CA1047615A - Surface acoustic wave filter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A surface acoustic wave filter having a,piezo-electric substrate with a major surface, input and output transducers which are formed on the major surface. Finger electrodes of one of the transducers are made as step type ones to divide the surface wave propagation path into a plurality of surface wave propagation channels. The phase difference of channels and the length of the steps of the finger electrodes are determined to minimize the multiple re-flection echo between the input and output transducers. Some ripples in the pass band are improved.

Description

BACKGROUND OF THE INVENTION

Field o~ the Invention This invention relates generally to a sur~ace acoustic wave filter device. and is directed more particularly to a surface acoustic wave filter device in which the ripples in a pass band are suppressed by means of a step type transducer.
Description of the Prior Axt In general, since a filter device utilizing a surface acoustic ware can produce a coherent ultrasonic wave and detect the same due to lts inter-digital transducer Or the transversal type, it is considered that the above îilter device is used as a linear phase band pass filter or a low ripple band pass filter which ser~es as a band pass filter for an intermediate frequency signal in an FM radio receiver or as a band pass filter îor a video intermediate fre-quency signal in a tele~ision receiver. From the practical point of ~riew, how-ever, ~arious echo components caused therein become obstacles and hence ripples are caused in the frequency characteristics and phase characteristics in pass band. The cause for distorting the phase is essentially due to rnu1tiplerellection echos between inputs and outputs Or the sur~ace acoustic wa~e filter.
~a~

- 2-:'~
.

- . , : ~ ~ . , , , . . . : .
.: : , ~ .: . , . , ., : ~

: . . ,. :. ' ,., ' , . : . ,: :
:: . . ~ ~ . . . :

lD47tilS
BRIEF DESCRIPTION OF THE DRAWINGS
.
Fig,. 1 is a schematic diagram of a surface acoustic wave filter used ~or esplain the present invention;
Fig, 2 is a schematic plan view showing an embodiment of the surface acoustic wave filters according to the present invention;
Fig. 3 is a schematic plan view show;ng another-embodiment of the surface acoustic wa1re filters accordîng to the present inYention; and Fig. 4 is a graph showing the frequemcy response characteristics of the surface acoustic wave filter of the invention shown in Fig. 3. ~` -I`here are tollowing two components in the reSlection echo o~
the sur ace acoustic wave on the interdi5ital transducer. One of the component~

~ .

..

. ~ .
., .

::
' .: .. .
. - 2a-is an electrical reflection due to the acousto-electric regeneration through a terminal load, and the other component is a mechanical reflection due to the acoustic wave impedance mismatch between metallized and unmetallized sec-tions of the transducer Such a multiple reMection echo E will be now described on a surface acoustic wave filter shown in Fig. 1. In Fig. 1, reference numeral 1 designates a piezo-electric substrate with a ma;jor surface which is made of such, for example, as lead zirconate titanate ceramics (PZT)7 lithium niobate (I,iNbO3) or bismuth germanium oxide (Bi1 2GeO2~). On one surface or the major surface of the piezo-electric substrate 1, there are located two interdigital transducers 2 and 3 apart at a predetermined distance in opposed relation.
Each of the interdigital transducers 2 and 3 consists oï a plurality of trans ducer elements. In this case, the arranging pitch oî the stripes Or the inter- -digital transd~lcers 2 and 3 is determined in response to the center frequency l 5 f Or a usecl frequency band. One of the interdigital transdllcers or the inter-digital transducer 3 in this example is divided in the beam width direction of the surface acoustic wave into a plurality of transducer elements to form a plurality of surface acoustic wave propagation channels 3a, 3b, .., 3k, .....
The lengths of the electrodes of the plurality of transducer elements corres-ponding to the respective surface acoustic wave propagation channels 3a, 3b, ,. . 3k, ... or the widths of the latter are taken as W1, W2, ... Wl, ...~.
The phase shirts or deviations of the electrodes of the respective surface acoustic wave propagation channels 3a, 3b, ... 3lc, . . from a reïerence line A, which is parallel to the interdigital ransducer 2, are taken as ~1 ' 2' ~ k~ ... In this case, the phase shifts ~ 2~ .. Gk- .. . are equal to - the phase differences between the lengths of the respective channels and an n~th wave length (n being a positive integer) at the center frequency under the resonance state. The mechanical reflection coefficients due to the acoustic impedance mismatch between metallized and unmetallized sections of the trans-ducers 3 and 2 are taken as R and R~, and the electrical renec-tion coeîficients ' ., , '' , .' ~7~5 ~ue to the electro-mechanical regeneration oi the tranducers 3 and 2 are taken Q and Q~. At this time~ the multiple rellection echo E (triple transit echo) is expressed as ~ollows:
E = B [RRI ~k Wk ej3~k ) + (R'Q ~ RQ') ( k Wk ei2~ak ~
; x (~ W ej~k ) + QQI(~ W~ ej~k)3~ ,-,.,,,,,......... ,,,(1) where B represents a constant.
In Fig 1. re~erence numerals 4a and 4b designate input te~ : :
nals. and 5a and 5b output terminals, respectively.
In order to cancel such a multiple reilection echo, there has been :
known a method as shown in Fig. 1 in the art in which the interdigital trans- :
ducer 3 is employed. The finger electrodes of the interdigital transducer 3 are divided into a plurality oS electrode elements or transducer elernents in the beam width direction (in the direction perpendicular to the beam propagation direction of a surSace acoustic wave) to form a plurality of surface acoustic wave propagation channels and to give a phase difference l~ ~ between the ad-jacent ones, whereby the echoi which is caused by the mechanical reilection :.
due to the acoustic inpedance mismatch between metalli~ed and unmetalliz;ed sections of the transducer, is suppressed (rerer to the gazette o~ the Japaness Patent App1ication laid open No. 107156/1974 to Nippon Electric and laid open date October 11, 1974).
. o With this prior method~ the echo component expressed by the first term of the equation (1) can be suppressed, but the echo components e:c-pressed by the second and third terms of equation (1) can not be suppressed any and hence the multiple rellection echo E can not bé suppressed sufSicien?ly.As a result, ripples appear in the frequency and phase characteristics in pass . band~ snd hence ~uch a prior art method is not so practical.
' . . ' . , . :' OBJECTS AND SUMMARY OF THE INVENTION

