Description

SAW filter having reduced triple transit response Field of the invention

The present invention relates to SAW (Surface Acoustic Wave) filters in which triple transit response suppression is enhanced and which comprises a small number of additional finger elements.

Description of the related art

The article "A Triple Transit Suppression Technique" by

Kentaro Hanmar and Bill J. Hunsinger, Coordinated Science

Laboratory, University of Illinois at Urbana Campaign, 61801, refers to the problem of triple transit responses in radio frequency filters working with surface acoustic waves.

Analyzing and suppressing techniques improving SAW transducer reflections are described and their effect is verified.

Conventional filter arrangements working with surface

acoustic waves and comprising an input transducer and an output transducer, both being arranged between two reflector elements within an acoustic track suffer from triple transit response. In general, input transducers convert radio

frequency signals by means of finger shaped electrode

structures into surface acoustic waves propagating within an acoustic track at the surface of a piezoelectric substrate. Output transducer transform surface acoustic waves in radio frequency signals by virtue of an analog electrode finger structure. Electrode Fingers not only generate radiofrequency signals from surface acoustic waves. They also regenerate surface acoustic waves from the produced radio frequency signals. The electrode finger structures not only convert surface acoustic waves into radio frequency signals but also reflect a certain percentage of the surface acoustic waves. Thus, a small but finite amount of surface acoustic wave energy is emitted by the input transducer, reflected by the output transducer, re-reflected by the input transducer and finally converted to a radio frequency signal by the output transducer. There is, of course, a phase delay between the primary radio frequency signal and the signal due to

reflection and re-reflection. The accordingly named triple transit response superimposes the original signal. It is a source for passband ripple when the respective SAW device is used within a bandpass filter.

Such triple transit spurious responses are an intrinsic problem of ordinary SAW devices and it is an object of the present invention to provide an SAW filter having reduced spurious triple transit responses.

Summary of the invention

The present invention provides SAW filters comprising an input transducer having a source center and an output

transducer having a drain center. The source center can be thought of as an imaginary line being perpendicular to the direction of propagation of the emitted surface acoustic waves that effectively acts as the excitation center. Analog, the drain center can be thought of an imaginary line

perpendicular to the propagation of the surface acoustic wave that effectively acts as the center converting surface acoustic waves into a radio frequency signal. In reality, both - the input transducer and the output transducer - comprise a plurality of structured "interdigitated" (i. e. comb shaped) electrode fingers where the finger pitch equals mainly λ/4 and where λ is the wave length of the surface acoustic wave.

The input transducer is arranged in an acoustic track on a piezoelectric substrate. The output transducer is also arranged within the acoustic track on the piezoelectric substrate .

Further, first reflecting means and second reflecting means are arranged within the acoustic track on the piezoelectric substrate. The input transducer and the output transducer are arranged between the first reflecting means and the second reflecting means. The first reflecting means and the second reflecting means reflect surface acoustic waves.

The source center is located at a position STDi along a longitudinal direction of the acoustic track and the drain center is located at a position DTDo along the longitudinal direction .

The first and the second reflecting means are arranged relative to the source center and relative to the drain center in such a way that the triple transit acoustic waves are canceled at least partially by canceling acoustic waves that are emitted by the source center, at least partially reflected by the second reflecting means, and at least partially re-reflected by the first reflecting means, where the triple transit acoustic waves are generated by the input transducer and two times reflected by the output transducer and the input transducer respectively. The canceling waves and the triple transit waves have a phase difference between 90° and 270°.

The idea of the present invention is to use additional reflection in order to create canceling acoustic waves that have a phase difference between 90° and 270° relative to the triple transit response signal. As a result, there will be negative interference between the canceling wave and the triple transit waves. Of course, it is preferred to have a phase difference of exactly 180° where the negative

interference is most efficient.

Canceling acoustic waves are reflected at the first

reflecting means and re-reflected at the second reflecting means. The canceling acoustic waves enter the drain center and cancel the triple transit waves.

