JPH08191229A - Vertically coupled double mode saw filter - Google Patents

Abstract

PURPOSE: To attain a wide-band and low-loss vertically coupled double mode SAW filter having a filter characteristic capable of sufficiently increasing the quantity of attenuation in the vicinity of a high frequency side by suppressing spurious radiation appearing in a high frequency side attenuation band of a conventional vertically coupled double mode SAW filter using primary and tertiary modes. CONSTITUTION: In the vertically coupled double mode SAW filter using the primary and tertiary modes, the inter-center intervals of the innermost electrode digits opposed to a center IDT 1 and both side IDTs 3 are different on right and left sides, approximately one is nλ/2 and the other is (2n-1)λ/4 (λ is the wavelength of an excited SAW, n=1, 2,...).

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 double mode SAW filter which has a wide band low loss and a large stop band attenuation required for mobile communication and the like.

[0002]

2. Description of the Related Art Conventionally, a unidirectional ID has been used for a wide band low loss SAW filter in a high frequency region of VHF to UHF band.
A transversal SAW filter using T, a multi-electrode SAW filter, and a resonator SAW filter have been used.

The transversal type SAW filter using the unidirectional IDT has the drawbacks that it requires a phase shifter and is difficult to manufacture, and it is difficult to completely eliminate the directional loss and it is still difficult to reduce the loss. Was enough. Further, the multi-electrode SAW filter has a drawback that the directional loss is reduced but the chip size is increased, and more spurious due to internal reflection of the IDT appears, resulting in insufficient attenuation in the stop band.

Next, the resonator type SAW filter is superior to the above two methods in that the vibration energy of the SAW is confined in the IDT so that there is no directional loss and the loss can be essentially reduced. Ladder type SA for resonator type SAW filter
There are W filters and dual mode SAW filters,
The ladder-type SAW filter needs to be constructed with many resonators in order to obtain a high attenuation gradient, and the design is complicated, and the electrode film thickness must be extremely thick in order to eliminate the influence of the SAW resonator spurious. It has the drawback of not becoming.

On the other hand, two IDTs are arranged close to each other along the propagation direction of the SAW, and the SAW that excites them is I.
The first-order mode and 2 that occur when acoustic coupling is performed between DTs
A longitudinally coupled double mode SAW filter utilizing two vibration modes of the next mode has a simple structure and is excellent in low loss.

Further, three IDTs are arranged close to each other along the propagation direction of the SAW, and two vibration modes, a first-order mode and a third-order mode, generated when the SAWs excited by these acoustically couple with each other, are generated. Vertically coupled dual mode SA used
The W filter has a wider band than the filter using the first-order mode and the second-order mode.

Moreover, in the filter, the inventors of the present invention have disclosed the central IDT and the IDs on both sides according to Japanese Patent Laid-Open No. 5-267990.
The distance between the centers of the innermost electrode fingers facing T is approximately λ / 4.
It has been disclosed that a wider band and lower loss longitudinally coupled double mode SAW filter can be obtained.

However, as shown in FIG. 4, the frequency attenuation characteristics of this longitudinally coupled double-mode SAW filter are such that sufficient attenuation can be obtained at the low frequency side near the pass band, while comparison is made at the high frequency side. Large spurious is generated. This is spurious due to the frequency characteristics of the transversal SAW filter determined by the logarithm of the input / output IDT, and it has been difficult to suppress it in the past. Due to this spurious, for example, a filter that requires a high amount of attenuation in the vicinity of the high frequency band, such as a 930 MHz band SAW filter for an American pager, has a drawback that desired attenuation characteristics cannot be obtained and cannot be put to practical use.

[0009]

An object of the present invention is to suppress spurious appearing in the high-side attenuation band of the conventional longitudinally coupled double-mode SAW filter using the first-order mode and the third-order mode as described above, and to reduce the amount of attenuation near the high-frequency side. It is an object of the present invention to provide a wide band low loss longitudinally coupled double mode SAW filter having a sufficiently large filter characteristic.

[0010]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, 1 according to the present invention
Vertically coupled dual-mode SAW using second- and third-order modes
In the filter, the centers of the innermost electrode fingers of the central IDT and the IDTs on both sides facing each other differ from each other on the left and right sides, and in particular, one is nλ / 2 and the other is (2n-1) λ / 4 (λ is The wavelength of the excited SAW, n = 1, 2, ...) is adopted.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described in detail with reference to the embodiments shown in the drawings. FIG. 1 is a configuration diagram of an electrode pattern of a longitudinally coupled dual mode SAW filter using a first-order mode and a third-order mode according to the present invention.
An input IDT ₂ for exciting the SAW, output IDTs 3 and 3 for receiving the SAW, and reflectors 4 and 4 on both sides thereof are arranged on the surface of the iTaO ₃ substrate 1 so that the propagation direction is the X-axis direction. In FIG. 1, when the center-to-center spacings of the innermost electrode fingers facing the input IDT2 at the center and the output IDT3 at both sides are L ₁ and L ₂ , one is left in a periodic arrangement with L ₁ = λ / 2, and the other is Is set to L ₂ = λ / 4 and is shifted from the periodic arrangement by λ / 4.

