Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-06-29
2003-05-27
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R
Reexamination Certificate
active
06570470
ABSTRACT:
This application is based on application Nos.2000-199132, 2000-365616 filed in Japan, the content of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter comprising a plurality of surface acoustic wave electrodes arranged on a piezoelectric substrate in a ladder fashion, and more particularly to a surface acoustic wave filter featuring an improved shoulder characteristic curve on a lower frequency of the passband of the filter.
2. Description of the Related Art
As a high-frequency bandpass filter for use in mobile communications devices, the surface acoustic wave filter has been known which comprises a plurality of surface acoustic wave electrodes arranged on the piezoelectric substrate. For instance, there has been disclosed in Japanese Unexamined Patent Publication No.5(1993)-183380 a surface acoustic wave filter wherein a plurality of surface acoustic wave electrodes are arranged on a piezoelectric substrate to define a ladder filter circuit thereon.
FIG. 8
is a schematic circuit diagram for illustrating the surface acoustic wave filter disclosed by the aforesaid prior art.
This surface acoustic wave filter
510
comprises a rectangular piezoelectric substrate
520
, on which resonators
530
,
540
,
550
,
560
comprised of surface acoustic wave electrodes are arranged.
As seen in
FIG. 8
, the resonators
530
,
540
are connected in series to form a series arm between an input terminal
570
and an output terminal
580
(hereinafter, the resonators
530
,
540
each referred to as “a series resonator” and collectively referred to as “a series resonator group”). On the other hand, the resonators
550
,
560
are electrically disposed in parallel between the series arm and a ground electrode
590
(hereinafter, the resonators
550
,
560
each referred to as “a parallel resonator” and collectively referred to as “a parallel resonator group”). The series resonators
530
,
540
and the parallel resonators
550
,
560
are alternatively arranged between the input terminal and the output terminal. The parallel resonators are connected to the ground electrode
590
via inductors
555
,
565
, respectively.
The series resonator
530
and the parallel resonator
550
are paired to define a one-stage ladder filter. Likewise, the series resonator
540
and the parallel resonator
560
are paired to define a one-stage ladder filter.
The surface acoustic wave filter
510
operates as follows.
FIG. 9
is a diagram for explaining a configuration of the surface acoustic wave electrode constituting each of the series or parallel resonators
530
-
560
.
FIG. 9
schematically illustrates only an electrode portion of a one-port surface acoustic wave resonator.
In
FIG. 9
, indicated at
700
is a surface acoustic wave electrode. The electrode
700
has a configuration wherein an interdigital transducer (IDT)
710
is sandwiched between reflectors
720
,
730
.
The IDT
710
has a configuration wherein a comb electrode
710
a
having a plurality of electrode fingers
711
and a comb electrode
710
b
having a plurality of electrode fingers
712
are arranged in such a manner that the electrode fingers
711
are interdigitated with the electrode fingers
712
. In this configuration, the comb electrode
710
a
is connected between an input electrode and an output electrode whereas the comb electrode
710
b
is connected to the ground electrode, for example.
A surface acoustic wave is excited by a signal inputted to the IDT
710
of the surface acoustic wave electrode
700
thus configured. The excited wave is reflected by the reflectors
720
,
730
to form a standing wave which is trapped between the reflectors
720
,
730
. Thus, the surface acoustic wave electrode
700
operates as a resonator having a high Q value. As is well known, the surface acoustic wave electrode
700
has such impedance characteristics that there exists a pole lowered in impedance by a resonant frequency while there appears a pole increased in impedance by an antiresonant frequency.
The surface acoustic wave filter
510
having the series and parallel resonators
530
-
560
of such a configuration utilizes the impedance characteristics of the surface acoustic wave electrode
700
for obtaining a passband of a desired bandwidth. More specifically, the resonant frequency of the series resonators
530
,
540
and the antiresonant frequency of the parallel resonators
530
,
540
are substantially set equal to each other thereby to match the output/input impedance with the characteristic impedance near these frequencies. Thus is established the passband.
Particularly in the ladder filter circuit, the surface acoustic wave electrode
700
has a predetermined impedance characteristic. Therefore, the filter circuit presents a very high impedance near the antiresonant frequency of the series resonators
530
,
540
while presenting a very low impedance near the resonant frequency of the parallel resonators
550
,
560
. Taking advantage of such a characteristic, the ladder filter circuit can obtain a wide filter characteristic ranging from a stop band on a high-frequency side of a passband to a stop band on a low-frequency side of the passband.
As an approach to permit such a ladder filter circuit to achieve an increased attenuation at an attenuation pole, there has been disclosed a resonator provided with an LC circuit comprised of a surface acoustic wave electrode (see, for example, Japanese Unexamined Patent Publication No.9(1997)-232906). Alternatively, a technique has been disclosed wherein the length of a wire connected to an external element is changed to vary the inductance of the wire itself thereby changing the position of an attenuation pole for adjustment of the attenuation thereof (see, for example, Japanese Unexamined Patent Publication No.11(1999)-55067).
Recently, however, the standards for efficient utilization of radio waves have been established for the mobile communications systems. For instance, the US PCS Standards defines the receive band in the range of 1930 to 1990 MHz and the transmission band in the range of 1850 to 1910 MHz so that a gap between the passbands of a transmitting filter and a receiving filter is 20 MHz. Thus, the transmission band is formed adjacent to the receive band. Therefore, the receiving filter is required to ensure a sufficient amount of attenuation and a low insertion loss in the passband while maintaining the wideness of the passband.
This requires a steep characteristic curve on the lower side (left-shoulder side) of the receive band, the curve extending from the stop band to the passband. Unfortunately, neither of the techniques disclosed in Japanese Unexamined Patent Publications Nos.9(1997)-232906 and 11(1999)-55067 is capable of achieving the filter characteristics featuring the increased passband and the steep characteristic curve on the left-shoulder side thereof.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a surface acoustic wave filter capable of achieving a sufficiently steep characteristic curve on a lower side of a passband while maintaining the wideness of the passband without use of any specific LC-circuit.
The surface acoustic wave filter according to the invention comprises a series resonator group including a plurality of surface acoustic wave electrodes formed on a surface of a piezoelectric substrate and electrically disposed in series between an input terminal and an output terminal; and a parallel resonator group including a plurality of surface acoustic wave electrodes individually electrically disposed in parallel between an input- or output-terminal of the individual electrodes of the series resonator group and a ground electrode, wherein a resonant frequency formed by a part of the surface acoustic wave electrodes among the surface acoustic electrodes of the parallel resonator group is lower than an antiresonant frequency formed by the other surface acoustic wave electrodes of the prarallel resonator group,
Maehara Hiroaki
Mizutani Yuuji
Hogan & Hartson LLP
Kyocera Corporation
Summons Barbara
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