Transversely coupled resonator type surface acoustic wave...

Details Transversely coupled resonator type surface acoustic wave... Transversely coupled resonator type surface acoustic wave...

2000-02-14

2002-03-05

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Plural channel systems

Having branched circuits

C333S193000, C333S195000, C310S31300R, C310S31300R

Reexamination Certificate

active

06353371

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resonator type surface acoustic wave filter used as, for example, a bandpass filter or other type of filter. More specifically, the present invention relates to a resonator type surface acoustic wave filter including a plurality of transversely or longitudinally coupled surface acoustic wave resonators which generates and uses a surface acoustic wave that has a Shear Horizontal (SH) wave as its main component, such as, a Bleustein-Gulyaev-Shimizu (BGS) wave.
2. Description of the Related Art
Various conventional surface acoustic wave filters are known bandpass filters. For example, a transversely coupled surface acoustic wave filter using a Rayleigh wave as a surface acoustic wave, and transversely coupled and longitudinally coupled resonator filters using a Rayleigh wave are already known bandpass filters.
The transverse type surface acoustic wave filter is constructed such that a fixed distance separates a pair of interdigital transducers (IDTs) from each other on a surface acoustic wave substrate.
Further, in the transversely coupled resonator filter using a Rayleigh wave, a plurality of surface acoustic wave resonators include a plurality of IDTs that are provided on a surface acoustic wave substrate with the plurality of surface acoustic wave resonators coupled in the transverse direction. In the transversely coupled resonator filter, reflectors are located on both sides of an area occupied by the IDTs.
Recently, an edge reflection type surface acoustic wave filter using and SH type surface acoustic wave such as a BGS wave has been developed because the filter has the advantage of being compact.
In a transversely coupled surface acoustic wave filter using the Rayleigh wave, because two sets of IDTs are separated by a fixed distance along the direction of the surface-wave transmission on a surface acoustic wave substrate, the size of the filter cannot be reduced and is instead much larger. Also, the insertion loss is very large.
In a transversely coupled resonator filter using a Rayleigh wave, because the electromechanical coupling coefficient K is small, the filter does not have sufficient bandwidth, and although the loss can be reduced, the aperture of the electrode fingers of the IDT is about 10&lgr; and the coupling length is about 1.5&lgr; assuming that the wavelength of the surface acoustic wave is B. Additionally, reflectors, as described above, are required. Accordingly, the size of the filter cannot be reduced and made compact.
Further, in an edge reflection type surface acoustic wave resonator using a SH type surface acoustic wave, a resonator with a wide bandwidth is obtained, which is due to the relatively large electromechanical coupling coefficient K, so that no reflectors are required. Further, the edge reflection type resonator is favorable because it can be made compact. However, up until now, when a SH type surface acoustic wave was used, the construction of a filter by coupling the plurality of surface acoustic wave resonators was considered to be impossible.
SUMMARY OF THE INVENTION
To overcome the problems of the related art described above, preferred embodiments of the present invention provide a resonator type surface acoustic wave filter constructed so as to make use of a SH type surface acoustic wave, particularly a BGS wave, and which is miniaturized and compact size with very small insertion loss.
A transversely coupled resonator type surface acoustic wave filter according to a preferred embodiment of the present invention includes a surface acoustic wave substrate having opposing first and second end surfaces, first and second surface acoustic wave resonators on the surface acoustic wave substrate, and which utilizes a surface acoustic wave having an SH wave as a main component, and the first and second surface acoustic wave resonators are coupled to constitute a stage of the transversely coupled resonator filter, wherein the first and second surface acoustic wave resonators include first and second interdigital transducers, the first and second interdigital transducers defined by a first bus bar, a second bus bar and a common bus bar all arranged so as to be substantially parallel to each other and extending in a direction of transmission of the surface acoustic wave, the common bus bar is located between the first and second bus bars, and a plurality of electrode fingers are connected to the first and second bus bars at a first end of the electrode fingers and extended towards the common bus bar, a plurality of electrode fingers are connected to the common bus bar at a first end of the electrode fingers and such that a first set of electrode fingers of the common bus bar are extended towards the first bus bar and a second set of electrode fingers are extended towards the second bus bar so that the first and second sets of electrode fingers of the common bus bar interdigitate with the electrode fingers of the first and second bus bars, wherein the surface acoustic wave substrate has a relative dielectric constant E along the direction of
11
of the surface acoustic wave substrate such that E=∈
s
11
/∈
0
, and where E is in a range of about 0 to about 3000, wherein an aperture y of the interdigital transducer is defined as an amount of overlap between the electrode fingers of the common bus bar with the electrode fingers of the first and second bus bars normalized by a wavelength &lgr; of the surface acoustic wave, and wherein the filter is constructed such that y satisfies a formula:
0.945+5.49×exp(−E/366)≧y≧2.46×exp(−E/219)  (1)
Also, when K is an electromechanical coupling coefficient of the surface acoustic wave substrate, the filter is preferably arranged such that F=K
2
·∈
s
11/∈
0
, and F is in the range of about 0 to about 250 and the aperture y of the interdigital transducer satisfies the following formula:
1.40+4.14×exp(−F/46)≧y≧0.25+0.97×exp(−F/42)  (2)
Further, in a transversely coupled resonator type surface acoustic wave filter, preferably, the coupling length x, which is normalized by the wavelength &lgr; of the surface acoustic wave, satisfies the following formula:
0.71+1.72×exp(−E/251)≧×≧0.045+0.16×exp(−E/418)  (3)
Preferably, if the filter is constructed so that F=K
2
·∈
s
11
/∈
0
is in the range of about 0 to about 250 then the coupling length x preferably satisfies the following formula:
0.452+1.953×exp(−F/49.56)≧×≧0.269×exp(−F/32)  (4)
Further, one bus bar of each of the first and second surface acoustic wave resonators is made a common bus bar and the width W of the common bus bar preferably satisfies the following formula:
0.32+0.853×exp(−E/222)≧W≧0.017+0.157×exp(−E/245)  (5)
Also, if the filter is constructed so that K
2
·∈
s
11
/∈
0
is in the range of about 0 to about 250, then the width W of the common bus bar of the first and second surface acoustic wave resonators preferably satisfies the following formula:
0.22+0.84×exp(−F/43)≧W≧0.03+0.14×exp(−F/21)  (6)
Further, the filter is preferably constructed so that the gap width G between the common bus bar and the tip of a plurality of electrode fingers extending from the other bus bars meets the following formula:
1.19+4.51×10
−4
×E−1.34×10
−6
×E
2
≧G≧−0.115+0.29×exp(−E/1150)  (7)
Also, if the filter is constructed so that F=K
2
·∈
s
11
/∈
0
is in the range of about 0 to about 250, then the gap width G preferably meets the following formula:
1.125−0.003×F−1.016×10
−6
×F
2
≧G≧−0.107+0.26×exp(−F/250)  (8)
Moreover, the outside gap G′ is also pre

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