JPH09130203A - Double stage cascade connection/cascade coupling duplex mode saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce group delay deviation without increasing insertion loss while keeping an out-of-band attenuation amount high by deviating the pitch of the electrode-finger of a 1st surface acoustic wave(SAW) filter and the pitch of the electrode-finger of a 2nd SAW filter. SOLUTION: On a piezoelectric substrate 1, a 1st interdigital transducer 7 and 2nd interdigital transducers 8 and 9 are arranged along the propagation direction of SAW to form the SAW filters by providing reflectors 10 and 11. Besides, on the same board 1, 2nd cascade link duplex mode SAW filters 13 in the configuration similar to that of the 1st SAW filters are formed in parallel arrangement and serially connected. Since a pitch P1 of the electrode-finger of the 1st SAW filter 12 is made mutually different from a pitch P2 of the electrode-finger of the 2nd SAW filter 13, group delay deviation can be reduced without increasing the insertion loss.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to at least two interdigital transducers (IDs) on a piezoelectric substrate.
T) is arranged along the surface acoustic wave (SAW) propagation direction, and two longitudinally coupled dual-mode SAW filters each having a reflector on both sides thereof are arranged in parallel on the piezoelectric substrate. The present invention relates to a two-stage cascade-connected longitudinally-coupled dual-mode SAW filter in which longitudinally-coupled dual-mode SAW filters are cascade-connected, and particularly, to reduce the group delay deviation without increasing the insertion loss while keeping the out-of-band attenuation large. The present invention relates to a two-stage cascade connection longitudinally coupled dual mode SAW filter capable of

[0002]

2. Description of the Related Art Conventionally, a surface acoustic wave (hereinafter SAW) for exciting or receiving three interdigital transducers (hereinafter abbreviated as IDT) on a piezoelectric substrate is used.
(Hereinafter abbreviated) are arranged close to each other along the propagation direction, and reflectors are further provided on both sides thereof to substantially confine the vibration energy of the excited SAW within the three IDTs, and acoustically couple these vibrations between the IDTs. A so-called longitudinally coupled dual-mode SAW that forms a bandpass filter by utilizing two vibration modes of a first order and a third order generated by
A filter has been proposed (for example, Japanese Patent Laid-Open No. 5-5).
5872, JP-A-5-267990, JP-A-5-3
No. 35881). FIG. 11A is a schematic configuration diagram showing the basic configuration of such a conventional longitudinally coupled dual mode SAW filter. That is, as shown in FIG. 11A, in this longitudinally coupled dual-mode SAW filter, three IDTs 2, 3 and 4 are arranged closely on the piezoelectric substrate 1 along the SAW propagation direction. The reflectors 5 and 6 are arranged on both sides of 3, 4 respectively. Note that FIG.
(B) shows the distribution of vibrational energy in the longitudinally coupled dual mode SAW filter having the above configuration.

[0003]

Here, in order to increase the amount of out-of-band attenuation, the longitudinally coupled dual-mode SAW filter is connected in a multistage cascade as shown in FIG.
However, in this method (two-stage cascade connection vertical-coupling dual-mode SAW filter), the out-of-band attenuation amount increases while the group delay deviation in the pass band increases. There was a flaw. FIG.
It is a figure which shows the filter characteristic of the above-mentioned conventional two-stage cascade connection longitudinal coupling dual mode SAW filter.
The figure shows the case where Y rotation cut X direction propagation LiTaO ₃ is used. In the conventional example shown in FIG. 12, the pitches P1 and P2 of the electrode fingers of each vertically coupled dual mode SAW filter are P1 = P2. Further, in the above-mentioned conventional two-stage cascade connection vertical coupling dual mode SAW filter,
In order to reduce the group delay deviation, there is also a method of reducing the number of the reflectors to reduce the degree of energy confinement in the first and third vibration modes, but this also causes an increase in insertion loss. There was a flaw. The present invention has been made in view of the above circumstances, and is capable of reducing the group delay deviation without increasing the insertion loss while maintaining a large out-of-band attenuation, and is a two-stage cascade connection vertical coupling duplex. An object is to provide a mode SAW filter.

