JP2000196399A - Surface acoustic wave filter and resonator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the blocking attenuating amount in the neighborhood of the low pass side of a passage band by constituting the electrode fingers of a reflector to be shorter as being away from IDT electrodes at both sides becomes longer in the reflector in a vertical coupled multiplex mode SAW filter constituted by means of arranging the plural IDT electrodes along the propagating direction of surface wave and by means of arranging the reflectors at both sides on a piezoelectric substrate. SOLUTION: In a double mode SAW filter, the three IDT electrodes 2, 3 and 4 are adjacently arranged along the propagating direction of surface wave and the reflectors 5a and 5b are arranged on both sides on the main surface of the piezoelectric substrate 1. The shapes of the reflectors 5a and 5b are constituted of a normal structure and a wedge shape structure. The shapes of the reflectors 5a and 5b in a part being adjacent to the IDT electrodes 2, 3 and 4 are adopted as a certain number of normal types by which surface wave is nearly perfectly reflected and the length of the electrode fingers is made to be shorter as the distance from the IDT electrodes 2, 3 and 4 becomes larger. Thus, ripples superimposed on the main lobe and side lobes are suppressed and standardized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter and a resonator, and more particularly, to a surface acoustic wave filter with improved rejection attenuation near the lower side of the passband and a spurious on the lower frequency side of the resonance frequency. Related resonators.

[0002]

2. Description of the Related Art In recent years, SAW devices have been widely used in the field of communications, and have been used particularly in mobile phones and the like because of their excellent features such as high performance, small size, and mass productivity. Figure 6
(A) is a plan view showing a configuration of a conventional first- and third-order longitudinally-coupled double-mode SAW filter using a first- and third-order longitudinal mode (hereinafter, referred to as a dual-mode SAW filter). ,
While three IDT electrodes 12, 13, and 14 are arranged close to each other along the propagation direction of the surface wave on the main surface of the piezoelectric substrate 11,
Grating reflectors (hereinafter, referred to as reflectors) 15a and 15b are provided on both sides thereof.
Each of the IDT electrodes 12, 13, and 14 is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and one of the IDT electrodes 12 has an input terminal IN.
And the other comb electrode is grounded. On the other hand, IDT
One of the comb electrodes 13 and 14 is connected to each other and connected to the output terminal OUT, and the other comb electrode is grounded.

As is well known, the operation of the dual mode SAW filter shown in FIG.
A plurality of surface waves excited by the reflectors 15a, 15
b, acoustic coupling is performed, and two longitudinal resonance modes of primary and tertiary order are strongly excited by the electrode pattern. Operates as a mode SAW filter. It is well known that the pass bandwidth of the dual mode SAW filter is determined by the frequency difference between the primary resonance mode and the tertiary resonance mode.

FIG. 6B shows a dual mode S shown in FIG.
FIG. 2 is a plan view showing a configuration of a two-stage cascade-type double-mode SAW filter in which two AW filters are cascade-connected. The filter requires a steep attenuation gradient near a passband or has a large guaranteed attenuation. It is used when Figure
7 (a) is a 36 ° Y-cut X-propagation LiTaO3 substrate 11
On the top, 15.5 pairs of IDT electrodes 12, 9.5 pairs of IDT electrodes 13 and 14, and 200 reflectors 15 a and 15 b respectively
This is the characteristics of a two-stage cascade-type double-mode SAW filter with a cross length of 60λ and an electrode thickness of 8% λ. The horizontal axis represents frequency (MHz), and the vertical axis represents insertion loss (Loss). it's shown. FIG. 7 (b) is an enlarged view of the vicinity of the lower side of the pass band in FIG. 7 (a). The attenuation of the stop band in the 50MHz frequency region is shifted to the lower frequency side from the lower end of the pass band. FIG. Small period ripples are superimposed on large main lobes and side lobes of the IDT electrodes 12, 13 or 14.

