JPH04373304A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To obtain the filter of a wide pass band without narrowing the cross width of an interdigital transducer by adjacently arranging three interdigital transducers on a piezoelectric substrate in a specified state and electrically parallelly connecting two outside interdigital transducers each other. CONSTITUTION:Three surface acoustic wave resonators arranging grating reflectors on the both sides of interdigital transducers 1-3 are adjacently arranged in a direction orthogonal to the propagating direction of surface acoustic waves excited by the interdigital transducers 1-3 so that the propagating directions of surface acoustic waves can be parallel. The outside interdigital transducers 2 and 3 are electrically parallelly connected. Thus, it is possible to utilize the resonance frequencies of plural various oscillation modes caused by the acoustic coupling of waves generated in the direction orthogonal to the propagating direction of surface acoustic waves excited by the interdigital transducers 1-3.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrowband multimode resonator filter using surface acoustic waves.

0002

[Prior Art] Conventional filters using surface acoustic wave resonators can realize narrow-band, low-loss filters in the range of several 10 MHz to 1 GHz, so they are widely used in communication equipment such as pagers and cordless telephones. It has been used as a front-end filter. For the piezoelectric substrate of such a surface acoustic wave resonator filter, a quartz crystal plate having a delay time temperature coefficient of 0 is often used from the viewpoint of reliability. However, since the electromechanical coupling coefficient of the quartz crystal plate is small, the actual situation is that the pass band width cannot be made sufficiently wide. Therefore, Figure 3
As shown in Figure 2, two resonators are arranged in parallel and close to each other, and two different vibration modes are resonated due to acoustic coupling of waves generated in a direction perpendicular to the propagation direction of surface acoustic waves excited by an interdigital transducer. A multi-vibration mode filter was used to achieve a wide band.

[0003] In such a multiple vibration mode filter,
The symmetric zero-order vibration mode shown in Fig. 3(a) and the symmetrical zero-order vibration mode shown in Fig. 3(b)
The antisymmetric zero-order vibration mode shown in is used, and the intersection width (hereinafter simply referred to as intersection width) is normalized by the surface acoustic wave wavelength so that the resonance frequencies of these two vibration modes are close to each other.
interdigital transducers are arranged, and
As shown in Figure 2, the passband of the filter was expanded to about twice that of a single vibration mode to achieve a wider band.

[0004] The resonance frequency difference between the two vibration modes of a surface acoustic wave filter based on this principle depends mainly on the intersection width. FIG. 2 shows the relationship between the normalized resonance frequency difference and the intersection width of each vibration mode when an aluminum electrode (film thickness/surface acoustic wave wavelength=2%) is provided on a quartz substrate.

For example, in order to set the normalized frequency difference between the symmetrical 0th order and the antisymmetrical 0th order to 0.05%, the intersection width W should be set to 12.
5λ (λ is the surface acoustic wave wavelength) to configure the surface acoustic wave filter.

[0006]

[Problems to be Solved by the Invention] In a multi-vibration mode filter having a configuration in which two resonators are arranged in parallel and close to each other as shown in FIG. It is necessary to widen the frequency difference between the two resonators (the 0th order and the antisymmetric 0th order), and to do so, it is necessary to narrow the normalized intersection width of the two resonators.

For example, in a multi-vibration mode filter having a configuration in which two resonators are arranged close to each other in parallel as shown in FIG.
The antisymmetric zero-order normalized frequency difference needs to be 0.1%. For this purpose, it is necessary to set the intersection width to 7λ as shown in FIG. In this way, when the intersection width is narrowed, the surface acoustic wave confinement effect inside the resonator is reduced, resulting in a decrease in the Q value and an increase in insertion loss as a surface acoustic wave filter. . Also, if you narrow the intersection width,
The impedance of the interdigital transducer also increases, and in order to match it with an external circuit, a matching circuit with a high conversion ratio must be constructed. As a result, a problem occurred in that the insertion loss in the matching circuit also increased.

