Surface acoustic wave filter capable of widening a bandwidth

In a surface acoustic wave filter utilizing at least two longitudinal modes, input and output interdigital transducers are mounted on a piezoelectric substrate so as to oppose each other. First and second grating reflectors are mounted on the piezoelectric substrate outside the input and the output interdigital transducers, respectively. Each of the input and the output interdigital transducers comprises a plurality of electrode digits which are equal to N in number. Each of the electrode digits is made of a metal material having a discontinuous impedance coefficient K related to acoustic impedance. The surface acoustic wave filter is structured so that a product of the number N and the discontinuous impedance coefficient K is not smaller than 0.55.

BACKGROUND OF THE INVENTION 
This invention relates to a surface acoustic wave filter utilizing at least 
two longitudinal modes which are different from each other and which are 
excited in the same direction as a propagation direction of a surface 
wave. 
As an example of a surface acoustic wave filter, a surface acoustic wave 
filter of a longitudinal mode resonator type is disclosed in an article 
which is contributed by MASAKI TANAKA et al to Fifteenth EM Symposium, 
Mar. 12, 1986, pages 5-9, and which has a title of "Narrow Bandpass Double 
Mode SAW Filter". Such a surface acoustic wave filter comprises a 
piezoelectric substrate, input and output interdigital transducers which 
are mounted on the piezoelectric substrate such that the output 
interdigital transducer is located opposite to the input interdigital 
transducer, and first and second grating reflectors which are mounted on 
the piezoelectric substrate. The first grating reflector opposes the input 
interdigital transducer at an opposite side of the output interdigital 
transducer. The second grating reflector opposes the output interdigital 
transducer at an opposite side of the input interdigital transducer. 
The input interdigital transducer comprises a plurality of input electrode 
digits which intersect one another. The input interdigital transducer 
converts an input electric signal into an input surface acoustic wave as a 
propagated surface acoustic wave. Similarly, the output interdigital 
transducer comprises a plurality of output electrode digits which 
intersect one another. The output interdigital transducer receives the 
propagated surface acoustic wave as a received surface acoustic wave and 
converts the received surface acoustic wave into an output electric 
signal. 
In such a surface acoustic wave filter, a surface wave excited by the input 
and the output electrode digits is confined between the first and the 
second grating reflectors. At this time, a cavity having a predetermined 
length is formed between the first and the second grating reflectors and 
the cavity causes a standing wave. Generally, the surface acoustic wave 
filter utilizes a basic or fundamental mode together with a primary or a 
secondary mode of the standing wave. Such a surface acoustic wave filter 
is called a surface acoustic wave filter of a double mode type. The basic, 
the primary, and the secondary modes have basic, primary, and secondary 
resonance frequencies, respectively. The primary mode, the secondary mode, 
and the like are collectively called a higher-order mode. As well known in 
the art, a resonance frequency of the standing wave is determined by the 
predetermined length of the cavity. A bandwidth of the surface acoustic 
wave filter is determined by a frequency interval between the basic 
resonance frequency and the primary or the secondary resonance frequency. 
In order to widen the bandwidth of the surface acoustic wave filter, it is 
necessary to reduce the number of the electrode digits of the input and 
the output interdigital transducers. There is, however, a restriction on 
widening the bandwidth of the surface acoustic wave filter caused by an 
increase in insertion loss. In other words, the surface acoustic wave 
filter of a double mode type has problems of increased impedance and 
degraded out of band attenuation resulting from widening the bandwidth. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a surface acoustic 
wave filter which is capable of widening a bandwidth. 
Other objects of this invention will become clear as the description 
proceeds. 
