Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-03-08
2004-02-17
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S133000
Reexamination Certificate
active
06693501
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus, such as a SAW filter, and more particularly, to a SAW apparatus having a balanced-to-unbalanced conversion function and having different impedance characteristics at an input side and an output side of the SAW apparatus. The present invention also relates to a communications unit including the above-described SAW apparatus.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and weight of portable communications units, such as cellular telephones. One method to decrease the size and weight is to reduce the number and the size of the individual components of communications units. Additionally, components having composite functions are being developed.
In view of this background, as a SAW filter used in an RF stage, a balanced filter provided with a balanced-to-unbalanced conversion function (so-called “balun function”) or a multi-band filter provided with a plurality of pass bands is used.
For example, Japanese Unexamined Patent Application Publication No. 10-117123 discloses a balanced SAW filter unit which utilizes an unbalanced input and a balanced output by combining SAW filters whose transmission phase characteristics are 180° out of phase with each other. The configuration of the SAW filter unit disclosed in the above publication is shown in FIG.
9
.
The SAW filter unit shown in
FIG. 9
includes longitudinally coupled SAW filters
511
and
512
. The longitudinally coupled SAW filter
511
is formed by cascade-connecting SAW filter devices
501
a
and
501
b
in two stages via connecting patterns
520
and
521
. The longitudinally coupled SAW filter
512
is formed by cascade-connecting a SAW filter device
501
c
and a SAW filter device
502
, having transmission phase characteristics are 180° out of phase with each other, in two stages via a connecting pattern
521
and a connecting pattern
522
.
In the above-described SAW filter unit, the input terminals of the SAW filter devices
501
a
and
501
c
are respectively connected to an unbalanced terminal
503
via wires
530
and
531
. The SAW filter devices
501
b
and
502
are connected in series with each other via a wire
505
, and are also respectively connected to output balanced terminals
504
via wires
532
and
533
. Terminals
506
are ground terminals, which are respectively connected to the SAW filters devices
501
a
and
501
c
via wires
534
and
535
.
An example of the multi-band filter is disclosed in Japanese Unexamined Patent Application Publication No. 10-341135 in which a plurality of SAW filter devices (SAW filters) are arranged on a piezoelectric substrate to obtain a plurality of pass bands.
In the above-described SAW filters having composite SAW filter devices, the plurality of SAW filter devices are provided on the same piezoelectric substrate. In order to reduce the size of such SAW filters, two or more SAW filter devices are disposed side by side in a SAW propagating direction.
However, such an arrangement of SAW filter devices causes ripples in the pass band of a transmission signal due to an influence of bulk waves.
In RF filters used in cellular telephones, the transmission characteristics in the pass band should be flat. The generation of ripples must be prevented to produce flat transmission characteristics.
One method of preventing the generation of ripples is to scatter bulk waves by roughening the reverse surface of a piezoelectric substrate (forming undulations by sandblasting). According to this method, however, the piezoelectric substrate is often broken or warped due to heat or stress applied to a wafer during processing.
In the above-described known SAW filters, the generation of ripples in the pass band is prevented by displacing a plurality of SAW filter devices such that they are not disposed in the corresponding SAW propagating directions. Such an arrangement of the SAW filter devices, however, increases the size of the substrate. Accordingly, there is a demand for a more compact SAW filter (SAW apparatus) in which ripples caused by bulk waves are reduced without performing sandblasting.
SUMMARY OF THE INVENTION
In order to overcome the above-described problems, preferred embodiments of the present invention provide a SAW apparatus including a first SAW filter device having a plurality of interdigital electrodes (interdigital transducers, which are hereinafter referred to as “IDTs”) provided on a piezoelectric substrate along a direction in which a SAW propagates, and a second SAW filter device including a plurality of IDTs provided on the piezoelectric substrate along a direction in which a surface acoustic wave propagates. The propagation path of the first SAW filter device partially overlaps the propagation path of the second SAW filter device. When the center-to-center distance between the first SAW filter device and the second SAW filter device is indicated by d
1
, and when the thickness of the piezoelectric substrate is indicated by t, the first SAW filter device, the second SAW filter device, and the piezoelectric substrate are arranged to satisfy the condition d
1
≦2.3×t or d
1
≧2.8×t.
With this arrangement, the above-described SAW apparatus is provided with, not only a filtering function having a specific pass band, but also composite multiple functions by providing a plurality of SAW filter devices.
In the above-described configuration, since the first SAW filter device, the second SAW filter device, and the piezoelectric substrate are arranged to satisfy the condition d
1
≦2.3×t or d
1
≧2.8×t, an influence of bulk waves caused by the excited SAW is effectively prevented between the first and second SAW filter devices.
Accordingly, ripples caused by bulk waves occurring in the transmission signal in the pass band are greatly reduced while achieving a reduced size and multiple functions of the SAW apparatus. Thus, flatter filtering characteristics are obtained such that the transmission characteristics in the pass band are greatly improved.
Additionally, according to the above-described configuration, an influence of bulk waves is prevented without performing sandblasting (to roughen the reverse surface of the piezoelectric substrate). Thus, breakage or warpage of the piezoelectric substrate caused by roughening the piezoelectric substrate is prevented, thereby decreasing the number of defective components produced in the manufacturing process.
In the aforementioned SAW apparatus, the transmission amplitude characteristics of the first SAW filter device in the pass band are preferably substantially the same as those of the second SAW filter device in the pass band. The transmission phase characteristics of the first SAW filter device are preferably substantially 180° out of phase with those of the second SAW filter device. An input terminal or an output terminal is an unbalanced terminal, and the other terminals are balanced terminals.
With the above arrangement, since the transmission phase characteristic of the first SAW filter device is about 180° out of phase with that of the second SAW filter device, a balun function is provided, thereby providing multiple functions for the SAW apparatus.
In the aforementioned SAW apparatus, the center frequency of the first SAW filter device is preferably different from that of the second SAW filter device such that the SAW apparatus functions as a multi-band filter. With this arrangement, the SAW apparatus is provided with composite multiple functions.
Another preferred embodiment of the present invention provides a SAW apparatus including first through fourth SAW filter devices, each having a plurality of IDTs provided on a piezoelectric substrate along a direction in which a SAW propagates. The first SAW filter device and the third SAW filter device are cascade-connected, and the second SAW filter device and the fourth SAW filter device are cascade-connected. The propagation path of the first SAW filt
Sawada Yoichi
Takasaki Hiroshi
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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