Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-07-27
2003-05-27
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06570471
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No. 2000-296671 filed in Sep. 28, 2000 and 2001-120011 filed in Apr. 18, 2000, whose priorities are claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and more particularly, it relates to a ladder type surface acoustic wave filter having plural surface acoustic wave resonators arranged in a series-arm and a parallel-arm.
2. Description of the Related Art
A ladder type surface acoustic wave (hereinafter referred to as SAW) filter using plural SAW resonators has been known as a band pass filter (see, for example, Journal of Institute of Electronics, Information and Communication Engineers A, Vol. J 76-A, No. 2, pp. 245-252 (1993)).
FIG. 12
shows a constitutional diagram of a conventional ladder type SAW filter.
A ladder type SAW filter has such a constitution that series-arm SAW resonators S
1
an S
2
are arranged between an input terminal Ti and an output terminal To of a piezoelectric substrate
1
, and parallel-arm SAW resonators P
1
and P
2
are arranged between the input and output terminals and a ground terminal G. SAW resonators S
1
, S
2
, P
1
, P
2
, are generally called one-part SAW resonators.
FIG. 13
shows a constitutional diagram of a one-part SAW resonator. A one-part SAW resonator has such a constitution that an interdigital transducer (hereinafter referred to as IDT)
2
for electrically exciting SAW and reflectors
3
-
1
and
3
-
2
arranged on a propagation path of the SAW for trapping the excited SAW in the IDT are formed on the piezoelectric substrate
1
.
The IDT
2
is formed with a large number of electrode fingers arranged in a comb form with a constant period (pi).
The reflectors
3
-
1
and
3
-
2
each is formed with a large number of grating electrode fingers
3
-
3
arranged with a constant period (pr) and is also referred to as a grating reflector.
In the IDT, the SAW is excited by two adjacent electrode fingers each extending downward and upward as one unit.
A ladder type SAW filter is designed in such a manner that a resonance frequency fr of the series-arm SAW resonators S
1
and S
2
substantially agrees to an antiresonance frequency fa of the parallel-arm SAW resonators P
1
and P
2
.
FIG.
14
(
a
) is a graph showing a pass characteristic diagram of a ladder type SAW filter, and FIG.
14
(
b
) is a graph showing single impedance characteristic diagrams at this time of the series-arm SAW resonators S
1
an S
2
and the parallel-arm SAW resonators P
1
and P
2
. The graph g
1
in FIG.
14
(
b
) is an impedance characteristic of the series-arm SAW resonators S
1
an S
2
, and the graph g
2
is an impedance characteristic of the parallel-arm SAW resonators P
1
and P
2
. In the graph g
1
of the series-arm SAW resonators, the frequency where the impedance becomes minimum is the resonance frequency frs, and the frequency where the impedance becomes maximum is the antiresonance frequency fas.
FIG. 15
is an explanatory diagram of frequency characteristics demanded for a band pass filter, such as a ladder type SAW filter.
The characteristic values herein include demanded pass band widths (BW
1
and BW
2
), attenuation level (ATT
1
and ATT
2
) defined by the specification, and attenuation band widths (BWatt
1
and BWatt
2
).
A ratio (BW
1
/BW
2
) of the band widths BW
2
and BW
1
at a certain attenuation level is referred to as shape factor. The closer the shape factor to 1, it is better and referred to as a high shape factor. The shape factor of the ladder type SAW filter is substantially decided by the frequency difference between the resonance frequency fr and the antiresonance frequency fa.
That is, the steepness of the inclination from the attenuation region on the lower frequency side to the pass region depends on the frequency difference (&Dgr;fp in FIG.
14
(
b
)) between the resonance frequency frp and the antiresonance frequency fap of the parallel-arm resonators P
1
and P
2
, and the smaller the &Dgr;fp is, the steeper the inclination is. The steepness of the declination from the pass region to the attenuation region on the higher frequency side depends on the frequency difference (&Dgr;fs in FIG.
14
(
b
)) between the resonance frequency frs and the antiresonance frequency fas of the series-arm resonators S
1
and S
2
, and the smaller the &Dgr;fs is, the steeper the declination is.
However, the &Dgr;fp and the &Dgr;fs are substantially decided by the electromechanical coupling factor of the used piezoelectric substrate 1, and do not vary by changing the number of pairs of the electrode fingers and the aperture length of the IDT. In this connection, Japanese Unexamined Patent Publication No. Hei 11(1999)-163664 discloses a SAW filter having &Dgr;fp and &Dgr;fs that are made small by periodically withdrawing the electrode fingers of the IDT. The &Dgr;fp and the &Dgr;fs can be adjusted with a simple constitution by using the withdrawing method, so as to realize a ladder type SAW filter having a desired pass band width and steepness of the edge parts of the pass region.
Furthermore, Japanese Unexamined Patent Publication No. 2000-315931 discloses a SAW resonator having electrode fingers connected to a positive potential and a negative potential that are inverted each other, so as to increase the steepness of the filter characteristics in the vicinity of the pass band.
However, in the case where the electrode fingers of the IDT are periodically withdrawing, both the &Dgr;fp and the &Dgr;fs become small, and there is a problem in that the capacitance of the IDT is decreased by the withdrawing although a filter having a high shape factor can be realized.
The decrease in capacitance of the IDT brings about mismatch of input and output impedance, which adversely affects the characteristics of the SAW filter.
In general, enlargement of the aperture length of the IDT or increase of the number of pairs of the electrode fingers of the IDT is conducted to compensate the decrease in capacitance of the IDT. However, the enlargement of the aperture length increases the size of the IDT, which restricts the demand in miniaturization of the filter chip size made upon utilization in portable phones.
For example, when 20% of the electrode fingers are withdrawed from the whole electrode fingers of the IDT, the capacitance of the IDT is decreased by 40%, which brings about enlargement of the IDT size for compensating the capacitance of about 1.67 times. This does not match the demanded specification of realizing a compact SAW filter.
On the other hand, when the electrode pair is inverted, the decrease in capacitance of IDT can be suppressed low, and the &Dgr;fp and the &Dgr;fs can be made small, so as to realize a SAW filter of a high shape factor having a smaller size than those made by the withdrawing method. However, it is necessary that the positions where the inverted regions are provided are carefully considered as in the withdrawing method. Because when the electrode pair is inverted periodically, there is a high possibility that plural numbers of spurious occur outside the pass band of the filter, so as to fail to satisfy the demanded specification. Furthermore, when several numbers or more of the electrodes are continuously inverted, the effect of minifying the &Dgr;fp and the &Dgr;fs is impaired to bring about necessity of increasing the number of the inverted regions, and as a result the size of the filter is increased.
SUMMARY OF THE INVENTION
The invention relates to a surface acoustic wave filter comprising a piezoelectric substrate having formed thereon a plurality of surface acoustic wave resonators electrically connected in a ladder form for exciting a surface acoustic wave, wherein each of the surface acoustic wave resonators is composed of an interdigital transducer and reflectors arranged closely on both sides of the interdigital transducer in a direction parallel to a propa
Ikata Osamu
Inoue Shogo
Matsuda Takashi
Tsutsumi Jun
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Summons Barbara
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