US 3845419 A
Acoustic surface wave devices having transducers exhibiting a weighted conversion loss versus frequency characteristic without apodization are disclosed wherein equally overlapped interdigitated electrodes are disposed on a substrate of piezoelectric material in such a manner that the phase change between adjacent electrodes is pi radians for a predetermined dispersion characteristic. Fresnel ripple reduction is also provided by additional sets of electrodes with wider and narrower spacing at the low and high frequency ends, respectively, of the transducers.
Description (OCR text may contain errors)
United States Patent [1 1 Nudd [451 Oct. 29, 1974 ACOUSTIC SURFACE WAVE DEVICE  Inventor: Graham R. Nudd, I355 Brinkley Ave., Los Angeles, Calif. 90049  Filed: Jan. 2, 1974 [2|] Appl. No: 430,379
Related US. Application Data  Continuation-impart of Ser. No. 347,288, April 2,
 US. Cl 333/30 R, 3l0/9.8, 333/72 [5 1] Int. Cl. H03h 9/26, H03h 9/32, H0lv 7/00  Field of Search 333/30 R, 72; 3l0/8.l,
 References Cited OTHER PUBLICATIONS Smith et al.,lEEE Trans. on Microwave Theory and Techniques, Vol. M'I'll7, No. 11, Nov. i969, pp. 871-872.
Primary Examiner-Archie R. Borchelt Assistant ExaminerMarvin Nussbaum Attorney, Agent, or Firm-W. H. MacAllister; John Holtrichter, Jr.
[5 7] ABSTRACT 2 Claims, 7 Drawing Figures Propagation Direction PMENIEDBU 29 I974 SHEU 2 (I 4 i nuNT PAIENTEflums m4 3.845419 SNEH Mi 4 F ig. 40.
E lectrode Length Fig. 4b.
Synchronous Frequency 1:;lanodn ai5; M
Electrode Spacing Electrode Number PMENTED 0H 2 9 I914 SIIEEIMIF 4 ACOUSTIC SURFACE WAVE DEVICE This is a continuation-in-part application of patent application Ser. No. 347,288, filed Apr. 2, 1973, now abandoned.
The invention herein described was made in the course of or under a contract with the United States Air Force.
BACKGROUND OF THE INVENTION The background of the invention will be set forth in two parts.
1. Field of the Invention This invention relates to acoustic surface wave devices and more particularly to improved electroacoustic transducer structures for such devices.
2. Description of the Prior Art With the advancement of modern technology, there has arisen the problem of an ever-increasing requirement for efficient acquisition and processing of immense quantities of data in very short periods of time. in the communications field, for example, these problems concern the filtering, amplifying, and storing of received signals and also the processing and recognition of signals of a desired and known form.
For many years there has been an interest in elastic wave propagation devices, developed in what is generally known as microwave acoustic technology, for solving the aforementioned problems. In the earlier part of this work, the focus of attention of most workers in the field was concentrated on the phenomenon of bulk elastic waves, at acoustic or sound frequencies, propagating totally inside solids. For example, devices were constructed which employed bulk elastic waves for the storage or delay of signals. in these early delay lines, electrical signals were converted to elastic waves, usually by piezoelectric crystals, which propogated in the elastic solid and then reconverted to electrical form by a second transducer.
The advantage of sound frequency energy for these applications in solids is related to the excellent transmission characteristics of acoustic media and to the relatively low propagation velocity of approximately five orders of magnitude less than that of the speed of light or that of electromagnetic waves. As an example, an elastic wave resonator operating at a given frequency is typically l00,000 times smaller than an electromagnetic wave resonator for the same frequency, and the high Q of acoustic media allows delay times of about I times that possible with low-loss electromagnetic waves.
Most of the effort until recently has been associated with realizing bulk acoustic wave devices such as delay lines and amplifiers consisting of a crystalline block with opposite flat and parallel surfaces to which opposing piezoelectric transducers are attached. An input transducer converts an electrical signal to acoustic energy which is beamed through the medium to an output transducer. However, in most typical bulk devices it is almost impossible to tap, switch, vary the delay, vary the amplitude, or otherwise manipulate the acoustic energy during transit through the solid. Consequently, the use of these devices has been generally limited to passive devices.
This undesired restriction of access to the elastic wave has led to investigations of the acoustic waves that can be propagated along the boundary surfaces of solids. This phenomenon was first described by Lord Rayleigh in an article entitled On Waves Propagated Along the Plane Surface of an Elastic Solid," Proceedings, London Mathematics Society, Vol. 17, pp. 4-! 1, Nov. 1885. Devices utilizing such surface waves have the advantage of allowing easy access at all times to the propagating acoustic energy, to sample it, and to modify and interact with it. it should therefore be evident that this permits the realization of a wider range of devices than with bulk waves.
