GB2044569A - Surface acoustic wave device having asymmetrical amplitude characteristics - Google Patents
Surface acoustic wave device having asymmetrical amplitude characteristics Download PDFInfo
- Publication number
- GB2044569A GB2044569A GB7939311A GB7939311A GB2044569A GB 2044569 A GB2044569 A GB 2044569A GB 7939311 A GB7939311 A GB 7939311A GB 7939311 A GB7939311 A GB 7939311A GB 2044569 A GB2044569 A GB 2044569A
- Authority
- GB
- United Kingdom
- Prior art keywords
- acoustic wave
- surface acoustic
- electrode
- wave signal
- electrode fingers
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 238000010897 surface acoustic wave method Methods 0.000 title claims abstract description 58
- 230000000644 propagated effect Effects 0.000 claims description 2
- 239000000758 substrate Substances 0.000 description 7
- 238000001914 filtration Methods 0.000 description 5
- 238000004519 manufacturing process Methods 0.000 description 5
- 238000000034 method Methods 0.000 description 3
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 2
- 229910052782 aluminium Inorganic materials 0.000 description 2
- 238000005070 sampling Methods 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical compound [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 description 1
- 238000000059 patterning Methods 0.000 description 1
- 230000001902 propagating effect Effects 0.000 description 1
- 238000003786 synthesis reaction Methods 0.000 description 1
- 230000002194 synthesizing effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
- H03H9/02—Details
- H03H9/125—Driving means, e.g. electrodes, coils
- H03H9/145—Driving means, e.g. electrodes, coils for networks using surface acoustic waves
- H03H9/14517—Means for weighting
- H03H9/1452—Means for weighting by finger overlap length, apodisation
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
- H03H9/46—Filters
- H03H9/64—Filters using surface acoustic waves
- H03H9/6423—Means for obtaining a particular transfer characteristic
Landscapes
- Physics & Mathematics (AREA)
- Acoustics & Sound (AREA)
- Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)
Abstract
A surface acoustic wave device having an asymmetrical filter characteristic comprises a first interdigital transducer 2 for converting an electric signal to a surface acoustic wave signal, a medium 1 for propagation of the surface acoustic wave signal, and a second interdigital transducer 4 for converting the surface acoustic wave signal to an electric signal. At least one of the transducers is apodized differently in its two halves as defined by a centre line A-A' to form two interdigital electrode patterns which are separately determined to correspond to two functions out of phase by pi /2 and which are derived from a desired asymmetrical amplitude characteristic by Fourier transform. The two functions are synthesized in the single interdigital transducer 2 by forming therein two sets of electrode fingers having overlap envelopes 2', 2'' corresponding to the two functions, respectively. An electroconductive member 5 is provided in the surface acoustic wave propagation path such that a surface acoustic wave excited by one set of electrode fingers is out of phase by pi /2 relative to a surface acoustic wave excited by the other set. <IMAGE>
Description
SPECIFICATION
Surface acoustic wave device having asymmetrical amplitude characteristics
This invention relates to a surface acoustic device having interdigital electrodes of a novel structure for realizing asymmetrical filtering characteristics.
According to one aspect of this invention there is provided a surface acoustic wave device comprising a first interdigital electrode for converting an electric signal to a surface acoustic wave signal, a medium for propagation of the surface acoustic wave signal from the first electrode and a second interdigital electrode for converting the surface acoustic wave signal to an electric signal, at least one of the first and second electrodes being apodized such that overlap lengths between adjacent two electrode fingers are variable, wherein the apodized electrode includes first and second regions of the electrode fingers the boundary between which is parallel to the direction of propagation of the surface acoustic wave signal, the overlap length
Wn of the electrode fingers in the first region and the overlap length Wn' of the electrode fingers in the second region being defined by the following equations (1) and (2), respectively: n #
Wn # h # - # ---------------- (1) 2fo 2#fo
n 1 #
Wn' # h# + - # ---------- (2)
2fo 4fo 2#fo 1 where n = -N, -N+1, ....., 0, 1, 2, ....., M
h(t) = a(t) cos#ot + b(t)sinw0t,
s'(#') = s(#' + #o),
#'(#') = #(#' + #o), #' = #-#o, #2' = #2-#o, #1'= #1-#o s(#) represents desired amplitude characteristic of the device, #(#) desired phase characteristic of the device, # angular frequency, fo center frequency, wo center angular frequency (2rfo), co and w2 desired angular frequencies defining angular frequency band width w2-w1, and # #
- - 2 # # # - 2 , and wherein a phase difference generating member is provided in the propagation path of the surface acoustic wave signal so that a surface acoustic wave signal excited by the electrode fingers in one of the two regions and propagated a predetermined distance is out of phase by 7r/2 relative to a surface acoustic wave signal excited by those in the other region.
