Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-08-09
2004-06-29
Wamsley, Patrick (Department: 2819)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06756867
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter having filtering characteristics and a balance-to-unbalance conversion function, and to a communication apparatus including such a surface acoustic wave filter.
2. Description of the Related Art
Recently, there has been significant technological progress in reducing the size and weight of cellular telephones. In order to achieve these results, in addition to reducing the number and size of components, multi-function components are being increasingly developed. In view of such a background, recent research has been actively conducted on surface acoustic wave (SAW) filters having a balance-to-unbalance conversion function, i.e., a so-called “balun” function, which filters are used in the RF (radio frequency) stage of cellular telephones. Such SAW filters are used mostly for GSM (global system for mobile communications) compliant devices.
If a balanced line such as a twin lead feeder is directly coupled to an unbalanced line such as a coaxial cable, an unbalanced current flows, thereby, undesirably causing the feeder itself to operate as an antenna. A circuit called a balun is thus required for preventing the unbalanced current in order to match the balanced line to the unbalanced line.
Many patent applications pertaining to SAW filters having a balance-to-unbalance conversion function have been filed. A well-known SAW filter having a balance-to-unbalance conversion function and an input impedance and output impedance which are substantially equal is illustrated in FIG.
11
.
In the SAW filter shown in
FIG. 11
, a comb-like electrode (also called an interdigital transducer, hereinafter referred to as an “IDT”)
102
is disposed on a piezoelectric substrate
100
. IDTs
101
and
103
are positioned on the left and right sides of the IDT
102
, respectively, in the surface acoustic wave (SAW) propagation direction.
Reflectors
104
and
105
for reflecting a surface acoustic wave in order to improve the conversion efficiency are further arranged in the SAW filter in such a manner that they sandwich the IDTs
101
,
102
, and
103
therebetween from the left and right. Balanced signal terminals
106
and
107
are connected to sets of electrode fingers of the center IDT
102
, respectively. An unbalanced signal terminal
108
is connected to one set of electrode fingers of each of the IDTs
101
and
103
.
The SAW filter of this type is called a three-IDT longitudinally coupled resonator SAW filter having a balance-to-unbalance conversion function by performing conversion between the IDTs
101
,
102
, and
103
using a surface acoustic wave.
In a SAW filter having a balance-to-unbalance conversion function, it is required that the transmission characteristics in the pass band between the unbalanced signal terminal
108
and each of the balanced signal terminals
106
and
107
have equal amplitude characteristics and 180°-out-of-phase characteristics.
The characteristic of the balanced signal terminals
106
and
107
having equal amplitude characteristics and the characteristic of the balanced signal terminals
106
and
107
being 180° out of phase are referred to as “amplitude balance” and “phase balance”, respectively.
The amplitude balance and the phase balance are defined as follows. If the SAW filter having a balance-to-unbalance conversion function is a three-port device consisting of, for example, a first port corresponding to an unbalanced input terminal and second and third ports corresponding to balanced output terminals, the amplitude balance=[A], where A=|20log(S
21
)|−|20log(S
31
)|, and the phase balance=B−180, where B=|∠S
21
−∠S
31
|. In the above equations, S
21
denotes the transfer coefficient from the first port to the second port, S
31
denotes the transfer coefficient from the first port to the third port, and ∥ indicates an absolute value.
In balance between balanced signal terminals, ideally, the amplitude balance and the phase balance, as defined above, are 0 dB and 0 degree, respectively, in the pass band of a SAW filter.
However, the SAW filter in the related art shown in
FIG. 11
has a problem in that balance between the balanced signal terminals
106
and
107
is reduced.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a balance-to-unbalance conversion function and having high balance between balanced signal terminals, and also provide a communication apparatus including such a novel SAW filter which achieves excellent high transmission characteristics.
According to a preferred embodiment of the present invention, a SAW filter includes a piezoelectric substrate, a filter unit provided on the piezoelectric substrate and having an odd number of at least three IDTs arranged along a SAW propagation direction, each IDT having a pair of electrode finger sets. Balanced signal terminals are connected to the pair of electrode finger sets of the center IDT in the IDTs, and an unbalanced signal terminal is connected to one electrode finger set of each of at least two end IDTs of the IDTs. The end IDTs are different from the center IDT, and are located on ends of the filter unit. The other electrode finger set of each of the at least two end IDTs is connected to a ground. The ground for at least one of the at least two end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT.
With this configuration, the balanced signal terminals are connected to the pair of electrode finger sets of the center IDT positioned at the center of the IDTs, and the unbalanced signal terminal is connected to one electrode finger set of each of at least two end IDTs that is different from the center IDT of the IDTs, thereby implementing a balance-to-unbalance conversion function. Furthermore, the ground for at least one end IDT of the end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT, thereby making it possible to improve balance between the balanced signal terminals.
In the SAW filter, a signal line which connects the unbalanced signal terminal to the end IDTs may be arranged so as to surround one of the balanced signal terminals, and a ground line may be provided between the signal line and the balanced signal terminal surrounded by the signal line. This allows the bridge capacitance inserted between the signal line and the balanced signal terminal to be reduced by the ground line, thereby making it possible to improve balance between the balanced signal terminals.
According to another preferred embodiment of the present invention, a SAW filter includes a piezoelectric substrate, a plurality of filter units provided on the piezoelectric substrate, each unit having an odd number of at least three IDTs along a SAW propagation direction, and each IDT having a pair of electrode finger sets. Balanced signal terminals are connected to the pair of electrode finger sets of a first center IDT which is located at the center of a first filter unit of the plurality of filter units, and an unbalanced signal terminal is connected to one electrode finger set of a second center IDT which is located at the center of a second filter unit, which is different from the first filter unit of the plurality of filter units. One electrode finger set of each of the end IDTs which are different from the first and second center IDTs of the plurality of filter units is connected to a ground, and the end interdigital transducers are located on ends of the first and second filter units. The ground for at least one of the end IDTs is electrically isolated on the piezoelectric substrate from the ground for the other end IDT.
With this unique configuration, the plurality of filter units increase the out-of-passband attenuation, and improve filtering characteristics. Furthermor
Keating & Bennett LLP
Murata Manufacturing Co. LTD
Wamsley Patrick
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