As shown in FIG. 7B, the IDT electrodes 12, 1
Small periodic ripples superimposed on the relatively large period main lobes and side lobes due to 3, 14
This is caused by the reflection characteristics formed by a and 15b. To briefly explain the reflection characteristics of the reflector, that is, the reflection coefficient,
When the normalized frequency f / f ₀ normalized by the frequency f ₀ determined by the wavelength of the reflector and the phase velocity of the surface wave is plotted on the horizontal axis, and the reflection coefficient 縦 軸 is plotted on the vertical axis, the reflection coefficient の of the reflector becomes as shown in FIG. As shown in a), a stop band is formed near a normalized frequency f / f ₀ of 1 and periodic waves are formed on both sides of the stop band. This characteristic is generally called a stop band characteristic. Fig. 8
(B) is a diagram in which the reflection characteristics when the number of two reflectors is set to 100 and 120, respectively, are superimposed, and periodic ripples that occur a lot on both sides of the stop band, that is, peaks or valleys are shown. It can be seen that the frequency differs depending on the number of reflectors, and the curve of the combined reflection coefficient approaches the average value of peaks and valleys. Fig. 8 (c) shows that the number of the two reflectors is 120
In this example, the reflection coefficients when the center frequencies of the stop bands are slightly different are superimposed and displayed. This characteristic also shows that the frequency of a large number of periodic peaks or valleys is different from each other, and that the curve of the combined reflection coefficient approaches the average value of the peaks and valleys.

When setting various parameters of the dual mode SAW filter, if the filter is selected so that the pass band of the filter coincides with a flat portion where the reflection coefficient of the stop band characteristic formed by the reflector is close to 1, the filter is inserted. It is well known that losses are minimized. The filtering characteristic of the dual mode SAW filter depends on the transversal characteristic formed by the IDT electrode, and the pass band characteristic is affected by its main lobe characteristic and the attenuation region is affected by its side lobe characteristic. A large number of periodic small ripples formed by the reflector are superimposed on the main lobe and the side lobe to form a stop band attenuation characteristic of the dual mode SAW filter. In general, the number of reflectors is selected to be at least one order of magnitude larger than the number of IDT electrode pairs, so that the periodicity formed by the reflector exhibits a smaller periodicity than that of the IDT electrode, and the main lobe and the It will be superimposed on the side lobe.

Japanese Patent Application Laid-Open No. H10-261935 discloses a means for improving the deterioration of the attenuation of the stop band due to a ripple having a small period superimposed on the main lobe and the side lobe. That is, there are means for changing the number of electrode fingers of each of the reflectors (reflectors) arranged on both sides of the IDT electrode, means for changing the thickness of each electrode of the reflector, and the like. FIG. 9A uses the same parameters as those shown in FIG. 7, and utilizes the characteristics of the reflector shown in FIG. 8B to increase the number of four reflectors to 170 and 180, respectively. 19 shows the filtering characteristics of a two-stage cascade-type double-mode SAW filter having 190, 200, and 200 filters. FIG. 9B is a diagram illustrating the attenuation of the stop band near the low band side of the pass band in FIG. 9A with the frequency enlarged.

Next, FIG. 10 (a) uses the same parameters as those shown in FIG.
The ratio Lt / Lr of the wavelength λidt of the DT electrode is 0.970, respectively.
FIG. 9 is a diagram showing the filtering characteristics of a two-stage cascade-connected double-mode SAW filter in the case of 5, 0.9715, 0.9725, and 0.9735.
FIG. 10B is a diagram illustrating the attenuation of the stop band near the lower side of the pass band in FIG. Figures 9 and 1
As is clear from Fig. 0, the characteristics of a conventional two-stage cascaded dual-mode SAW filter using four reflectors of the same number (Fig. 7)
It can be seen that the small amount of ripple superimposed on the side lobe near the low band side of the passband is suppressed, and thus the amount of rejection attenuation is improved.

[0009]

However, a two-stage cascade-type double mode S using a conventional reflector having the same configuration is used.
Example of AW filter (FIG. 7), example using means for varying the number of four reflectors (FIG. 9), or example using means for varying the wavelength of four reflectors and IDT electrodes (FIG. 9) As shown in FIG.
− CDMA transmission channel 906MHz ± 19MHz and reception channel 851
In the region adjacent to MHz ± 19 MHz, for example, there is a problem that the TX filter cannot satisfy the required attenuation of 40 dB in the frequency range from 832 MHz to 870 MHz. SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and has as its object to provide a dual mode SAW filter with improved rejection attenuation near the lower side of the passband.