Surface waves, in contrast to bulk waves, are localized to the surface of solids. The typical particle motion is elliptical, and the amplitude decays exponentially into the body of the medium. As to the phase velocity. the speed of a surface wave is approximately ninety percent that of the bulk shear wave in most media. Probably the medium most widely used at the present time is one of several piezoelectric materials.
The basic building blocks of all surface wave devices is the acoustic surface wave delay line which includes spaced transducers disposed on a surface wave supporting medium. The transducers allow transition from normal electric circuitry into the acoustic domain. Where the medium is a non-piezoelectric material such as sapphire, for example, the transducer generally utilizes a piezoelectric slab bonded to a wedge-shaped block of material that is in turn bonded at an adjacent surface to the surface of the surface wave medium. Electrical energy is coupled to the piezoelectric slab which then generates bulk waves in the wedge-shaped block to excite Rayleigh waves along the outer surface of the non-piezoelectric surface wave medium.
In more common designs, a piezoelectric material such as lithium niobate or quartz, for example, is used as the surface wave medium. In this case. the transducers most commonly in use are of the interdigital type consisting of a series of conductive electrodes that form a pattern which is disposed on the piezoelectric substrate surface. The interdigital or interdigitated trans ducer is a two-terminal device having two separate arrays of metal strips resembling interleaved fingers and converts electrical signals into surface waves and also converts surface wave energy incident thereon into electrical signals. In the case of an input transducer, an incoming electrical signal is converted by the transducer into a time-dependent space-varying electric field pattern which, in turn, generates an acoustic surface wave directly on the substrate through the electrostatic action of the piezoelectric material.
The frequency response characteristic of interdigital transducers having equal finger spacing throughout is relatively narrow, and in order to provide a useful device in many important applications, such as filters, several techniques have been developed to broaden the band widths from the characteristic sinMfl/Af, where A is a constant depending upon the number of electrodes in the array. One such technique well known in the art is to vary the finger spacings across the transducer, most often in accordance with the formula T,, T, +kn for finger position, where T, is the position of the nth finger, T is the position of the first finger, and K is the constant depending on the time band width product. However, this configuration results in a tapered frequency response due to a varying number of active fingers (N throughout the band, and the increased length a of the fingers in wave length at the higher frequency.
A widely used technique to produce a broader response of an interdigital surface wave transducer is to construct an array with a linear frequency modulated (LFM) characteristic providing the wide band width and then to apodize the elements within the array to provide the desired spectral weighting. This technique is described in many articles on the subject, one of which is Acoustic Surface Wave Filters" by R. H. Tancrell and M. G. Holland, in the Proceedings of the IEEE, Vol. 59, pp. 393409, March 1971.
Although providing the desired response, apodization can cause problems because of the non-uniform acoustic beam width it produces and the variation in defraction losses across the band. In this type structure, acoustic beam width varies with frequency, which in many applications, is very undesirable. As to the problem of defraction, it causes varying loss with length of delay so that apodized transducer design (if equalized for diffraction losses) can only be used for a particular delay and a new design must be made for any other delay. The maximum variation in insertion loss in such devices is limited by the ratio of the apertures within the array, and the reduction in aperture associated with apodization causes inefficient use of the array, as well as the aforementioned phase front variations which must be corrected by dummy electrodes, and the like.
The aforementioned technique of using apodization to provide a transducer having a spectral-weighted, wide band width characteristic does not deal with another undesirable feature, namely that of a relatively high pass-band ripple associated with a finite structure.
In the past, this problem has been dealt with by the use 1 SUMMARY OF THE INVENTION In view of the foregoing factors and conditions characteristic of the prior art, it is a primary object of the present invention to provide an acoustic surface wave device having a novel transducer structure and thereby not being subject to the disadvantages enumerated above.
It is another objective of the present invention to provide an acoustic surface wave device having a trans ducer structure utilizing a technique that relies on varying the number of effective transducer elements (i.e., those that are synchronous or near synchronous) as a function of frequency to provide a desired conversion loss variation.
It is also another object of the present invention to provide an acoustic surface wave device having a transducer structure that reduces pass-band ripple, without the use of apodization techniques.
In accordance with an embodiment of the present invention, an acoustic surface wave device having a flat band response includes a substrate of piezoelectric material capable of propagating acoustic surface wave energy, and transducer means including at least one interdigitated-type electro-acoustic transducer having a plurality of equally-overlapped electrodes for providing a dispersion expressed by successive ones of said electrodes being spaced at intervals of 1r radians such that I (r,,) (1,, 1r with l 21r If df/dr dldl.
