Multi-longitudinal mode coupled saw filter

Details Multi-longitudinal mode coupled saw filter Multi-longitudinal mode coupled saw filter

2000-04-13

2002-01-01

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S196000, C310S31300R, C310S31300R

Reexamination Certificate

active

06335667

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave (hereinafter referred to as SAW) filter which has a third interdigital transducer to control a displacement state of a vibration mode in a multi-longitudinal-mode-coupled resonator type SAW filter that utilizes the surface acoustic wave.
2. Description of Related Art
The construction of a conventional multi-longitudinal-mode-coupled resonator type SAW filter having two interdigital transducers (hereinafter referred to as IDTs) is disclosed in Japanese Unexamined Patent Publication No. 61-285814, and a conventional multi-longitudinal-mode-coupled resonator type SAW filter having three IDTs is disclosed in Japanese Unexamined Patent Publication No. 1-231417.
These conventional arts have the problem that a plurality of longitudinal resonance modes (respectively simply designated S
0
, A
0
, and S
1
) in use are difficult to independently control to equalize resonance amplitudes in two or three resonance modes and to set a passband width to a desired width.
SUMMARY OF THE INVENTION
The present invention is intended to resolve such a problem, and it is an object of the present invention to realize a multi-longitudinal-mode-coupled resonator type SAW filter that permits a passband width to be relatively easily set, and has flat transmission characteristics with equalized insertion losses in the vicinities of upper end frequency and lower end frequency thereof.
A multi-longitudinal-mode-coupled resonator type SAW filter of a first exemplary embodiment includes, on a piezoelectric substrate, a first IDT for exciting a surface acoustic wave, a second IDT for receiving the surface acoustic wave excited by the first IDT, a third IDT, interposed between the first and second IDTs, for controlling the amplitude of the excited surface acoustic wave, and a pair of reflectors which are arranged with the first, second and third IDTs therebetween in the direction of propagation of the surface acoustic wave,
wherein the reflectors, the first IDT, the second IDT and the third IDT are formed of parallel metallic conductors that are periodically arranged on the piezoelectric substrate,
the distance between the reflector and the closet parallel conductor of the first IDT is set to be equal to the width of a spacing of the first IDT with one period length thereof having alternating spacing and line, the distance between the reflector and the closest parallel conductor of the second IDT is set to be equal to the width of a spacing of the second IDT with one period length thereof having alternating spacing and line,
each of the period length PT1of the parallel conductors of the first IDT and the period length PT2 (=PT1) of the parallel conductors of the second IDT is set to be shorter than the period length PT3 of the parallel conductors of the third IDT (PT3 >PT1, PT2), each of the period lengths PT1 and PT2 is set to be shorter than the period length PR of the parallel conductors of the reflectors,
the first, second and third IDTs have a total reflective coefficient &Ggr; set to be 10>&Ggr;>0.8, and are energy trapped type resonators, and
a two-longitudinal-mode-coupled resonator filter is formed of a fundamental wave symmetrically longitudinal mode S
0
which has a displacement amplitude function generally symmetrical with respect to a central position in the direction of propagation X in which the surface acoustic wave is standing, and a fundamental wave anti-symmetric mode A
0
which has a displacement amplitude function generally anti-symmetrical with respect to the central position.
In accordance with the first exemplary embodiment, the third IDT is arranged between the first and second IDTs, which are input and output electrodes of the filter, and the IDTs have their own frequencies independently set therein. In this arrangement, an insertion loss and frequency locations of two independent natural vibration modes S
0
and A
0
which vibrate in the longitudinal direction in a standing wave fashion are controllable.
With the relationship of PT1, PT2, and PT3 set in accordance with the first exemplary embodiment, the insertion loss of the A
0
mode is reduced, thereby equalizing the insertion losses of both modes. A two-longitudinal-mode-coupled resonator filter having flat transmission characteristics and a bandwidth of 1000 to 1500 ppm results.
In accordance with the first exemplary embodiment, (PR−PT1)/PR=1&egr; to 1.7&egr; and (PR−PT3)/PR=0 to 0.8&Ggr; hold where &Ggr; is a deviation (f
RO
−f
TO
)/f
RO
between a center frequency f
RO
and a frequency f
TO
where in the setting of the period lengths PT1, PT2, and PT3, f
RO
is the reflective wave center frequency of the reflector, and f
TO
is the frequency at which the first IDT, the second IDT, and the third IDT, when considered as a unit, exhibit a maximum reflective conductance G.
With these ranges set, the filter increases energy trapped in the longitudinal direction in the A
0
mode having a peak displacement amplitude in the vicinity of the center of each of IDT 1 and IDT 2, and reduces the insertion loss, thereby equalizing the insertion loss to the insertion loss of the S
0
mode. A two-longitudinal-mode-coupled resonator filter having flat transmission characteristics and a bandwidth of 1000 to 1500 ppm results.
In accordance with the first exemplary embodiment, the sum of pairs of electrode fingers M=M1+M2+M3 falls within a range from 140 to 180 where M1 is the number of pairs of electrode fingers of the first IDT, M2 is the number of pairs of electrode fingers of the second IDT, and M3 is the number of pairs of electrode fingers of the third IDT, M1 is set to be equal to M2, and DIV falls within a range from 2.1 to 2.4 where DIV is defined as M1=M/DIV.
The number of pairs of the electrode fingers of each IDT is set in this way, and a frequency difference between the S
0
mode and the A
0
mode is set to within a range from 1000 to 1500 ppm. A two-longitudinal-mode-coupled resonator filter having this bandwidth results.
In accordance with the first exemplary embodiment, a cross bus bar conductor
1
and a cross bus bar conductor
2
, each connected to ground potential, are respectively arranged between the first IDT and the third IDT, and between the second IDT and the third IDT, the total width D1 of the width of the cross bus bar conductor
1
and the widths of the spacings on both sides of the cross bus bar conductor
1
is n&lgr;+(¼)&lgr; or n&lgr;+(¾)&lgr;(n=0, 1, 2, . . . ) where &lgr; represents the wavelength of the surface acoustic wave, the total width D2 of the width of the cross bus bar conductor
2
and the widths of the spacings on both sides of the cross bus bar conductor
2
is n&lgr;+(¼)&lgr; or n&lgr;+(¾)&lgr; (n=0, 1, 2, . . . ) where &lgr; represents the wavelength of the surface acoustic wave, and each of D1 and D2 is within a range of 20 to 100 &mgr;m or within a range of 2&lgr; to 3&lgr;.
There are times when an unknown vibration mode in a frequency region higher than that of the S
0
mode is generated with resonance modes not limited to the S
0
and A
0
modes, depending on the values of D1 and D2.
Such an unknown vibration mode is not generated if D1 and D2 are set up as described above, and a regular two-longitudinal-mode-coupled resonator filter constructed of S
0
and A
0
results. If each of D1 and D2 falls within a range from 20 to 100 &mgr;m or within a range from 2&lgr; to 3&lgr;, the realization of a passband of 1000 to 1500 ppm is not affected.
A multi-longitudinal-mode-coupled resonator type SAW filter of a second exemplary embodiment in this application includes, on a piezoelectric substrate, a first IDT for exciting a surface acoustic wave, a second IDT for receiving the surface acoustic wave excited by the first IDT, a third IDT, interposed between the first and second IDTs, for controlling the amplitude of the excited surface acoustic wave, and a pair of

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