Surface acoustic wave filter utilizing a transmission line...

Details Surface acoustic wave filter utilizing a transmission line... Surface acoustic wave filter utilizing a transmission line...

2000-09-05

2002-06-11

Lee, Benny (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S195000

Reexamination Certificate

active

06404302

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a surface acoustic wave filter which is used for cellular phones and other mobile communication equipment.
BACKGROUND OF THE INVENTION
FIG. 10
shows a circuit diagram of a conventional SAW (surface acoustic wave) filter.
FIG. 10
shows SAW resonant elements
101
,
102
, and
103
which are disposed on a piezoelectric substrate, and input terminal
104
and output terminal
105
. Here, three resonators configure a band-stop filter.
FIG. 11
shows characteristics of the band-stop filter configured by three resonators which are disposed on a piezoelectric substrate made of LiTaO3 (lithium tantalate) using conventional circuit structure.
FIG. 12
shows an equivalent circuit diagram of the SAW resonant element shown in FIG.
10
. In
FIG. 12
, reference number
111
is a parallel capacitor C
0
, reference number
112
an equivalent series inductor L
1
, reference number
113
an equivalent series capacitor C
1
, and reference number
114
an equivalent series resistor R
1
.
But the material property of the piezoelectric substrate shown in
FIG. 12
mentioned above determines a physical constant, or capacitance ratio &eegr;=C
0
/C
1
. A series resonance frequency Fs and a parallel resonance frequency Fp of a SAW resonator element (for LiTaO3, &eegr;=about 13) are
F
S
=1/(2&pgr;(
L
1
×C
1
)
0.5
)
F
p
=1/(2&pgr;(
L
1
×C
1
×C
2
/(
C
1
+C
2
))
0.5
)
respectively. Therefore, because capacitors C
1
and C
2
share a relation determined by the capacitance ratio &eegr;, a frequency separation between the series resonance frequency and the parallel resonance frequency of the SAW resonant element as electrical characteristics can not be set arbitrarily. As a result, the attenuation characteristics and the pass characteristics—both are the important filter characteristics—also have a limitation as shown in FIG.
11
.
DISCLOSURE OF THE INVENTION
An object of the present invention is to attain a SAW filter which can set attenuation characteristics and pass characteristics arbitrarily.
In order to attain the object, the SAW filter of the present invention comprises comb-shaped electrodes facing each other disposed on the same piezoelectric substrate, at least two resonators with an input and output terminals, and an equivalent transmission line which is connected between an output terminal of the first resonator and an input terminal of a second resonator. The phase of the transmission line can be adjusted from 5 degrees to 175 degrees arbitrarily at the series resonance frequency of the resonators.
The band-stop filter or the band-pass filter based on the present invention can set the attenuation characteristics and the pass characteristics arbitrarily.


REFERENCES:
patent: 5499002 (1996-03-01), Kinsman
patent: 5905418 (1999-05-01), Ehara et al.
patent: 43-2402 (1968-01-01), None
patent: 43-7739 (1968-03-01), None
patent: 51-142246 (1976-12-01), None
patent: 61-193501 (1986-08-01), None
patent: 7-15279 (1995-01-01), None
patent: 722892 (1995-01-01), None
patent: 7-231241 (1995-08-01), None
patent: 07-263995 (1995-10-01), None
patent: 10-13187 (1998-01-01), None
patent: 10-65489 (1998-03-01), None
patent: 10-93376 (1998-04-01), None
patent: 10-270976 (1998-10-01), None
Japanese search report for PCT/JP99/06272 dated Feb. 22, 2000.
English translation of Form PCT/ISA/210.

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