Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1998-12-15
2001-03-27
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S133000, C310S31300R
Reexamination Certificate
active
06208223
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) receiving filter to be used in a radio frequency (RF) filter that is used in compact mobile communication devices such as cellular phones and so forth.
2. Description of Related Art
Recently the development of compact-size and light-weight mobile communication device terminals such as cellular phones and so forth has been progressed. Along with the development, compact size and high performance of parts to be used therein are required, and so is the development of SAW elements.
Since a SAW separator is a device that will greatly contribute to making an RF portion compact, there is a great demand for its practical applications.
An RF portion shown in
FIG. 7
comprises an antenna
101
, an ANT terminal
102
that is connected to the above antenna, a transmitting filter
103
and a separator line
107
that are respectively connected to the above ANT terminal, a transmitting terminal (Tx terminal)
104
that is connected to the above transmitting filter, an electric power amplifier
105
that is connected to the above transmitting terminal, a receiving filter
108
that is connected to the above separator line, and a receiving terminal (Rx terminal)
109
that is connected to the above receiving filter.
The SAW separator
110
comprises the transmitting filter
103
, the receiving filter
108
and the separator line
107
, and such a SAW separator are required to have its characteristics including compact size, low insertion loss of a pass band, large attenuation in an attenuation band, and so on.
Since the SAW separator
110
is arranged at output line of the electric power amplifier
105
at the final stage, electric power load from 1 to 2 W is imposed on the transmitting filter
103
, while electric power load around 1 W is imposed on the receiving filter
108
, therefore, excellent electric power-resisting property is required to the SAW separator.
Conventionally, a dielectric wave separator has been used in such a field and apt to make volume large, which has been a problem with the prior art.
In the transmitting filter
103
and the receiving filter
108
, used in general is a ladder type filter as shown in FIG.
8
. The filter is equipped with series arm SAW resonators S
1
, S
2
and S
3
, and parallel arm SAW resonators P
1
, P
2
, P
3
and P
4
. The series arm SAW resonator S
1
at a first stage has interdigitated width of 50 &mgr;m and number of pairs of 100, the series arm SAW resonator S
2
at a second stage has interdigitated width of 50 &mgr;m and number of pairs of 100, and the series arm SAW resonator S
3
at a third stage has interdigitated width of 50 &mgr;m and number of pairs of 100. The parallel arm SAW resonator P
1
at a first stage has interdigitated width of 70 &mgr;m and number of pairs of 70, the parallel arm SAW resonator P
2
at a second stage has interdigitated width of 110 &mgr;m and number of pairs of 80, the parallel arm SAW resonator P
3
at a third stage has interdigitated width of 110 &mgr;m and number of pairs 80, and the parallel arm SAW resonator P
4
at a fourth stage has interdigitated width of 70 &mgr;m and number of pairs of 70.
FIG. 9
is a diagram showing a symbol of each of the respective SAW resonators (S
1
-S
3
, P
1
-P
4
) in
FIG. 8
, and
FIG. 10
is a diagram showing a lumped parameter equivalent circuit therein. These respective resonators are arranged with their comb-shape electrodes or interdigital electrodes mutually engaged as shown in FIG.
11
.
In
FIG. 11
, reference numerals
10
a
and
10
b
show comb-shape type electrodes or interdigital electrodes, and they are engaged with each other, and reference numeral
12
shows a pair of finger electrodes, while the code D shows a interdigitated width of the finger electrodes.
Usually, each of the resonators has finite Q, and there will occur electric power loss that decreases this Q value. A resistance component that causes this electric power loss generates heat when electric power is supplied to the resonator, and this heat causes damage on electrodes. This fact is known to those skilled in the art. Especially in the case of a receiving filter of a SAW separator, attention should be given to heat generating at electrodes of a parallel arm at a first stage.
In the circuit structure shown in
FIG. 8
, in order to prevent the above damage, a method wherein the number of pairs of the input stage resonator S
1
of series arm is made larger than the number of each of other series arm resonators S
2
and S
3
is known in the field. (Refer to, for example, Japanese Unexamined Patent Publication (TOKKAIHEI) No.6(1994)-29779.)
However, in the above receiving filter of conventional SAW separator, no measures are taken on the number of pairs in parallel arm resonators. Accordingly, when electric power is applied to a parallel arm resonator at a first stage, heat is generated or released in electrodes at the first stage, and the heat causes damage on electrodes, which has been a problem to the conventional SAW separator.
For example, a receiving filter of the structure shown in
FIG. 8
has been employed in a SAW separator. When electric power is applied to the SAW separator, a transmitting filter and a receiving filter employed in the SAW separator are broken.
A ladder type filter shown in
FIG. 8
is well known as one wherein comb-shape electrodes are melted off owing to heat generated by current flowing through series arms (S
1
, S
2
, S
3
) and parallel arms (P
1
, P
2
, P
3
, P
4
). Especially in operating a cellular phone, parallel arms in a receiving filter of a SAW separator generate more heat than series arms. The parallel arm at a first stage generates heat most among the respective parallel arms, whereby electrodes of parallel arms are broken, and consequently the receiving filter is broken. Accordingly, measures to enhance electric power-resisting properties of parallel arms (P
1
, P
2
, P
3
, P
4
) have been required.
In view of the above circumstances, an object of the present invention is to provide a receiving filter of a SAW separator with high resistance to electric power and with high performance by preventing temperature thereof from rising.
SUMMARY OF THE INVENTION
In order to achieve the above object, according to a first aspect of the present invention, there is provided a receiving filter that is used in a SAW separator and that comprises a ladder type filter, wherein
the ladder type filter has plural stages of series arms and plural stages of parallel arms wherein each of the input side stages thereof is the first stage and each of the output side stages thereof is the last stage,
each of the series arms and parallel arms comprises a SAW resonator,
the parallel arm at the first stage comprises one SAW resonator as a parallel arm resonator, and
the number of pairs of finger electrodes of the parallel arm resonator at the first stage is made larger than the number of pairs of finger electrodes of each of the parallel arm resonators at other stages.
According to a second aspect of the present invention, there is provided a receiving filter that is used in a SAW separator and that comprises a ladder type filter, wherein
the ladder type filter has plural stages of series arms and plural stages of parallel arms wherein each of the input side stages thereof is the first stage and each of the output side stages thereof is the last stage,
each of the series arms and parallel arms comprises a SAW resonator,
the parallel arm at the first stage comprises one SAW resonator as a parallel arm resonator, and
the interdigitated width of finger electrodes of the parallel arm resonator at the first stage is made longer than intergigitated width of finger electrodes of each of the parallel arm resonators at other stages.
According to a third aspect of the present invention, there is provided a receiving filter that is used in a SAW separator and that comprises a ladder type filter, wherein
the ladder type filter has plural stages of series arms and plural stages of
Fujita Yoshiaki
Komazaki Tomokazu
Shimamura Hajime
Umezawa Masaaki
Burdett James R.
Frank Robert J.
Lee Benny
Oli Electric Industry, Co., Ltd.
Summons Barbara
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