Abstract:
A surface wave device (SWD) programmable to produce any one of a plurality of phase coded SWD digital delay line configurations from a single transducer pattern characterized by parallel transducer electrode arrays to provide for each of two opposite phase alternatives of acoustic surface waves being propagated. Each transducer comprises pairs of interdigital taps, each tap characterizing a single binary bit. Transducer phase coding is accomplished by severing in selected taps all metalized fingers of like polarity from their common conductor bar. Critical configuration tolerances prohibitive to programmed operations are thus obviated.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a programmable surface wave device (SWD) to form any one of a plurality of phase coded (SWDs) surface wave devices from a single production unit. 
     PRIOR ART 
     Early acoustic devices, called volume-wave devices, made use of either longitudinal or shear waves that passed through the interior of a solid material and were accessible only at the ends of the solid. Such devices had a low wave velocity resulting in compact embodiments, and a low propagation loss making possible high Q acoustic filters. The later development of acoustic surface wave devices made available the additional advantages provided by Rayleigh waves, which are accessible along the surface of a solid rather than only at the ends. 
     An acoustic surface wave device (SWD) is a passive electro-acoustic device comprising metalized interdigital transducer elements formed on the surface of a piezoelectric substrate, typically either quartz or lithium niobate. Electrical energy is converted into acoustic energy and reconverted into electrical energy by the interaction of the transducer elements with the substrate properties, with most of the acoustic energy being confined to a region within one wavelength of the surface of the substrate. 
     The acoustic or Rayleigh wave thus generated propagates along the surface of the substrate with a velocity five orders or magnitude smaller than the velocity of electromagnetic waves in free space, thus making available the use of an SWD in digital delay line applications. Because the acoustic surface waves are easily accessible, signals with different delay times may be sampled at intermediate points along their propagation path. Thus, a phase-coded signal may be tapped by various sampling elements of a transducer which are configured to be synchronously additive only when a specifically coded waveform is presented to the transducer. So configured, the transducer is said to constitute a matched filter. Such configurations have found significant use in signal processing wherein a phase coded input signal is applied to an input transducer and, in response, an identification signal is generated by the output transducer. 
     A further advantage of surface wave devices is the simplicity of the fabrication process for interdigital transducers. Basically, it involves optically polishing a single surface of a piezoelectric substrate, depositing a metal film upon the polished surface, and then forming transducers using standard photolithography techniques such as have been highly developed for the semiconductor industry. The process is inexpensive, highly repeatable, and can be used to produce filters for the VHF and UHF ranges, where other filter technologies have very limited capabilities. 
     Where production facilities are directed to the manufacture of a wide variety of transducer patterns. however, economy may be improved by producing a single basic pattern that may readily be altered or programmed after basic manufacture rather than manufacturing a multiplicity of different patterns. 
     Interdigital transducer patterns embodied in phase coded SWDs may comprise numerous spaced taps, each consisting of a few fingers configured to produce phase reversed output signals to effect coding. Prior methods of coding the transducers entailed use of individual photo masks for each code cutting away portions of each finger of each tap of a transducer. The metalized fingers are positioned very close together. Alterations thereof with a scribe, an electron beam or other conventional means necessitated critical tolerance levels prohibitive to an automated system. 
     The present invention is directed to a method and system compatible with programmable automatic alteration systems by which a transducer electrode mask effectively is altered tap wise rather than finger by finger. The critical tolerance required in cutting each finger of each transducer thus is obviated. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a programmable processor where surface waves are generated in a piezoelectric substrate which is provided with means thereon to produce surface waves of like character in the substrate in two paths. The surface waves in one path are characterized by a phase opposite the waves in the second path. A first output transducer array on the substrate samples the waves in the first path at spaced time points along the first path. The output transducer array on the substrate simultaneously samples the waves in the second path at time points in the second path corresponding to the time points along the first path. Means are provided to combine the signals from the output transducers. In encoded form, elements of the two output arrays are selectively disabled to provided an output from elements at like points from only one of the two output transducers. More particularly, output transducers are formed from spaced interdigitated pairs of bit taps, each comprising metalized fingers extending transversely from two parallel conducting bars. Selected taps of the transducer, one set from each pair, may then be disabled by severing metalized fingers of like polarity from their common conductor bar. When the transducer is energized, reversals of polarity in the output signal are present in coding, depending upon the pattern of the disabled sets. 
     In one aspect of the invention, electrode arrays are provided having two separate transducer bit patterns for each phase alternative of a phase coded SWD, one of which is disabled. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a functional block diagram of a label system embodying the present invention; 
     FIG. 2 is a detailed perspective view of the phase-coded SWD of FIG. 1; and 
     FIGS. 3a-3c illustrate an impulse response diagram of the SWD of FIGS. 1 and 2. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGURE 1 
     Referring to FIG. 1, a polyphase phase-shift-keyed (PSK) surface wave device (SWD) 10 is shown interposed between a receiving antenna 11 and a transmitter antenna 14. This system forms a coded transponder. More particularly, receiving antenna 11 is connected by way of a channel 12 to the input of PSK SWD 10, the output of which is connected by way of a channel 13 to transmitter antenna 14. 