One of the major features of this invention lies in that the two sets of interdigital electrode fingers are in a specific structure for realizing asymmetrical amplitude characteristics for the surface acoustic wave device characteristic which can obviate the prior art disadvantages described below and which can be materialized with a fixed space between electrode fingers of the interdigital electrode and with great freedom of design and manufacture process.
More particularly, two interdigital electrode patterns are separately determined in a manner well known per se which correspond to two orthogonal functions being out of phase by 7r/2 which are.derived from the desired filtering characteristic through Fourier transform. The two functions are synthesized in a unitary interdigital electrode on a piezoelectric substrate by forming therein two sets of electrode fingers having the overlap length patterns corresponding to the two functions, respectively. An electroconductive member is then provided in a surface acoustic wave propagation path on the surface of the piezoelectric substrate so that a surface acoustic wave excited by one set of electrode fingers is out of phase by m/2 relative to a surface acoustic wave excited by the other set.
The invention will now be described by way of example only with reference to the accompanying drawings, in which:
Figure 1 is a top schematic view of a prior art, typical surface acoustic wave device;
Figure 2 is a top view of a surface acoustic wave filter as a first embodiment of surface acoustic wave device in accordance with the invention; and
Figure 3 is a top view of a surface acoustic wave filter as a second embodiment of surface acoustic wave device in accordance with the invention.
In general, a surface acoustic wave device, especially highlighted as a surface acoustic wave filter, comprises as shown in Fig. 1 a piezoelectric substrate 1, an input electrode 2 for converting an electric signal into a surface acoustic wave signal, a propagation path 3 for the surface acoustic wave signal, and an output electrode 4 for restoring the surface acoustic wave signal to an electric signal, the input and output electrodes and the propagation path being formed on one surface of the piezoelectric substrate. With such a surface acoustic wave device, as taught by C. S. Hartmann et al, in IEEE Trans. Vol.MTT-21, No. 4 162 (1973), a desired filtering characteristic can be obtained by varying the overlap length and the spacing in the interdigital electrode However, a surface acoustic wave device with an asymmetrical amplitude characteristic produced according to the prior art tends to have irregularly narrow spaces between the electrode fingers in the interdigital electrode, with the consequent failure of products to short-circuit, thus resulting in the reduced production yield rate. To eliminate such a disadvantage, Yoshikawa et al (Japanese Patent Application Laid-Open No. 46350/'74) proposed a triple electrode filter with two input electrodes and one output electrode or one input electrode and two output electrodes.This approach succeeded in substantially solving the problem of filtering characteristics but raised practical problems, specifically of design and manufacture, such as complexity of the whole structure, difficulties for manufacture process due to the complexity, increased wire bonding steps due to increase in the number of electrodes, increased chip size and so on. Accordingly, it has urgently been desired to provide a further improved surface acoustic wave device.
Prior to describing a preferred embodiment of the invention, the theoretical ground thereof will first be described.