Accordingly,. ii is an object o~ ~he present invention to provid~
,, an improYed surtace acoustic WaYe Mter. ~:

"., :": ' ~ `~4 ~b~L5 It is another o~.jec-t of the invention to provide a surface acoustic wave filter in ~ch r~pples in pass~-band are suppressed.
~ccording to an aspect of the present invention there is prov~ded a surface acoustic wave filter comprising:
a. a piezo-electric su~str~te having a major surface;
b. input and output transducers formed on the major sur-face of said su~strate, each of said transducer consisting .::
of a plurality of transducer elemen-ts; and lQ c. a plurality of surface acoustic wave propagation : channels formed on said substrate between said input and output transducer; in which,.when the width of a channel is taken as Wk, and the phase difference between the length of the channel k and n times the wave length at the center frequency is taken as ~k ~n being a positive ~nteger~, the values of ~k and Wk are selected to satisfy : th-~ following condition:
.
IRR t~ Wke;3~k~t~Q ~R Q~ t~Wkej29k) (~Wkej9k)+QQ (~Wke;~k)3¦
2Q ~10-2 ~ ~Wkei ak ~

.
where R and R' are, respectively, mechanical re~lection coeffi- ~ :
cients of said input and output transducers, Q and Q' are, . respectively, electrical reflection coefficients of said input . and output trans~ucers, and ~kWk is normallized as ~kWk=l, the :~
transducer elements of said input transducer being located close ~~.
together ~nd electrically aonnected, and said plurality of .~:
: surface acoustic wave proFagation channels being formed without an overlapping are~, ~fie additional and other objects, features and advantages of t~e pxesent invention will become apparent from the following . descrpt~on taken in conjunction with ..the accompanying drawings.
: -5- :

- . : . . '': , ' .'' , . '''. ' .. ` .' :

DE;SCRI3PTION OF THE PREFÆRRED EMBODIMENTS : .
' ' ' - ' An embodiment of the suriace acoustic wave filters according to the present invention will be. hereinbelow described with reference to Fig. 2 O in which the parts corresponding to those of Fig 1 are marked with the cor-responding reference numerals and letters . ~ .
In the embodiment of Fig. 2, lithium ~niobate ~LiNbO3) is empl~yed ~. :
as the pieso-electrio substrate 1 which is used as a y-cut, 2;-direction pro~
pagation (the surface ;s perpendicular to the y-axis, and the direction of the surface acoustic wave propagation is paral1el to the z-axis~. In this case, ~- the effective electro-mechanical coupling coefficient is taken as 4.5 ~. In this en~bodiment~ further such an interdigital transducer 2 is errployed that the nuri~ber of its finger-shaped electrode.pairs is 15J and such an interdigital transducer 3 is empl~yed that the number of its finger~shaped electrode pairs :.
~zO is 11 and the width of the finger-shaped electrodes is selected equal to the `. . distance between the adjacent ones Between the transducers 2 and 3 there is ~:
formed a surrace acoustic wave propagation channel.6 on the substrate 1. The . .
interdigital transducer ~ is also divided into two transducer elements to fo~n two surface acoustic wave propagation channels 3a and 3b In this case~ the : .
phase. shifts e1 and e2 of the finger-shaped electrodes ol the suriace acoustic wa~e propagation channels 3a and 3b from the reference line A are selected . ::
as--0 and ~ 32 = ~)~ respectively, where the reference line A is . :~
the center line between the finger-shaped electrodes of the surrace acbustic wave propopagation channels 3a and 3b~ and the lengths of the ~i~ger-shaped .':

: ~ ' .. ' ., " ~ ' ' '.'. ~''.~.
" ~ ' .
. :

, ., . . . . . . . . . : .

