JP3419949B2 - Vertically coupled dual mode SAW filter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SAW filter, and more particularly to a longitudinally coupled dual mode (leaky) SAW filter having a wide band low loss and a large stop band attenuation on the high band side.

[0002]

2. Description of the Related Art As a wide band low loss filter used in a high frequency range from VHF to UHF, a transversal SAW filter using a unidirectional IDT or a SAW filter using a multi IDT has been widely used. The former has the advantage that the degree of freedom in designing the frequency characteristics of the filter is high, but it requires the addition of a phase shifter to the filter element (group type unidirectional ID
In the case of using T or three-phase unidirectional IDT) There is a defect that causes a problem in manufacturing yield, and the excited SA
The directional loss of W was not small, and the loss of the filter was not satisfactory.

On the other hand, the SA using the latter multi-IDT
If 9 sets or more of IDTs are arranged in the W filter, the directional loss becomes 1 dB or less and the low loss of the filter can be realized.
There is a defect that not only the filter element size becomes large due to the increase in the number of electrode pairs but also many spurs due to the increase in the reflection of SAW inside the IDT appear and the filter stop band attenuation amount becomes insufficient.

As a method of solving these problems, a highly coupled piezoelectric substrate, 64 ° Y-cut X-propagation LiNbO.
_There is a first-order and third-order longitudinally coupled dual-mode SAW filter utilizing ₃ and, for example, in JP-A-5-267990, each IDT
Λ / 4 (where λ is the ID
It is disclosed that the T electrode finger period) has a wide band. This first-order, third-order longitudinally coupled dual-mode SAW filter has an S that the IDT excites or receives as shown in FIG.
Three IDTs are arranged close to each other along the propagation direction of the AW, and reflectors are provided on both sides of them to confine the excited SAW in the IDT and use the waves of the first and third modes generated by the acoustic coupling between the IDTs. In the SAW filter, the IDT interval is set as described above.

However, this first-order third-order dual mode S
As shown in the filter characteristic diagram of FIG. 4, the AW filter has a defect that a sufficient amount of stopband attenuation on the high frequency side of the pass band cannot be obtained. This is a property inevitably caused by the transversal property of IDT, and until now, no suitable method for solving it has been found.

Therefore, when used as an RF filter for transmission / reception of a 900 MHz band mobile telephone, for example, there is a problem that the attenuation amount on the high frequency side of the pass band does not satisfy the specifications because the channels are very close to each other.

SUMMARY OF THE INVENTION The present invention has been made to eliminate the drawbacks of the first-order, third-order, longitudinally-coupled dual-mode SAW filters as described above, and has a wide band low loss and a high band side out-of-band attenuation amount. The purpose is to provide a large SAW filter.

SUMMARY OF THE INVENTION To achieve the above object, a first-order third-order longitudinally coupled dual-mode SAW filter according to the present invention comprises three interdigital transducers (I) on a piezoelectric substrate.
DT) electrodes are arranged along the propagation direction of surface acoustic waves (SAW) excited or received by these IDTs, and reflectors are further provided on both sides of the electrodes, so that the vibration energy of the excited SAWs is substantially distributed within the three IDTs. In a dual-mode filter that uses first-order and third-order vibration modes generated by acoustic coupling between the IDTs of these vibrations, (1) Center-to-center spacing of innermost electrode fingers where the IDTs face each other, Is approximately (2n + 1) λ / 4 (where λ is the above I
A space is provided as the electrode finger period of DT), n gratings are arranged at equal intervals in the space, and (2) the piezoelectric substrate is a 64 ° Y-cut X-direction propagation Li having a large coupling coefficient.
The NbO ₃ substrate or (3) the piezoelectric substrate is a 36 ° Y-cut X-direction propagation LiTaO ₃ substrate having a large coupling coefficient.

The present invention will be described in detail below with reference to the drawings showing the embodiments and the experimental results.

FIG. 1 (a) is a diagram conceptually showing an electrode pattern of a first-order / third-order longitudinally coupled dual-mode SAW filter showing an embodiment of the present invention.

In the figure, reference numeral 1 denotes a piezoelectric substrate. On the surface of the piezoelectric substrate 1, an input IDT 2 is provided near the center along the SAW propagation direction, and output IDTs 3 and 3 are provided on both sides of the IDT 2.
Gratings 5 and 5 are respectively arranged between T2 and IDT3, and reflectors 4 and 4 are provided on both sides of these gratings. SAW excited by the input IDT2 is reflected by the reflectors 4 and 4 to IDTs 2 and 3, First-order and third-order longitudinally coupled dual-mode SAW, which is confined in the region of 3 and utilizes the waves of the first- and third-order modes generated by acoustic coupling between the IDTs.
It is a filter.

For example, as a piezoelectric substrate, 36 ° Y cut X
A space is provided by using propagation LiTaO ₃ and the center-to-center spacing l of the innermost electrode fingers of the IDT ₂ and the IDT ₃ facing each other is set to 9λ / 4, and the center-to-center distances of the four gratings are approximately equal to the center of the space. All of them are connected to the ground terminal of the input IDT 2 via the bus bar. With such a configuration, FIG.
The following filter characteristics are obtained. The figure shows the filter characteristics in the 1.5 GHz band when the input IDT is 27.5 pairs, the output IDT is 16.5 pairs, the number of reflectors is 300, and the grating period is 9 / 20λ.