In describing the gist of this invention, it should be understood that a 
surface acoustic wave filter utilizes at least two longitudinal modes 
which are different from each other and comprises a piezoelectric 
substrate, an input interdigital transducer supplied with an input 
electric signal and mounted on the piezoelectric substrate for converting 
the input electric signal into an input surface acoustic wave as a surface 
acoustic wave propagated thereon. The input interdigital transducer 
comprises a plurality of input electrode digits which are equal to N in 
number where N represents a positive integer greater than two and which 
intersect one another. The surface acoustic wave filter further comprises 
an output interdigital transducer mounted on the piezoelectric substrate 
and located opposite the input interdigital transducer and comprises a 
plurality of output electrode digits which are equal to N in number and 
which intersect one another. The output interdigital transducer receives 
the propagated surface acoustic wave as a received surface acoustic wave 
and converts the received surface acoustic wave into an output electric 
signal. The surface acoustic wave filter still further comprises a first 
grating reflector mounted on the piezoelectric substrate and located 
opposite to the input interdigital transducer at an opposite side of the 
output interdigital transducer and a second grating reflector mounted on 
the piezoelectric substrate and located opposite the output interdigital 
transducer at an opposite side of the input interdigital transducer. Each 
of the input and the output electrode digits is made of metal material 
having a discontinuous impedance coefficient K related to acoustic 
impedance. The filter is structured so that a product of the number N and 
the discontinuous impedance coefficient K is not smaller than 0.55.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, two types of conventional surface acoustic wave 
filters will be described in order to facilitate an understanding of this 
invention. 
In FIG. 1(a), the surface acoustic wave filter is of the type which is 
disclosed in a reference cited before, namely, "Narrow Bandpass Double 
Mode SAW Filter", and utilizes a basic mode and a primary mode of a 
standing wave. The surface acoustic wave filter comprises a piezoelectric 
substrate 10 which is formed by an ST cut crystal substrate. On the 
piezoelectric substrate 10, input and output interdigital transducers 11 
and 12 are mounted so that the output interdigital transducer is 12 
located opposite to the input interdigital transducer 11. First and second 
grating reflectors 16 and 17 are mounted on the piezoelectric substrate 10 
so that the first grating reflector 16 opposes the input interdigital 
transducers 11 at an opposite side of the output interdigital transducer 
12 and that the second grating reflector 17 opposes the output 
interdigital transducer 12 at an opposite side of the input interdigital 
transducer 11. 
The input interdigital transducer 11 has an input electrode pattern 
comprising an input electrode 11-1 connected to an input terminal 13 and 
an input earth electrode 11-2 which is grounded or earthed. Between the 
input electrode 11-1 and the input earth electrode 11-2, a plurality of 
input electrode digits 11-3 are disposed in parallel along a predetermined 
direction so that the input electrode digits 11-3 intersect one another. 
Each of the electrode digits 11-3 has a uniform length. The input 
interdigital transducer 11 is supplied with an input electric signal 
through the input terminal 13 and converts the input electric signal into 
an input surface acoustic wave (SAW). The input surface acoustic wave is 
propagated on a surface of the piezoelectric substrate 10 as a propagated 
surface acoustic wave along the predetermined direction. 
The output interdigital transducer 12 has an output electrode pattern which 
is similar to the input electrode pattern of the input interdigital 
transducer 11. Namely, the output electrode pattern comprises an output 
electrode 12-1 connected to an output terminal 14, an output earth 
electrode 12-2 which is grounded or earthed, and a plurality of output 
electrode digits 12-3 which are disposed in parallel along the 
predetermined direction so that the electrode digits 12-3 intersect one 
another. 
The propagated surface acoustic wave is received by the output interdigital 
transducer 12 as a received surface acoustic wave. The output interdigital 
transducer 12 converts the received surface acoustic wave into an output 
electric signal. The output interdigital transducer 12 outputs the output 
electric signal through the output terminal 14. 
The first and the second grating reflectors 16 and 17 are for confining a 
surface wave excited by the input and the output electrode digits in order 
to generate a standing wave in a cavity formed between the first and the 
second grating reflectors 16 and 17. For this purpose, the first grating 
reflector 16 comprises a plurality of reflector electrodes 16-1 which are 
disposed in parallel and which are electrically connected in parallel. 
Similarly, the second grating reflector 17 comprises a plurality of 
reflector electrodes 17-1 which are disposed in parallel and which are 
electrically connected in parallel. 
