Surface acoustic wave filter having input and output transducers with different aperture lengths

A surface acoustic wave filter is provided which performs balanced signal/unbalanced signal conversion. The input and output impedances are different from each other. In a surface acoustic wave filter of the longitudinal mode type, a signal is applied to two terminals of an input interdigital transducer, or a signal is output from two terminals of an output interdigital transducer, thereby consituting a balanced type filter. The aperture length of the input interdigital transducer is different from that of the output interdigital transducer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a surface acoustic wave filter, and particularly 
to a surface acoustic wave filter which is used in a high-frequency 
region. 
2. Related art of the Invention 
Recently, studies on a surface acoustic wave device which can be used in a 
filter have been intensively conducted. In order to cope with recent 
development of mobile communications and increase of the frequency, 
particularly, development of a surface acoustic wave device has been 
vigorously made. 
Conventionally, several kinds of filters which are configured by a surface 
acoustic wave device and which are used in the high-frequency band, 
particularly in the barnd of the hundreds of megahertz are known. Typical 
examples of such filters are as follows: a filter which is of the 
so-called ladder type and configured by plural surface acoustic wave 
resonators, such as that disclosed in Japanese patent publication (Kokai) 
No. SHO52-19044; a filter which is of the so-called multielectrode type, 
such as that disclosed in Japanese patent publication (Kokai) No. 
SHO58-154917; and a filter which is of the so-called longitudinal mode 
type and in which surface acoustic wave resonators are juxtaposed and 
coupling among the resonators are used, such as that disclosed in Japanese 
patent publications (Kokai) Nos. HEI32-222512, SHO61-230419, and 
HEI1-231417. Most of these filters handle an unbalanced signal and are set 
to have a characteristic impedance of 50 ohms in order to meet user 
requests. 
Recently, there is a movement afoot to balance a high-frequency circuit in 
order to improve the performance of the circuit. In a high-frequency 
circuit of a portable phone or the like, for example, an unbalanced 
circuit is used in the high-frequency side and a balanced circuit in the 
low-frequency side. When a circuit is balanced, merits can be produced 
that the resistance to noise is improved and that the circuit can be 
driven at a lower voltage. However, such a balanced configuration requires 
circuit parts to be ready for a balanced circuit. In an unbalanced type 
circuit of the prior art, parts have an input/output impedance of 50 ohms. 
When a circuit is balanced, however, the impedance is not always 50 ohms. 
In a transitional period from an unbalanced circuit to a balanced circuit, 
particularly, parts are necessary which have unbalanced terminals at the 
input and balanced terminals at the output. An example of such parts is a 
balun. A balun has a configuration which can control the impedances of the 
balanced and unbalanced terminals. When a circuit of the prior art, i.e., 
an unbalanced circuit is to be used, however, a balun is not necessary. 
When a balun is used for deforming a circuit of the prior art into a 
balanced circuit, therefore, defects such as the increased cost of parts 
for the balun and the increased area required for mounting the balun are 
produced. 
SUMMARY OF THE INVENTION 
To comply with this, the inventors have thought that such a function or 
that performed by a balun may be added to a surface acoustic wave filter 
which is conventionally used. First, a modification of a filter of the 
longitudinal mode type has been invented as a system which receives an 
unbalanced signal and outputs a balanced signal or in reverse receives a 
balanced signal and outputs an unbalanced signal. Usually, a filter of the 
longitudinal mode type is configured as shown in FIG. 10. Reference 
numerals 1003-1 and 1003-2 designate reflectors, reference numeral 1004 
designates one of the two input terminals and reference numberal 1005 
designates one of the two output terminals. All of an input interdigital 
transducer 1001 and output interdigital transducers 1002-1 and 1002-2 are 
always used while the other input terminal and the other ouput terminal 
are grounded (in other words, both the input and the output handle an 
unbalanced signal). From the principle of generation of a surface acoustic 
wave, the electric signal/surface acoustic wave conversion ought to 
function even when signals which are shifted in phase from each other by 
180 degrees are respectively applied to the two terminals or a balanced 
signal is input to the terminals. Also in the surface acoustic 
wave/electric signal conversion, it is considered that, in a propagating 
surface acoustic wave, the phases at two terminals of an interdigital 
transducer are shifted in phase from each other by 180 degrees and a 
balanced signal is output. When such a configuration is adopted, 
therefore, it is possible to attain a configuration in which the input 
signal is an unbalanced signal and the output signal is a balanced signal, 
or vice versa. 