A Fourier transform h(t) of a given filtering characteristic f(w) is written as, h( t ) 1 rm 1 2"r = 2rr dw
1 1
- # S(#)e@ @@@@
2# -# d# ---- (1) where f(#): s(#)e-@#(#) s(w): desired amplitude characteristics of the filter (w): desired phase characteristic of the filter.Equation (1) can be reduced to,
h(t) = a(t) cos#ot + b(t) sin#ot ----------- (2) where,
s'(#') = s(#' + #o) ----------------------- (5), #'(#') = #(#' + #o) ----------------------- (6), #' = # - #o ----------------------------- (7), #o = 2#fo (center angular frequency) #2' = #2 - #o #1' = #1 - #o, and #1 and #2 are desired angular frequencies of the filter.Also, assumed herein are s(#)=s( - #) and #(#)= -#( - #).
To sample equation (2), it is a conventional practice to determine an overlap length Wn of one electrode finger set and an overlap length Wn' of the other electrode finger set respectively pursuant to the following equations (8) and (9), n n
Wn # h# # = (-1)n # a# # ------------- (8)
2fo 2fo n 1 n 1
Wn' # h# + # = (-1)n # b# + # -- (9) 2fo 4fo 2fo 4fo where n = -N, -N+1,. .,0,1,2,. .,M.
Then, two interdigital electrode finger patterns are separately prepared in accordance with equations (8) and (9) and thereafter arranged on the substrate such that a surface acoustic wave signal excited by one interdigital electrode is out of phase by #/2 relative to a surface acoustic wave signal excited by the other. This prior art arrangement is, however, disadvantageous as described above.
Thus, according to the invention, one set of electrode fingers having the overlap length Wn pursuant to equation (8) and the other having the overlap length Wn' pursuant to equation (9) are formed in a unitary interdigital electrode to create the #/2 phase difference, and an electroconductive member having a width as measured in the surface acoustic wave signal propagating direction which width corresponds to #o/4 (#o: wave length of the surface acoustic wave signal) is disposed in the propagation path for one set of electrode fingers, thereby eliminating the prior art defects.
In a preferred embodiment of the invention as shown in Fig. 2, two sets of electrode fingers of an interdigital electrode are synthesized or coupled such that the upper-half region and the lower-half region joined at center line A-A', for example, have the overlap length defined by W0 and that defined by Wn', respectively.
This idea can be further developed by effecting sampling pursuant to the following equations (10) and (11) to provide an interdigital electrode having substantially the same W and Wn' which is easy to pattern.
In other words, when the Fourier transform function h(t) is sampled precisely for determining the polarity for the electrode finger arrangement in the course of determination of the electrode overlap length and polarity, it is most likely to occur that the polarity of the interdigital electrode fingers be inverted at a position where the overlap length is minimum. In such a case, the technique illustrated in Fig. 2 may not be always useful. In one aspect of the invention, the sampling may be carried out according to the following equations (10) and (11) so that the maximum positions and minimum positions defined by Wn and Wn' in the two electrode finger sets are almost identical also in this case. Thus, troubles heretofore encountered in the determination of the electrode finger porality is removed to facilitate the patterning of the electrode fingers.
Wn # h# 2f@ 2#f@
0
n #
= a# - # # cos(n# = #)
2fo 2#fo
n #
+ b# - # # sin(n# = #) ----- (10)
2fo 2#fo
Wn' # h# + - # 2fo 4fo 2#fo
n 1 #
= a# + - # # cos{(n+)# - #}
2fo 4fo 2#fo
n 1 #
+ b# + - # # sin{(n+)# - #}
2fo 4fo 2#fo where - < # < - 2 2
Fig. 3 shows a second embodiment having interdigital electrode patterns obtained by replacing 8 in equation (10) and (1 1) with n/4. It is to be understood from Fig. 3 that a lefthand interdigital electrode has a compled electrode finger pattern wherein the upper-half region is substantially the same as the lower-half region.
To synthesize the sets of electrode finger patterns, an overlap length distribution containing values of the lengths pursuant to equations (10) and (11) with respect to positions (corresponding to n) of the electrode fingers of the interdigital electrodes, so to speak, a histogram of frequency distribution is first established and the values appearing along the common abscissa of the histogram are coupled to thereby constitute a unitary interdigital electrode. Upon synthesis, those values are multiplied by proportional constants which are eventually matched with an impedance of the coupled interdigital electrode and an impedance of the external circuit for determining a final pattern of the electrode fingers.