[0004]

To achieve the above object, the present invention provides a surface acoustic wave (SAW) for exciting or receiving at least two interdigital transducers (IDTs) on a piezoelectric substrate. The vibration energy of the excited SAW is substantially confined in the three IDTs, which are arranged along the propagation direction, and reflectors are provided on both sides of the reflectors, and the primary of the vibrations is generated by acoustic coupling between the IDTs. And two longitudinally coupled dual-mode SAW filters utilizing two vibration modes of third and third are arranged in parallel on the piezoelectric substrate, and these two longitudinally coupled dual-mode SAW filters are cascaded to form a two-stage cascaded vertical coupling. In the dual mode SAW filter, one of the two longitudinally coupled double mode SAW filters is a longitudinally coupled double mode SAW filter. If filter of the pitch of the electrode fingers P1, the other longitudinally coupled double mode pitch of the electrode fingers of the SAW filter was P2, the relationship between the P1 and P2, characterized in that the P1 ≠ P2.

Another feature is that the electrode finger pitches P1 and P2 are
And the relationship of 1.002 ≦ P1 / P2 ≦ 1.005. Another feature is the electrode finger pitch P1.
And P2 is P1 / P2 = 1.0036. Another feature is that the piezoelectric substrate is 3
6 ° Y rotation cut X direction propagation LiTaO ₃ was used.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below based on the illustrated embodiments. FIG. 1 is a schematic configuration diagram of a two-stage cascade connection longitudinally coupled dual mode SAW filter according to the present invention. As shown in FIG. 1, this two-stage cascade connection longitudinally coupled dual mode SAW filter includes a piezoelectric substrate 1 on which a first interdigital transducer (IDT) 7 and two second IDTs 8 and 9 are surface acoustic waves. The reflectors 1 are arranged along the propagation direction of (SAW) and are provided on both sides of the second IDTs 8 and 9.
0 and 11 are provided to form a first longitudinally coupled dual mode SAW filter 12, and a second longitudinally coupled dual mode SAW filter 13 having the same configuration as the first SAW filter 12 is provided on the piezoelectric substrate 1. And the first SAW filter 12 and the second SAW filter 13 are connected in parallel in a parallel arrangement. Here, the first SAW filter 12 is formed.
Pitch P1 of electrode fingers of the filter 12 and the second SAW
The pitch P2 of the electrode fingers of the filter 13 is different from each other (P1 / P2 ≠ 1).

By making the pitch P1 and the pitch P2 different from each other, it is possible to reduce the group delay deviation without increasing the insertion loss while maintaining a large out-of-band attenuation amount. The reason why a heavy mode SAW filter can be achieved will be described below. Here, the group delay deviation means the difference between the maximum value and the minimum value within the band defined by the individual standard. First, if two-stage vertically coupled SAW filters having the same electrode finger pitch are cascade-connected, the out-of-band attenuation increases, but one vertically coupled double-mode SAW filter and the other vertically coupled double-mode SAW filter. Since the frequencies at which the group delay time of the mode SAW filter becomes maximum match, the group delay deviation in the pass band becomes large. That is, the group delay time of one longitudinally coupled dual mode SAW filter is shown in FIG.
In the case shown in FIG. 2A, if two SAW filters are vertically connected, the group delay time will be the sum of those shown in FIG. 2A, and the maximum points will coincide. (B)
As shown in, the group delay deviation in the pass band becomes large.

Therefore, the pitch P of each SAW filter is
By shifting 1 and P2, as shown in FIG. 3 (a), the frequency at which the group delay time becomes maximum shifts.
As shown in (b), a filter having a small group delay deviation can be constructed. Here, the relationship between the pitch P1 of the electrode fingers of the first SAW filter 12 and the pitch P2 of the electrode fingers of the second SAW filter 13 is 1.002 ≦ P
It is desirable that 1 / P2 ≦ 1.005, and especially P1
Most preferably, /P2=1.0036. FIG.
Is a two-stage cascade connection longitudinally coupled dual mode SAW filter having the configuration shown in FIG.
2 = 1.0036, and the piezoelectric substrate 1 is 36 ° Y
FIG. 6 is a diagram showing a filter characteristic when a rotation cut X-direction propagation LiTaO ₃ is used, and FIG. 5 shows a relationship between the filter characteristic (shown by a solid line) according to the present embodiment of FIG. 3 and the pitch under the same conditions. FIG. 7 is a diagram in which P1 and P2 are set to 1 (in the case of the conventional example) and the filter characteristics (shown by a dotted line) are overlapped and compared. However, if P1 / P2 ≠ 1, the pass band width becomes narrower, so in the example of FIG.
The pass band width is widened by decreasing the number of IDT logarithms compared to the above example.

