United States Patent ['11 3,582,837

[72] Inventor Adrian J. DeVries 3300,739 l/l967 Mortley ...... 333/30 Elmhurst, III. 3,360,749 I 2/1967 Sittig ...... 333/30 [2l] Appl. No. 681,524 3,070,761 l2/I962 Rankin 333/30 [22] Filed Nov. 8,1967 2,00l,387 5/l935 Hansell ...... 333/72 [45] Patented June 1,1971 [73] Assignee Zenith Radio Corporation Primary Examiner-Herman Karl Saalbach Chicago, Ill. Assistant Examiner-C. Baraff Attorney-Francis W. Crotty

[54] SIGNAL FILTER UTILIZING FREQUENCY DEPENDENT VARIATION OF INPUT IMPEDANCE 0F ONE-PORT TRANSDUCER 14 Claims, 3 Drawing Figs. ABSTRACT: A body of piezoelectric material propagates acoustic surface waves. A frequency-selective transducer is [52] U.S. CI...... 333/72, coupled to a surface of the body to interact with the surface 333/30 waves. The transducer has a pair of terminalsa'n'dlin operation ' [5 1] Int. Cl...... “03h 9/32 exhibits at those terminals an impedance that has a signi?cant [50] Field of Search ...... 333/30, 72, resistive component to signals of a predetermined frequency 6; 310/8, 9; 340/16 but which has a relatively insigni?cant resistive component to [56] References Cited signals of frequencies within a desired operating range but dif fering from that predetermined frequency. Input signals are UNITED STATES PATENTS fed to the transducer while output signals are derived 3,401,360 9/1968 SchuItz-DuB0is..... 333/30 therefrom. The system constitutes a ?lter which need have but 3,289,114 11/1966 Rowen ...... 333/30 a single transducer element coupled to the acoustic waves.

non

Load ‘ PATENTEU'JUN Han 3.682.837

Lodd

Balanced . Q Pushi-Pull Amplifier

‘inn. -r . Adrlon. J. INVEN'I‘ORZDevrles i?mBY - ' 3,582,837 1 2 SIGNAL FILTER UTILIZING FREQUENCY-DEPENDENT single ended or unbalanced. The secondary or output winding VARIATION ‘OF INPUT IMPEDANCE OF ONE-PORT 13 of transformer 12 is tapped at its center point 14 and that ' TRANSDUCER point is returned to ground. One of the ends of winding 13 is This invention pertains to signal translation systems. More connected to a terminal 15 ofa transducer 16, while the other particularly, it relates to solid-state tuned circuitry including end of winding I3 is coupled through a capacitor 17 to the an acousto-electric ?lter which involves interaction between a other terminal 18 of transducer I6. A load I9 is coupled transducer coupled to a piezoelectric material and acoustic between ground and a point intermediate capacitor I7 and waves propagated in that material. terminal 18. With equal impedances presented by transducer In copending application Ser. No. 582,387, filed Sept. 27, 16 and capacitor 17, balanced or push-pull signals are desired 1966‘, now abandoned I disclosed and claimed a number of 10 across the ends of secondary winding 13. Of course, balanced different embodiments of acousto-electric devices in which operation also can be obtained‘ with unequal impedances by acoustic surface waves propagating in a piezoelectric material correspondingly moving the position of tap I4. interact with transducers coupled to the surface waves. In Transducer 16 is coupled to one major surface of a each of the embodiments particularly disclosed in that appli piezoelectric body or substrate 20. The transducer is con cation, surface waves launched in the body of piezoelectric structed of two comblike electrodes 21 and 22, the stripes of material are caused in one manner or another, to interact with one comb electrode being interleaved with the stripes of the a second transducer space along the surface from the ?rst. In other comb electrode. The electrodes are formed of a material the simplest case, the ?rst transducer is coupled to a source of such as gold which may be vacuum deposited upon the plane signals while the second transducer is coupled to a load, the 20 surface of a polished piezoelectric body of a material such as signal energy being translated by the acoustic waves between PZT or quartz. The distance between the centers of two con the two transducers. In actual practice, the acoustic-wave path secutive stripes is one-half of the acoustic wavelength, in the length between the two transducers introduces a signi?cant material of substrate 20, of a signal wave for which it is desired delay in translation of the signals. Such delay may be undesire to achieve maximum response. able in certain applications. In addition, re?ection of acoustic 25 It is known that a transducer, composed of interleaved wave energy may occur from the second or output transducer combs of conducting stripes or “teeth" to which are fed alter back to the ?rst, and such reflection can undesirably alter the nating electric potentials, when coupled to a piezoelectric overall response of such devices. medium produces acoustic surface waves on the medium It is, accordingly, a general object of the invention to pro which, in the simpli?ed isotropic case of a ceramic poled per vide a new and improved signal translation system of the pendicularly to the surface, travel at right angles to the stripes. aforesaid acousto-electric variety which overcomes the at 30 Conversely, the acoustic surface waves on the medium in least sometimes undesircable features of the prior acousto electric systems. teract with the electrode array and are converted back to an electrical signal. Thus, in operation, direct piezoelectric sur It is another object of the present invention to provide a new and improved acousto-electric signal translation system in face-wave transduction is accomplished by the spatially which the active portion of the system is but a two-terminal periodic interdigital electrodes of transducer 16 and speci? cally by the periodic electric fields created between those device. _ “ ‘ It is a further object of the present invention to provide a electrodes in response to a signal from source 10 ofa frequen new and improved acousto_electric signal translation system cy such that the wavelength of the acoustic waves corresponds that is especially adaptable to fabrication utilizing integrated to the center-to-center spacing of two alternate teeth or circuit techniques. 