The surface acoustic wave device in accordance with the foregoing teachings may be used, as an example, for an intermediate frequency surface acoustic wave filter for use in a television receiver set, as will be described hereinafter.
Fig. 2 shows a surface -acoustic wave device embodying the invention in the form of a surface acoustic wave filter. In the figure, a piezoelectric substrate 1 is made of lithium niobate which is of 127.8 rotation Y-cut. Input and output electrodes 2 and 4 have about 0.8 jbm thick, aluminum vapor deposited films. Specifically, the input electrode 2 includes 50-pair apodized electrode fingers (reduced in number for simplified illustration), and the output electrode 4 includes 1 5-pair (also reduced in number for simplified illustration) normal electrode fingers with their space and overlap length made constant. The input electrode 2 has a resultant envelope of the overlap length as a result of synthesizing two overlap length envelopes 2' and 2" for 0 = 0 in equations (10) and (11) about center line A-A'.Reference numeral 5 represents an electroconductive member of aluminum for generating the phase difference. The member 5 has, in this embodiment, a width of 570 m as measured in the direction perpendicular to the direction of the surface acoustic wave propagation and a length of about 17 m##o/4 (#o: center frequency) as measured in the direction of the surface acoustic wave propagation so that a surface acoustic wave signal excited by one set of electrode fingers contributing to the envelope 2' is out of phase by #/2 relative to a surface acoustic wave signal excited by the other set contributing to the envelope 2".
Fig. 3 shows a second embodiment of the invention wherein an input electrode 2 is arranged for # = #/4 in equations (10) and (11) and a phase difference generating member 5 extends transversely of a piezo-electric substrate 1 to also act as an earth electrode for preventing electric noise which may otherwise appear due to direct coupling of an electric signal between input and output electrodes 2 and 4. Except these points, the second embodiment is the same as the first embodiment of Fig. 2.
It will be appreciated that the above described teachings of the invention may be applied to the output electrode.
As has been described, the invention ensures the provision of the surface acoustic wave device with electrode fingers of a fixed space therebetween which can readily realize the asymmetrical frequency characteristics. Namely, since the two sets of electrode fingers of a fixed space having different frequency characteristics are formed in the unitary interdigital electrode, it is possible to prevent decrease in the production yield rate due to inter-electrode finger shortcircuiting which is caused across irregularly narrowed space between electrode fingers in the prior art device and to decrease the number of bonding sites for connection to the external circuit with consequent miniaturization of the chip size.
Claims (4)
1. A surface acoustic wave device comprising a first interdigital electrode for converting an electric signal to a surface acoustic wave signal, a medium for propagation of the surface acoustic wave signal from the first electrode and a second interdigital electrode for converting the surface acoustic wave signal to an electric signal, at least one of the first and second electrodes being apodized such that overlap lengths between adjacent two electrode fingers are variable, wherein the apodized electrode includes first and second regions of the electrode fingers the boundary between which is parallel to the direction of propagation of the surface acoustic wave signal, the overlap length W, of the electrode fingers in the first region and the overlap length Wnl of the electrode fingers in the second region being defined by the following equations (1) and (2), respectively: n #
Wn # h # - # ---------------- (1) 2fo 2#fo n 1 #
Wn' # h# + - # ---------- (2) 2fo 4fo 2#fo 1 where n = -N, -N+1, , 0, 1, 2 ....., M
h(t) = a(t) cos#ot + b(t) sin#ot,
s'(#') = s(#' + #o),
#'(#') = #(#' + #o), #' = # - #o, #2' = #2 - #o, #1' = #1 - #o s(#) represents desired amplitude characteristic of the device, #(#) desired phase characteristic of the device, # angular frequency, fo center frequency, #o center angular frequency (2#fo), #1 and #2 desired angular frequencies defining angular frequency band width #2 - #1, and # #
- - < # < - 2 2 and wherein a phase difference generating member is provided in the propagation path of the surface acoustic wave signal so that a surface acoustic wave signal excited by the electrode fingers in one of the two regions and propagated a predetermined distance is out of phase by #/2 relative to a surface acoustic wave signal excited by those in the other region.
