Surface acoustic wave device

In a preferred embodiment, a surface acoustic wave device comprises a piezoelectric substrate, a first electrode formed on the substrate and acting to convert an electrical signal into a surface acoustic wave, and second and third electrodes formed on the substrate and acting to convert the surface acoustic wave into an electrical signal. The second and third electrodes are disposed at different distances from the first electrode and connected electrically with each other. Electrode fingers of the second electrode are arranged in a relationship of the opposite polarity with respect to those of the third electrode so as to cause peaks in the constant frequency interval multi-peak frequency characteristics of the device acting as a filter to take place at frequencies which are (integer+1/2) times the constant frequency interval .DELTA.f. In another preferred embodiment, a plurality of surface acoustic wave device units are formed on a common piezoelectric substrate. Respective units include a set of first, second and third electrodes and operate at different frequency bands which partly overlap. The distance between the first electrode and the second electrode or the third electrode is determined in the respective units such that the delay time due to the propagation of the surface acoustic wave between the input and output electrodes is equal in the respective units and the output voltage of the output electrode is in phase in the respective units, thereby providing accurate peak frequencies over the whole frequency band at which the surface acoustic wave device operates.

This invention relates to surface acoustic wave devices and more 
particularly to a surface acoustic wave device having 
constant-frequency-interval multi-peak frequency characteristics. The 
device may include a plurality of device units each of input and output 
electrodes formed on a common piezoelectric substrate wherein the units 
operate at different frequency bands and adjacent frequency bands partly 
overlap so that a suitable arrangement of the input and output electrodes 
is required for obtaining highly accurate multi-peak frequency 
characteristics. 
A surface acoustic wave device generally has a configuration as shown in 
FIG. 1. More particularly, on a piezoelectric substrate 3 are formed an 
input electrode (first electrode) 2 which converts an electric signal from 
a signal source 1 into a surface acoustic wave, an output electrode 
(second electrode) 4, and another output electrode (third electrode) 5 
connected electrically in parallel with the former output electrode 4, the 
output electrodes 4 and 5 acting to convert the surface acoustic wave 
propagating along the piezoelectric substrate 3 to an electric signal. The 
electric signal derived from these output electrodes 4 and 5 is supplied 
to a load 6. 
The center frequency at which the device operates is determined by the 
pitch between electrode fingers 2c and 2d which are respectively integral 
with opposing bus bars 2a and 2b of the input electrode 2, the pitch 
between electrode fingers 4a and 4b of the output electrode 4, and the 
pitch between electrode fingers 5a and 5b of the output electrode 5, the 
electrode fingers 4a and 5a being integral with a common bus bar 7a and 
the electrode fingers 4b and 5b being integral with an opposing common bus 
bar 7b. In general, the aforementioned pitches are all equal. The device 
has such a transfer function H(.omega.) that a comb-like characteristic 
having peaks or bottoms spaced by a frequency interval of .DELTA.f=Vs/L, 
where L is a distance between the output electrodes 4 and 5 and Vs is a 
velocity of the surface acoustic wave, is superimposed on a characteristic 
which is determined by a product F(.omega.).times.G(.omega.) of a 
frequency response F(.omega.) of the input electrode 2 and a frequency 
response G(.omega.) of the output electrodes 4 and 5 of the same 
configuration. 
To detail the comb-like characteristic, because of the distance L between 
the bus output electrodes, the delay time between the output electrode 5 
and the input electrode 2 is larger than the delay time between the output 
electrode 4 and the input electrode 2 by a difference of .tau.=L/Vs. 
Therefore, for a frequency f of a high frequency signal supplied to the 
input electrode 2 from the signal source 1, a phase difference .phi. 
between the output electrodes 4 and 5 amounts to 2.pi.f.tau.. Thus, for a 
frequency of f=n/.tau. with which the phase difference .phi. is n times 
2.pi., i.e., 2.pi. (n: integer) output voltages of the output electrodes 4 
and 5 become in phase to produce a resultant high frequency voltage 
output. Conversely, for a frequency of f=(n+1/2)/.tau., output voltages of 
the output electrodes 4 and 5 have the same amplitude but are out of phase 
so that the resultant output becomes zero. Consequently, the frequency 
characteristics have a number of peaks spaced by a frequency interval of 
.DELTA.f=1/.tau.. 