40 stripes. This piezoelectric coupling to the surface waves oc A signal translation system constructed in accordance with curs when the strain components produced by the electric the present invention includes a body of piezoelectric material fields in the piezoelectric substrate are substantially matched that is propagative to acoustic surface waves. Coupled to a to the strain components associated with the surface-wave mode. , surface of the body is a frequency-selective transducer which 45 interacts with the acoustic surface waves. The transducer has Source 10, for example a television receiver, produces a a pair of terminals which in operation exhibit an impedance range of signal frequencies. But due to the selective nature of that has a signi?cant resistive component to signals of a the arrangement, including transducer 16, only a signal of a predetermined frequency but has a relatively insignificant re particular frequency and its intelligence-carrying sidebands sistive component to signals of frequencies within a chosen are converted to surface-wave energy. The result is that the range but differing from that predetermined frequency. Input single ?lter element, composed of transducer 16 and signals are fed to the transducer by means coupled to its ter piezoelectric substrate 20, constitutes a selective-filter device. minals, and signals are derived from the transducer by means A signi?cant resistive component of impedance, to be also coupled to those terminals. discussed more particularly hereafter, is exhibited across ter The features of‘the present invention which are believed to 55 minals 15 and 18 during operation when the signals from be novel are set forth with particularity in the appended source 10 are of the frequency of maximum response which claims. The organization and manner of operation of the in occurs when the acoustic wavelength approaches twice the vention, together with further objects and advantages thereof, center-to-center tooth separation. On the other hand, the im may best be understood by reference to the following descrip pedance exhibited across terminals 15 and 18 has a relatively tion taken‘in- conjunction with the accompanying drawing, in insigni?cant resistive component and becomes essentially a the several ?gures of which like reference numerals identify capacitive reactance for signals from source 10 of frequencies like elements and in which: within a desired operating range but differing from that FIG. I is a schematic diagram, including a perspective view predetermined maximum response frequency. of an acoustic-electric ?lter element, of one embodiment of a It will be observed that source ‘10 is passively coupled to ter signal translation system; 65 minals 15 and 18 for feeding the input signal to transducer 16. FIG. 2 is a schematic diagram, partially in block diagram Similarly, load 19 is passively coupled to those same terminals form, of an alternative embodiment of a signal-translation for deriving signals from transducer 16. In‘ the particular ar system; and rangement of FIG. 1, the overall translation system is in the FIG. 3 is a schematic diagram of a circuit equivalent to a form of a bridge. That is, transducer 16 and its associated portion of the systems of FIGS. 1 and 2. 70 piezoelectric substrate 20 constitute a ?rst arm of the bridge. In FIG. I, a source I0 of signals which may occur at a plu The second bridge arm is capacitor 17, while the two sections rality of different frequencies is coupled across the input wind of winding 13 on either side of tap‘ l4 constitute the third and ing ll of a transformer 12. One of the pair of leads coupling fourth arms of the bridge. Inpu‘t source 10, for the illustrated source 10 to winding I1 is connected to a plane of reference equal-impedance case, is coupled by winding 11 equally to potential, here shown as ground, so that this input network is 75 those third and fourth arms, while load 19 is coupled between 3,582,837 3 . 