It is apparent that the input and output impedances depend on the product 
of the apeture length of an interdigital transducer and the number of 
pairs. As the product is larger, the impedances are lower, and, as the 
product is smaller, the impedances are higher. However, the number of 
pairs of an interdigital transducer is closely related to the band width 
of a filter. When the number of pairs is made smaller, for example, the 
band of a filter tends to be wider, and, when the number is made larger, 
the band of a filter tends to be narrower. In an actual use, the necessary 
band width is previously determined. In view of this, the number of pairs 
has a very small degree of freedom. When the aperture length of an input 
interdigital transducer is made different from that of an output 
interdigital transducer, or when the apeture length of one or both of the 
interdigital transducers is weighted, it is possible to control the input 
and output impedances. 
In this way, according to the invention, a surface acoustic wave filter 
which is suitable for a balanced type circuit can be obtained. 
Furthermore, a surface acoustic wave filter in which the input and output 
impedances can be set to be different values can be obtained.

PREFERRED EMBODIMENTS 
Hereinafter, embodiments of the invention will be described. 
Embodiment 1 
Embodiment 1 of the invention will be described. A lithium niobate 
(LiNbO.sub.3) substrate of 64.degree.-Y-cut and X-propagation is used as 
the piezoelectric substrate, and a filter is configured using the 
substrate. FIG. 1 shows the electrode configuration. The reference numeral 
101 designates an input interdigital transducer, 102-1 and 102-2 designate 
output interdigital transducers, 103-1 and 103-2 designate reflectors, 
104-1 and 104-2 designate input terminals, and 105-1 and 105-2 designate 
output terminals. In the figure, for the sake of simplification, the 
interdigital transducers and the reflectors are shown in a reduced number. 
The thus configured filter was connected to a measuring apparatus by using 
baluns 201-1 and 201-2 as shown in FIG. 2, and the characteristics of the 
filter were then measured. The baluns were used because of the following 
reason. A usual measuring apparatus is used for measuring an unbalanced 
signal and cannot measure a balanced signal. Therefore, the unbalanced 
signal/balanced signal conversion must be performed by using baluns. As a 
result of the measurement, the characteristics of the filter are 
substantially equal to those in the case of an unbalanced signal in the 
prior art. Similarly, also in the configuration shown in FIG. 3, the 
characteristics of the filter are measured, using balum 301-1 with the 
result that the characteristics of the filter were substantially equal to 
those in the case of an unbalanced signal in the prior art. FIG. 11 shows 
characteristics in the unbalanced signal/unbalanced signal case in the 
prior art, and FIG. 12 shows characteristics in the balanced 
signal/unbalanced signal case shown in FIG. 3. 
Embodiment 2 
Next, Embodiment 2 of the invention will be described. A lithium niobate 
(LiNbO.sub.3) substrate of 64.degree.-Y-cut and X-propagation is used as 
the piezoelectric substrate, and a filter is configured using the 
substrate. FIG. 4 shows the electrode configuration. The reference numeral 
401 designates an input interdigital transducer, 402 designates an output 
interdigital transducer, 403-1 and 403-2 designate reflectors, 404 
designates an input terminal, and 405 designates an output terminal. In 
the figure, for the sake of simplification, the interdigital transducers 
and the reflectors are shown in a reduced number. The aperture length of 
the output interdigital transducer 402 is smaller than that of the input 
interdigital transducer 401. In the filter, naturally, the output 
impedance is higher than the input impedance. Specifically, the filter is 
designed so that the input impedance is 50 ohms and the output impedance 
is 100 ohms. 
An impedance matching circuit was connected to the output of the filter, 
and the characteristics of the filter were then measured. As a result, 
although the characteristics are slightly inferior to those of a filter of 
the prior art, the characteristics sufficiently satisfy the requirements 
for a filter. 
Embodiment 3 
Next, Embodiment 3 of the invention will be described. A lithium niobate 
(LiNbO.sub.3) substrate of 64.degree.-Y-cut and X-propagation is used as 
the piezoelectric substrate, and a, filter is configured using the 
substrate. FIG. 5 shows the electrode configuration. The reference numeral 
501 designates an input interdigital transducer, 502 designates an output 
interdigital transducer, 503-1 and 503-2 designate reflectors, 504-1 and 
504-2 designate input terminals, and 505-1 and 505-2 designate output 
terminals. In the figure, for the sake of simplification, the interdigital 
transducers and the reflectors are shown in a reduced number. The aperture 
length of the output interdigital transducer 502 is smaller than that of 
the input interdigital transducer 501. In the filter, naturally, the 
output impedance is higher than the input impedance. Specifically, the 
filter is designed so that the input impedance is 50 ohms and the output 
impedance is 100 ohms. 
The thus configured filter was connected to a measuring apparatus by using 
baluns 601 and 602 as shown in FIG. 6, and the characteristics of the 
filter were then measured. The balun 601 performs the balanced 
signal/unbalanced signal conversion of 50 ohms, and the balun 602 conducts 
the conversion between an unbalanced signal of 50 ohms and a balanced 
signal of 100 ohms. The baluns were used because of the following reason. 
A usual measuring apparatus is used for measuring an unbalanced signal and 
cannot measure a balanced signal. Therefore, the unbalanced 
signal/balanced signal conversion must be performed by using baluns. As a 
result of the measurement, the characteristics of the filter are 
substantially equal to those in the case of an unbalanced signal in the 
prior art. 
Embodiment 4 
Next, Embodiment 4 of the invention will be described. A lithium niobate 
(LiNbO.sub.3) substrate of 64.degree.-Y-cut and X-propagation is used as 
the piezoelectric substrate, and a filter is configured using the 
substrate. FIG. 7 shows the electrode configuration. The reference numeral 
701 designates an input interdigital transducer, 702-1 and 702-2 designate 
output interdigital transducers, 703-1 and 703-2 designate reflectors, 704 
designates an input terminal, and 705 designates an output terminal. In 
the figure, for the sake of simplification, the interdigital transducers 
and the reflectors are shown in a reduced number. The aperture length of 
the input interdigital transducer 701 is weighted. The aperture lengths of 
portions of the input interdigital transducer 701 which are nearest to the 
output interdigital transducers 702-1 and 702-2 are equal to those of the 
output interdigital transducers 702-1 and 702-2. This is conducted in 
order to allow a surface acoustic wave to efficiently propagate between 
the input and output interdigital transducers. As a result, the product of 
the apeture length and the number of pairs in the output interdigital 
transducers 702-1 and 702-2 are larger than those in the input 
interdigital transducer 701. In other words, in the filter, the output 
impedance is higher than the input impedance. The filter is designed so 
that the input impedance is 50 ohms and the output impedance is 100 ohms. 
An impedance matching circuit was connected to the output of the filter, 
and the characteristics of the filter were measured. As a result, although 
the characteristics are slightly inferior to those of a filter of the 
prior art, the characteristics sufficiently satisfy the requirements for a 
filter. 
In the embodiment, the aperture length of the input interdigital transducer 
701 is weighted. Alternatively, the apeture lengths of the output 
interdigital transducers 70)2-1 and 702-2 may be weighted, or the aperture 
lengths of all the input and output interdigital transducers may be 
weighted. The manner of weighting is not restricted to the shape shown in 
the figure and weighting may be conducted in any manner. 
Embodiment 5 
Next, Embodiment 5 of the invention will be described. A lithium niobate 
(LiNbO.sub.3) substrate of 64.degree.-Y-cut and X-propagation is used as 
the piezoelectric substrate, and a filter is configured using the 
substrate. FIG. 8 shows the electrode configuration. The reference numeral 
801 designates an input interdigital transducer, 802-1 and 802-2 designate 
output interdigital transducers, 803-1 and 803-2 designate reflectors, 
804-1 and 804-2 designate input terminals, and 805-1 and 805-2 designate 
output terminals. In the figure, for the sake of simplification, the 
interdigital transducers and the reflectors are shown in a reduced number. 
The aperture length of the input interdigital transducer 801 is weighted. 
The aperture lengths of portions of the input interdigital transducer 801 
which are nearest to the output interdigital transducers 802-1 and 802-2 
are equal to those of the output interdigital transducers 802-1 and 802-2. 
This is conducted in order to allow a surface acoustic wave to efficiently 
propagate between the input and output interdigital transducers. As a 
result, the product of the apeture length and the number of pairs in the 
output interdigital transducers 802-1 and 802-2 is larger than that in the 
input interdigital transducer 801. In other words, in the filter, the 
output impedance is higher than the input impedance. The filter is 
designed so that the input impedance is 50 ohms and the output impedance 
is 100 ohms. 
The thus configured filter was connected to a measuring apparatus by using 
baluns 901 and 902 as shown in FIG. 9, and the characteristics of the 
filter were then measured. The balun 901 performs the balanced 
signal/unbalanced signal conversion of 50 ohms, and the balun 902 conducts 
the conversion between an unbalanced signal of 50 ohms and a balanced 
signal of 100 ohms. The baluns were used because of the following reason. 
A usual measuring apparatus is used for measuring an unbalanced signal and 
cannot measure a balanced signal. Therefore, the unbalanced 
signal/balanced signal conversion must be performed by using baluns. As a 
result of the measurement, the characteristics of the filter are 
substantially equal to those in the case of an unbalanced signal in the 
prior art. 
In the embodiment, the aperture length of the input interdigital transducer 
801 is weighted. Alternatively, the aperture lengths of the output 
interdigital transducers 802-1 and 802-2 may be weighted, or the aperture 
lengths of all the input and output interdigital transducers may be 
weighted. The manner of weighting is not restricted to the shape shown in 
the figure and weighting may be conducted in any manner. 