As shown in FIGS. 4 and 5, the group delay deviation is 840 when the conventional structure is used, that is, when P1 / P2 = 1.
ns, but 720 when P1 / P2 = 1.0036
It is about 120 ns smaller than ns. Further, although the conventional method of reducing the number of reflectors to reduce the group delay deviation has led to an increase in insertion loss, the use of the configuration according to the present invention reduces the group delay deviation without increasing the insertion loss. be able to. Further, it is obvious that the group delay deviation can be further reduced by slightly increasing the insertion loss by using the structure according to the present invention together with the conventional method of reducing the number of reflectors to reduce the group delay deviation.

8 and 9 show the SA shown in FIG.
P1 / P2 under the same conditions as the filter characteristics (shown by the solid line) when P1 / P2 = 1.005 in the W filter
It is the figure which overlapped and compared with the filter characteristic (indicated by a dotted line) in the case of 2 = 1 (in the case of the conventional example). As shown in FIG. 8, the filter characteristic of P1 / P2 = 1.005 is P
Although the group delay deviation in the pass band is smaller than that in the case of 1 / P2 = 1, the pass band width is narrowed and the stop band attenuation is also small as shown in FIG. . This tendency becomes more remarkable as P1 / P2 becomes larger than 1.005.

Next, the filter characteristics of the two-stage cascade connection vertical coupling dual mode SAW filter having the configuration shown in FIG. 1 when the pitch relationship is P1 / P2 = 1.002 are shown in FIGS. It will be described with reference to FIG. FIGS. 6 and 7 show P in the SAW filter shown in FIG.
The filter characteristic when 1 / P2 = 1.002 (shown by the solid line) and the filter characteristic when P1 / P2 = 1 (in the case of the conventional example) under the same conditions (shown by the dotted line) are overlapped. It is the figure which compared. As shown in FIG. 7, P1 / P
In the case of 2 = 1.002, the deterioration of the stopband attenuation amount as in the case of P1 / P2> 1.005 is not seen. Further, as shown in FIG. 6, the pass band width is almost the same as in the case of P1 / P2 = 1, but the group delay deviation is not so small and the effect is small. This tendency becomes more remarkable as P1 / P2 becomes smaller than 1.002. Therefore, as described above, the pitch relationship P1 / P2 is 1.002 ≦ P1 / P2 ≦ 1.0.
It is desirable to set it to 05, and in particular, P1 / P2 = 1.00
36 is most desirable. In the description of the present invention, a piezoelectric material using 36 ° Y rotation cut X-direction propagation LiTaO ₃ has been described as an example, but the present invention is not limited to this and other piezoelectric materials may be used. As a material, for example, ST-cut quartz X-112 ° Y
LiTaO ₃ , 64 ° Y rotation cut X direction propagation LiNb
It may be applied those suitable for constituting the SAW filter of the O ₃ and the like. Further, although the central first IDT 7 is used as the input / output terminal in the embodiment shown in FIG. 1, the input / output terminal may be obtained from the second IDTs 8 and 9 on both sides as shown in FIG. Further, in the above embodiment, the three IDTs 7, 8 and 9 are arranged between the reflectors 10 and 11, and the vertical coupling SAW filter utilizing the first and third vibration modes has been described, but the present invention is not limited to this. Not limited to
For example, it may be applied to a longitudinally coupled dual-mode SAW filter in which two IDTs are arranged close to each other between reflectors and the primary and secondary vibration modes are used.

[0012]

As described above, according to the present invention, in the two-stage cascade connection longitudinally coupled dual mode SAW filter, the pitch P1 of the electrode fingers of the first SAW filter and the pitch of the electrode fingers of the second SAW filter. Since P2 is shifted, the group delay deviation can be reduced without increasing the insertion loss while keeping the out-of-band attenuation large.