4 - the point intermediate the first and second arms and the point As a matter of coarse approximation, transducers I6 and 31 intermediate the third and fourth arms. may each be represented electrically by the parallel combina At the resonant frequency of transducer 16, when as men tion of a capacitance C, with a with a series circuit containing tioned a signi?cant resistive component of impedance appears a resistor, an inductor and a capacitor. Referring to FIG. 3 and between terminals 15 and 18, the bridge is unbalanced and a limiting the consideration to the generation of surface waves, useful fraction of the input signal is transferred to load I9. On the equivalent circuit is shown as capacitor C0 in parallel with the other hand, when the frequency of the signals from source the series arrangement of inductor L,, capacitor C,, and re 10 departs from that resonant frequency by a sufficient sistor R,. Of these, capacitor C0 is a parameter referred to as amount in relation to the equivalent 0 of the transducer, the the clamped capacitance of the transducer because it is the impedance presented across terminals 15 and I8 is primarily capacitance measured with inhibition of all mechanical mo capacitive. By assigning a value to capacitor 17 substantially tion. At the region of maximum efficiency of surface-wave equal to that capacitance presented between terminals l5 and transduction, or maximum response of the transducer, the 18, the transfer of energy to load 19 is substantially reduced in branch network L,, C,, Rl is resonant and represents essen the off-resonance condition. Considerations determining the tially a resistive impedance. This is the significant resistive Q of the transducer are recited in my aforesaid copending ap 15 component of impedance referred to above and in the ap pllcation. pended claims. Its signi?cance decreases at frequencies differ In a typical embodiment, utilizing quartz as the piezoelec ing from the resonant or maximum response frequency in a tric material of substrate 16, the stripes of transducer 16 are manner analogous to the impedance change of a tuned circuit approximately 0.7 mil wide and are separated by 0.7 mil for a which, as is well understood, is a function of the Q of the 40 megahertz application. The width of the comb structure, device. Outside the region of maximum efficiency of surface from top to bottom of FIG. I, is approximately 0.4 inch. This wave transduction, the transducer impedance is primarily structure acts as a single-tuned circuit with a resonant represented by the capacitance C,,. In the system of FIG. 1 as frequency of 40 megahertz, the resonant frequency being shown, then, it is primarily the clamped capacitance Co which determined by the spacing of the stripes, as pointed out 25 is balanced against capacitor I7; this approximation assumes hereinbefore. that capacitance Co is substantially greater than the value of The potential developed between any given pair of stripes the capacitance represented by the capacitor in the paralleled produces two waves traveling along the surface of substrate 20 equivalent series circuit so that, below resonance, the net in opposing directions perpendicular to the stripes for the capacitive impedance is essentially determined by capacitance isotropic case (as when using PZT) and in any event generally 30 C,,. Any difference between the values of capacitor 17 and the laterally away from the stripes. When the distance between clamped capacitance of transducer 16 may be compensated the stripes isone-half of the acoustic wavelength at the desired by changing the position of tap 14. In the FIG. 2 system, either input frequency, or an integral multiple thereof, a maximum both transducers should be selected to have like values of Co of acoustic-wave development occurs. For increased selectivi or any difference should be compensated by purposefully un ty, additional electrode stripes are added to produce a longer 35 balancing amplifier 30. Further with reference to FIg. 2, possi comb pattern of the type depicted. ble capacitance variations between the two transducers due to Because waves propagate outwardly away from transducer fabrication tolerances may be avoided by disposing both trans I6, the assembly includes means located in the path of those ducers I6 and 31 on the same substrate at the same time. waves in order to attenuate them. To this end, the opposing The choice of frequencies of maximum response for trans end surfaces 23 and 24 of body 20 are shaped to have an ir 40 ducers l6 and SI is dictated by the response characteristic. regular contour. Consequently, the waves upon re?ection desired of the signal translating device. The individual from the end surfaces are scattered and thereby attenuated in multiple re?ections. Hence, the interaction between trans response of each such transducer is similar to that of a crystal filter but, as pointed out above, each transducer has a readily ducer I6 and the acoustic surface waves in body 20 is primari- ' adjustable equivalent 0 as determined by the selected ly effective only with those wave as they originally are 45 developed. Interaction with waves which otherwise might be frequency of maximum response of each transducer and their re?ected from planar end surfaces, and which would include equivalent Qs. phase-delayed signal information, is minimized and is essen With respect to both FIGS. 1 and 2, it should be noted that the particular surface wave transducers employed also tially prevented. generate bulk waves in the shear and longitudinal modes. Such In the system of FIG. 2, transformer 12 of FIG. I is replaced 50 by a push-pull ampli?er 30 the output of which is balanced bulk waves are developed at frequencies above the surface with respect to ground. This alternative input arrangement wave frequencies. For example, with the transducer designed likewise may be employed in the system of FIG. 1, or the input to produce surface waves at 10 megahertz, bulk shear waves arrangement of FIG. 1 may be used iri FIG. 2. Also in the are developed in the region of 20 megahertz while bulk lon system of FIG. 2, capacitor 17 is replaced by a second selec 