Longitudinal coupled multiple mode surface acoustic wave filter

Details Longitudinal coupled multiple mode surface acoustic wave filter Longitudinal coupled multiple mode surface acoustic wave filter

2002-02-14

2004-12-07

Summons, Barbara (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S193000, C310S31300R

Reexamination Certificate

active

06828879

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface acoustic wave filter (thereinafter to be referred to as an SAW filter).
2. Description of Prior Art
An SAW filter is a small and thin filter utilizing a surface acoustic wave and is in practical use as a key device for mobile communication equipments such as cellular phones. With the recent demands for a miniaturized, high-frequency cellular phone requiring low power consumption for a long-duration of call, there has been a growing demands for higher frequency SAW filters with lower insertion loss and higher power handling capability.
FIG. 22
is a schematic plan view illustrating a longitudinal coupled double mode SAW filter utilizing a conventional primary-third order longitudinal mode. The SAW filter in
FIG. 22
shows also an unbalanced input and unbalanced output type SAW filter. On a main surface of a piezoelectric substrate, as shown in
FIG. 22
, there are provided three IDTs (Interdigital Transducer)
61
,
62
,
63
closely disposed to each other along a propagation direction of a surface wave, with grating reflectors (hereinafter to be referred to as reflector)
64
a
,
64
b
disposed on both sides of these IDTs. The IDTs
61
,
62
,
63
are structured by comb electrodes
61
b
,
61
c
,
62
b
,
62
c
,
63
b
,
63
c
having a plurality of electrode fingers
61
a
,
62
a
,
63
a
, with each electrode finger inserted into a space of the other comb electrode. A comb electrode
61
b
of the IDT
61
is connected with an input terminal In and the other comb electrode
61
c
is grounded. A comb electrode
62
b
of the IDT
62
and a comb electrode
63
b
of the IDT
63
are mutually connected with each other and are connected to an output terminal Out. The other comb electrode
62
c
of the IDT
62
and the other comb electrode
63
c
of the IDT
63
are mutually connected with each other and are grounded.
The SAW filter shown in
FIG. 22
operates as follows as known. A plurality of surface waves excited by the IDTs
61
,
62
,
63
are trapped between the reflectors
64
a
and
64
b
, and a coupling is generated among the IDTs
61
,
62
, and
63
. As a result, two longitudinally coupled resonance modes of primary and third-order are excited strongly through the electrode pattern, and the filter works as a double mode SAW filter utilizing these two modes by giving appropriate terminations. It is known that a pass-band width of the double mode SAW filter is proportional to a frequency difference between the primary-order resonance mode and the third-order resonance mode.
FIG. 23
shows another longitudinal coupled double mode SAW filter with an unbalanced input and balanced output configuration. On a main surface of a piezoelectric substrate, as shown in
FIG. 23
, there are provided three IDTs
71
,
72
,
73
closely disposed to each other along a propagation direction of a surface wave, with reflectors
74
a
,
74
b
disposed on both sides of these IDTs. The IDTs
71
,
72
,
73
are structured by comb electrodes
71
b
,
71
c
,
72
b
,
72
c
,
73
b
,
73
c
having a plurality of electrode fingers
71
a
,
72
a
,
73
a
, with each electrode finger inserted into a space of the other comb electrode. A comb electrode
71
b
of the IDT
71
and a comb electrode
72
b
of the IDT
72
are mutually connected with each other and are connected with an input terminal In. The other comb electrode
71
c
of the IDT
71
and the other comb electrode
72
c
of the IDT
72
are mutually connected with each other and are grounded. A comb electrode
73
c
of the IDT
73
is connected with an output terminal Out
1
and the other comb electrode
73
b
is connected with an output terminal Out
2
.
FIGS. 24 and 25
show filtering properties of the longitudinal double mode SAW filter comprising a middle IDT including 27 electrode fingers, side IDTs including 19 electrode fingers each and reflectors including 120 electrode fingers each on a 36-degree Y-cut X-propagation LiTaO
3
substrate, and assuming an aperture length of 80 &mgr;m and a wavelength &lgr; of 2.14 &mgr;m.
FIG. 24
shows properties of the unbalanced input and unbalanced output type SAW filter in FIG.
22
.
FIG. 25
shows properties of the 50 &OHgr; unbalanced input and 50 &OHgr; balanced output type SAW filter in
FIG. 23. A
horizontal axis indicates frequency (GHz), takes 1.925 at the middle and is marked in 0.375 GHz increments. A vertical axis indicates an insertion loss and is marked in 10 dB decrements.
Even though the unbalanced input and balanced output type SAW filter can be provided with two output terminals Out, as can be seen from
FIGS. 24 and 25
, stop-band attenuation of the SAW filter is lower than that of an unbalanced input and unbalanced output type SAW.
The present invention was made to solve the above problem and has an objective to provide an unbalanced input and balanced output type SAW filter capable of obtaining stop-band attenuation approximately equivalent to that of an unbalanced input and unbalanced output type SAW filter.
Also the present invention has an objective to provide an SAW filter which can be fabricated readily when the width between an input IDT and an output IDT is less than &lgr;/2.
SUMMARY OF THE INVENTION
This invention was made to provide a longitudinal coupled multiple mode SAW filter comprising a plurality of IDTs disposed along a propagation direction of a surface wave on a piezoelectric substrate. In the SAW filter, an IDT is flanked on either side by IDTs, one of comb electrodes of IDT in the middle is connected with an input terminal, other comb electrode of the middle IDT is grounded, one of comb electrodes of IDT on each side of the middle IDT is connected with an output terminal, other comb electrodes of the side IDT are grounded. Electrode fingers are so arranged that adjacent electrode fingers of the middle IDT and the IDT on one side makes a connection between terminals or a connection between grounds and the other adjacent electrode fingers of the middle IDT and the IDT on the other side makes a connection between a terminal and a ground.
The SAW filter can be so structured that the output terminals are formed facing one direction and the grounds are also established facing one direction.
Also the SAW filter may be so structured that the output terminals are mutually formed in the opposite directions.
With the above mentioned structure, an unbalanced input and balanced output SAW filter can improve stop-band attenuation.
Also, the SAW filter of the present invention comprises an resonator, which includes one or a plurality of IDTs for exciting and receiving a surface acoustic wave and has antiresonant frequency approximately equivalent to cut-off frequency on the high-pass side of the longitudinal coupled multiple mode SAW filter. The resonator is connected with the input terminal of the SAW filter in series and is provided with an input terminal at the IDT.
The longitudinal coupled multiple mode SAW filter of the present invention comprises two reflectors on both sides of a plurality of IDTs along the propagation direction of the surface wave. These reflectors may reflect the surface wave and trap oscillation energy of the surface wave in between.
The resonator is a one-port resonator and may comprise two reflectors on both sides of one or a plurality of IDTs along the propagation direction of the surface wave. These reflectors may reflect the surface acoustic wave.
When alternating voltage is applied between the input terminal and the output terminal, stress is created between adjacent electrode fingers of the one-port SAW resonator on the piezoelectric substrate by the piezoelectric effect and the surface acoustic wave is excited. By reflecting the exciting surface acoustic wave between two reflectors, the oscillation energy of the wave is trapped between the reflectors, as a result standing wave is generated. The one-port resonator has antiresonant frequency. By connecting the longitudinal coupled multiple mode SAW filter and the one-port resonator in series and match

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