Such multi-peak may be utilized for a channel selecting device of a 
television receiver, for example. Specifically, it can be utilized in a 
frequency synthesized channel selecting device in which the output of a 
local oscillator which is scanning its output frequencies is delivered to 
the surface acoustic wave device (comb-like characteristic filter), the 
number of peaks which are delivered as the output of the filter during the 
scanning are counted, and the local oscillator stops its scanning when the 
counted peaks coincide with a preset frequency. 
Where the surface acoustic wave device is desired to operate over a wide 
frequency band, for example, at UHF band (530-830 MHz), a single surface 
acoustic wave device has difficulty in covering the band as a whole. 
Therefore, the band is divided into, for example, two sub-bands i.e., a 
higher sub-band and a lower sub-band, and two surface acoustic wave device 
units respectively associated with the two sub-bands are formed on a 
single substrate with their outputs connected in parallel. FIG. 2 shows a 
prior art example of this type which comprises one surface acoustic wave 
device unit consisting of first, second and third electrodes 21, 31 and 41 
taking charge of the higher band, UHFHi, and the other surface acoustic 
wave device unit consisting of first, second and third electrodes 22, 32 
and 42 for the lower band, UHFLo. The pitch between electrode fingers of 
the UHFHi electrodes is smaller than the pitch between electrode fingers 
of the UHFLo electrodes. The first electrodes (input electrodes) 21 and 22 
for the respective sub-bands have common bus bars 81 and 82 of which bus 
bar 81 is connected to a common UHF input terminal 9 and bus bar 82 is 
grounded. Similarly, the second electrodes (output electrodes) 31 and 32 
and the third electrodes (output electrodes) 41 and 42 for the respective 
sub-bands have common bus bars 83 and 84 of which bus bar 83 is connected 
to an output terminal 10 and bus bar 84 is grounded. 
In the prior art surface acoustic wave device as shown in FIG. 2, since the 
distance between the input and output electrodes is optionally determined 
and different for the higher and lower sub-band device units, the delay 
time of the surface acoustic wave due to the propagation thereof from the 
input electrode to the output electrode in one device unit does not 
coincide with that in the other device unit with a result that undesired 
multi-peak characteristics are created corresponding to the difference in 
the delay time. These undesired characteristics are superimposed on the 
desired constant-frequency-interval comb-like characteristics. In a 
frequency band portion at which the output level of the one sub-band 
device unit approximates the output level of the other sub-band device 
unit at the overlapped frequencies in, that is, the two sub-bands, the 
aforementioned phenomenon is eminent. 
In conjunction with FIG. 2, when taking for example x.sub.2 representative 
of the center to center distance between the input electrode 21 and the 
output electrode of the higher sub-band unit being 556 .mu.m, x.sub.1 
representative of a similar distance for the lower sub-band unit being 
1365 .mu.m, a piezoelectric substrate of lithium niobate (LiNbO.sub.3) 
being of y-axis cut and z-axis propagation, and the distance between the 
output electrodes 31 and 41 (32 and 42) being 1740 .mu.m, a multi-peak 
characteristics of 4.43 MHz frequency interval is then partly superimposed 
on a multi-peak characteristic of 2 MHz frequency interval which covers 
530 to 830 MHz, within a frequency band covering 630 to 680 MHz at which 
the two sub-band units have substantially equal output levels. As a 
result, it is impossible to obtain the multi-peak, 2 MHz frequency 
interval characteristic over the whole frequency band. 
The prior art surface acoustic wave device of FIG. 1 manifesting the 
multi-peak frequency characteristic will be again discussed with reference 
to FIG. 3. As shown in FIG. 3, the second electrode 4 has electrode 
fingers 4a to 4d and the third electrode 5 has electrode fingers 5a to 5d. 