As is apparent from the figure, the out-of-band attenuation on the high band side of the pass band is improved by 5 dB compared to the filter characteristic of the conventional configuration of FIG. This enables 1.5G for mobile phones
The characteristics applicable to the Hz band RF filter can be obtained.

FIG. 1B shows another embodiment of the present invention 1
It is a figure which shows notionally the electrode pattern of a 3rd order vertical coupling dual mode SAW filter.

In the figure, reference numeral 1 is a piezoelectric substrate, and an input IDT is formed on the surface of the piezoelectric substrate 1 along the SAW propagation direction.
2 and the output IDTs 3 and 3, and the gratings 5 and 5 are arranged between the IDTs 2 and 3, and reflectors 4 and 4 are provided on both sides of the gratings to confine the excited SAW in the IDTs 2, 3 and 3 and between the IDTs. It is a SAW filter that utilizes the waves of the first and third modes generated by the acoustic coupling of.

For example, as a piezoelectric substrate, 36 ° Y cut X
A space is provided by using propagation LiTaO ₃ and the center-to-center spacing l of the innermost electrode fingers of the IDT ₂ and the IDT ₃ facing each other is 17λ / 4, and the center-to-center distances of the eight gratings are approximately equal to the center of the space. So that
Both are connected to the ground terminal of the input IDT 2 via the bus bar. Due to such a configuration, FIG.
A filter characteristic as shown in (b) is obtained. It should be noted that in the figure, the input IDT 27.5 pair, the output IDT 16.5 pair, the number of reflectors 300, and the filter characteristic in the 1.5 GHz band when the grating period is 17 / 36λ are shown in FIG. The out-of-band attenuation on the high band side of the pass band is improved by 10 dB as compared with the filter characteristic of the conventional configuration. This makes it possible to obtain characteristics applicable to the 1.5 GHz band RF filter for mobile phones.

In the above, the present invention is applied to a 36 ° Y piezoelectric substrate.
The example applied to the cut X-propagation LiTaO ₃ has been described, but the present invention is not limited to this, and any first-order / third-order longitudinally coupled dual-mode SAW filter using any piezoelectric substrate may be used. Is applicable, for example
45 ° X-cut Z-propagation Li ₇ B ₄ O ₇ , ST-cut quartz,
Especially, since the coupling coefficient is large, it is easy to obtain a wide band.
° Y cut X propagation LiTaO ₃ or 64 ° Y cut X
Suitable for propagating LiNbO ₃ .

In the above embodiment, the case where the center-to-center spacing of the innermost electrode fingers is 9λ / 4 and 17λ / 4 has been described, but the present invention is not limited to this.
The center-to-center spacing of the innermost electrode fingers is (2n + 1) λ / 4,
It has been confirmed that the same effect can be obtained by setting the number of gratings to n. Further, in the above embodiment, the case where all the gratings are connected to the ground electrode of the input IDT has been described, but it is well known that it is not always necessary to connect the gratings to each other or to the ground.

Since the present invention is configured as described above, it is remarkable in increasing the stopband attenuation amount on the high frequency side of the pass band of the RF filter of the public communication radio equipment of about 900 MHz to 2 GHz. Be effective.

[Brief description of drawings]

1A and 1B are first and second first-order and third-order longitudinally coupled dual-mode SAW filters according to the present invention, respectively.
Diagram showing the electrode pattern of the example

FIGS. 2A and 2B are FIGS. 1A and 1B, respectively.
Of the filter characteristics when the RF is applied to the RF filter

FIG. 3 is a configuration diagram showing an electrode pattern of a conventional first-order third-order longitudinally coupled dual-mode SAW filter.

FIG. 4 is a diagram showing a filter characteristic in which a conventional first-order third-order longitudinally coupled dual-mode SAW filter is applied to an RF filter.

[Explanation of symbols]

1 ... Piezoelectric substrate 2 ... Input IDT 3 ... Output IDT 4 ... Reflector 5 ... Grating

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Claims (3)

(57) [Claims] 1. Interdigital transducer (IDT) electrodes are arranged on a piezoelectric substrate along the propagation direction of a surface acoustic wave (SAW) excited or received by these IDTs, and reflectors are provided on both sides thereof. , Excited SA
In a dual-mode filter that substantially confines the vibrational energy of W in the three IDTs and utilizes the first- and third-order vibrational modes generated by the acoustic coupling between the IDTs of these respective vibrations, the IDTs are separated from each other. Set the center-to-center spacing of the innermost facing electrode fingers to (2n + 1)
A space is provided as λ / 4 (where λ is the electrode finger period of the IDT), and n spaces (where n = 1, 2, 3, ...
・ ・) A longitudinally coupled dual-mode SAW filter characterized by arranging the gratings of () at equal intervals. 2. The piezoelectric substrate having a large coupling coefficient of 64 ° Y
The longitudinally coupled dual mode SAW filter according to claim 1, wherein the substrate is a cut X-direction propagating LiNbO ₃ substrate. 3. The piezoelectric substrate having a large coupling coefficient of 36 ° Y
The longitudinally coupled dual mode SAW filter according to claim 1, wherein the substrate is a LiTaO ₃ substrate that propagates in the cut X direction.