 megahertz) oscillator and amplifier 
connected to an antenna through a mixer. The mixer allows the pulse 
generator to modulate the radio frequency generator so that bursts of 
radio waves are transmitted through the antenna. The duration and 
periodicity of such bursts are controlled by the pulse generator. For 
example, the electromagnetic signal produced at the transmitting antenna 
may be a burst one microsecond in duration at a carrier frequency of 
several hundred megahertz with a repetition rate in the kilohertz region. 
The transmitted electromagnetic signal is detected nearly instantaneously 
at the coded identifier which is held in proximity to the antenna. This 
identifier is a passive transponder which is coded to give a 
characteristic reply signal. The transponder has a small antenna of its 
own. The electromagnetic signal detected by the transponder antenna is 
converted into an acoustic signal by an input transducer. Several output 
transducers receive this acoustic signal during the delay period and the 
delayed acoustic signal is reconverted into an electromagnetic pulsed 
radio frequency signal at time intervals determined by location of the 
transducers on the transponder. This signal is then retransmitted by the 
transponder antenna and detected by a receiving antenna. The receiving 
antenna may be separate from the transmitting antenna or the same antenna 
may be used for both transmitting and receiving, as shown in FIG. 1. This 
system of a common antenna for both transmitted and received 
electromagnetic signals is well known in the field of radar. 
The received signal consists of a train of radio frequency pulses, their 
number being dependent upon the number of output transducers in the device 
and their time interval determined by the location of such output 
transducers. For example, for every pulse originally received by the 
transponder at a rate of one kilohertz (i.e. once every millisecond), 
there will be detected at the receiver a train of pulses spaced about a 
microsecond apart with their precise number varying from about 10 to 100 
depending upon size of the device and the total number of output 
transducers. 
The signal arriving at the receiving antenna consists of two parts. The 
antenna will receive the unmodified, high-energy transmitted radio 
frequency burst. In addition, the signal modified and retransmitted by the 
transponder will also be received. An electronic switch is provided to 
attenuate the transmitted high-energy signal in synchronism with the pulse 
generator so that the high-energy signal does not saturate the receiver. 
The output of the electronic switch is a radio frequency pulse train. It 
has a periodicity equal to the frequency of the pulse generator and 
consists of a radio frequency pulse coincident with the transmitted burst, 
followed by a number of pulses whose number is dependent upon the number 
of output transducers in the coded transponder. The output of the 
electronic switch is illustrated schematically in FIG. 2. 
For decoding purposes it is necessary to know the frequency and maximum 
number of pulses in the pulse train. Assuming for purposes of illustration 
that the maximum number of pulses is fixed at 10 and that the pulses occur 
at evenly spaced intervals (e.g. 2 microseconds), there exist 2.sup.10 = 
1024 unique binary code patterns which may be designed into the 
transponder. Therefore, 1024 different transponders can be fabricated, 
each having a different code pattern. 
The receiver is a standard radio frequency receiver which detects pulses 
and converts them into logic level computable signals, preserving the 
pattern and spacing of the pulses. Output of the receiver for the binary 
code pattern 1011010111 is illustrated schematically in FIG. 3. 
Once received, the pulse train is amplified and detected to provide a 
repeated train of a video pulse pattern. This pulse pattern is matched in 
the decoder with a number of stored pulse patterns for appropriate 
identification and recording. 
The decoder monitors received pulse patterns, identifies them, optionally 
records each pattern, and answers queries. For example, in a limited 
access system the bearer of a transponder may or may not perform some 
other function according to the code pattern generated by his transponder. 
The decoder will answer with a yes or no regarding the bearer's ability to 
access an area or another system. The decoder includes two components: a 
synchronous pattern receiver and an analyzer. 
The synchronous pattern receiver monitors the receiver output and converts 
the time serial pattern received into a parallel pattern using a 
serial-to-parallel converter. 
The signals illustrate schematically in FIG. 4 are used in the synchronous 
pattern receiver. Processing of these signals is explained with reference 
to FIG. 5. The pulse pattern (PP) will be at logic 0 prior to arrival of 
the synchronizing pulse as will be the sampling signal (SPL). A sampling 
clock (CS) runs in synchronism with PP at 0.5 megahertz. The time at which 
CS is logic is centered at the middle of the received pulses and of 
shorter duration (e.g. .+-. 250 nanoseconds). When the synchronizing pulse 
is received, SPL (the output of the J-K master/slave flip/flop) is set to 
logic 1. This enables the decimal counter to begin counting and 
simultaneously, through the feedback of SPL into the AND gate at the J 
input, allows SPL to be reset 10 clock periods later using the carry out 
of the decimal counter. 
The signal SPL can be used to sample PP and store the received pulse 
pattern, without the synchronizing pulse, in the serial-to-parallel 
converter. In addition, SPL can be used to indicate that a valid pattern 
(i.e. not being sampled) is present in the serial-to-parallel converter. 
This received pattern P=P.sub.9 P.sub.8 . . . P.sub.1 P.sub.0, can be 
displayed, recorded, (on computer or human readable output) or transmitted 
directly to a computer for analysis. 
An analyzer is provided to give a yes or no (logic 1 or 0) for each pattern 
received, as shown in FIG. 6. (Note: we assume throughout that P=000 . . . 
00 is an invalid pattern since it will be produced in the absence of a 
transponder). While any type of combinational logic could be designed to 
produce this answer, the simplest is the most universal, namely a random 
access memory. Assume the memory has 2.sup.10 one-bit words. In each word 
is stored a logic one or zero. The pattern P can be used as an address to 
produce the value of the word thus producing a logic 1 or 0 for each of 
the possible patterns, as shown in FIG. 6. 
This logic 1 or 0 can be used to allow or deny access to the card bearer or 
to permit or not permit him to perform any functions. 
With this formulated technique in mind several other options become 
available. For example, if 2 bit words are used, 2.sup.2 = 4 categories of 
answers are available. These could be assigned to be permission granted, 
permission not granted, invalid or stolen card, card not issued. The last 
two categories would be useful for retrieving stolen or lost cards. 
Generally speaking, most analyzers would use read-only memory (ROM) for 
non-votability in the absence of power. If this is the case, the nt 
pattern for each category would be relatively fixed. However, 
field-programmable read-only memories (PROM) could be used giving the 
owner of the analyzer the ability to modify the category to which any card 
belongs in the event of stolen or lost cards or change of access 
privileges. 
Finally all of the above can be accomplished through the use of 
programmable logic arrays (PLA) or field programmable logic arrays (FPLA) 
in which case fewer than 2.sup.10 words need be used but with the same 
number of bits per word. Of course, any pattern not programmed would give 
a no answer to the query. 
A preferred embodiment of a surface acoustic wave identifier of the 
invention is described with reference to FIG. 7. The identifier 20 
includes a surface acoustic wave delay line 21 imprinted on a 
piezoelectric substrate 22 approximately the same size as a credit card. A 
radio signal emitted by a transmitter (not shown) is received by a 
receiving antenna or first antenna means 23 and applied to an input 
transducer 24. The delay line also includes five linearly arranged output 
transducers 25a, 25b. 25c, 25d, 25e connected to a retransmitting antenna 
or second antenna means 30. Optionally and preferably, the receiving 
antenna 23 and retransmitting antenna 30 may be combined into a single 
antenna, as shown in FIG. 8. 
In the preferred embodiment illustrated the transducers 24, 25 are metal 
interdigital electrodes etched upon the surface of a quartz substrate 22. 
The input transducer 24 converts a received radio frequency signal into a 
surface acoustic wave of the same frequency but of greatly reduced 
(100,000 times) wavelength as compared with electromagnetic waves. For a 
basic clock frequency of 70 megahertz, electrodes within each transducer 
are spaced apart a distance of 0.0238 mm. This is one-half the wave length 
of a 70 megahertz wave on the quartz surface. 
Surface acoustic waves generated by the input transducer 24 propagate along 
the delay line 21. In the preferred embodiment shown in FIG. 7 the delay 
line 21 is provided with five rubber pads or pad means 35a, 35b, 35c. 35d, 
35e, corresponding to each of the five output transducers 25a, 25b, 25c, 
25d, 25e. These rubber pads 35 are used to apply pressure in zones between 
the individual output transducers 25, thereby disrupting transmission of 
surface acoustic waves distal to the zone in which pressure has been 
applied. In the preferred embodiment illustrated in FIG. 7 pressure has 
been applied to the middle pad 35c, thereby preventing transmission of a 
surface acoustic wave to the three output transducers 25c, 25d, 25e, 
located distally of the middle pad 35c. This results in emission of the 
binary reply code pattern 11000 by the delay line 21 and retransmission 
antenna 30. 
The reply code pattern thus retransmitted is picked up by a remote receiver 
(not shown) for further processing, as described above by reference to 
FIG. 1. When pressure is removed from any given pad, communication is 
restored between the input transducer 24 and output transducers 25 located 
distally of that pad 35. It is readily apparent that the pad means 35 
constitute programming means by which the reply code pattern of the delay 
line 21 may be modified manually without resort to any electrical 
connections. 
Even though the input signal and the output reply may utilize separate 
antennas, it is likely that interaction will occur between the received 
and transmitted signals when the input and output transducers are arranged 
as shown in FIG. 7. Hence the output transducers 25 will inevitably 
generate surface acoustic waves propagating in both directions along the 
substrate 22. Such interaction materially restricts the number of 
available codes. 
An arrangement by which this objectionable interaction may be eliminated is 
shown in FIG. 8. As depicted schematically, an input transducer 24 is 
provided with a very large aperture and the output transducers 25 are each 
several times smaller. Because surface acoustic waves are generated 
perpendicular to the parallel electrodes along the delay line 21, and 
because the output transducers are positioned in a staggered array to 
avoid interaction with surface acoustic waves generated by other output 
transducers, there is no interaction between the output transducers 25. 
This avoids reduction in the number of available code patterns. The output 
coded reply pattern is retransmitted through the antenna 23. 
A second embodiment of an identifier of the invention is illustrated in 
FIG. 9. This identifier is a microacoustic shear bulk wave device 40 
including an ST quartz substrate 41 having a principal surface 42 and an 
opposed, parallel second surface 43. Both surfaces 42, 43 are smooth, 
polished surfaces in the preferred embodiment shown. 
An input interdigital transducer 44 is supported on the principal surface 
42, and output transducers 45a, 45b, 45c, 45d spaced at intervals along 
the principal and second surfaces, as illustrated. This device is coded by 
omitting or disconnecting output transducers, or by substituting rubber 
pressure pads (not shown) for one or more of the output transducers 45. 
In this bulk device 40 the input transducer 44 includes interdigital 
aluminum electrode pairs 50 etched upon the principal surface 42, as shown 
in FIG. 10. A 6.4 micron thickness layer 51 of RF sputtered ZnO lies over 
the transducer electrodes 50. Other piezoelectric substances such as 
aluminum nitrate may be substituted for the ZnO layer. 
A thin layer 52 of conducting aluminum is deposited over the ZnO layer 51. 
This film 52 must be sufficiently thin to avoid excessive mechanical 
loading of the ZnO layer. In the preferred embodiment illustrated this 
aluminum film 52 has a thickness of about 0.1 micron. 
A second type of microacoustic shear bulk wave identifier is illustrated in 
FIG. 11. Here a thin conductive aluminum plane 60 is interposed between 
the principal surface 42 and ZnO layer 51. Interdigital aluminum 
electrodes 50 are deposited over the ZnO layer 51. 
At boundaries represented by polished surfaces 42, 43 of the substrate 41, 
the shear bulk wave is reflected and continues along a bouncing path 
indicated in FIG. 9. The distance between the input transducer 44 and the 
most distal output transducer 45d in FIG. 9 is 5.08 mm. The output 
transducers 45a, 45b, 45c, 45d are identical in structure to the input 
transducer 44. 
The shear bulk wave is propagated at an angle .theta. to the principal 
surface 42. This angle .theta. varies, depending upon thickness of the ZnO 
layer and periodicity of the input transducer 44. In the preferred 
embodiment described herein and illustrated in FIGS. 9 and 10, 
.theta.=58.3.degree.. 
It should be noted that the function of the ST quartz in the substrate 41 
is to provide a substrate with small dispersion or beam spreading over a 
wide frequency range. The fact that ST quartz is crystalline and 
piezoelectric is only incidental and plays no role in transduction. A 
similar device has been constructed with a substrate of fused quartz, 
which is neither crystalline nor piezoelectric. It is believed that most 
homogeneous substances, including glass, will perform as an effective 
substrate in a microacoustic shear bulk wave device. 
While the foregoing invention has been described by reference to several 
preferred embodiments, it will be understood that numerous changes and 
modifications therein may be made without departing from the spirit and 
scope of the invention.