In one embodiment, the input transducer has reflection structures that form the first reflecting means. The first reflecting means have their reflection center at a position RTDi along a longitudinal direction of the acoustic track. The output transducer has reflection structures that form the second reflecting means with their reflection center at a position RTDo along a longitudinal direction of the acoustic track. Then, the positions STDi, RTDo, RTDi and DTDo are chosen to fulfill the following requirement:

(i+1/4) * λ < |STDi-RTDo| + | RTDi-RTDo | + | RTDi-DTDo | - 3* I STDi-DTDo I < (i+3/4) * λ where i is an integer number and λ is the wave length of the acoustic waves. In the equation, | STDi-RTDo | represents the distance between the source center of the input transducer and the reflection center of the reflection structure of the output transducer (second reflecting means). |RTDi-RTDo| represents the

distance of the reflection center of the first reflecting means (the reflection structures of the input transducer) and the reflection center of the second reflecting means (the reflection structures of the output transducer). |RTDi-DTDo| represents the distance between the reflection center of the first reflecting means and the drain center of the output transducer. Thus, | STDi-RTDo | + | RTDi-RTDo | + | RTDi-DTDo | represents the distance the canceling waves have to pass before arriving at the drain center of the output transducer. I STDi-DTDo I represents the distance between the source center of the input transducer and the drain center of the output transducer. 3* | STDi-DTDo | , thus, represents the distance the triple transit acoustic waves cover before arriving at the drain center of the output transducer. Thus, the above equation states that the phase difference between the

canceling waves and the triple transit waves is in-between 90° ( [i+1/4] * λ) and 270° ([i+3/4] * A) .

The reflection structures may be additional finger electrodes within the respective transducer that do not attribute to the source center. They may or may not be electrically connected to the bus bar of the respective transducer.

In one embodiment, the SAW filter comprises a first reflector forming the first reflecting means and a second reflector forming the second reflecting means. The first reflector is arranged within the acoustic track and has a reflection center at a position RRl along the longitudinal direction of the acoustic track. The second reflector also is arranged within the acoustic track and has a reflection center at a position RR2 along the longitudinal direction of the acoustic track :

The positions RRl and RR2 are chosen in such a way that the following equation is fulfilled.

(i+1/4) * λ < |RRl-STDi| + |RRl-DTDo| - ( | STDi-RR2 | + | DTDo- RR2 I ) < (i+3/4) * λ where i is an integer number and λ is the wave length of the acoustic waves. The input transducer and the output

transducer may be arranged between the first reflector and the second reflector. The sum (|RRl-STDi| + |RRl-DTDo|) represents the length within the acoustic path an acoustic wave has to travel after being emitted by the source center, reflected by the first reflector and absorbed by the drain center of the output transducer. In contrast, the sum ( | STDi- RR2 I + I DTDo-RR2 I ) represents the length of an acoustic path of an acoustic wave that is emitted by the source center of the input transducer, reflected by the second reflector and absorbed by the drain center of the output transducer. In other words, the first reflector and the second reflector are positioned within the acoustic track in such a way that additional reflections of surface acoustic waves occurring at these reflecting elements reflect waves that cancel each other and, thus, may not contribute to spurious signals in the output transducer.

In one embodiment, the SAW filter comprises a first reflector forming the first reflecting means within the acoustic track with the reflection center at the position RRl along the longitudinal direction of the acoustic track and a second reflector forming the second reflecting means within the acoustic track with the reflection center at the position RR2 along the longitudinal direction of the acoustic track where

(i+1/4) * λ < |RR1 - STDi I + | RR1 - RR2 | + | DTDO - RR2 | - 3*|STDi - DTDo I < (i+3/4) * λ

In other words: the position of the first reflector and the second reflector are chosen in such a way that surface acoustic waves emitted by the source center, reflected by the first reflector, re-reflected by the second reflector and finally absorbed by the drain center of the output transducer (which waves are themselves another kind of spurious triple transit signals) will eliminate the conventional triple transit signal produced by reflections at the source center and the drain center.