The arrangement of the electrode fingers 4a to 4d is in relationship of 
the same polarity with respect to the arrangement of the electrode fingers 
5a to 5d. In other words, both the arrangements are symmetrical with 
respect to the propagation of the surface acoustic wave. 
This symmetry can mathematically be expressed as, 
##EQU1## 
where V represents a resultant output of the surface acoustic wave device, 
f a frequency of the input signal, x.sub.1 a distance between the first 
electrode 2 and the second electrode 4, x.sub.2 a distance between the 
first electrode 2 and the third electrode 5, .lambda. a wave length of the 
surface acoustic wave, and A an amplitude of the surface acoustic wave. 
By setting velocity v of the surface acoustic wave such that 
.DELTA.f=v/(x.sub.2 -x.sub.1) and neglecting the time terms, equation (1) 
can be reduced to, 
##EQU2## 
where A' is a constant. 
Consequently, the resultant output has a frequency characteristic having 
peaks at f=n.multidot..DELTA.f (n: integer) and .DELTA.f corresponds to a 
frequency interval between the peaks. 
With this configuration, when taking .DELTA.f=2 MHz, for example, the peaks 
take place at even (even number in mega-Hertz unit) frequencies such as 
150 MHz, 152 MHz, -- but do not at odd frequencies such as 151 MHz, 153 
MHz, --. For .DELTA.f=1 MHz, the peaks can occur at odd frequencies. But 
the half value of .DELTA.f approximately doubles the chip size of the 
device. 
An object of this invention is therefore to obtain 
constant-frequency-interval multi-peak frequency characteristics which 
frequency interval is different from that of the prior art device, by 
maintaining the chip size as it is in the prior art device, thereby 
extending areas in which the surface acoustic device is employed. 
Another object of this invention is to eliminate the aforementioned prior 
art disadvantages and to provide a surface acoustic device in which the 
peak frequencies can be preset accurately.

Referring now to FIG. 4, there is shown a first embodiment of the 
invention. In FIG. 4, like reference characters designates like elements 
in FIG. 1. As shown, a second output electrode 4 has electrode fingers 4a 
to 4d and a third output electrode 7 has electrode fingers 7a to 7d. What 
is different from FIG. 1 is that the electrode fingers 7a to 7d of the 
third electrode 7 are in a relationship of opposite polarity with respect 
to the electrode fingers 4a to 4d of the second electrode 4. The term 
"opposite polarity" used in this specification will now be explained with 
reference to FIG. 4. Assuming that the load 6 is substituted by a signal 
source, the phase of a surface acoustic wave signal produced between the 
fingers 4a and 4c of the electrode 4 is different by 180.degree. from 
(opposite to) that of a surface acoustic wave signal produced between the 
fingers 7a and 7c of the electrode 7 which is connected with the electrode 
4 by means of the common bus bar. This is because the fingers4a and 7a 
have one polarity (due to one common bus bar connection) and the fingers 
4c and 7c have the opposite polarity (due to the other common bus bar 
connection) while those fingers in the electrodes 4 and 7 which are 
nearest to the electrode 2 are the fingers 4a and 7c having opposite 
polarities and those fingers in the electrodes 4 and 7 which are next 
nearest to the electrode 2 are the fingers 4c and 7a also having opposite 
polarities. In other words, the arrangement of the electrode fingers 7a to 
7d and the arrangement of the electrode fingers 4a to 4d are asymmetrical 
with respect to the propagation of the surface acoustic wave. 
In particular, the electrode fingers 7a and 7b of the third electrode 7 are 
placed toward the wave front of the propagating surface acoustic wave 
relative to the electrode fingers 7c and 7d, respectively, in contrast to 
the corresponding electrode fingers 4a and 4b of the second electrode 4, 
thus ensuring the opposite polarity relationship. The distance between 
electrode finger 2a of the first electrode (input electrode) and the 
electrode finger 4d is x.sub.1 and the distance between the electrode 
finger 2a and the electrode finger 7d is x.sub.2, as in FIG. 3. 