[Brief description of the drawings]

FIG. 1 is a two-stage cascade connection vertical coupling dual mode S according to the present invention.
It is a schematic block diagram of an AW filter.

FIGS. 2A and 2B are explanatory diagrams of group delay time when two conventional SAW filters are vertically connected. FIG.

3 (a) and 3 (b) are explanatory diagrams of a group delay time when the pitch is shifted according to the present invention.

4 is a diagram showing filter characteristics when the pitch relationship is P1 / P2 = 1.0036 in the SAW filter shown in FIG.

FIG. 5 shows the present embodiment (P1 / P2 = 1.0) shown in FIG.
FIG. 14 is a diagram in which the filter characteristics related to the case of 036) and the filter characteristics of the conventional example (when P1 / P2 = 1) shown in FIG.

FIG. 6 shows an embodiment (when P1 / P2 = 1.002)
FIG. 6 is a diagram in which the filter characteristics related to (1) and the filter characteristics of the conventional example (when P1 / P2 = 1) are superimposed and compared.

FIG. 7: This embodiment (when P1 / P2 = 1.002)
FIG. 6 is a diagram in which the filter characteristics related to (1) and the filter characteristics of the conventional example (when P1 / P2 = 1) are superimposed and compared.

FIG. 8 shows an embodiment (when P1 / P2 = 1.005)
FIG. 6 is a diagram in which the filter characteristics related to (1) and the filter characteristics of the conventional example (when P1 / P2 = 1) are superimposed and compared.

FIG. 9 shows an embodiment (when P1 / P2 = 1.005)
FIG. 6 is a diagram in which the filter characteristics related to (1) and the filter characteristics of the conventional example (when P1 / P2 = 1) are superimposed and compared.

FIG. 10 is a schematic configuration diagram of a modification of a two-stage cascade connection vertical coupling dual mode SAW filter according to the present invention.

11 (a) and (b) are conventional longitudinally coupled dual mode SAs.
It is a schematic structure figure of a W filter, and a figure showing the vibration energy.

FIG. 12 Conventional two-stage cascade connection longitudinally coupled dual mode SAW
It is a schematic block diagram of a filter.

13 is a diagram showing filter characteristics of the SAW filter shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate, 2, 3, 4, 7, 8, 9 ... Interdigital transducer (IDT), 5, 6, 10, 11 ... Reflector, 12, 13 ... Longitudinal coupling dual mode SAW filter, P1, P2 ... Pitch of electrode fingers,

Claims (4)

[Claims] 1. At least two interdigital transducers (IDTs) are arranged on a piezoelectric substrate along a propagation direction of a surface acoustic wave (SAW) for exciting or receiving the IDTs, and reflectors are provided on both sides thereof. A longitudinally coupled dual-mode SAW filter is provided that substantially confines the vibration energy of the excited SAW in the plurality of IDTs and utilizes two vibration modes generated by acoustic coupling between the IDTs of the respective vibrations. Two pieces are arranged in parallel on the piezoelectric substrate and these two longitudinally coupled dual modes S
A two-stage cascade connection longitudinally coupled dual mode SAW filter in which AW filters are cascaded, wherein one of the two longitudinally coupled dual mode SAW filters is longitudinally coupled dual mode SA.
When the pitch of the electrode fingers of the W filter is P1 and the pitch of the electrode fingers of the other longitudinally coupled dual mode SAW filter is P2, the relation between P1 and P2 is P1 ≠ P2. Two-stage cascade connection longitudinally coupled dual mode SAW filter. 2. The two-stage cascade connection vertical coupling dual mode according to claim 1, wherein the relationship between the electrode finger pitches P1 and P2 is 1.002 ≦ P1 / P2 ≦ 1.005. SAW filter. 3. The two-stage cascade connection vertical mode dual mode SAW filter according to claim 1, wherein the relationship between the electrode finger pitches P1 and P2 is P1 / P2 = 1.0036. 4. The two-stage cascade connection vertical coupling dual mode SAW filter according to claim 1, 2 or 3, wherein 36 ° Y rotation cut X direction propagating LiTaO ₃ is used as the piezoelectric substrate.