In FIG. 1(b), the surface acoustic wave filter is of the type disclosed in 
an article which is contributed by TADASHI KANDA et al to THE TRANSACTIONS 
OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, 
1988, page 1-240, and which has a title of "A Method for Widening the 
Bandwidth of SAW Resonator Filter by Utilizing Longitudinal Inharmonic 
Modes". The surface acoustic wave filter utilizes the basic mode and a 
secondary mode of the standing wave and comprises a piezoelectric 
substrate 20, an input interdigital transducer 21 mounted on the 
piezoelectric substrate 20, and first and second output interdigital 
transducers 22 and 23 which are mounted on the piezoelectric substrate 20 
on both sides of the input interdigital transducer 21. First and second 
grating reflectors 26 and 27 are mounted on the piezoelectric substrate 20 
so that the first grating reflector 26 opposes the first output 
interdigital transducer 22 at an opposite side of the input interdigital 
transducer 21 and that the second grating reflector 27 opposes the second 
output interdigital transducer 23 at an opposite side of the input 
interdigital transducer 21. 
The input interdigital transducer 21 has an input electrode pattern 
comprising an input electrode 21-1 connected to an input terminal 24 and 
an input earth electrode 21-2 which is grounded. Between the input 
electrode 21-1 and the input earth electrode 21-2, a plurality of input 
electrode digits 21-3 are disposed in parallel along a predetermined 
direction with the input electrode digits 21-3 inter-secting one another. 
The input interdigital transducer 21 is for converting an input electric 
signal supplied through the input terminal 24 into an input surface 
acoustic wave. The input surface acoustic wave is propagated on the 
surface of the piezoelectric substrate 20 as a propagated surface acoustic 
wave. 
Each of the first and the second output interdigital transducers 22 and 23 
has an output electrode pattern which is similar to the input electrode 
pattern of the input interdigital transducer 21. With regard to the first 
output interdigital transducer 22, the output electrode pattern comprises 
a first output electrode 22-1, a first output earth electrode 22-2 which 
is grounded, and a plurality of first output electrode digits 22-3 which 
are disposed in parallel along the predetermined direction with the first 
output electrode digits 22-3 inter-secting one another. Similarly, the 
output electrode pattern of the second output interdigital transducer 23 
comprises a second output electrode 23-1, a second output earth electrode 
23-2 which is grounded, and a plurality of second output electrode digits 
23-2 which are disposed in parallel along the predetermined direction with 
the second output electrode digits 23-3 inter-secting one another. The 
first and the second output electrodes 22-1 and 23-1 are commonly 
connected to an output terminal 25. 
Like the first and the second grating reflectors 16 and 17, the first 
grating reflector 26 comprises a plurality of reflector electrodes 26-1 
which are disposed in parallel and which are electrically connected in 
parallel while the second grating reflector 27 comprises a plurality of 
reflector electrodes 27-1 which are similar to the reflector electrodes 
26-1. 
The propagated surface acoustic wave is received by the first and the 
second output interdigital transducers 22 and 23 as first and second 
received surface acoustic waves. Each of the first and the second output 
interdigital transducers 22 and 23 converts the first and the second 
received surface acoustic waves into first and second output electric 
signals. The surface acoustic wave filter outputs an output electric 
signal through the output terminal 25. 
In the surface acoustic wave filter illustrated in FIG. 1(a), the surface 
wave excited by the input and the output electrode digits is confined 
between the first and the second grating reflectors 16 and 17. At this 
time, the cavity having a predetermined length is formed between the first 
and the second grating reflectors 16 and 17 and the cavity causes the 
standing wave. The basic, the primary, and the secondary modes have basic, 
primary, and secondary resonance frequencies, respectively. As well known 
in the art, a resonance frequency of the standing wave is determined by 
the predetermined length of the cavity. A bandwidth of the surface 
acoustic wave filter is determined by a frequency interval between the 
basic resonance frequency and the primary or the secondary resonance 
frequency. 