This asymmetry can be expressed as, 
##EQU3## 
where V.sub.1 represents a resultant output of the electrodes 4 and 7, f a 
frequency of the input signal, .lambda. a wave length of the surface 
acoustic wave, and B an amplitude of the surface acoustic wave. 
By setting velocity v of the surface acoustic wave such that 
.DELTA.f=v/(x.sub.2 -x.sub.1) and neglecting the time terms, equation (3) 
can be reduced to, 
##EQU4## 
where B' is a constant. 
Consequently, the resultant output V.sub.1 has a frequency characteristic 
having peaks at f.sub.o =(n+1/2).DELTA.f, n being integer. 
To describe details of the structure embodying the invention, reference is 
made to FIG. 5 which illustrates a surface acoustic wave device used as a 
filter having constant-frequency-interval multi-peak frequency 
characteristic suitable for automatic channel selection of a tuner section 
of a television receiver. The filter has a second electrode 4 and a third 
electrode 7, both having the same arrangements of electrode fingers as 
those shown in FIG. 4, surface acoustic wave absorbers 8 disposed adjacent 
to a first electrode 2 and the third electrode 7 at opposite ends of a 
piezoelectric substrate 3, and a shield electrode 9 of metallic thin film 
interposed between the first electrode 2 and the second electrode 4 to 
suppress direct waves occuring between the input and output electrodes. 
Assuming that the electrodes 2, 4 and 7 are 10-pair unapodized electrodes 
having center frequencies 113 MHz, the distance L between the output 
electrodes 4 and 7 is 1741 .mu.m, the filter provides 
constant-frequency-interval multi-peak frequency characteristics of 2 MHz 
interval having peaks at a 101MHz, 103 MHz, 105 MHz, --. 
In the example of FIG. 5, the second and third electrodes 4 and 7 are 
disposed on the same side of the first electrode 2. Alternatively, the 
first electrode may be interposed between the second and third electrodes. 
It is also possible that the input electrical signal fed to the second and 
third electrodes 4 and 7 and delivered out of the first electrode 2. 
Furthermore, the electrodes 4 and 7 may be connected in series. 
FIG. 6 shows a second embodiment of the invention wherein, like the prior 
art of FIG. 2, UHF band (530-830 MHz) is divided into two sub-bands, one 
surface acoustic wave device unit consisting of a set of an input 
electrode 21 and output electrodes 31 and 41 taking charge of one sub-band 
and the other surface acoustic wave device unit consisting of a set of an 
input electrode 22 and output electrodes 32 and 42 taking charge of the 
other sub-band are formed on a common substrate 3, and input and output 
terminals 9 and 10 common to both the sub-bands are provided. For the 
lower sub-band, the input electrode 22 comprises a 61.5-pair apodized 
electrode, and the output electrodes 32 and 42 comprise a 7-pair 
unapodized electrode having a center frequency of 602.5 MHz. For the 
higher sub-band, the input electrode 21 and output electrodes 31 and 41 
each comprise a 5-pair unapodized electrode having a center frequency of 
752.0 MHz. The substrate 3 made of LiNbO.sub.3 of Y-axis cut and Z-axis 
propagation is provided with the electrodes of 0.1 .mu.m thick aluminum. 
The distance between the output electrodes is 1740 .mu.m. 
In this embodiment, for distance L.sub.2 between the leading electrode 
finger of the input electrode 21 and the trailing electrode finger of the 
output electrode 31 and for distance L.sub.1 between the leading electrode 
finger of the input electrode 22 and the trailing electrode finger of the 
output electrode 32, the difference in distance (L.sub.2 -L.sub.1) was set 
to be 177.4 .mu.m so that the delay time due to the propagation of the 
surface acoustic wave from the input electrode 21 to the output electrode 
31 in the higher sub-band unit is made equal to the delay time due to the 
propagation of the surface acoustic wave from the input electrode 22 to 
the output electrode 32 in the lower sub-band unit, and output voltages of 
the output electrodes 31 and 32 become in phase. As a result, undesired 
constant-frequency-interval multi-peak characteristics due to the delay 
time of the surface acoustic wave propagation between the input and output 
electrodes were eliminated and highly accurate peak frequencies covering 
530 to 830 MHz could be obtained. 