In one embodiment, the SAW filter comprises a first reflector as the first reflecting means with a reflection center at the position RR1 along the longitudinal direction, and a second reflector as the second reflecting means with a reflection center at the position RR2 along the longitudinal section. The positions RR1, STDi, DTDo, and RR2 are chosen in such a way that the following equation is fulfilled:

(i+1/4) * λ < |RR1 - STDi I + | RR1 - RR2 | + | RR2 - DTDo | - ( I STDi-RTDo I + I RTDi-RTDo I + I RTDi-DTDo I ) < (i+3/4) * λ In other words: acoustic waves being generated by the source center and being reflected by the first reflector, then being re-reflected by the second reflector and then being absorbed by the drain center ( | RR1 - STDi | + | RR1 - RR2 | + | RR2 - DTDo I ) cancel acoustic waves that are generated by the source center, reflected by the reflection structures of the output transducer, and are re-reflected by the reflection structures of the input transducer and that are absorbed by the drain center ( | STDi-RTDo | + | RTDi-RTDo | + | RTDi-DTDo | ) .

However, it may be preferred that the above signals are mainly in-phase: (i-1/4) * λ < |RR1 - STDi I + | RR1 - RR2 | + | RR2 - DTDo | - ( I STDi-RTDo I + I RTDi-RTDo I + I RTDi-DTDo I ) < (i-3/4) * λ

In one embodiment, the SAW filter has its drain center and its source center at positions in such a way that surface acoustic waves that are generated by the source center, that are reflected at the drain center at the position RTDo, that are re-reflected at the reflection structures forming the first reflection means at the position RTDi, and that

absorbed at the drain center travel along a path whose length complies mainly with the product of an integer plus one half and the wave length λ:

(i+1/4) * λ < I STDi - RTDo I + | RTDi - RTDo | + | RTDi - DTDo | < ( j +3/4) * λ

In one embodiment, the input transducer of the SAW filter comprises a bi-directional transducer that emits between 60% and 80% of the SAW energy towards the output transducer and that emits between 40% and 20% of the SAW energy towards the opposite direction.

Such a transducer design is mainly used for decreasing the bidirectional loss by emitting energy mainly in the direction towards the output transducers. Then, of course, spurious primary acoustic waves that - when they are re-reflected at reflecting structures - can cause less spurious responses due to different and unwanted phase delay times.

In one embodiment, the reflection capability of the

reflection structures of the input and the output transducer and the reflection capability of the first and the second reflector are chosen to cancel triple transit signals by combining canceling signals of two different paths:

This first canceling path consists of

the route from the position of the source center of the input transducer towards the reflection center of the first

reflector plus

the route from the reflection center of the first reflector towards the reflection center of the second reflector plus the route from the reflection center of the second reflector towards the drain center of the output transducer.

Its length is | RR1 - STDi | + | RR1 - RR2 | + | DTDO - RR2 | . The other canceling path consists of

the route from the position of the source center of the input transducer towards the reflection center of the output transducer plus

the route from the reflection center of the output transducer towards reflection center of the input transducer plus the route from the reflection center of the input transducer towards the drain center of the output transducer.

Its length is | STDi-RTDo | + | RTDi-RTDo | + | RTDi-DTDo | . Then, the signals of the first canceling path have a signal strength and a phase SI according to

Sl=CCl * exp {j*(|RRl - STDi | + | RR1 - RR2 | + | DTDO - RR2|)*k}.

The signals of the other canceling path have a strength and a phase S2 according to S2=CC2 * exp { j * ( I STDi-RTDo | + | RTDi-RTDo | + | RTDi-DTDo | ) *k} .

The Triple Transit Signals have a strength and a phase according to S3=CC3 * exp {j*(3*|STDi - DTDo|)*k}.

SI, S2, and S3 are complex values having an amplitude

representing the strength and a phase. CCl, CC2, cc3 determine the reflection strength.

The phases of SI, S2, and S3 are optimized by selecting the position of the respective reflectors. Further, CCl, CC2, cc3 are determined to fulfill the equation S1+S2+S3 = 0; i. e. S2=-(S1+S2) .

It is preferred that cc2 is as small as possible.

In one embodiment, the SAW filter comprises input transducers or output transducers with reflecting fingers that are not electrically connected with the bus bar of the respective transducer . Such floating electrodes allow it for transducers to emit different percentages of energy towards the two different directions. In the extreme case where all energy is emitted in one direction, a unidirectional transducer in gained.