In accordance with another embodiment of the invention which is subject to a linear f.m. signal f(r) bt where b is a constant and having a phase characteristic of the form @(t) btl2, an acoustic surface wave device includes a substrate of piezoelectric material capable of propagating acoustic surface wave energy and at least one electro-acoustic transducer having a unitimpulse response M!) alt) e and including a main set of interdigitated comb-shaped electrodes disposed on the substrate, the electrodes all being of equal length and adjacent ones of which overlap each other an equal amount. The transducer exhibits a relatively broad pass band generally defined between frequencies F l and F those electrodes synchronous to frequencies between F, and F being disposed such that the phase change between the adjacent electrodes of 1 0,.) l (l,, is 1r radians, the position of the nth electrode being given by T,, T, kn. The transducer also includes additional sets of interdigitated comb-shaped electrodes of equal length and overlap and are disposed on the substrate at the ends of the main set of electrodes, those of the electrodes of the additional sets that are synchronous to frequencies below F being positioned according to the function T, T gkn such that g is less than I and those above F, being positioned according to the function if}, T gkn such that g is more than l.
The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by making reference to the following description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like elements in the several views.
BRIEF DESCRIPTION OF THE DRAWINGS:
FIG. 1 is a schematic representation of a portion of an acoustic wave device illustrating an electroacoustic transducer in accordance with one embodiment of the present invention;
FIG. 2 is a graphical representation of the insertion loss characteristic of the device of FIG. 1;
FIG. 3 is a schematic representation of a portion of an acoustic wave device in accordance with another embodiment of the present invention;
FIGS. 4A and B are graphical representations illustrating the relationship between transducer electrode lengths and synchronous frequency against electrode position in the transducer of FIG. 3;
FIG. 5 is a graphical representation showing the relationship between electrode spacing as against electrode number in the transducer of FIG. 3; and
FIG. 6 is a graphical representation illustrating the conversion loss characteristic of the device shown in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Referring now to the drawings and more particularly to FIG. 1, there is shown a portion of an acoustic surface wave device 11 having an electro-acoustic transducer l3 bonded, deposited or otherwise disposed to the surface of a substrate 15 of piezoelectric material capable of propagating surface wave energy, such as lithium niobate or quartz, for example. One or more such transducers 13 may be provided and each has a unit-impulse response of h(t) =a(t)e"". This is a constant amplitude [0(1)] and phase I (t)] relationship characteristic of all transducers of the type described. Since this characteristic is well known in the art, it will not be described in detail, but references on the subject may be consulted such as, for example, the book Information, Transmission, Modulation, and Noise" by Mischa Schwartz, Chapter 2, page 82, 2nd Edition, published by McGraw-Hill Book Co., New York, London, I970.
The transducer 13 includes interdigitated combshaped electrodes I7 and 19 extending transversely from respective electrode base portions 21 and 23. All the electrodes are of equal length and adjacent ones of the electrodes overlap each other an equal amount. In order to provide a synchronous, non-dispersive unapodized array with a constant and relatively wide aperture, the spacing between any two adjacent electrodes is M2 (where A is the synchronous wavelength), or phase increments of 1r radians.
The abovedescribed structure utilizes the novel technique of varying the number of transducer elements which are synchronous in any given frequency band, to provide the desired conversion loss against frequency characteristic. This technique for controlling the insertion loss of a surface wave acoustic transducer as a function of frequency avoids apodization and its associated problems and may provide a relatively flat band-pass filter of constant aperture, as exemplified by the graphical representation shown in FIG. 2. A more detailed description of the above-mentioned technique may be obtained by making reference to a publication by the inventor in an article entitled Technique for Varying the Conversion Loss Against Frequency of a Surface-Wave Transducer Without Apodization," published in Electronic Letters, Vol. 8, No. l4, l3 July I972.
It will be noted that in equation (4) of the abovecited article that the relative conversion efficiency at frequency w, (T(w)), is stated in terms of the number of effective fingers Mm) and the angular frequency (0(0) Z-rrj) as T,,(w) KFw Mw), where both K and F are arbitrary constants. Accordingly, in terms of linear frequency this can be expressed as T0) cont Nmj In following the basic concept as set forth in this cited article, the desired conversion loss vs. frequency, ll TU) ll may be provided by designing an array having a dispersion of where N is the total number of fingers or transducer elements, and where f,,, and f are, respectively, the high and low frequencies of the band pass B. Since the fingers or electrodes must be placed at phase increments of 1r radians (as noted above), the phase may be obtained by double integration of the dispersion. Thus. the delays to each electrode I may be determined by noting that I (r,,) b(r,, 1r, where (r) 21:- ff df/d: dtdt.
As noted in the Electronics Letters article, an approach which can sometimes be employed is to apodize only at the ends of the array (if at all) to reduce the path-band ripple associated with a finite structure, as indicated by reference numeral 3] in FIG. 2. It is there suggested that less than 10 percent of the fingers or electrodes at the extreme ends are tapered to prevent sharp discontinuities and the associated Fresnel-type ripple. All other electrodes in the transducer are of constant length and equal to the optimum for the device.
