Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-09-14
2002-09-17
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S358000
Reexamination Certificate
active
06452306
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave device and a piezoelectric substrate used therefor.
DESCRIPTION OF THE PRIOR ART
In recent years, various kinds of mobile communication terminal devices, including cellular telephones, have come into widespread use. It is highly desirable to reduce this kind of terminal equipment in size and weight for enhanced portability. In order to reduce the size and weight of terminal devices, their electronic parts must be substantially reduced in size and weight. For this reason, surface acoustic wave devices enabling size and weight reduction, namely, surface acoustic wave filters, are often used for high- and intermediate-frequency parts of terminal devices. Such devices are formed with an inter-digital electrode for exciting, receiving, reflecting and propagating surface acoustic waves on the surface of a piezoelectric substrate thereof.
Among characteristics important to a piezoelectric substrate used for surface acoustic wave devices are surface wave velocity (SAW velocity), temperature coefficient of center frequency in the case of filters or of resonant frequency in the case of resonators (the temperature coefficient of frequency: TCF), and electromechanical coupling factor (k
2
). The characteristics of typical piezoelectric substrates currently known for surface acoustic wave devices are set forth below in Table 1. For details regarding these characteristics, reference should be made to Yasutaka SHIMIZU, “Propagation characteristics of SAW materials and their current application”, the Transactions of The Institute of Electronics, Information and Communication Engineers A, Vol. J76-A, No.2, pp. 129-137 (1993). Hereinafter, the piezoelectric substrates for surface acoustic wave devices are referred to using the designations in Table 1.
TABLE 1
SAW
Propagation
Velocity
K
2
Symbol
Composition
Cut Angle
Direction
(m/s)
(%)
128LN
LiNbO
3
128°
X
3992
5.5
-Rotated Y
64LN
LiNbO
3
64°-Rotated Y
X
4742
11.3
36LT
LiTaO
3
36°-Rotated Y
X
4212
4.7
LT112
LiTaO
3
X
112°
3288
0.64
-Rotated Y
ST
Quartz
ST
X
3158
0.14
Quartz
Crystal
Crystal
As can be seen from Table 1, currently known piezoelectric substrates are divided into the group including 128LN, 64LN, and 36LT which have high SAW velocities and high electromechanical coupling factor and the group including LT112 and ST quartz crystal which have low SAW velocities and low electromechanical coupling factor. The piezoelectric substrates which belong to the group with high SAW velocity and high electromechanical coupling factor (128LN, 64LN, and 36LT) are used for surface acoustic wave filters of high-frequency parts of terminal devices. The piezoelectric substrates which belong to the group with low SAW velocity and low electromechanical coupling factor (LT112 and ST quartz crystal) are used for surface acoustic wave filters of intermediate-frequency parts of terminal devices.
Various systems are practically employed all over the world for mobile communications devices, typically cellular telephones, and are all used at frequencies of the order of 1 GHz. Therefore, filters used for high-frequency parts of terminal devices have a center frequency of approximately 1 GHz. A surface acoustic wave filter has a center frequency substantially proportional to the SAW velocity of the piezoelectric substrate used and almost inversely proportional to the width of electrode fingers formed on the substrate. To enable such filters to be operated at high frequencies, therefore, it is preferable to utilize substrates having high SAW velocities, for instance, 128LN, 64LN, and 36LT. Also, a wide passband width of 20 MHz or more is required for filters used as high-frequency parts. To achieve such wide passband, however, it is essential for the piezoelectric substrate to have a large electromechanical coupling factor k
2
. For these reasons, much use is made of 128LN, 64LN, and 36LT.
On the other hand, mobile communication terminal devices use an intermediate frequency in the 70 to 300 MHz band. When a filter having a center frequency in this frequency band is constructed using a surface acoustic wave device, if the aforementioned 128LN, 64LN, or 36 LT is used as the piezoelectric substrate, the widths of the electrode fingers formed on the substrate have to be much larger than those of the aforementioned filter used as a high-frequency part.
More specifically, the following equation (1) roughly applies to the relationship among the width d of an electrode finger of a surface acoustic wave transducer that forms a surface acoustic wave filter, the center frequency f
0
of the surface acoustic wave filter, and the SAW velocity V of the piezoelectric substrate used.
f
0
=V
/(4
d
) (1)
If a surface acoustic wave filter having a center frequency of 1 GHz is constructed on the assumption that the SAW velocity is 4000 m/s, the width of the electrode finger thereof is calculated from the equation (1) to be
d=
4000 (m/s)/(4×1000 (MHz))=1 &mgr;m
On the other hand, when an intermediate-frequency filter having a center frequency of 100 MHz is constructed using this piezoelectric substrate having a SAW velocity of 4000 m/s, the width of the electrode finger required for this is given by
d=
4000 (m/s)/(4×100 (MHz))=10 &mgr;m
Thus, the required width of the electrode finger is ten times as large as that for the high-frequency part filter. A large width of the electrode finger means that the surface acoustic wave intermediate-frequency filter itself becomes large. Therefore, in order to make a surface acoustic wave intermediate-frequency filter small, it is necessary to use a piezoelectric substrate having a low SAW velocity V, as can be appreciated from the equation (1).
For this reason, LT112 and ST quartz crystal, whose SAW velocities are low, are used for the piezoelectric substrates of surface acoustic wave intermediate-frequency filters. ST quartz crystal is particularly suitable because the primary temperature coefficient of frequency TCF is zero. Because the electromechanical coupling factor k
2
of ST quartz crystal is low, only a filter having a narrow passband is achievable. However, because it is a function of the intermediate-frequency filters to pass signals through a single narrow channel, the fact that the ST quartz crystal has a small electromechanical coupling factor has caused no problem.
In recent years, however, digital mobile communication systems have been developed and put into practical use. These systems have won very rapid acceptance because of their ability to make effective use of frequency resources, compatibility with digital data communications, and so on. The passband of the digital system is very wide, for instance, several hundred KHz to several MHz. In the case where an intermediate-frequency filter having such a wide passband is constructed using a surface acoustic wave device, it is difficult to use an ST quartz crystal substrate. In order to further reduce the size of mobile communication terminals for enhanced portability, it is required to reduce the mounting area of surface acoustic wave intermediate-frequency filters. However, because the SAW velocities of ST quartz crystal and LT112, which are considered to be suitable for surface acoustic wave intermediate-frequency filters, are over 3100 m/sec, further minimization is difficult.
As explained above, when surface acoustic wave devices for intermediate-frequency are constructed using piezoelectric substrates having high electromechanical coupling factor such as 128LN, 64LN, and 36LT, the device size must be large since the SAW velocities of the substrates are high, although a wide passband can be obtained. On the other hand, when surface acoustic wave devices for intermediate-frequency are constructed using piezoelectric substrates having low SAW velocities such as ST quartz crystal and LT112 in order to reduce the device size, a wide passband cannot be obtained since the electromechanical coupling factors of the substrates are low. Thus, su
Inoue Kenji
Kawasaki Katsumi
Morikoshi Hiroki
Sato Katsuo
Brown & Raysman Millstein Felder & Steiner LLP
Medley Peter
Ramirez Nestor
TDK Corporation
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