In one embodiment, an SAW filter comprises reflecting fingers that are electrically connected to ground.

Such finger electrodes are always connected to a well defined potential. This makes it easier to handle them in computer simulations .

In one embodiment, the input transducer or the output trans ^¬ ducer comprises two fingers per λ/2 that are electrically connected to the same bus bar.

Compared to standard SAW transducers, the number of fingers is doubled. Transducers comprising two fingers per λ/2 establish electrode fingers that have a higher reflectivity for surface acoustic waves arriving at the transducer

fingers .

In one embodiment, the SAW filter comprises reflectors having two fingers per λ.

In one embodiment, the SAW filter comprises two fan shaped reflectors. Further, the input transducer and the output transducer may also be fan shaped. The two reflectors, the input transducer and the output transducer fulfill the requirement:

0.01 < AL/L < 0.2, where L is the mean length L1+L2/2 and AL is the difference in length in lateral direction of the transducer or

reflector: AL=L2-L1. Here, LI is the length in lateral direction on one side of the transducer or reflector and L2 is the length in lateral direction on the other side.

In one embodiment, the SAW filter comprises finger structures that are arranged in the acoustic track between the input transducer and the output transducer and the finger

structures act as an acoustic lens.

Fan shaped reflectors or fan shaped input or output

transducers act as mirrors or acoustic lenses that will effectively deflect surface acoustic waves. Finger structures that act as acoustic lenses and that are arranged between input and output transducers can be used to correct this deflection .

Brief description of the drawings

The present invention will become fully understood from the detailed description given herein below and the accompanying drawings. In the drawings:

FIG. 1 illustrates a conventional arrangement of surface acoustic wave transducers being arranged within an acoustic track between reflector elements,

FIG. 2 illustrates a possible arrangement of the source center with regard to the input transducer and the drain center with regard to the output transducer and reflection centers with regard to the input and/or output transducer and/or to the first and second reflectors, illustrates possible paths between reflection centers and/or drain or source centers as propagation paths for surface acoustic waves, illustrates a transducer structure having two fingers per λ/2, illustrates a transducer structure comprising reflecting means having two fingers per λ/2, illustrates a reflector element having two fingers per λ, illustrates the reduced passband ripple within the insertion loss characteristic due to the reflecting means , illustrates the impedance behavior of matrix element Sn of an according resonator or filter, illustrates the reflection coefficient according to matrix element Sn,

illustrates the impedance behavior of the matrix element S22, illustrates the reflection coefficient according to matrix element S22,

FIG. 12 illustrates the temporal response of two standard surface acoustic wave filters and of a surface acoustic wave filter with reduced triple transit response,

FIG. 13 illustrates a fan shaped transducer.

Detailed description

FIG. 1 schematically illustrates the arrangement of two transducers TD within an acoustic track AT on a piezoelectric substrate PS. Both transducers are arranged between two reflectors R.

FIG. 2 illustrates an input transducer TDi and an output TDo being arranged between two reflectors R. The input transducer TDi comprises a source center STDi at a position STDi along a longitudinal direction LD of the acoustic track. It further comprises a reflection center at a position RTDi along the longitudinal direction of the acoustic track. The source center can be thought of being the reference location with regard to the phase information for emitted surface acoustic waves. The reflection center can be thought of being the reference location regarding phase information regarding reflected surface acoustic waves. This means that emitted surface acoustic waves can be regarded as being emitted directly at the source center and reflected surface acoustic waves can be regarded as being reflected directly at the reflection center. Accordingly, DTDo is the drain center of the output

transducer and RTDo is the reflection center of the output transducer. The reflection center RTDi and the source center STDi may coincide in the local position but that is not a necessity. Analogously, the drain center DTDo and the

reflection center RTDo of the output transducer may coincide in the local position but that also is not a necessity. Further, each reflector has a reflection center. RR1 is the reflection center of the ^x left' reflector and RR2 is the reflection center of the ^x right' reflector.