55 gitudinal waves are produced at a frequency in the neighbor tive device 31 constructed in the same manner as the com hood of 30 megahertz. Such bulk waves travel into the body of bination of transducer 16 and piezoelectric substrate 20. That the substrate material at an angle to the surface. is device 31 likewise is composed of an electrode array af?xed In the frequency ranges at which such bulk waves are to a surface of piezoelectric materialpropagative of acoustic generated, the transducer impedance also exhibit a significant surface waves, and the array is composed of interleaved 60 resistive component. In effect, then, transducer 16 in FIGS. I combs of conductive elements with the center-to-center spac and 2 is more completely described by the equivalent circuit ing between those elements being effectively one»half the shown in FIG. 3. As discussed above, this circuit includes the length of the acoustic surface waves at a selected frequency. clamped capacitance Co paralleled by the series combination In this case, that selected frequency to which the second array of an inductor L,, a capacitor C, and a resistor R1, the series is resonant is somewhat different from the predetermined 65 combination exhibiting resonance at the point of maximum frequency to which transducer 16 is selective in order to attain surface-wave transduction. At the same time, the circuit a condition’ of unbalance in the bridge at a desired operating further includes, also in parallel with Co, another series com frequency range to effect a transfer of signal energy to load bination L2, C2 and R, together with a still further series com 19. At a frequency separated from the frequency of maximum bination L3, Ca and R3. The ?rst of these added pair of series system response, the impedances of transducers I6 and 31 are 70 combinations exhibits resonance at the point of maximum at least nearly the same and the bridge is substantially development of the bulk shear waves and the second exhibits balanced, resulting in but a low output level to load 19. On the its resonance at the point of maximum generation of bulk lon other hand, in the region of maximum response of both trans gitudinal waves. The arrangements of FIGS. 1 and 2 contem ducers, the bridge is highly unbalanced and a substantial out plate operation in the frequency range associated with surface put signal is ‘developed across load 19. 75 wave generation. 3,582,837 5 6 Because of the existence of these additional resonant cir input transducer; coupled to said terminals for feeding cuits, it will be observed that the bridge network of FIG. 1 ac‘ signals to said transducer; tually will be unbalanced at undesired operating frequency and output means coupled to said terminals for deriving ranges unless capacitor 17 is replaced by a more complicated signals from said transducer. network. While such compensating may be unnecessary in 5 2. A system as de?ned in claim 1 in which said transducer is many applications. when desired capacitor 17 may be an electrode array composed of interleaved combs of conduc replaced by an electrical network like that of FIG. 3, except tive elements with the center-to-center spacing between said for the omission of L,, Cl, and R1, to obtain essentially elements being effectively one-half the length of the acoustic complete compensation. On the other hand, partial compen surface waves at said predetermined frequency, said terminals sation may be all that is desired in some cases so that only one being coupled individually to respective ones of said combs. of the network branches of elements Lz-cgR2 or L,_c,R, of 3. A system as de?ned in claim 1 in which said input means FIG. 3 is included. in any case, the elements of the compensa includes an amplifier producing two signals of opposite phase tory network are chosen so that the equivalent circuit com~ and of respective amplitudes effecting balance between said ponents of FIG. 3 are simulated as closely as desired outside signals at said predetermined frequency. the range of surface-wave frequencies. it maybe noted further 4. A system as de?ned in claim 1 in which said input means that the arrangement of FIG. 2 automatically achieves a high includes a balance push~pul| ampli?er. degree of compensation because transducer 31 is preferably 5. A system as de?ned in claim 1 in which said input means identically formed as, but exhibits a slightly different frequen includes means for converting unbalanced input signals to signals of balanced character. cy of maximum response than, transducer 16. 20 It will be observed that the overall circuitry of the systems of 6. A system as de?ned in claim 1 in which said system is ar FIGS. 1 and 2 resembles approaches utilized with simple ranged to include a four-armed bridge with said transducer piezoelectric crystal ?lters in which advantage is taken of the constitutinga ?rst arm of said bridge, a capacitive reactance series resonance, exhibited by an ordinary piezoelectric element constituting a second arm of said bridge coupled at one end of said ?rst arm, and a pair of impedance elements in crystalline element at a given frequency, to afford a selective 25 response. However, those ?lters exhibit a very. narrow pass dividually constituting respective third and fourth arms of said band. in contrast, utilization of the surface-wave interaction bridge and coupled in series between the other ends of said