2. A surface acoustic wave device according to Claim 1 wherein the electrode finger structure is such as to satisfy 0 = 0.
3. A surface acoustic wave device according to Ciaim 1 wherein the electrode finger structure is such as to satisfy # 0= -.
4. A surface acoustic wave device constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Fig. 2 or Fig. 3 of the accompanying drawings.
4
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP13887778A JPS5566120A (en) | 1978-11-13 | 1978-11-13 | Elastic surface wave device |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2044569A true GB2044569A (en) | 1980-10-15 |
| GB2044569B GB2044569B (en) | 1983-02-23 |
Family
ID=15232194
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB7939311A Expired GB2044569B (en) | 1978-11-13 | 1979-11-13 | Surface acoustic wave device having asymmetrical amplitudecharacteristics |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS5566120A (en) |
| DE (1) | DE2945643A1 (en) |
| GB (1) | GB2044569B (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0260656A3 (en) * | 1986-09-19 | 1989-05-24 | Hitachi, Ltd. | Unidirectional surface acoustic wave device |
| GB2306821A (en) * | 1995-11-03 | 1997-05-07 | Advanced Saw Prod Sa | Electro-acoustic devices |
| US5867075A (en) * | 1995-03-31 | 1999-02-02 | Siemens Aktiengesellschaft | Surface wave dual converter for a dual filter in particular for television sets |
| US6067449A (en) * | 1995-12-21 | 2000-05-23 | Nokia Mobile Phones Limited | Antenna selection control circuitry |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5836012A (en) * | 1981-08-27 | 1983-03-02 | Seiko Epson Corp | surface acoustic wave device |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3818379A (en) * | 1972-12-04 | 1974-06-18 | Hughes Aircraft Co | Acoustic surface wave device |
| FR2319245A1 (en) * | 1975-07-24 | 1977-02-18 | Thomson Csf | PERFECTED TRANSDUCER FOR SURFACE WAVE FILTER WITH ASYMMETRICAL TRANSFER FUNCTION AND FILTER CONTAINING SUCH A TRANSDUCER |
| DE2615719A1 (en) * | 1976-04-09 | 1977-10-27 | Siemens Ag | Filter based on surface wave principle - has weighted and non-weighted transducers giving mean frequency with linear phase characteristic |
-
1978
- 1978-11-13 JP JP13887778A patent/JPS5566120A/en active Pending
-
1979
- 1979-11-12 DE DE19792945643 patent/DE2945643A1/en active Granted
- 1979-11-13 GB GB7939311A patent/GB2044569B/en not_active Expired
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0260656A3 (en) * | 1986-09-19 | 1989-05-24 | Hitachi, Ltd. | Unidirectional surface acoustic wave device |
| US5867075A (en) * | 1995-03-31 | 1999-02-02 | Siemens Aktiengesellschaft | Surface wave dual converter for a dual filter in particular for television sets |
| GB2306821A (en) * | 1995-11-03 | 1997-05-07 | Advanced Saw Prod Sa | Electro-acoustic devices |
| GB2306821B (en) * | 1995-11-03 | 2000-05-31 | Advanced Saw Prod Sa | Electro-acoustic device |
| US6268782B1 (en) | 1995-11-03 | 2001-07-31 | Micronas Semiconductor Sa | Electro-acoustic device with a variable acoustic wave velocity piezoelectric substrate |
| US6067449A (en) * | 1995-12-21 | 2000-05-23 | Nokia Mobile Phones Limited | Antenna selection control circuitry |
Also Published As
| Publication number | Publication date |
|---|---|
| DE2945643A1 (en) | 1980-06-19 |
| JPS5566120A (en) | 1980-05-19 |
| GB2044569B (en) | 1983-02-23 |
| DE2945643C2 (en) | 1988-08-11 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |