Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2000-02-02
2001-10-30
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C333S194000, C310S31300R
Reexamination Certificate
active
06310524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to SAW resonator filters used as band filters and other filters, for example, in mobile communications equipment. More specifically, the present invention relates to edge reflection type longitudinally coupled SAW resonator filters, each of which has a multiple-stage structure in which a single surface acoustic wave substrate is used to define a plurality of longitudinally coupled resonator filter units of an edge reflection type.
2. Description of the Related Art
There have been proposed various surface acoustic wave filters as band filters used in mobile communications equipment. Among such surface acoustic wave filters, longitudinally coupled SAW resonator filters using longitudinal-mode couplings have filter characteristics achieved by coupling two kinds of resonant modes including symmetric modes and asymmetric modes generated by interdigital transducers (hereinafter referred to as IDTs).
In SAW resonator filters, oscillations caused by other modes such as higher modes occur outside of a pass band as spurious responses. Such spurious responses appear in a different manner depending on the configurations of IDTs aligned in a surface acoustic wave propagating direction.
When attenuation outside of the pass band is not sufficiently increased in a SAW resonator filter, a method for connecting a plurality of SAW resonator filters in series is used. However, when the configurations of the plurality of the connected SAW resonator filters are exactly the same, frequency characteristics in the SAW resonator filter units are equal. As a result, since the frequency positions where the above-described spurious responses occur are the same, the spurious responses cannot be significantly attenuated.
Thus, in order to solve the above problems, a SAW resonator filter, which is shown in
FIG. 6
of the present application, is disclosed in Japanese Unexamined Patent Publication No. 9-294049.
In a SAW resonator filter
51
, two SAW resonator filter units
53
and
54
are disposed on a surface acoustic wave substrate
52
. In the SAW resonator filter unit
53
, two IDTs
55
and
56
are aligned in a surface acoustic wave propagating direction. In addition, reflectors
57
and
58
are disposed in the surface acoustic wave propagating direction on both sides of the region where the IDTs
55
and
56
are disposed.
Similar to the SAW resonator filter unit
53
, the SAW resonator filter unit
54
is arranged such that reflectors
61
and
62
are disposed on both sides of the region where IDTs
59
and
60
are disposed.
The IDT
56
and the IDT
58
are electrically connected by a connecting conductive portion
63
. However, the number of pairs of electrode fingers of the SAW resonator filter unit
53
is different from that of the SAW resonator filter unit
54
.
In other words, in the SAW resonator filter
51
, although the two longitudinally coupled SAW resonator filter units
53
and
54
are connected in series, the numbers of pairs of the electrode fingers of the IDTs of the SAW resonator filter units
53
and
54
are different. As a result, in this structure, the frequency position of a spurious response outside of the band exhibited in the frequency characteristics of the SAW resonator filter unit
53
is different from that of a spurious response outside of the band exhibited in the frequency characteristics of the SAW resonator filter unit
54
. Therefore, it is possible to reduce the magnitude of a spurious response in the frequency characteristics of the overall SAW resonator filter
51
.
The above-described SAW resonator filter
51
, which is disclosed in Japanese Unexamined Patent Publication No. 9-294049, utilizes a Rayleigh wave as one of the surface acoustic waves. Recently, an edge reflection type surface acoustic wave device utilizing an SH-type surface acoustic wave such as a BGS (Bleustein-Gulyaev-Shimizu) wave or a Love wave, as alternatives to the Rayleigh wave, have been studied and developed. In other words, in the edge reflection type surface acoustic wave equipment, reduction in the size of the equipment can be achieved, since it is not necessary to include reflectors on both sides of IDTs.
Accordingly, when a SAW resonator filter defined by connecting the above-described two SAW resonator filter units in series is disposed in the edge reflection type surface-acoustic-wave equipment, a compact SAW resonator filter can be obtained.
However, it is significantly difficult to produce a plurality of edge reflection type SAW resonator filter units by using a single surface acoustic wave substrate and make the numbers of pairs of electrode fingers of IDTs of the SAW resonator filter units different, as described in Japanese Unexamined Patent Publication No. 9-294049. In other words, although it is necessary to form an end surface for reflecting a surface acoustic wave on both sides of IDTs in the edge reflection type SAW resonator, when the numbers of pairs of the electrode fingers of the IDTs of the plurality of the SAW resonator filter units are different, the positions of the end surfaces of the SAW resonator filter units are different from each other.
Therefore, with these conventional devices, processing of the surface acoustic wave substrate is very difficult and a precision in forming the end surfaces is reduced. As a result, the filter characteristics are greatly deteriorated.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide an edge reflection type longitudinally coupled SAW resonator filter in which a spurious response outside of a pass band is effectively suppressed and minimized without losing an advantage in an edge reflection type SAW resonator filter such as easy reduction in the size thereof, so that excellent filter characteristics are achieved.
In an edge reflection type longitudinally coupled SAW resonator filter in accordance with a preferred embodiment of the present invention, the plurality of the longitudinally coupled resonator filter units are provided on the single surface acoustic wave substrate. In this case, since the plurality of the longitudinally coupled resonator filter units have equal numbers of pairs of electrode fingers, the distances between the first end surfaces and the second end surfaces where a surface acoustic wave is reflected are equal in the longitudinally coupled resonator filter units. Accordingly, even though the plurality of the longitudinally coupled resonator filter units are provided on the single surface acoustic wave substrate, the first and second end surfaces are easily produced by cutting or other processes and methods.
Furthermore, in at least one of the plurality of the longitudinally coupled resonator filter units, the ratio of the numbers of pairs of the electrode fingers between the plurality of the IDTs is different from the ratios of the numbers of pairs of the electrode fingers between the plurality of the IDTs in the other longitudinally coupled resonator filter units. As a result, spurious responses occurring in the frequency characteristics of the longitudinally coupled resonator filter units are distributed. Therefore, spurious responses are greatly suppressed and minimized. As a result, an edge reflection type longitudinally coupled SAW resonator filter has excellent filter characteristics.
When each of the longitudinally coupled resonator filter units has a first IDT and a second IDT, and in at least one of the longitudinally coupled resonator filter units, the ratio of the numbers of pairs of the electrode fingers between the first IDT and the second IDT is different from the ratios of the numbers of pairs of the electrode fingers between the first IDTs and the second IDTs in the other longitudinally coupled resonator filter units, the positions where spurious responses occur are different as described above. As a result, spurious responses are greatly suppressed. In addition, since each of the longitudinally coupled resonator filter units has a
Ago Junya
Horiuchi Hideya
Kadota Michio
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Pascal Robert
Summons Barbara
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