FIG. 3 illustrates possible paths for surface acoustic waves that are emitted by the source center and that are absorbed at least partially by the drain center. The standard path StP only comprises the distance between the source center of the input transducer STDi and the drain center DTDo of the output transducer. This distance is the reference distance when concerning phase differences between the primary signal and spurious triple transit signals.

TT denotes the path of triple transit signals: surface acoustic waves are emitted at the source center STDi of the input transducer and are reflected by the finger electrodes of the output transducer at the position of the drain center of the output transducer DTDo. The signals are then re- reflected at the source center STDi of the input transducer and are - finally - absorbed by the drain center of the output transducer. As a result, the phase difference between the primary signal according to the standard path Stp and the triple transit path TT is equivalent to two times the

distance between the source center and the drain center of the input and the output transducer.

CaPl denotes a first path for canceling acoustic waves canceling the triple transit response at least partially: CaPl is the path of surface acoustic waves that are emitted by the source center of the input transducer STDi and are reflected by reflection structures forming the second

reflection means at a reflection center at a position RTDo of the output transducer. The reflected acoustic waves are then re-reflected by reflection structures forming the first reflection means at a position RTDi of the input transducer. The re-reflected surface acoustic waves are then finally absorbed by the drain center of the output transducer. The phase difference of signals according to the first canceling path CaPl and the surface acoustic waves of the triple transit path TT correspond to the sum of the difference | RTDi - STDi I + I DTDo - RTDo | . It is preferred that the phase difference between canceling acoustic waves according to the first canceling path CaPl and the triple transit path TT is an odd multiple of λ/2 where λ is the wave length of the surface acoustic wave.

CaP2a denotes a possible path of surface acoustic waves that are emitted by the source center of the input transducer towards a first reflector having a reflection center at a position RR1. The acoustic waves are then reflected by the first reflector and are absorbed by the drain center of the output transducer. Such surface acoustic waves usually add spurious responses to the signal traveling the standard path Stp. In order to eliminate these spurious responses, it is preferred that further surface acoustic waves travel along a path CaP2b, i.e. being emitted by the source center of the input transducer being reflected by the second reflector with a reflection center at a position RR2 and finally are

absorbed by the drain center of the output transducer. Thus, it is preferred that the path length of the path CaP2a plus the path length of path CaP2b is an even multiple of the wave length λ of the acoustic waves. CaP3 further denotes an acoustic path for surface acoustic waves that is well suited to cancel triple transit responses: at first, surface acoustic waves are emitted by the source center of the input transducer towards the first reflector at a longitudinal position RR1 in an opposite direction with regard to the longitudinal direction. The surface acoustic waves are reflected at least partially and arrive at the second reflector at a longitudinal position RR2 and are again reflected. The re-reflected surface acoustic waves then are absorbed by the drain center of the output transducer at the position DTDo. It is preferred that the path length of canceling path CaP3 and the triple transit path TT have a phase difference between 90° and 270°.

In other words, there is a first canceling path CaPl that can cancel triple transit responses of triple transit path TT and there is another canceling path CaP3 that can cancel spurious triple transit responses according to the triple transit path TT. As the additional reflecting means at positions RTDi and RTDo may in addition to the source center and to the drain center act as reflecting means for surface acoustic waves, the reflecting means within the transducers and the first and the second reflector are positioned in such a way that detrimental reflection at the reflection means within the transducers cancel each other according to the canceling paths CaP2a and CaP2b.

FIG. 4 illustrates a transducer having two fingers EF per λ/2 where λ is the wave length of an acoustic wave AW. FIG. 5 illustrates a transducer comprising additional

reflecting fingers RF for reflecting acoustic waves AW. The transducer has two fingers per λ/2 and the reflection means comprising the reflection fingers have two reflection fingers RF per λ/2.

FIG. 6 illustrates an acoustic reflector R having two

reflection fingers RF per λ. An acoustic wave AW is indicated as arriving at the reflector from the right side. A certain percentage of the acoustic wave can pass the reflector without being reflected while the major part of the surface acoustic wave is reflected. The reflection behavior is indicated by an amplitude that is decreasing across the reflector .