~7~
electrodes are selected as W1 and W2 (W1 ~ W2), respectively. In this case, : ~-if W1 and W2 are nomalized as W1 -~ W2 = 1 J W1 and W2 can be expressed as follow s:
W1 = sin2 ¢`/2 W2 = cos2 ~/2 In this case, the main signal SM ;s expressed as follows:
SM = B ~ Wk ei ls) = B (cos ~ -~ i cos ~ sin ~ ) Meantime, the multiple reflection or triple transit echo E is expressed as follows:
E = B ~RRI (~ Wk e;3~kj ~ (RIQ + RQ~ Wk eJ2~k) X (~ Wk e;~k~ ~ QQI (~ Wk e;~k)3 ~ :
a B LRR~ ( COS 3~ ~ i cos 0 sin 3~ (RQI ~ RIQ) X (cos 2~ -~ i cos 0 sin 2~) (cos 0 -~ :i cos 0 sin ~) -~ QQ~ ( cos ~ -~ i cos QS sin ~)3 ~
In this caseJ the mechanical reflection coefficient R due to the . acoustic impedance mismatch between metallized and unmetallizecl sections of the transducer is expressed as follows:
R = i tan h (N~) .
where N represents the number of finger-shaped electrode pairs of the inter-digital transduce:r~ and- ~;is ?~ in the reflection coefficient of the surface acous-tic wave due to the acoustic impedance mismatch per one pair of the finger-shaped electrode pairs of the interdigital transducer i sin ~ and its transmis-sion coefficient cos ~ and is determined by the substance, the surface dlrectlonof the substrate and surface acoustic wave propagation direction, respectively Further, the electrical reflection coefficient Q due to the electro-mechanical regeneration is expressed by the following equation when it is assumed that the conversion efficiency from the surface acoustic wave to the electrical signal is taken as L, the conversion efficiency from the electrical signal to the aurace acoustlc wave as M, -the admittance of the transducer as .

.

:~ 7 :

Yin = Gin -~ j Bin and the admittance of a load as Yex = Gex ~ j Bex, respectively, Q LM
Yin ~ Yex Under the resonance state at the center frequency, Yin~= Yex (where Yin is the complex conjugate of Yex), so that the electrical reflection coefîicient Q is expressed as follows:
1 + i sin h (N~) ~ 2 cos h (I~) In the present invention, the ratio of E by SM (E/SM) can be expressed based upon the above equations as follows:

E ¦RRI (~k Wk ei3~k) ~ (R~Q ~ RQ~ {Wkej2~) (7Wkei~k) SM ¦ ~l Wk e -~ QQt (~c Wl ei~l~)3¦
.
¦ RR' ¦cos 3~ ~ i cos ~ sin 3~ (RQI ~R~Q) (cos2t9~icos~sin2~) I ~ cos ~ -~ i cos ~) sin ~) l x(cos ~ -~ icos ¢` sin ~ QQ~ ¦cos ~ -~ i cos ~ sin ~3)31 Now, when the values of ~ and ~, which would minimize the above equation, are obtained by a so-called trial and error method, ~ = 2.26 and ~ =
0.819 ~45 as SE` = 0.37 x 10 8 in the embodiment of Fig. 2, so that W1: W2=
0 85: 0.15.
In fact, if the following condition is satisfied, such a filter can be used practically.

M
As described above~ according to the present invention, the surface acoustic wave filter, which can suppress the multiple reflection or triple transit echo E to such an extent that the ratio E/SM is smaller than 10 2, is obtained In other words, such a surface acoustlc wave filter, which can - suppress surface acoustic wave echos caused by both the mechanical and elec-trical reflection, is obtained.

'.:
:~' :`.
` .

,.

~7~315 In the embodiment of the invention shown in Fig 2, the inter-digital transducer 3 is divided into two transducer elements to form two sur-face acoustic wave propagation channels 3a and 3b, but it will be easily understood that even when the interdigital transducer 3 is di~ided into more than three transducer elements to form more than three surface acoustic wave propagation channels, the same effects can be jperformed.
Fur-ther, when the surface acoustic wave f-ilter of the invention is used as a delay element, the multiple reflection echo can be, of course, suppressed effectively.
It is also possible that the input and output transducers~ which form a plurality of surface acoustic wave propagation channels, are arranged close with each other to form the propagation channels separately but in paral-lel with one another and the electrodes of the respective input and output trans-ducer elements are electrically connected.
Fig, 3 show another embocliment Or the present ;nvention in which the reference numerals similar to those used in Figs. 1 ancl 2 designate similarelements.
In the embodiment of Fig. 3, the surface acoustic wave propaga-tion channel 6 is divided into three channels 6b, 6a1 and 6a2 which are selecteddifferent in width~ and the surface acoustic wave propagation channel 6b whose width is smallest, is located except both side ends of the surface acoustic wavepropagation channel 6. That is, in this example, the surface acoustic wave propagation cl1annel 6b of smallest width is located along the center of the chan_ nel 6 and surrace acoustic wave propagation chanllels 6a1 and 6a2~ whose widths are wider than that of the channel 6b, are loca~ed at the both sides of the center channel 6b.
With the embodiment shown in ~ig. 3~ the surface acoustic wave beams produced in the center surface acoustic wave propagation channel 6b are affected by diffraction, but all the beams can be incident upon t~e transducer without departing from the opposed transducers. Meantime, the surface acous-tic wave beams produced in the surface acoustic wave channels 6a1 and 6a2 f .. .. , ~ ' .

; ~ 9 _ ~,, greater width are subjected to diffraction and scattering a~
their ends, but the energy of the diffraction and scattering : is small as compared with the total energy of the channels 6al and 6a2. Therefore, the diffraction and scattering energy does not a~fect performance significantly.
For this reason, the embodiment of Fig. 3 can suppress the multiple re~lection echo more effectively and hence it may be appreciated that the phase can be compensated for more correctly -Fig. 4 is a graph showing the frequency characteristics of the surface acoustic wave filter of the invention shown in Fig. 3 when~Wk is normalized as ~Wk = 1 and the widths of the surface acoustic wave propagation channels 6a1~ 6b and 6a2 are selected as 2 5 ~ 0.15 and 2 5 ~ respec-tively.
It will be apparent from the graph of Fig. 4 that the ripples are `i much reduced or improved near the center frequency of pass band as compared with the prior art.
The embodiment of Fig. 3 is such an example that the surface acoustic wave propagation channel is divided into three channels of different widths and the channel of the smallest width is located between the other chan-nels, but it wil1 be apparent that even if the surface acoustic wave propagation -channel is divided into more than three channels of different widths, the samè : ~
effect can be performed by locating-the channel of the smallest width at a ;
position except both side ends. - -' ., . ',.'' ' . :
It will be apparent that many modifications and variat;ons could be-made by one skilled in the art withDut departing from the spirits or scope of the novel concepts of the present invention, so that the spirits or scope o~
the invention should be d~termined by the appended claims ' . ' ' . .
, .~. "... .
.
, ' ' ' '' ', " ' , ' '. .
' ~ 10- .
- .

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED, ARE DEFINED AS FOLLOWS:

1. A surface acoustic wave filter comprising:
a. a piezo-electric substrate having a major surface;
b, input and output transducers formed on the major sur-face of said substrate, each of said transducer consisting of a plurality of transducer elements; and c. a plurality of surface acoustic wave propagation channels formed on said substrate between said input and output transducer; in which, when the width of a channel k is taken as Wk, and the phase difference between the length of the channel k and n times the wave length at the center frequency is taken as .theta.k (n being a positive integer) the values of .theta.k and Wk are selected to satisfy the following condition:

were R and R' are, respectively, mechanical reflection coeffi-cients of said input and output transducers, Q and Q' are, respectively, electrical reflection coefficients of said input and output transducers, and .SIGMA.kWk is normallized as .SIGMA.kWk=1, the transducer elements of said input transducer being located close together and electrically connected, and said plurality of sur-face acoustic wave propagation channels being formed without an overlapping area.

2. A surface acoustic wave filter as claimed in claim 1, in which said input and output transducers are formed integrally and at least one of them has a step type configuration.

3. A surface acoustic wave filter as claimed in claim 2, in which the channel having the smallest width Wk is located in the surface acoustic wave propagation channel in a position removed from the sides thereof.

4. A surface acoustic wave filter as claimed in claim 1, in which said piezo-electric substrate comprises a lithium niobate single crystal of y cut and Z direction propagation.

5. A surface acoustic wave filter as claimed in claim 1, in which each of said input and output transducers includes two pairs of transducer elements.

6. A surface acoustic wave filter as claimed in claim 5, in which said transducer elements are made such that said phase difference satisfies .theta. ?45° and the widths W1 and W2 of said two channels satisfy W1:W2?0.85:0.15, respectively.