     An incoming signal by way of example may be a linear FM or &#34;chirp&#34; signal. It may comprise 2 or 3 sine wave cycles of high frequency. The incoming signal is received by antenna 11 and applied along channel 12 to the SWD 10. In response thereto, a phase coded signal is generated by SWD 10 and applied to transmitter antenna 14 by way of channel 13. Thus, upon receiving a signal, SWD 10 generates a coded signal which is transmitted by antenna 14. The coded signal may uniquely identify an article, vehicle or location with which SWD 10 is associated. 
     FIGURE 2 
     A more detailed illustration of an embodiment of SWD 10 is shown in FIG. 2. Each phase alternative is characterized by one of two parallel transducer configurations. More particularly, an input transducer 15 acts in combination with a coded output transducer 17, and an input transducer 16 acts in combination with a coded output transducer 18 to form a biphase SWD. 
     An upper conductive bar 15a of transducer 15 is connected by way of a bus 19 to a lower conductor bar 16b of transducer 16, and by way of channel 12 to receiving antenna 11. In addition, a lower conductive bar 15b of transducer 15 is connected to both an upper conductive bar 16a of transducer 16 and to an electrode 20 at ground potential. 
     Electrode 20 in turn is connected to a lower conductive bar 17b of a transducer 17 and to an upper conductive bar 18a of a transducer 18. Further, an upper conductive bar 17a of transducer 17 is connected by way of an electrode path 21 to a lower conductive bar 18b of transducer 18. Bar 17a is also connected by way of channel 13 to transmitting antenna 14. 
     In FIG. 2, metalized fingers extend transversely from the bars 15a, 15b, 16a, 16b, 17a, 17b, 18a and 18b of transducers 15-18. The fingers are clustered in groups of four, with each group constituting a single interdigitated bit tap. Input transducers 15 and 16 thus are dual tap transducers comprising taps 50 and 51, respectively. Transducer 17 has four active taps 30 interspersed with as many disabled taps 31 to effect phase reversals in an electrical output signal. Similarly, transducer 18 comprises active taps 40 and selectively disabled taps 41. Each of the taps of transducer 17 are paired in space with a tap from transducer 18. 
     In accordance with the invention, taps 31 and 41 are disabled one per pair by severing metalized fingers of like polarity from their common conductor bar. By this means, transducers 17 and 18 are coded tap by tap rather than by individual finger alterations. 
     In operation, an RF signal is received by antenna 11 and applied by way of channel 12 and electrode 19 to input transducers 15 and 16. Electric fields thus generated in the piezoelectric substate between the interdigital fingers of each transducer cause corresponding strains in the surface of the substrate underlying each signal tap. These strains propagate away from the taps of transducers 15 and 16 in the form of acoustic Rayleigh waves, which are sampled by taps 30 and 40 of coded transducers 17 and 18, respectively. The outputs of transducers 17 and 18 are then combined by way of electrode path 21 and channel 13 for use as a uniquely coded signal. 
     FIGURES 3a-3c 
     To more clearly illustrate the phase relationships involved in the operation of the transducer configuration of FIG. 2, reference is made to the impulse response diagrams of FIGS. 3a-3c. 
     The simplest, most direct method for evaluating matched filters is to observe their impulse response, which is a time-reversed copy of the corresponding input signal. To simulate an impulse, a pulse narrower than one half cycle of the center frequency is used. Such a signal is represented in FIG. 3a by a wave in envelope 60 applied by way of channel 12 and path 19 to transducers 15 and 16, respectively. 
     Since the fingers of taps 50 and 51 are alternately connected to the upper and lower conductor bars of transducers 15 and 16, respectively, a signal voltage impressed upon the electrodes causes the strains induced between adjacent fingers to be opposite in phase. The resultant waveforms are illustrated as acoustic waveforms 61 and 62 of FIG. 3a. 
     Acoustic waveform 61 is induced by transducer 15 and consists of a plurality of pulses, with each pulse having two negative excursions and one positive excursion. By way of contrast, acoustic waveform 62 is generated by transducer 16 and consists of a plurality of pulses having two positive excursions and one negative excursion. Waveforms 61 and 62 have an opposite phase relationship because of the structural difference between transducers 15 and 16. 
     As an acoustic wave of waveform 61 propagates across the substrate, it is sampled by taps 30 of transducer 17, thereby producing an output electrical signal of waveform 63. Concurrently, an acoustic wave having waveform 62 is sampled by taps 40 of transducer 18 to produce an output electrical signal of waveform 64. Output signals 63 and 64 are then combined by way of electrode path 21 and channel 13. The resultant is then transmitted as a PSK waveform 65 by way of antenna 14. The sudden reversals in polarity caused by disabled taps 31 and 41 divide waveform 65 into segments which may represent a &#34;1&#34; or a &#34;0&#34; in a binary code as illustrated by binary sequence 66, FIG. 3c, wherein a &#34;0&#34; represents a phase reversal. 
     In accordance with the present invention, there is provided a programmable SWD compatible with automated system tolerance levels to form, from a single master design, transducer electrode masks for a polyphase PSK SWD. By providing pairs of electrode sets at each bit location the spacing are such that electrodes depending from bar 17a or 18b may readily be severed without damage to any operating part of the structure. Each device to be programmed may be manually altered or may be placed in a machine programmed to slice or remove portions of arrays 31 and 41 to render them ineffective. It will be appreciated that antennas 11 and 14 may comprise a single unit serving as both receiver and transmitting antenna. Further, an FM system may be employed as well as the PSK system and may be selected where maximizing input energy in the substrate is desired. 
     Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.