FIG. 7 illustrates the frequency dependent matrix element S21 of an according surface acoustic wave filter comprising canceling means for canceling triple transit response. For comparison only, a filter characteristic of a conventional SAW filter is shown also. It is clearly seen that the filter having reduced triple transit signals comprises less pass band ripple.

FIG. 8 illustrates the frequency dependent impedance

represented by matrix element Sn. Matrix element Sn denotes the reflection coefficient of one of input and output port of a two-port SAW filter. It can clearly be seen that the SAW filter is well adapted to the desired matching impedance at the center of the smith chart. There is mainly no detrimental impedance mismatch caused by the further triple transit canceling means. FIG. 9 represents the frequency dependent reflection

coefficient according to matrix element Sn for both filters with and without triple transit suppression means. It can clearly be seen that the triple transit canceling means have no detrimental effect on the reflection

coefficient .

FIG. 10 illustrates the frequency dependent impedance

behavior of the other of first and second port of a

respective SAW filter having triple transit suppression means compared to filters having no triple transit suppression means. It can again clearly be seen that there is no

detrimental impedance detuning caused by the triple transit canceling means.

Analog to FIG. 9, FIG. 11 illustrates the reflection

coefficient of the respective other port of a surface

acoustic wave having triple transit suppression means

compared to the surface acoustic wave filter having no triple transit canceling means.

FIGs. 8 to 11 clearly show that triple transit canceling means do neither negatively affect impedance behavior nor negatively affect the reflection coefficient of the filter.

FIG. 12 illustrates the temporal response of surface acoustic wave filters comprising triple transit suppression means according to the invention compared to conventional surface acoustic wave filters. The peak "M0" denotes the primary pulse hitting the drain center or activating the drain center approximately at t = 30 ]is . The peak "Ml" denotes the triple transit responses of two conventional filters at

approximately t = 0.85 \is having an insertion loss of -40 dB compared to an SAW device comprising triple transit

suppression means showing an insertion loss of -50 dB at the first triple transit peak "Ml". Peak "M2" denotes a second triple transit response at

approximately T = 1.45 ]is .

Thus, from FIGs. 7 and 12, it can clearly be seen that the pass band ripple (FIG. 7) and the triple transit response (FIG. 12) is reduced resulting in an improved filter

characteristic .

FIG. 13 illustrates a fan shaped transducer having a width W perpendicular to the longitudinal direction and having a shorter length L2 in parallel to the longitudinal direction. The length LI denotes the longer length on the other side of the transducer. The distortion of the transducer is given by the ratio (LI - L2)/[ (L1 + L2)/2] . The present invention comprises means for canceling spurious triple transit responses including means for canceling detrimental responses caused by the said means. Canceling means may be additional reflecting fingers within a

transducer structure or additional reflectors within the acoustic path or reflectors in the acoustic path at a special longitudinal position.

The basic concept does not depend on details concerning the reflecting means. Other reflecting means as, for example, finger electrodes within the surface of the piezoelectric substrate or edges of the piezoelectric substrate or areas of different metallization may also act accordingly as

reflecting means. Further, the invention is not restricted by the embodiments or the accompanying figures. Thus, numerous variations departing from the figures are possible without departing from the invention.

List of reference signs

PS: piezoelectric substrate

R: reflector

TD: transducer

AT: acoustic track

LD: longitudinal direction

RR1 : position of reflection center of a first reflector RTDi : position of reflection center of reflection structures of the input transducer

STDi: position of the source center of the input transducer DTDo: position of the drain center of the output transducer RTDo: position of the reflection center of the output transducer

RR2 : position of reflection center of the second reflector

Stp: standard path

TT: triple transit path

CaPl : first canceling path

CaP2a: additional canceling path

CaP2b: second additional canceling path

CaP3 : third canceling path

AW: acoustic wave

EF: electrode finger

RF: reflecting finger

IL: insertion loss

SCll: Smith chart showing input impedance

Refill: reflection coefficient of input port

SC22: Smith chart showing impedance behavior of the output port

Refl22: reflection coefficient of output port

TR: temporal response

M0, Ml, M2 : marks for primary signal, first triple transit signal, second triple transit signal FT: fan shaped transducer

W: width of transducer

LI, L2 : length of different sides of a fan shaped transducer