JP3181090B2 - Vertically coupled dual mode leaky 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, excellent steepness, and large stop band attenuation.

[0002]

2. Description of the Related Art Conventionally, a unidirectional ID has been used for a wideband low-loss SAW filter in a high frequency range of a VHF to UHF band.
A transversal type SAW filter using T and a multi-transducer type SAW filter are widely used. Transversal SA using one-way IDT
The W filter requires a phase shifter and has a drawback that it is difficult to manufacture, and it is difficult to completely eliminate directional loss, and low loss is still insufficient. Next, the multi-transducer type SAW filter has a large chip size and a large number of bonding wires, and has a large IDT.
There is a drawback that many spurious components due to internal reflection appear, and the attenuation of the stop band is insufficient.

On the other hand, a SAW resonator filter, in particular, two IDTs are arranged close to each other along the propagation direction of the SAW.
A longitudinally-coupled dual-mode SAW filter using two vibration modes, a first-order mode and a second-order mode, generated when the SAWs excited by these are acoustically coupled to each other between the IDTs, achieves the above-described two modes in order to realize low loss. Better than the method. The pass band width of this longitudinally-coupled dual-mode SAW filter has been considered to be difficult to apply to a wide-band filter because of the limitation of the capacitance ratio of the resonator. And large 64 ° Y-cut X-propagation LiNbO
Wide-band low-loss longitudinally coupled dual-mode leaky SA having a fractional bandwidth of 2.5% or more using SAW propagating through _three substrates
W filter was invented. (Japanese Patent Application No. 2-337281)

Further, for example, in a 900 MHz band RF filter used for a mobile phone or a car phone, the relative bandwidth is 3 to 4%.
In this case, three IDTs are arranged close to each other along the propagation direction of the aforementioned 64 ° Y-cut X-propagation LiNbO ₃ substrate, and the SAWs excited by these are acoustically coupled between the IDTs. 1 that occurs when
When the two vibration modes, the second mode and the third mode, are used, the frequency difference between the modes is widened as compared with the filter using the first mode and the second mode, and a filter having a wide enough pass band width for practical use is obtained. Clarified that it can be configured.

[0005] However, 900 MHz for mobile phones and the like
In a band RF filter, for example, in the North American specification (AMPS), a pass band width of 25 MHz is required, whereas a filter for transmission and reception is separated by only 45 MHz. When the above-described longitudinally-coupled dual-mode leaky SAW filter using the first-order mode and the third-order mode is applied to the 900 MHz band RF filter for the cellular phone, a relatively steep shoulder characteristic is obtained in the low frequency side, and the stopband attenuation is also reduced. On the other hand, on the high frequency side, the spurious characteristic of the longitudinally coupled dual mode filter causes the stopband attenuation to be insufficient and the steepness of the shoulder characteristic to be insufficient.

As a method for solving this problem, it is conceivable that the filters are connected in cascade in multiple stages to increase the amount of attenuation in the stop band and to sharpen the shoulder characteristics. The band becomes narrow and it is not practical. Therefore, for example, even if the insertion loss is reduced to a level that can be practically used by connecting two vertically coupled dual-mode leaky SAW filters in cascade, the attenuation in the high-frequency side is insufficient and the demand cannot be satisfied. there were.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned disadvantage of the conventional longitudinally coupled dual mode leaky SAW filter, in which the stop band attenuation on the high frequency side is insufficient. It is another object of the present invention to provide a longitudinally coupled dual mode leaky SAW filter having a filter characteristic having a large steepness and a sufficiently large stopband attenuation.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, a longitudinally coupled dual mode leaky SAW filter according to the present invention has an IDT electrode finger width (line width) h and an interval width (space width) a of 0. 3 ≦ h / (h + a) <0.5.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. FIG. 1 is a configuration diagram of an electrode pattern of one embodiment of a longitudinally coupled dual mode leaky SAW filter according to the present invention. An input IDT is provided on the surface of a piezoelectric substrate 1 and output IDTs 3 and 4 are arranged on both sides thereof. Reflectors 5 and 5 are arranged on both sides thereof. This IDT cycle is represented by L _T (= SA
The wavelength λ of W), the line width is h, and the space width is a.
[Where L _T = 2 (h + a)] With this configuration, the first mode and the third mode generated by acoustic coupling between IDTs
Coupled dual mode leaky S using two resonance modes
An AW filter can be realized.

Next, FIG. 2 shows that the line width h and the space width a of the IDT are equal to each other in FIG.
/(H+a)=0.5] is a diagram showing a frequency attenuation characteristic when a conventional longitudinally coupled dual mode leaky SAW filter with cascade connection of two stages is set, where the substrate is 64 ° Y-cut X-propagation LiNbO _3. , Input IDT is 16 pairs, output ID
T is 10 pairs each, the number of reflectors is 250 on both sides, the intersection length is 40λ, the electrode is aluminum and the film thickness is 0.2 μm. As is clear from the figure, a large spurious component 6 appears near the high frequency side of the passband.
S) In the case of a mobile phone, a pass band width of 25 MHz is required, whereas a transmission / reception frequency interval is 45 MHz, so ± 45 MHz from the center frequency as shown by hatching in FIG.
Sufficient attenuation is required at frequencies apart by z. However, as described above, the attenuation was deteriorated due to the large spurious component 6 near the high frequency side of the passband, and the standard could not be satisfied.

