Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-01-14
2004-11-16
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S133000, C333S195000
Reexamination Certificate
active
06819203
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) filter devices including a plurality of SAW elements, and in particular, to a SAW filter device in which a plurality of SAW resonators define parallel arm resonators and series arm resonators such that a ladder circuit is provided.
2. Description of the Related Art
Various types of SAW filters according to the prior art have been proposed in which a series arm SAW resonator is provided in a series arm between an input and an output of the SAW filter and a parallel arm SAW resonator is provided in a parallel arm between the series arm and the ground potential. SAW filters having a ladder circuit configuration have low loss and broad bandwidth. Accordingly, SAW filters are used as bandpass filters, and are widely used as cellular-phone bandpass filters.
According to Japanese Unexamined Patent Application Publication No. 5-183380, as shown in
FIG. 29
, the matching of a ladder type SAW filter is improved by connecting unit filters
501
and
502
as indicated by broken lines such that the configurations of the unit filters
501
and
502
are symmetrical. A unit filter is a circuit portion including a series arm resonator and a parallel arm resonator. In the SAW filter shown in
FIG. 29
, three unit filters
501
to
503
are connected. A combination of the resonators of the unit filters
501
and
502
is represented by the circuit diagram shown in FIG.
30
.
In the circuit configuration shown in
FIG. 30
, series arm resonators S
1
and S
2
are connected between an input
504
and an output
505
, and parallel arm resonators P
1
and P
2
are provided in two parallel arms. Where the capacitance of the parallel arm resonator P
1
on the input side is represented by Cp
1
, and the capacitance of the parallel arm resonator P
2
in the parallel arm located between the series arm resonators S
1
and S
2
is represented by Cp
2
, the ratio Cp
2
/Cp
1
=2 exists.
Japanese Unexamined Patent Application Publication No. 11-55067 discloses a structure in which the ratio Cp
2
/Cp
1
is at least 2 and an inductor that is connected to an end of a parallel arm resonator P
2
positioned between series arm resonators has a capacitance that is greater than that of an inductor connected to another parallel arm resonator. With this structure, attenuation in a frequency domain (frb to fra) lower than a pass band is achieved, as shown in FIG.
31
.
It is common knowledge that, in a ladder SAW filter according to the prior art which includes a plurality of series arm resonators and a plurality of parallel arm resonators, the capacitances Cp
1
and Cp
2
of the parallel arm resonators may be set such that the ratio Cp
2
/Cp
1
=2 is obtained where impedance matching is important, and to ensure attenuation in a range lower than a pass band the ratio Cp
2
/Cp
1
may be set to at least 1:2, and an inductor is connected to an end of a parallel arm resonator in a parallel arm located between the series arm resonators having a capacitance larger than that of an inductor connected to another parallel arm resonator.
A case in which the ratio Cp
2
/Cp
1
=2 as described in Japanese Unexamined Patent Application Publication No. 5-183380 has a problem in that the steepness of filter characteristics in a range lower than in the pass band deteriorates. The steepness of the filter characteristics in the lower range is indicated by a frequency distance &Dgr;f between attenuation values A to B in FIG.
32
. The smaller the frequency distance &Dgr;f, the higher the selectivity.
As is disclosed in Japanese Unexamined Patent Application Publication No. 11-55067, where the ratio Cp
2
/Cp
1
is at least 2 and an inductor that is connected to an end of a parallel arm resonator provided between series arm resonators has an inductance that is greater than that of an inductor connected to another parallel arm resonator, attenuation is enhanced in a range lower range than the pass band, as described above. Also, the steepness of the filter characteristics in the lower range of the pass band is enhanced, although this feature is not clearly described in Japanese Unexamined Patent Application Publication No. 11-55067.
However, it is found that the structure disclosed in Japanese Unexamined Patent Application Publication No. 11-55067 causes deterioration in attenuation in a range higher than the pass band.
SUMMARY OF THE INVENTION
To overcome the above-described problems, preferred embodiments of the present invention provide a ladder circuit type SAW filter device that has sufficient attenuation in each stop band of ranges higher and lower than a pass band and has superior steepness of filter characteristics in a low frequency domain in the vicinity of the pass band, and a communication apparatus including such a novel SAW filter.
According to a preferred embodiment of the present invention, a ladder circuit type surface acoustic wave filter device includes a piezoelectric substrate, a plurality of parallel arm resonators and a plurality of series arm resonators provided on the piezoelectric substrate which are respectively defined by surface acoustic wave resonators, and a plurality of inductors connected in series to the parallel arm resonators. The parallel arm resonators include a first parallel arm resonator connected to one of an input end and an output end, and a second parallel arm resonator connected to a junction between two series arm resonators. The first parallel arm resonator and the second parallel arm resonator have a relationship represented by the following expression:
Cp
1
×
2
<
Cp
2
where Cp
1
represents the capacitance of the first parallel arm resonator, Cp
2
represents the capacitance of the second parallel arm resonator. The inductor connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator.
Preferably, the capacitance Cp
2
of the second parallel arm resonator is in a range represented by the following expression:
Cp
1
×
2
<
Cp
2
<
Cp
1
×10.
The resonance frequency of the second parallel arm resonator may be lower than the resonance frequency of the first parallel arm resonator.
The ladder circuit type surface acoustic wave filter device according to another preferred embodiment of the present invention further includes a package which contains surface acoustic wave elements in which the parallel arm resonators and the series arm resonators are provided on the piezoelectric substrate and a plurality of electrode pads connected to the series arm resonators or the parallel arm resonators, and bonding wires to connect the electrode pads to the series arm resonators or the parallel arm resonators. The length of the bonding wire connected to the second parallel arm resonator is substantially equal to or less than the length of the bonding wire connected to the first parallel arm resonator such that the inductor connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor connected to the first parallel arm resonator.
The ladder circuit type surface acoustic wave filter device according to another preferred embodiment further includes a package which contains surface acoustic wave elements in which the parallel arm resonators and the series arm resonators are provided on the piezoelectric substrate and a plurality of electrode pads connected to the series arm resonators or the parallel arm resonators, and bonding wires to connect the electrode pads to the series arm resonators or the parallel arm resonators. One end of the second parallel arm resonator is electrically connected to at least two electrode pads among the electrode pads which are connected to the ground potential such that the inductor which is connected to the second parallel arm resonator has an inductance that is substantially equal to or less than the inductance of the inductor
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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