Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2001-06-21
2004-08-24
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S195000, C310S31300R
Reexamination Certificate
active
06781478
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to longitudinally coupled resonator type surface acoustic wave filters. More particularly, the present invention relates to longitudinally coupled resonator type surface acoustic wave filters having balance-unbalance conversion functions.
2. Description of the Related Art
Recently, the sizes and weights of mobile phones have been greatly reduced. With such a trend, the numbers and sizes of components used in mobile phones have also been reduced while making such components more and more multifunctional.
With the above-described considerations, various kinds of mobile phones have been provided. In these mobile phones, surface acoustic wave filters are incorporated in the RF stages of the phones to have balance-unbalance conversion functions, or so-called balun functions.
FIG. 22
is a schematic plan view for illustrating the electrode structure of a conventional surface acoustic wave filter having a balance-unbalance conversion function.
In this filter, first, second and third IDTs
101
,
102
and
103
are arranged in a surface acoustic wave propagating direction. Reflectors
104
and
105
are arranged on each side of the surface acoustic wave propagating direction in the region where the IDTs
101
,
102
and
103
are arranged. When a wavelength determined by the electrode finger pitch of each of the IDTs
101
,
102
and
103
is &lgr;I, both a distance between the centers of the mutually adjacent electrode fingers of the IDTs
101
and
102
and a distance between the centers of the mutually adjacent electrode fingers of the IDTs
102
and
103
are 0.75 &lgr;I. The widths of the electrode fingers
109
and
110
of the ends of the IDT
102
are increased to reduce spaces between the IDTs. As a result, loss due to the irradiation of a bulk wave can be reduced. In
FIG. 22
, terminals
106
and
107
are balanced signal terminals, and a terminal
108
is an unbalanced signal terminal.
In such a surface acoustic wave filter having the balance-unbalance conversion function, regarding propagation characteristics in a pass band between the unbalanced signal terminal
108
and the balanced signal terminal
106
and between the unbalanced signal terminal
108
and the balanced terminal
107
, amplitude characteristics need to be equal and propagating signals need to be 180° out of phase with respect to each other. The condition in which the amplitude characteristics are equal is referred to as amplitude balance and the degree at which the propagating signals are 180° out of phase with respect to each other is referred to as phase balance.
The amplitude balance and the phase balance are defined as follows when the surface acoustic wave filter having the balance-unbalance conversion function is regarded as a device having three ports, such as the unbalanced input terminal as port
1
and the balanced output terminals as ports
2
and
3
.
Amplitude balance=|A|.
A=|20log S21|−|20log S31|.
Phase balance=|B−180|.
B=|∠S21−∠S31|.
In this definition, S21 denotes a transfer factor from the port
1
to the port
2
and S31 denotes a transfer factor from the port
1
to the port
3
.
Ideally, in the pass band of the filter, the amplitude balance needs to be 0 dB and the phase balance needs to be 0°. However, with the use of the structure shown in
FIG. 22
, when intending to obtain a filter having a balance-unbalance conversion function, since the IDT
102
has an odd number of electrode fingers, the number of electrode fingers connected to the balanced signal terminal
106
increases by one more than the number of electrode fingers connected to the balanced signal terminal
107
. As a result, there is a problem in that the degree of balance is deteriorated. The higher the central frequency of the filter is, the more noticeable the deterioration. Thus, like DCS filters and PCS filters, in a filter having a central frequency near 1.9 GHz, sufficient balance cannot be obtained.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally coupled resonator type surface acoustic wave filter having a balance-unbalance conversion function, in which the amplitude balance and the phase balance are greatly improved. In addition, preferred embodiments of the present invention provide a communication apparatus including such a novel longitudinally coupled resonator type surface acoustic wave filter.
According to a preferred embodiment of the present invention, a longitudinally coupled resonator type surface acoustic wave filter having a balance-unbalance conversion function includes a piezoelectric substrate, first, second and third IDTs arranged on the piezoelectric substrate in a surface acoustic wave propagating direction, the second IDT being positioned between the first and third IDTs and having an even number of electrode fingers.
According to another preferred embodiment of the present invention, a longitudinally coupled resonator type surface acoustic wave filter having a balance-unbalance conversion function includes first-stage and second-stage longitudinally coupled resonator surface acoustic wave filters longitudinally coupled to each other, each of the first-stage and second-stage filters having a piezoelectric substrate and first, second and third IDTs arranged on the piezoelectric substrate in a surface acoustic wave propagating direction, an unbalanced signal terminal connected to one end of the second IDT of the first-stage longitudinally coupled resonator surface acoustic wave filter, a first balanced signal terminal connected to one end of the second IDT of the second-stage longitudinally coupled resonator surface acoustic wave filter, a second balanced signal terminal connected to the other end of the second IDT of the second-stage filter, a first signal line connecting one end of the first IDT of the first-stage longitudinally coupled resonator surface acoustic wave filter and one end of the first IDT of the second-stage longitudinally coupled resonator surface acoustic wave filter, and a second signal line connecting one end of the third IDT of the first-stage longitudinally coupled resonator surface acoustic wave filter and one end of the third IDT of the second-stage longitudinally coupled resonator surface acoustic wave filter. In this filter, an electric signal propagating through the first signal line is 180° out of phase with an electric signal propagating through the second signal line.
In the filter according to this preferred embodiment of the present invention, the second IDT of at least one of the first-stage filter and the second-stage filter may have an even number of electrode fingers.
According to a third preferred embodiment of the present invention, a communication apparatus including the longitudinally coupled resonator type surface acoustic wave filter according to the above-described preferred embodiment of the present invention.
The above and other elements, characteristics, features, and advantages of the present invention will be clear from the following detailed description of preferred embodiments of the present invention in conjunction with the accompanying drawings.
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Ouchi Minefumi
Sawada Yoichi
Takamine Yuichi
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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