FIG. 3 is a graph showing experimental values of frequency attenuation characteristics when the ratio of the line width h to the space width a of the IDT is changed under the same conditions. As is clear from FIG.
Ratio of line width h and space width a of T [h / (h + a)]
The smaller the value, the smaller the spurious near the high frequency side of the pass band and the greater the attenuation. However, as described above, the conventional IDT
Is generally set to h: a = 1: 1. Particularly, in a SAW resonator or a leaky SAW resonator, when the line width is smaller than the space width, the Q of the resonator decreases. It was usual to make the line width as large as possible or even as much as possible. [0.5 ≦ h /
(H + a)]

On the other hand, the line width h of the IDT in the vertically coupled dual mode leaky SAW filter of the present invention is as follows:
The width is narrower than the space width a in order to reduce spurious near the high frequency side of the passband and increase the attenuation. [H / (h + a) <0.5] FIG. 4 is a diagram showing a change in the amount of attenuation near the high frequency side of the passband with respect to the ratio h / (h + a) of the line width h and the space width a of the IDT. , H
When / (h + a) is set to 0.47 or less, an attenuation of 25 dB or more that satisfies the above-mentioned mobile phone specification for North America (AMPS) is obtained. When the value of h / (h + a) is further reduced, the amount of attenuation increases, but the line width of the IDT becomes narrower, so that the ohmic cross increases and the impedance increases. FIG. 5 is a diagram showing a frequency attenuation characteristic when h / (h + a) = 0.20, and other conditions are the same as those in FIG. As shown in FIG. 5, when the line width is extremely narrow, a large deviation appears in the passband characteristic under the same termination condition (50Ω), and the practical use becomes difficult. In the case of the termination condition of 50Ω, the ratio of the line width h to the space width a such that the deviation in the pass band is small and the insertion loss does not deteriorate is around h / (h + a) = 0.30. Therefore, in order to obtain the attenuation near the high frequency side that satisfies the above-mentioned mobile phone specification for North America (AMPS) without significantly affecting the passband characteristics, the ratio of the line width h to the space width a h / ( h + a) is 0.3 ≦ h / (h +
It became clear that a) ≦ 0.47 was sufficient.

When the line width h is reduced, the influence of the mass load effect is reduced and the center frequency shifts to a higher value.
₁₂ ') there is a merit that it will also spread the pass band width wider stop band width increases. Although the present invention has been described above with reference to the specification of a mobile phone for North America (AMPS) as an example, the present invention is not limited to this. The relationship with the space width a is 0.3 ≦ h / (h +
a) A vertically coupled dual mode leaky SAW filter with <0.5 is shown. Further, the present invention uses a leaky SAW on a 64 ° Y-cut X-propagation LiNbO ₃ substrate,
Although the longitudinally coupled dual mode leaky SAW filter using two resonance modes of the second mode and the third mode has been described, similarly, two resonance modes of the first mode and the second mode may be used. ° Y cut X propagation LiNb
Obviously, other cuts and leaky SAW of the substrate may be used, such as O ₃ , 36 ° Y-cut X-propagation LiTaO _3, etc.

[0014]

Since the present invention is constructed as described above, it has a remarkable effect in obtaining a filter having a wide band, low loss and a large stop band attenuation in a high frequency region.

[0015]

[Brief description of the drawings]

FIG. 1 shows a longitudinally coupled dual mode leaky SAW according to the present invention.
The figure which shows the electrode pattern structure of a filter.

FIG. 2 is a diagram showing a frequency attenuation characteristic of a conventional longitudinally coupled dual mode leaky SAW filter [h / (h + a) = 0.5].

FIG. 3 is a diagram showing a change in frequency attenuation characteristics when h / (h + a) is changed in the longitudinally coupled dual mode leaky SAW filter of FIG. 1;

FIG. 4 is a diagram showing a change in a high-frequency side attenuation near a pass band when h / (h + a) is changed in the longitudinally coupled dual-mode leaky SAW filter of FIG. 1;

FIG. 5 is a diagram illustrating frequency attenuation characteristics when h / (h + a) = 0.20 in the longitudinally coupled dual mode leaky SAW filter of FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate 2 ... Central IDT 3, 4 ... Both-side IDT 5 ... Reflector 6 ... Spurious near high-pass side of a pass band

Continuation of the front page (56) References JP-A-1-212015 (JP, A) JP-A-2-194714 (JP, A) JP-A-63-173412 (JP, A) JP-A-62-199111 (JP) , A)

Claims (2)

(57) [Claims] An interdigital transducer (ID)
T) Leaky surface acoustic waves (SA) excited or received by electrodes
The input and output IDTs are arranged close to each other on the piezoelectric substrate along the propagation direction of W), and reflectors for confining the vibration of the leaky SAW and forming resonators are provided on both sides thereof, and acoustic coupling between the IDTs is provided. In a longitudinally coupled dual mode leaky SAW filter utilizing two generated resonance modes,
The relationship between the electrode finger width (line width) h and the interval width (space width) a of the IDT satisfies 0.3 ≦ h / (h + a) <0.5.
filter. 2. The longitudinally coupled double mode according to claim 1, wherein said IDT comprises three IDTs, and utilizes first and third order resonance modes generated by acoustic coupling between said IDTs. Leaky SAW filter.