FIG. 3 shows a configuration diagram of an electrode pattern of the conventional method according to the above-mentioned JP-A-5-267990, where L ₁ = L ₂ = λ / 4. 1 and 3, the IDT cycle is 4.38 μm and the central IDT is 31.5.
Pairs, on both sides IDT are each 21.5 pairs, crossing length is 32Ramuda, reflector number is present right respectively 180, the ratio of the IDT of the period (L _T) and the reflector of the period (L _R) is L _T / L _R = 0.992, the electrode is aluminum, and the film thickness is 0.175 μm (4% λ).

FIG. 4 shows the frequency attenuation characteristic of a structure in which two vertically coupled double mode SAW filters using the conventional first-order mode and third-order mode with this electrode pattern are cascade-connected. In FIG. 4, the center frequency (f ₀ ) is 930 MHz,
1 dB bandwidth 20 MHz, desired high frequency side f ₀ +15 MH
The amount of attenuation around z was 22 dB.

On the other hand, FIG. 2 shows the frequency attenuation characteristics of a structure in which filters of the electrode pattern of FIG. 1 according to the present invention are cascaded in two stages. F ₀ is the same 930 MHz, 1 dB bandwidth is 14 MHz, and the high frequency side is shown. Since an attenuation pole can be provided near f ₀ +15 MHz, an attenuation amount of 40 dB or more can be obtained. Bandwidth is slightly reduced, but the desired bandwidth is 3M
Since it is Hz, it sufficiently satisfies the requirement. The insertion loss is about 2.5 dB, which is unchanged. Therefore, the method of the present invention has a significant effect on a filter that requires an attenuation amount near the pass band and the high band side.

Although the present invention has been described above with the center-to-center spacing L ₁ = λ / 2, L ₂ = λ / 4 of the innermost electrode fingers of the input / output IDT, the input / output IDT intervals are separated by λ / 2. Even I
Although the coupling between DTs is weakened and the bandwidth is reduced, the effect of appearance of the attenuation pole is exactly the same.

The present invention also provides a 36 ° Y-cut LiTaO ₃
Although an example using a substrate has been described, the present invention is not limited to this substrate, and ST cut quartz, X cut LiTaO
₃ , 64 ° Y-cut LiNbO ₃ , 45 ° X-cut Li ₂ B ₄
It is obvious that the present invention can be applied to the case where another piezoelectric substrate such as O ₇ is used.

[0017]

Since the filter according to the present invention is constructed as described above, a remarkable effect is obtained in obtaining a filter having a wide band low loss in the high frequency region and a large attenuation on the high frequency side near the pass band. There is.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of an electrode pattern of a longitudinally coupled dual mode SAW filter using a first-order mode and a third-order mode according to the present invention.

FIG. 2 is a diagram showing frequency attenuation characteristics of an example of a longitudinally coupled dual mode SAW filter using a first-order mode and a third-order mode according to the present invention.

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

FIG. 4 is a diagram showing frequency attenuation characteristics of a conventional longitudinally coupled dual mode SAW filter using a first-order mode and a third-order mode.

[Explanation of symbols]

1 ... Piezoelectric substrate, 2, 3 ... IDT, 4 ... Reflector,
L ₁ , L ₂ ... Distance between centers of innermost electrode fingers

Claims (2)

[Claims] 1. A SAW excited by arranging three interdigital transducer (IDT) electrodes at the center and on both sides for exciting and receiving a surface acoustic wave (SAW) on a piezoelectric substrate and providing reflectors on both sides of these electrodes. In a longitudinally coupled dual mode SAW filter that confines the vibration energy of the two in the IDT and utilizes the waves of the first and third modes generated by the acoustic coupling between the IDTs. A longitudinally coupled dual-mode SAW filter, characterized in that the center-to-center spacing of the innermost facing electrode fingers is different on the left and right. 2. The centers of the innermost electrode fingers of the central IDT and the IDTs on both sides facing each other are approximately nλ / 2 on the one hand and (2n-1) λ / 4 (λ is the wavelength of the excited SAW) on the other hand. ，
2. The longitudinally coupled double mode SAW filter according to claim 1, wherein n = 1, 2, ...