[0008] The present invention does not require narrowing the crossing width, which causes a decrease in Q value and increases impedance.
The purpose of this invention is to provide a surface acoustic wave filter with a wide passband.

[0009]

[Means for Solving the Problems] The surface acoustic wave filter of the present invention has three interdigital transducers arranged on a piezoelectric substrate so that the propagation direction of the surface acoustic waves coincides with the propagation direction of the surface acoustic waves. Symmetrical in orthogonal directions 1
It is characterized in that it is arranged close to the position where the next vibration mode occurs, and the two outer interdigital transducers are electrically connected in parallel.

The surface acoustic wave filter of the present invention is composed of a combination of three interdigital transducers, and each interdigital transducer has grating reflectors arranged on both left and right sides through which the surface acoustic waves excited by each interdigital transducer propagate. It has a surface acoustic wave resonator structure. In addition, the three interdigital transducers each have a resonator structure, so that the propagation direction of the surface acoustic waves excited by each transducer is the same, and a symmetrical first-order vibration mode is generated in a direction orthogonal to the propagation direction of the surface acoustic waves. The outer interdigital transducers are placed close to the originating location and electrically connected in parallel.

The symmetrical first-order vibration mode is caused by the acoustic coupling of waves occurring in a direction orthogonal to the propagation direction of surface acoustic waves when the three interdigital transducers are viewed as one resonator. occurs. Furthermore, it is also possible to utilize resonance frequencies of a plurality of different vibration modes other than the symmetric first-order vibration mode.

In the surface acoustic wave filter of the present invention,
Among the three interdigital transducers, the middle interdigital transducer is used for input.
Outer interdigital transducers electrically connected in parallel can be used for output. Moreover, the reverse can also be used.

It is preferable that the intersection widths of the outer interdigital transducers electrically connected in parallel constituting the surface acoustic wave filter of the present invention are set to be substantially the same. Further, it is preferable that the intersection width of the central interdigital transducer is wider than the intersection width of the outer interdigital transducers.

[0014] The piezoelectric substrate of the surface acoustic wave filter of the present invention includes crystal, LiTaO3, Li2B4O7.
Alternatively, a LiNbO3 substrate or the like can be used. A crystal whose delay time temperature coefficient is 0 is preferable from the viewpoint of reliability.

[0015]Aluminum (Al) is also used as the electrode material.
, gold (Au), and other metals can be used. Furthermore, the present invention is applicable not only to surface acoustic waves but also to SSBW (SSBW) such as shear waves.
Surface Skimming Bulk Wave)
The present invention can also be applied to a resonator using a resonator, and the same effects as the present invention can be obtained.

[0016]

[Operation] By combining the three interdigital transducers as described above, the whole can be regarded as one resonator. As a result, it is now possible to utilize the resonant frequencies of multiple different vibration modes resulting from the acoustic coupling of waves generated in a direction perpendicular to the propagation direction of the surface acoustic waves excited by each interdigital transducer. Become.

Furthermore, in the antisymmetric zero-order vibration mode of the central interdigital transducer, no excitation or reception occurs because the charges cancel. As a result, it is possible to realize a multiple vibration mode filter that utilizes the combination of the symmetric zero-order vibration mode and the symmetric first-order vibration mode.

[0018]

[Embodiments] Hereinafter, embodiments of the present invention will be explained with reference to the drawings.

FIG. 1 shows an embodiment of the invention. Three surface acoustic wave resonators with grating reflectors arranged on both sides of the interdigital transducer are arranged in a direction perpendicular to the propagation direction of the surface acoustic waves excited by the interdigital transducer, and parallel to the propagation direction of the surface acoustic waves. They are placed in close proximity to each other.

The outer interdigital transducers 2 and 3 are electrically connected in parallel, with the central interdigital transducer 1 used for input and the outer interdigital transducers 2 and 3 used for output. Furthermore, the intersection widths of the outer interdigital transducers 2 and 3 were set to be approximately the same. Further, each grating reflector was used as a terminal for taking out an electrode.