FIG. 7 shows a third embodiment of the invention which has the same 
electrode arrangement as that of FIG. 6, that is, the same polarity 
relationship between the electrode fingers of the second electrode and 
those of the third electrode but additionally comprises a shield electrode 
11 interposed between the input and output electrodes and across the two 
surface acoustic wave units to attenuate the level of direct waves, and a 
grating reflector 12 (comprised of 200 grating conductors and having a 
center frequency of 522 MHz) interposed between the input electrode 22 and 
output electrode 32 of the lower sub-band unit to attenuate the lowermost 
frequency region of the frequency band. 
FIG. 8 shows a fourth embodiment of the invention which has substantially 
the same electrode arrangement as that of FIG. 7 except that the electrode 
fingers of the second electrode are arranged in the opposite polarity 
relationship with those of the third electrode. This embodiment 
additionally comprises a shield electrode 11 and a grating reflector 12 
which are identical with those of FIG. 7. 
In FIG. 9 showing a fifth embodiment of the present invention, the filter 
structure is basically identical with that shown in FIG. 8. The 
differences therebetween only lie in that the electrode 22' has another 
finger arrangement (e.g., the number of the fingers is different) and the 
arrangement of the fingers of the electrodes 32' and 42' are such as to 
deal with a surface acoustic wave signal the polarity of which is opposite 
to that of the surface acoustic wave signal dealt with by the electrodes 
32 and 42 shown in FIG. 8. Since the finger arrangement of the electrode 
22' is such as to produce a surface acoustic wave signal which will be 
received at the electrodes 32' and 42' with a phase or polarity opposite 
to that of the acoustic wave signal received at the electrodes 32 and 42 
in FIG. 8, the modified electrode structures 32' and 42' shown in FIG. 9 
is necessary for making the electric signals obtained from electrodes 31, 
41, 32' and 42' in phase with each other. 
In the foregoing embodiments with reference to FIGS. 6-9, the delay time 
between the input electrode and the output electrode close thereto in one 
device unit was made equal to the delay time between the input electrode 
and the output electrode close thereto in the other device unit and 
outputs from the close output electrodes in the two device units were 
rendered in phase. But, as will be clearly seen from the fundamental 
operational description with reference to FIG. 1, the elimination of 
difference in the delay time may be effected between the input electrode 
and the output electrode which is remote therefrom, and the establishment 
of the in-phase output may be effected in connection with the remote 
output electrode. The number of surface acoustic wave device units is not 
limited to two but it may be three or more. The second and third 
electrodes acting as the output electrode in the foregoing embodiments may 
alternatively be used as the input electrode. Also, the first electrode 
may be interposed between the second and third electrodes at different 
distances therefrom. Further, the electrodes 31 and 34 (32 and 42; 32' and 
42') may be connected in series. 
As described above, according to the invention, the second and third 
electrodes connected in parallel (or in series) with their electrode 
fingers arranged in the opposite polarity relationship ensure that peak 
frequencies can occur at (integer+1/2) times the frequency interval. 
Accordingly, for a frequency interval of 2 MHz, for example, it is 
possible to provide a filter having constant-frequency-interval multi-peak 
characteristic in which the magnitude of peaks is odd in mega-Hertz unit, 
without increasing the size of chip. Furthermore, according to the 
invention, since the distance between the input and output electrodes in 
the sub-band device units formed on a single substrate is determined such 
that the delay time(s) between the input and output electrodes are equal 
and the output voltages are in phase in the sub-band device units, 
accurate multi-peak frequencies covering the whole frequency band at which 
the sub-band units operate can be ensured, thereby extending areas in 
which the surface acoustic device is employed.