In order to widen the bandwidth of the surface acoustic wave filter, it is 
necessary to reduce the number of the electrode digits of the input and 
the output interdigital transducers. There is, however, a restriction on 
widening the bandwidth of the surface acoustic wave filter caused by an 
increase in insertion loss. In other words, the double mode type surface 
acoustic wave filter has problems of increased impendance and degraded out 
of band attenuation resulting from widening the bandwidth. These problems 
apply to the surface acoustic wave filter illustrated in FIG. 1(b). 
Referring to FIGS. 2 and 3, attention will be directed to a resonator type 
surface acoustic wave filter which does not use the grating reflector. On 
a piezoelectric substrate 30, input and output interdigital transducers 31 
and 32 are mounted so as to oppose each other. The input interdigital 
transducer 31 is connected to an input terminal 33 while the output 
interdigital transducer 32 is connected to an output terminal 34. An input 
electrode pattern of the input interdigital transducer 31 is made of a 
metal material. Similarly, an output electrode pattern of the output 
interdigital transducer 32 is made of the same metal material as the input 
electrode pattern of the input interdigital transducer 31. 
Each of the input and the output interdigital transducers 31 and 32 has a 
plurality of electrode digits which are equal to N in number and the metal 
material has a discontinuous impedance coefficient K related to acoustic 
impedance. The discontinuous impedance coefficient K is represented by an 
equation given by: 
EQU K=.vertline.(Zn/Zo)-1.vertline., 
where Zn represents the acoustic impedance of a portion on which the 
electrode is mounted, Zo represents the acoustic impedance of a free 
surface which does not include the electrode. 
In the example being illustrated, if a product of the number N and the 
discontinuous impedance coefficient K becomes large, impedance mismatching 
between the acoustic impedances Zn and Zo causes internal reflection in 
the input and the output interdigital transducers 31 and 32. As a result 
of the internal reflection, resonance phenomena occurs in the electrode 
digits. 
In FIG. 3, a first curve shown by a chain line represents a first 
attenuation versus frequency characteristic when the product N.multidot.K 
is equal to 0.33. A second curve illustrated by a dotted line represents a 
second attenuation versus frequency characteristic when the product 
N.multidot.K is equal to 0.55. A third curve shown by a real line 
represents a third attenuation versus frequency characteristic when the 
product N.multidot.K is equal to 0.78. As apparent from the first through 
the third attenuation versus frequency characteristics, when the product 
N.multidot.K becomes large, two resonance peaks appear under the influence 
of the internal reflection. Under the circumstances, it will be assumed 
that an acoustic reflection coefficient of the electrode digits is not 
smaller than 0.5. Therefore, it is necessary that the product N.multidot.K 
is not smaller than 0.55 in order to generate the internal reflection. 
Referring to FIGS. 4 and 5, the description will proceed to a surface 
acoustic wave filter according to a preferred embodiment of this 
invention. The surface acoustic wave filter is of a multiple mode type and 
comprises a piezoelectric substrate 40 which is formed by the ST cut 
crystal substrate. Input and output interdigital transducers 41 and 42 are 
mounted on the piezoelectric substrate 40 such that the output 
interdigital transducer 42 is located opposite to the input interdigital 
transducer 41. First and second grating reflectors 46 and 47 are mounted 
on the piezoelectric substrate 40 so that the first grating reflector 46 
opposes the input interdigital transducers 41 at an opposite side of the 
output interdigital transducer 42 and that the second grating reflector 47 
opposes the output interdigital transducer 42 at an opposite side of the 
input interdigital transducer 41. The input and the output interdigital 
transducers 41 and 42 and the first and the second grating reflectors 46 
and 47 are made of a metal material such as aluminum. 
Taking a point described in conjunction with FIGS. 2 and 3 into 
consideration, the output interdigital transducer 42 is spaced apart from 
the input interdigital transducer 41 by a first distance L1 as will later 
be described in detail. The first and the second grating reflectors 46 and 
47 are spaced apart from the input and the output interdigital transducers 
41 and 42, respectively, by a second distance L2. 