FIG.
(a), (b), and (c).

There are symmetric zero-order, anti-symmetric zero-order, and symmetric first-order vibration modes. However, in the resonator of this example,
Since the resonators of the interdigital transducers 2 and 3 have substantially the same crossing width, the resonators have a symmetrical structure with respect to the center line. Therefore, the antisymmetric zero-order vibration mode is not excited or received because the charge cancels in each interdigital transducer.

Therefore, with the resonator of this embodiment, it is possible to realize a filter that utilizes two vibration modes, symmetrical zero-order and symmetrical first-order.

In this embodiment, the intersection width W of the entire resonator is set to 1.
It was set to 5λ. In this case, as shown in Figure 2, the symmetry 0
Since the frequency difference between the symmetrical 0th-order vibration mode and the symmetric 1st-order vibration mode is 0.1%, it is possible to achieve a wider band than conventional surface acoustic wave filters that utilize symmetrical 0th-order and anti-symmetric 1st-order vibration modes. In order to achieve a frequency difference of 0.1% using the conventional method, it is necessary to set the crossing width to about 7λ. Therefore, the crossing width must be approximately half that of the present invention.

[0025] Normally, when the crossing width becomes less than 10λ, the Q value deteriorates rapidly. In the present invention, with a crossing width of 10λ or more,
Since the frequency difference can be ensured, there is no deterioration of the Q value and no increase in insertion loss as a filter. Furthermore, since the intersection width is wide, the impedance of the interdigital transducer does not become high, so a matching circuit with an external circuit can be easily created, and loss in this matching circuit does not increase.

Furthermore, in this embodiment, the central interdigital transducer 1 can be used for output, and the outer interdigital transducers 2 and 3 can be used for input.

[0027]

Effects of the Invention The surface acoustic wave filter of the present invention has three interdigital transducers arranged on a piezoelectric substrate so that the propagation directions of the surface acoustic waves coincide with each other and in a direction orthogonal to the propagation direction of the surface acoustic waves. Since the two outer interdigital transducers are placed close to the position where the symmetrical first-order vibration mode occurs, and the two outer interdigital transducers are electrically connected in parallel, a multi-vibration mode filter can be used to eliminate the symmetric zero-order vibration mode.
Two modes of vibration can be used: second order and symmetric first order. Therefore, it is possible to realize a filter with a wide passband, and also to widen the crossing width, so there is no deterioration of the Q value and no increase in insertion loss. Furthermore, since the impedance of the interdigital transducer does not need to be high, it becomes easier to create a matching circuit.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a surface acoustic wave according to an embodiment of the present invention;
FIG. 1(a) shows a symmetric zero-order vibration mode, FIG. 1(b) shows an anti-symmetric zero-order vibration mode, and FIG. 1(c) shows a symmetric first-order vibration mode.

FIG. 2 is a diagram showing the cross width dependence of the normalized resonance frequency difference of each vibration mode.

FIG. 3 is a diagram showing a conventional surface acoustic wave in which two resonators are arranged close to each other in parallel; FIG. 3(a) shows the symmetric zero-order vibration mode, and FIG. 3(b) shows the anti-symmetric zero-order vibration. FIG.

FIG. 4 is a diagram showing frequency characteristics of the surface acoustic wave shown in FIG. 3;

[Explanation of symbols]

1... Interdigital transducer 1, 2...
Interdigital transducer 2, 3...Interdigital transducer 3.

Claims (1)

[Claims] 1. Three interdigital transducers are arranged on a piezoelectric substrate so that the propagation directions of surface acoustic waves coincide with each other, and a symmetrical first-order vibration mode is generated in a direction orthogonal to the propagation direction of the surface acoustic waves. A surface acoustic wave filter characterized in that two interdigital transducers on the outside are arranged close to each other and electrically connected in parallel.