Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-06-12
2003-06-24
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S133000, C333S196000, C310S31300R
Reexamination Certificate
active
06583691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as a band-pass filter or other filter in an RF stage of a portable telephone or electronic apparatus, and more particularly, to a longitudinally coupled resonator type surface acoustic wave device having a plurality of interdigital transducers (IDTs) arranged in the propagation direction of a surface acoustic wave.
2. Description of the Related Art
As band-pass filters used in the RF stage of portable telephones, surface acoustic wave filters have been widely used. The band-pass filters are required to have a low loss, a high attenuation, and a wide band. Accordingly, various attempts to satisfy such requirements have been made in the surface acoustic wave filters.
For example, Japanese Unexamined Patent Application Publication No. 5-267990 discloses a method of increasing the band-width of a longitudinally coupled resonator type surface acoustic wave filter.
FIG. 31
shows a longitudinally coupled resonator type surface acoustic wave filter
101
disclosed in the Japanese Unexamined Patent Application Publication No. 5-267990. A distance Z (hereinafter, referred to as an IDT-IDT interval) between the centers of adjacent electrode fingers in neighboring IDT's shown in
FIG. 31
is set at about 0.25 times the wavelength &lgr;I which is determined by the pitch of the electrode fingers.
FIGS. 27 and 28
are graphs for illustration of increasing a band-width in this conventional technique.
FIG. 27
shows a relation between generated resonance mode frequencies.
FIG. 28
schematically shows the effective current distributions at the respective resonance mode frequencies.
In the method of the above-described conventional technique, a resonance mode (resonance mode indicated by the arrow C) having peaks in the intensity distribution of a surface acoustic wave and presented in the IDT-IDT interval areas, is utilized, in addition to the zero-order mode (resonance mode indicated by the arrow B), and the secondary mode (resonance mode indicated by the arrow A) to form a pass band. Accordingly, ordinarily, the IDT-IDT interval is set at 0.50 &lgr;I to prevent undesired radiation of a bulk wave. As seen in the above-description of the conventional technique, the band can be widened by setting the interval at 0.25&lgr;I.
FIGS. 29 and 30
illustrate changes in the resonance mode frequencies indicated by the arrows A to C, obtained when the IDT-IDT interval is varied. The results shown in
FIGS. 29 and 30
are obtained and ascertained when the impedance matching conditions are intentionally deviated. It should be pointed out that the results shown in
FIGS. 29 and 30
show relative changes in the resonance mode frequencies, not indicating the absolute positions of the accurate resonance mode frequencies.
FIG. 29
shows the shifts of the respective resonance mode frequencies on a basis of the zero-order mode frequency, caused when the IDT-IDT interval is varied, that is, changes in frequency difference between the respective resonance mode frequencies, on a basis of the resonance frequency in the zero-order mode.
FIG. 30
shows changes in amplitude level between the respective resonance mode frequencies. As seen in
FIGS. 29 and 30
, all of the resonance mode frequencies and the amplitude levels are changed when the IDT-IDT interval is varied.
In the above-described conventional technique, the IDT-IDT interval is adjusted so as to increase the pass bandwidth. This will be described with reference to FIG.
32
.
FIG. 32
is a schematic plan view illustrating one example of the conventional longitudinally coupled resonator type surface acoustic wave filter.
In this case, a surface acoustic wave filter
200
is produced by forming the respective electrodes made of Al on a 40±5° Y-cut X propagation LiTaO
3
substrate (not shown). The longitudinally coupled resonator type surface acoustic wave filter
200
has the configuration in which longitudinally coupled resonator type surface acoustic wave filter portions
201
and
202
are two-stage dependently connected. The surface acoustic wave filter portions
201
and
202
are configured in the same manner, and contains first, second, and third IDT's
205
to
207
, and reflectors
208
and
209
provided on both of the sides of the first, second, and third IDT's
205
to
207
. The surface acoustic wave filter portions
201
and
202
are designed in compliance with the following specifications.
electrode finger crossing-width: 43.41 &lgr;I (&lgr;I is the wavelength of a surface acoustic wave determined by the electrode finger pitch of an IDT)
the number of the electrode fingers of the IDT (the numbers are
25
,
31
, and
25
for the IDT's
205
,
206
, and
207
, respectively)
the wavelength &lgr;I of an IDT: 4.17 &mgr;m
wavelength &lgr;R for a reflector: 4.28 &mgr;m
the number of electrode fingers of a reflector: 100
the intervals D and E between the neighboring IDT's: 0.32 &lgr;I
the intervals between the IDT's and the reflectors: 0.50 &lgr;R
duty in an IDT: 0.73
duty in a reflector: 0.55
duty in a reflector: 0.55
electrode film-thickness: 0.08 &lgr;I
It should be noted that in this patent specification, all of the IDT-IDT intervals, the IDT—reflector intervals, and the intervals between adjacent electrode fingers are expressed as the distance between the centers of electrode fingers, respectively. The above-mentioned duty is defined as the ratio of the size in the width direction of an electrode finger along the surface acoustic wave propagation direction, based on the sum of the size in the width direction and the size of the space between the electrode finger and the adjacent electrode finger.
FIGS. 33 and 34
illustrate changes in characteristic of the surface acoustic wave transducer
200
shown in
FIG. 32
, obtained when the IDT-IDT intervals D and E are decreased by 0.005 &lgr;I as compared with the above-described design value to increase the bandwidth. The solid lines represents the characteristics obtained when the IDT intervals D and E are decreased, and the broken lines represent the characteristics obtained when the above specifications are applied.
FIG. 33
shows the frequency characteristics, and
FIG. 34
shows changes in VSWR.
As seen in
FIGS. 33 and 34
, the pass bandwidth which corresponds to the range of from the through-level to 4 dB lower than the through-level is increased by about 1 MHz. However, the flatness of the insertion loss within the pass band is deteriorated, and moreover, the VSWR is reduced by about 0.25.
On the other hand,
FIGS. 35 and 36
show changes in characteristics of the surface acoustic wave filter
200
, obtained when the IDT-IDT intervals D and E are increased by 0.003 &lgr;I as compared with the abode-described design values, so that the flatness of the insertion loss within the pass band is enhanced, and the VSWR characteristic is improved. The solid lines represent the characteristics when the IDT intervals D and E are increased, and the broken lines show the characteristics obtained when the above-described specifications are applied.
As seen in
FIGS. 35 and 36
, the flatness of the insertion loss and the VSWR within the pass band can be improved by increasing the IDT-IDT intervals, while the pass bandwidth becomes narrow by about 1 MHz. The reason lies in that when it is attempted to produce a desired characteristic by adjustment of the IDT-IDT intervals, all of the resonance modes are changed, as seen in
FIGS. 29 and 30
.
That is, since the three resonance modes can not be individually operated, characteristics satisfying all of the pass bandwidth, the flatness of the insertion loss within the pass band, the VSWR, and so forth can not be obtained.
SUMMARY OF THE INVENTION
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a longitudinally coupled resonator type surface acoustic wave filter which solves the above-described technical defects, and in which the arran
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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