[0010]

According to a first aspect of the present invention, there is provided a surface acoustic wave filter according to the present invention.
In a longitudinally coupled multi-mode SAW filter comprising a DT electrode and reflectors disposed on both sides thereof, the reflector is configured such that the electrode fingers of the reflector become shorter as the distance from the IDT electrodes at both ends increases. This is a surface acoustic wave filter characterized by the following. According to a second aspect of the present invention, there is provided a longitudinally coupled dual mode SAW filter comprising three IDT electrodes on a piezoelectric substrate along a propagation direction of a surface acoustic wave and reflectors on both sides of the IDT electrodes. ID of both ends
The surface acoustic wave filter is characterized in that the electrode finger of the reflector is shortened as the distance from the T electrode increases. According to a third aspect of the present invention, there is provided a surface acoustic wave resonator configured by arranging an IDT electrode and reflectors on both sides thereof along a propagation direction of a surface acoustic wave on a piezoelectric substrate, wherein the reflector is located at a distance from the IDT electrode. The surface acoustic wave resonator is characterized in that the electrode fingers of the reflector are shortened as the distance increases. The invention according to a fourth aspect is the surface acoustic wave filter according to any one of the first to third aspects, wherein the reflector comprises a regular-shaped portion and a wedge-shaped portion. The invention according to claim 5 is characterized in that the reflector is constituted by a regular portion and a semi-elliptical portion.
4. The surface acoustic wave filter according to 3. The invention according to claim 6 is the surface acoustic wave filter according to any one of claims 1 to 3, wherein the reflector includes a regular portion and a semicircular portion.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 is a plan view showing a configuration of a dual mode SAW filter according to the present invention, in which three IDT electrodes 2, 3, and 4 are closely arranged on a main surface of a piezoelectric substrate 1 along a propagation direction of a surface wave. 5 reflectors on each side
a and 5b are arranged and configured. The feature of the present invention is the IDT electrode
The reflectors 5a and 5b are arranged on both sides of 2, 3, and 4.
That is, the reflectors 5a and 5b have a regular structure and a wedge-shaped structure.

The present inventor prototyped a dual-mode SAW filter (first-order to third-order longitudinal mode) in order to develop an N-CDMA transmission / reception SAW filter, as shown in FIGS. 9 (b) and 10 (b). As described above, in the conventional dual mode SAW filter, in consideration of variations at the time of manufacturing, temperature, aging, etc.,
It was insufficient to meet the required 40 dB attenuation in the frequency range from 832 MHz to 870 MHz. Therefore, the dual mode S
The same number of reflectors on both sides of the AW filter
The low-pass-side attenuation in the passband was measured while gradually shaving off the end of the reflector from the end of the substrate using a laser.
It has been found that the attenuation characteristic on the low frequency side fluctuates when the shape of the tip of the reflector is made various shapes such as a wedge shape, a cutting knife shape, a semi-elliptical shape, and a semi-circular shape.

Therefore, as shown in FIG.
The normal type part α constituting the reflectors 5a and 5b of the AW filter,
FIG. 2 (a) shows the filtering characteristics when the number of α ′ is 150, the number of wedge-shaped portions β and β ′ is 50, and the other constants are the same as in FIG. The figure (b) is an enlarged view of the frequency in order to show the amount of stop attenuation near the lower side of the passband, and the shaded area indicated by the arrow R is 40d of the required standard of N-CDMA.
There is B. As is clear from this figure, FIG. 9 (b), FIG.
The small ripples superimposed on the main lobe and side lobes in (b) are greatly suppressed, sufficiently meeting the required standard of N-CDMA.

Next, FIG. 3 (a) shows that the number of normal-shaped parts α and α ′ constituting the reflectors 5a and 5b is 100 and the number of wedge-shaped parts β and β ′ is 100, respectively. Other constants are shown in Fig. 7.
It is a filtering characteristic when the same value is used. Also in this case, the small ripple superimposed on the main lobe and the side lobe is largely suppressed, and sufficiently satisfies the required standard of N-CDMA of 40 dB. FIG. 3 (c) is an enlarged view of the vicinity of the lower side of the passband. The depressed curve of the periodic ripple indicated by the arrow A is the characteristic shown in FIG. 7 (b), and the curve indicated by the arrow B is FIG. It is the figure which expanded and represented the characteristic near the passband of (b) further. It can also be seen from this figure that the small ripple caused by the reflector is greatly suppressed.

It is not clear why the reflectors 5a and 5b are composed of the normal type and the wedge type to suppress small ripples superimposed on the main lobe and the side lobe. It is presumed that reflections at frequencies away from the pass band cancel each other out and do not become standing waves, but exhibit an average value of peaks and valleys of ripples.