In accordance with another embodiment of the present invention, illustrated in FIG. 3, a technique is utilized which lowers the Fresnel-type ripple without the use of apodization at the extreme ends of the array. Fresnel ripple results from end effects in the array. In most cases it can be thought of as a mathematical consequence of the finite length of the array without regard to the particular positioning of the electrodes within the array.
Referring to FIG. 3, there is shown a second embodiment of the invention, designated 4], which includes a transducer 42 having fingers or electrodes 43 and 45 all of equal length extending from electrode base portions 47 and 49 respectively. Electrodes from opposite base portions overlap each other an equal amount to provide a relatively wide and constant aperture. It will be noted that the electrodes 43 and 45 in the central portion of the transducer 42 are spaced similarly to the electrodes 17 and 19 in the transducer 13 of FIG. I. This portion of the transducer 41 includes elements that are synchronous to frequencies between F, and F as are all the electrodes in the transducer [3. However, unlike the first described embodiment II, the transducer 42 in embodiment 4] is also provided with additional sets of electrodes, namely, electrodes 43' and 45' at the low frequency of F, end, and electrodes 43" and 45" at the high frequency or F, end of the interdigi tated array.
Where the array is coupled to a linear F.M. signal f(!) gbt, where b is a constant, the electrodes in the lower frequency set of additional electrodes have a spacing such that g is less than I, while those electrodes in the other set have a spacing such as g is more than I. It should of course be understood that the electrodes synchronous to frequencies between F, and F have a spacing such that g is equal to 1.
This relationship is graphically illustrated in FlGS. 4A and B where constant electrode length is shown for all electrode positions, while the rate of change of the synchronous frequency against electrode position changes in accordance with the teachings of the invention. Thus, the rate of change of the synchronous frequency of the electrodes between F, and F g is linear at a rate of b (g l while the rate is less than b below F, and greater than b above F Also, FIG. 5 is provided to show the relationship between electrode spacing and electrode position in the array of FIG. 3.
Thus it can be seen that the main concept of the first described embodiment of the invention has been extended to also reduce Fresnel-type ripple, without using the prior art technique of apodization. The convesion loss curve shown in FIG. 6 is provided to show, by comparison with the curve of PK]. 2, the reduction of ripple due to the additional electrodes in the array of FIG. 3. For the sake of simplicity, the transducer 42 of embodiment 41 is not shown disposed on a substrate. However, it should be realized that the array will be bonded or otherwise attached to a suitable piezoelectric substrate in order to practice the invention. It should further be understood that the materials used to fabricate the various embodiments of the invention are not critical and any material exhibiting desired characteristics may be substituted for those mentioned. For example, the transducer arrays may be fabricated from aluminum by means of deposition directly on the substrate surface. Accordingly, it should further be realized that although the present invention has been shown and described with reference to particular embodiments, various changes and modifications obvious to one skilled in the art to which the invention pertains are deemed to be within the spirit, scope, and contemplation of the invention.
What is claimed is: I. An acoustic surface wave device having a flat band response, comprising:
a substrate of piezoelectric material capable of propagating acoustic surface wave energy; and
transducer means including at least one interdigitated-type electro-acoustic transducer having a plurality of equally-overlapped electrodes for providing a dispersion expressed by where TU) is the relative conversion efliciency at frequency f, N, is the total number of transducer electrodes, B is the band pass having a highest synchronous frequency fl" and a lowest synchronous frequency f,,,, and I, and l,, are the delays to the nth and n-l transducer electrodes, successive ones of said electrodes being spaced at intervals of 1r radians such that 0(1.) l (t,, 1r with Mr) 21r If df/d! drdr, where 0 is the relative phase.
2. An acoustic surface wave device subject to a linear f.m. signal f(t) b! where b is a constant and having a phase characteristic of the form @(r) br /2, comprismg:
a substrate of piezoelectric material capable of propagating acoustic surface wave energy; and
at least one electro-acoustic transducer having a unitimpulse response M2) a(t)e" and including a main set of interdigitated comb-shaped electrodes disposed on said substrate, said electrodes all being of equal length and adjacent ones of which overlap each other an equal amount, said transducer exhibiting a relatively broad pass band generally defined between frequencies F, and F those electrodes synchronous to frequencies between F, and F 2 being disposed such that the phase change between said adjacent electrodes of Du l (t,, is 1r radians, the position of the nth electrode being given by T,, T, kn, said transducer also including additional sets of interdigitated combshaped electrodes of said equal length and overlap and dis posed on said substrate at the ends of said main set of electrodes, those of said electrodes of said additional sets that are synchronous to frequencies below F, being positioned according to the func tion T3 T, gkn such that g is less than l and those above F, being positioned according to the function T,,"' T} gkn such that g is more than 1.