principle employed by the circuitry of FIGS. 1 and 2 enables ?rst and second arms, said input means being coupled to said the attainment of an overall bandwidth through the entire third and fourth arms of said bridge and said output means being coupled between said one end of said ?rst and second signal-translation network comparable to the bandwidth of the 30 surface—wave transducer itself. That is, in FIG. 1 the overall arms and the junction of said third and fourth arms. bandwidth of the network is primarily a function of the band 7. A system as de?ned in claim 6 in which said second arm width of transducer 16 rather than of some characteristic of exhibits an impedance simulating the impedance exhibited by piezoelectric body 20. As pointed out above, the bandwidth of said transducer at frequencies other than said predetermined transducer 16 is selectively adjustable by means of the choice 35 frequency. 8. A system as de?ned in claim 7 in which said capacitive of the number of stripe elements in the electrode combs. reactive element comprises: Moreover. the bandwidth attainable may be adjusted either in a frequency-selective second transducer coupled to a body itially by the choice of the number of stripes to be provided or of piezoelectric material propagative of acoustic surface subsequently through a selective switching network to in 40 waves to interact with the acoustic surface waves therein, crease or decrease the number of stripes used at a given time. said second transducer having but a pair of terminals Having incorporated transducer 16 and its associated across which in operation is exhibited an impedance hav piezoelectric substrate 20 into a particularly advantageous ing a signi?cant resistive component to signals of a overall network arrangement, it will be apparent upon con selected frequency different from said predetermined siderationof the attributes of the ?lter device itself that it also 45 frequency and which becomes essentially capacitive to may be utilized in other networks. Such networks are known signals of frequencies departing from said selected as such but heretofore have made use of ordinary piezoelec frequency. _ tric crystalline elements in which selectivity or bandwidth is a 9. A system as de?ned in claim 8 in which the clamped function of the characteristics of the crystalline material ‘and capacitances of said ?rst and second transducers are the same. its dimensions. in any event, it is to be noted that the assembly 10. A system as de?ned in claim 6 in which said impedance is employed as a two~terminal device, as a result of which elements of said third and fourth arms are inductive. signal delay is minimized. At the same time, difficulty is 11. A system as de?ned in claim 6 in which, for signals de avoided with response to interaction with re?ected signal parting in frequency from said predetermined frequency by a energy. Moreover, the entire selective~?lter system is subject speci?c amount, the impedance of said capacitive reactance to fabrication as a single integrated circuit. 55 element in said second arm is substantially equal to the im ' While particular embodiments of the present invention have pedance exhibited across said terminals. been shown and described, it will be obvious to those skilled in 12. A system as de?ned in claim -1 in which said transducer the art that changes and modi?cations may be made without interacts with acoustic waves propagating generally in a given departing from the invention in its broader aspects. Ac direction along a surface of said body and in which said body cordingly, the aim in the appended claims is to cover all such 60 includes means located, in the path of and attenuative of said changes and modi?cations as fall within the true spirit and acoustic waves. scope of the invention. 13. A system as de?ned in claim 1 in which said transducer lclaim: interacts with acoustic waves propagating generally in a given l. A signal translation system comprising: direction along a surface of said body and in which‘ an end sur a body of piezoelectric material propagative of acoustic 65 face of said body oriented generally transverse to said waves; direction is of irregular contour. ‘ a frequency-selective transducer coupled to a surface of 14. A signal translation system comprises: said body to interact with acoustic waves having frequen a body of piezoelectric material propagative of acoustic cies within a particular range, said transducer having but waves; a pair of terminals which in operation exhibit an im 70 a ?rst frequency-selective transducer coupled to a ?rst sur pedance that has a signi?cant resistive component to face portion of said body to interact with a ?rst set of signals of a predetermined frequency within said range acoustic surface waves on said body; but has a relatively insigni?cant resistive component to a second frequency-selective transducer coupled to a signals within said range and differing from said predeter second surface portion of said body to interact with a mined frequency; 75 second set of acoustic surface waves on said body; 3,582,837 7 8 means for applying signals, in push-pull relation relative to a and means for deriving signals from a point in said series plane of reference potential, across said ?rst and second combination intermediate said ?rst and, second transdu transducers in series combination; cers and said plane of reference potential.