The input interdigital transducer 41 has an input electrode pattern 
comprising an input electrode 41-1 connected to an input terminal 43 and 
an input earth electrode 41-2 which is earthed. Between the input 
electrode 41-1 and the input earth electrode 41-2, a plurality of input 
electrode digits 41-3 are disposed in parallel along a predetermined 
direction with the input electrode digits 41-3 inter-secting one another. 
Each of the input electrode digits 41-3 has a predetermined width 
.lambda./4 and is formed at a predetermined interval of .lambda./4, where 
.lambda. represents a predetermined period, and has a uniform length. The 
output interdigital transducer 42 has an output electrode pattern which is 
similar to the input electrode pattern of the input interdigital 
transducer 41. Namely, the output electrode pattern comprises an output 
electrode 42-1 connected to an output terminal 44, an output earth 
electrode 42-2 which is carthed, and a plurality of output electrode 
digits 42-3 which are disposed in parallel along the predetermined 
direction with the output electrode digits 42-3 inter-secting one another. 
Each of the output electrode digits 42-3 has the predetermined width 
.lambda./4 and is formed at the predetermined interval of .lambda./4 and 
has the uniform length. In the example being illustrated, the number N of 
the input electrode digits 41-3 is equal to six and is equal to the number 
of the output electrode digits 42-3. 
The first grating reflector 46 comprises a plurality of reflector 
electrodes 46-1 which are disposed in parallel along the predetermined 
direction and which are electrically connected in parallel. Similarly, the 
second grating reflector 47 comprises a plurality of reflector electrodes 
47-1 which are disposed in parallel along the predetermined direction and 
which are electrically connected in parallel. 
More specifically, the first and the second distances L1 and L2 are defined 
as mentioned below. The input electrode pattern of the input interdigital 
transducer 41 includes a pair of input outermost electrode digits 41-3a 
and 41-3b while the output electrode pattern of the output interdigital 
transducer 42 includes a pair of output outermost electrode digits 42-3a 
and 42-3b. Each of the input and the output outermost electrode digits 
41-3a, 41-3b, 42-3a, and 42-3b has a center line extending along a 
longitudinal direction thereof. As illustrated in FIG. 5 in detail, the 
first distance L1 is equal to an interval between the center line of the 
input outermost electrode digit 41-3b and the center line of the output 
outermost electrode digit 42-3b. On the other hand, the first and the 
second grating reflectors 46 and 47 include first and second outermost 
electrodes 46-1a and 47-1a which are close to the input and the output 
outermost electrode digits 41-3a and 42-3a, respectively. Each of the 
first and the second outermost electrodes 46-1a and 47-1a has a center 
line extending along a longitudinal direction thereof. The second distance 
L2 is equal to an interval between the center line of the first outermost 
electrode 46-1a and the center line of the input outermost electrode digit 
41-3a and is equal to an interval between the center line of the second 
outermost electrode 47-1a and the center line of the output outermost 
electrode digit 42-3a. As shown in FIG. 5, the predetermined electrode 
period .lambda. is equal to twice an interval between two center lines of 
the two nearest electrode digits. 
The input interdigital transducer 41 is supplied with an input electric 
signal through the input terminal 43 and converts the input electric 
signal into an input surface acoustic wave. The input surface acoustic 
wave is propagated on a surface of the piezoelectric substrate 40 as a 
propagated surface acoustic wave along the predetermined direction. The 
propagated surface acoustic wave is received by the output interdigital 
transducer 42 as a received surface acoustic wave. The output interdigital 
transducer 42 converts the received surface acoustic wave into an output 
electric signal. The output interdigital transducer 42 outputs the output 
electric signal through the output terminal 44. 
In the surface acoustic wave filter illustrated in FIG. 4, if the product 
N.multidot.K mentioned in conjunction with FIG. 2 is not smaller than 
0.55, first through third longitudinal modes are excited. The first 
longitudinal mode is based on a first resonance Ra occurring between the 
input and the output interdigital transducers 41 and 42 within a stop band 
defined by the input and the output interdigital transducers 41 and 42. 
