Transverse double mode saw filter

Details Transverse double mode saw filter Transverse double mode saw filter

2000-12-21

2002-05-07

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S193000, C310S31300R, C310S31300R

Reexamination Certificate

active

06384698

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a resonator-type surface acoustic wave (SAW) filter using a surface acoustic wave, and particularly, relates to a transversally coupled double-mode SAW filter in which two SAW resonators are disposed transversally and in parallel.
2. Description of Related Art
Recently, SAW filters, especially transversally coupled multi-mode resonator-type SAW filters, have been widely used as the intermediate-frequency (IF) filters of mobile communication terminals such as portable telephone because they are small, low-loss, have a narrow-band passband characteristics and also less out-of-band unwanted modes. For example, in the transversally coupled double-mode SAW filter described in Japanese unexamined patent application No. S59-131213, two SAW resonators are disposed in parallel on a piezoelectric substrate in a direction perpendicular to the propagation direction of SAW which is excited by curtain-like electrodes of the SAW resonators, i.e., inter-digital transducer (IDT) electrodes, two independent inherent vibration modes originating in the acoustic coupling of SAW, i.e., a fundamental wave symmetric mode S
0
and a fundamental wave anti-symmetric mode A
0
are used, and the passband width is determined by the difference between their resonance frequencies f
1
, f
2
. Particularly, when a quartz crystal substrate with excellent frequency-temperature characteristics is used, a two-stage cascade connection filter is obtained which is in a planar element the size of 2 mm×6.5 mm and has excellent characteristics of ca. 700 ppm in specific bandwidth and 5 dB in insertion loss.
SUMMARY OF THE INVENTION
More recently, digital communication modes such as GSM mode and PHS mode seem to be adopted in a portable telephone with the digitalization of communication systems, thus an even smaller IF filter with a relatively broad passband width of 1,000 ppm has been required. In aforesaid conventional two transversally coupled double-mode SAW filters, however, the resonance frequency difference of the fundamental wave symmetric mode S
0
and the fundamental wave anti-symmetric mode A
0
could not be more than 700 ppm at a width dimension of 7 to 9&lgr; (&lgr;: the wavelength of SAW) of one SAW resonator which gives a filter impedance Z
0
of 600-800 &OHgr; considered appropriate before. In order to expand the passband width, the electrode finger cross width of IDT and the gap between SAW resonators must be narrowed, therefore such problems as an increase of insertion loss and deterioration of the degree of suppression of out-of-band frequency characteristics arise.
Moreover, if the IF filter is to be miniaturized, the sum M+N of the number of pairs of electrode fingers of IDT forming these SAW filters (M) and the number of conductors of one-side reflectors (N) needs to be decreased. For example, when they are housed in a container of 3.8×3.8 mm or less in plane size required, more recently, the size of elements is made to be 2×3 mm, therefore the sum M+N of the number of pairs of electrode fingers of the SAW filters (M) and the number of conductors of one-side reflectors (N) must be made to be about 200 or less. It lowers the Q value of the SAW resonators (the resonance sharpness), decreases the excitation intensity of resonance amplitude and displacement transmission coefficient and deteriorates the transmission characteristics of the SAW filters.
Furthermore, the thickness of electrode films should be thickened to remedy a deficiency in the number of conductors of the reflectors and improve the insertion loss, but it increases the spurious level of longitudinal and transversal higher modes and deteriorates the degree of suppression of the out-of-band frequency characteristics.
In Japanese unexamined patent publications No. H4-373304 and No. H9-93079, a transversally coupled triple-mode SAW filter in which three SAW resonators are disposed in a direction perpendicular to the propagation direction of SAW has been proposed to expand the passband without narrowing the electrode finger cross width of IDT. In this SAW filter, however, there is the problem that differences unavoidable in design between filter impedances of adjacent 1st and 2nd SAW resonators and the filter impedances of adjacent 2nd and 3rd SAW resonators occur, respectively, making it difficult to obtain practically good filter characteristics because this generates a ripple in the passband. Still more, in Clemens C. W. Ruppel et al.'s paper “SAW Devices for Mobile Communication Applications” (
The
26
th Symposium of Electric Society
&
Electronic Circuit Technical Committee
, page 129-130 (1997)), a transversally coupled multi-mode SAW filter which enables an increase in the frequency difference between two modes, i.e., S
0
(fundamental wave symmetric)—S
1
(primary symmetric) modes and A
0
(fundamental wave anti-symmetric)—A
1
(primary anti-symmetric) modes by using these modes and consequently expanding the passband width has been disclosed. However, this SAW filter practically has a dispersion in design and in manufacture. It is feared that this is the reason why the A
0
mode is excited to generate the ripple in the S
0
-S
1
mode and why the S
1
mode is excited to generate the ripple in the A
0
-A
1
mode. There is also concern that the construction of electrode patterns including the disposition of input/output terminals is complicated, therefore the size of filter cannot be fully miniaturized.
Accordingly, this invention was made to overcome aforethe previous problems, and its purpose consists in providing a transversally coupled multi-mode SAW filter which enables to seek the broadening of passband width and the miniaturization.
Moreover, the purpose of this invention consists of providing a transversally coupled multi-mode SAW filter which has excellent frequency stability and good S/N ratio, and is suited to the use as an IF filter of mobile communication terminals such as portable telephone.
The inventors of this application discovered that the vibration displacement of a so-called transversal mode and the resonance frequency thereof can be calculated by use of a theory described below. This transversal mode is an inherent vibration mode which exists depending on the length of SAW resonators in the cross direction (the Y-axis direction perpendicular to the propagation direction X axis of SAW), and the length in the cross direction generally means the electrode finger cross width WC of IDT, i.e., the dimension of a portion where electrode fingers of positive polarity and electrode fingers of negative polarity overlap each other in the cross direction.
As a method for simply calculating the vibrational displacement of SAW resonators in the cross direction, the inventors of this application have published a differential equation dominating the transversal modes in a paper “K-cut quartz crystal SAW resonators having dynamic and static zero-temperature coefficients at normal temperature” (Takagi, Momosaki et al.,
the
25
th EM Symposium of Electric Society
&
Electronic Circuit Technical Committee
, page 77-83 (1996)). This equation is described as the following Eq. (1).
a&ohgr;
0
2
(Y)V(Y),
YY
+{&ohgr;
0
2
(Y)}V(Y)=0  (1)
Here, &ohgr; is the angular frequency, &ohgr;
0
(Y) is the angular frequency of element in the region, a is the effective shear rigidity constant in the cross direction, V(Y) is the amplitude of displacement of surface acoustic wave in the cross direction, and Y is the Y-coordinate specified by the wavelength of the surface acoustic wave. Moreover, &ohgr;
0
(Y) is a quantity given by converting the speed of the surface acoustic wave at the coordinate Y to the angular frequency, and is called the frequency potential function.
It has been confirmed that this frequency potential function changes with a function of the thickness H(Y) of aluminum metal conductor film existing in the propagation path of a surface acoustic wave in the vicinity of operating points of SA

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