FIGS. 13 and 14 show other examples of weighting, respectively. FIG. 13 
shows a surface acoustic wave filter having a configuration in which the 
aperture length is smallest in the center portion of an input interdigital 
transducer 1301, and, as moving outward, the aperture length is gradually 
increased to a size which is equal to the aperture lengths of output 
interdigital transducers 1302-1L and 1302-2. In FIG. 13, reference 
numerals 1303-1 and 1303-2 designate reflectors. FIG. 14 shows a surface 
acoustic wave filter: having a configuration in which the aperture lengths 
of output interdigital transducers 1402-1 and 1402-1 are different from 
each other, and the aperture length of an input interdigital transducer 
1401 positioned between the| output interdigital transducers is weighted 
so as to be varied from the aperture length of the output interdigital 
transducer 1402-1 to that of the output interdigital transducer 1402-2. In 
FIG. 14, reference numerals 1403-1 and 1403-2 designate reflectors. In the 
same manner as Embodiment 4, the configuration of FIG. 13 may be modified 
so that one of input terminals 1304-1 and 1304-2, or one of output 
terminals 1305-1 and 1305-2 is grounded, or one of the input terminals and 
one of the output terminals are grounded, and an unbalanced signal is 
input or output. The configuration of FIG. 14 may be modified so that one 
of input terminals 1404-1 and 1404-2, or one of output terminals 1405-1 
and 1405-2 is grounded, or one of the input terminals and one of the 
output terminals are grounded, and an unbalanced signal is input or 
output. 
Embodiment 6 
FIG. 15 is a block diagram showing a high-frequency portion of a signal 
transmitting/receiving apparatus of Embodiment 6 of the invention. The 
embodiment is a configuration illustrating an application example of the 
surface acoustic wave filters described above. For example, such a filter 
can be used between an unbalanced circuit in a high-frequency portion of a 
signal transmitting/receiving apparatus such as a portable phone, and a 
balanced circuit in a low-frequency portion such as a mixer. Referring to 
FIG. 15, in the transmitter side, a mixer 2001 mixes a modulated signal 
from a controller 2008 with a local signal so as to obtain a signal of a 
desired transmitting frequency. Usually, the mixer output signal is a 
balanced signal. The mixer output signal is supplied to a transmitting 
amplifier (PA) 2003 through a filter 2002. The transmitting amplifier (PA) 
2003 is an unbalanced circuit. The transmitting amplifier (PA) 2003 
amplifies the signal and the amplified signal is output through an antenna 
duplexer 2004 and an antenna 2009. The antenna 2009, the antenna duplexer 
2004, and the transmitting amplifier (PA) 2003 constitute the transmitting 
means, and the mixer 2001 constitutes the frequency converting means. 
By contrast, in the receiver side, a signal received by the antenna 2009 
passes through the antenna duplexer 2004, and then amplified by a 
received-signal amplifier (LNA) 2005. Thereafter, a desired signal is 
extracted by a receiving filter 2006 and supplied to a receiving mixer 
2007. In the front and rear of the receiving filter 2006, the 
received-signal amplifier (LNA) 2005 is an unbalanced circuit, and the 
receiving mixer 2007 is a balanced circuit. The antenna 2009, the antenna 
duplexer 2004, and the received-signal amplifier (LNA) 2005 constitute the 
receiving means, and the receiving mixer 2007 constitutes the frequency 
converting means. 
Among the components of FIG. 15, the antenna 2009, the antenna duplexer 
2004, the transmitting amplifier (PA) 200:3, and the received-signal 
amplifier (LNA) 2005 exhibit inferior characteristics when they are 
configured in the form of a balanced circuit. These circuits in the form 
of a balanced circuit remain to require a considerable time period for 
practical use. By contrast, even now, many kinds of balanced circuits can 
be used as the mixers 2001 and 2007, and characteristics of a mixer of the 
balanced type are superior. The use of the filters of the embodiments as 
the above-mentioned filters will result in the use of more excellent parts 
or circuits. 
In Embodiment 6, an example in which the filter is used in a signal 
transmitting/receiving apparatus such as a portable phone has been 
described. It is a matter of course that an apparatus and location where 
the filter is used are not restricted to the above. 
In the embodiments described above, two or three interdigital transducers 
are disposed between the reflectors. It is a matter of course that 
interdigital transducers of a number which is larger than three can be 
used. 
In the embodiments described above, a lithium niobate (LiNbO.sub.3) 
substrate of 64.degree.-Y-cut and X-propagation is used as a piezoelectric 
member constituting a surface acoustic wave resonator. The invention is 
not restricted to this. It is a matter of course that, even when another 
kind of substrate such as a lithium tantalate (LiTaO.sub.3) substrate or a 
quartz substrate is used, the same effects can be attained.