The second longitudinal mode is based on a second resonance Rb occurring 
in the input interdigital transducer 41 or the output interdigital 
transducer 42 caused by the internal reflection in the electrode digits. 
The third longitudinal mode is based on a third resonance Rc occurring 
between the first and the second grating reflectors 46 and 47 out of the 
stop band. The first through the third resonances Ra, Rb, and Rc have 
first through third resonance frequencies, respectively. The surface 
acoustic wave filter utilizing the first through the third longitudinal 
modes may be called a triple mode type surface acoustic wave filter. 
Referring to FIG. 6, first through third curves C1, C2, and C3 correspond 
to the first through the third resonances Ra, Rb, and Rc, respectively, 
and represent first through third frequency fluctuation coefficients of 
the first through the third resonance frequencies, respectively. In this 
case, the second distance L2 is set at 0.7.lambda.. Each of the first and 
the third frequency fluctuation coefficients varies dependent upon the 
first distance L1. In particular, when the first distance L1 is reduced, a 
difference between the first and the second frequency fluctuation 
coefficients becomes large. Similarly, if the first distance L1 is 
increased, a difference between the second and the third frequency 
fluctuation coefficients becomes large. This means that the triple mode 
type surface acoustic wave filter requires a restricted range with respect 
to the first distance L1. 
Referring to FIG. 7, fourth through sixth curves C4, C5, and C6 correspond 
to the first through the third resonances Ra, Rb, and Rc, respectively, 
and represent first through third frequency fluctuation coefficients of 
the first through the third resonance frequencies, respectively. In this 
event, the first distance L1 is set at 0.4.lambda.. In FIG. 7, each of the 
first through the third frequency fluctuation coefficients has a small 
variation and is slightly influenced by the second distance L2. With 
respect to the second distance L2, the triple mode type curface acoustic 
wave filter may have a range wider than the restricted range of the first 
distance L1. 
Under first and second conditions described hereinunder, it is possible to 
provide the triple mode type surface acoustic wave filter by utilizing the 
first through the third longitudinal modes which are related to the first 
through the third resonances Ra, Rb, and Rc and which are different from 
one another. The first condition is defined by a first inequality given 
by: 
EQU (n/2-4/24).lambda..ltoreq.L1.ltoreq.(n/2-2/24).lambda., 
where n represents a positive integer. The second condition is defined by a 
second inequality given by: 
EQU (n/2-2/24).lambda..ltoreq.L2.ltoreq.(n/2+7/24).lambda.. 
Such a triple mode type surface acoustic wave filter has an output versus 
frequency characteristic as shown in FIG. 8. In FIG. 8, a center frequency 
of the stop band is equal to 250 MHz. A channel CH1 shows an enlarged part 
in the neighborhood of the stop band of a channel CH2. 
Under a third condition described hereinunder, it is possible to provide a 
double mode type surface acoustic wave filter by utilizing two 
longitudinal modes which are related to the first and the second 
resonances Ra and Rb and which are different from each other. The third 
condition is defined by a third inequality given by: 
EQU (n/2-1/24).lambda..ltoreq.L1.ltoreq.(n/2+7/24).lambda.. 
Such a double mode type surface acoustic wave filter has an output versus 
frequency characteristic as shown in FIG. 9. Like in FIG. 8, a center 
frequency of the stop band is equal to 250 MHz. A channel CH11 shows an 
enlarged part in the neighborhood of the stop band of a channel CH12. 
As apparent from the above description, the present invention is 
characterized by the product N.multidot.K, the first distance L1 between 
the input and the output interdigital transducers 41 and 42, and the 
second distance L2 between the input interdigital transducer 41 and the 
first grating reflector 46 and between the output interdigital transducer 
42 and the second grating reflector 47. 
While this invention has thus far been described in conjunction with a 
preferred embodiment thereof, it will readily be possible for those 
skilled in the art to put this invention into practice in various other 
manners.