FIG. 4 shows another embodiment of the present invention, in which only the shape of one reflector of the dual mode SAW filter is shown. The reflector shown in FIG. 3A is composed of a regular mold part α and a cut-out knife part γ. The reflector shown in FIG. 4 (b) is composed of a normal part α and a semi-elliptical part δ. As described above, the shape of the reflector adjacent to the IDT electrode is a normal type having a sufficient number to reflect the surface wave almost completely, and the length of the electrode finger becomes shorter as the distance from the IDT electrode increases. As a result, it is possible to suppress ripples superimposed on the main lobe and the side lobes and level the ripples.

FIG. 5 shows another embodiment. As shown in FIG. 4 (b), a reflector composed of 100 normal mold portions and 100 semi-elliptical portions δ is used on both sides of the IDT. The constant is an example of the filtering characteristic of the dual mode SAW filter having the same value as in FIG. Fig. 5 (a) shows the entire filter characteristics, and Fig. 5 (b)
Represents the attenuation characteristics of the stop band near the lower side of the pass band with the frequency enlarged. As is clear from this figure,
Small ripples on the main lobe and side lobes are sufficiently suppressed, sufficiently satisfying the N-CDMA standard of 40 dB indicated by the arrow R.

As described above, in the description of the dual mode SAW filter according to the present invention, the reflectors on both sides are described as having the same shape. However, similar effects can be obtained by making the shapes of the reflectors on both sides different from each other. Also, a primary-tertiary longitudinally coupled dual mode SAW
Although the present invention has been described with reference to a filter as an example, the present invention is not limited to this, and the present invention is not limited thereto.
It goes without saying that the present invention can be applied to a longitudinally coupled multi-mode SAW filter such as an AW filter. Further, it has been found that the present invention is also effective when applied to a laterally coupled multi-mode SAW filter.

Although the above description has been given of a multi-mode SAW filter, the present invention is also effective when applied to a SAW resonator composed of an IDT electrode and reflectors arranged on both sides thereof. That is, by forming the reflector from the normal type portion and the wedge type portion, it is possible to suppress spurious components on the low frequency side of the resonance frequency of the resonator.

[0020]

According to the present invention, as described above, the amount of stop attenuation near the low-pass side of the pass band of the dual-mode SAW filter can be improved.
If it is used for a TX or RF filter such as CDMA, it exhibits an excellent effect in improving the performance of the receiver.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration of a dual mode SAW filter according to the present invention.

FIG. 2 (a) shows 150 regular-shaped reflectors and 5 wedge-shaped reflectors.
FIG. 7B is a diagram illustrating the filtering characteristics of a double mode SAW filter with zero filters, and FIG. 8B is a diagram illustrating the amount of attenuation in the stop band near the low band side of the pass band.

FIG. 3 (a) shows 100 regular-shaped reflectors and one wedge-shaped reflector.
FIG. 7B is a diagram showing the filtering characteristics of the 00 dual-mode SAW filter, FIG. 7B is a diagram showing the amount of stop band attenuation near the low band side of the pass band, and FIG.

4 (a) is a plan view of a reflector composed of a regular mold part and a cut-out knife-shaped part, and FIG. 4 (b) is a plan view of a reflector composed of a regular mold part and a semi-elliptical part. FIG.

5A is a diagram showing a filtering characteristic of a dual mode SAW filter having 100 normal-shaped portions and 100 semi-elliptical portions of a reflector, and FIG. It is a figure showing a stop zone attenuation.

FIG. 6A is a plan view showing a configuration of a primary-tertiary-order longitudinally coupled double-mode SAW filter (double-mode SAW filter), and FIG. 6B is a configuration of a two-stage cascade-connected double-mode SAW filter. FIG.

FIG. 7A is a diagram illustrating filtering characteristics when the number of regular reflectors is set to 200, and FIG. 7B is a diagram illustrating a stop band attenuation near a low band side of a pass band.

8 (a) to 8 (c) show stop band characteristics of a reflector, FIG. 8 (a) shows 100 reflectors with a single reflector, and FIG.
The combination of the reflection coefficients of the two reflectors, and (c) is the combination of two reflection coefficients with different wavelengths of the 120 reflectors.

9A is a diagram illustrating a filtering characteristic of a dual mode SAW filter in which the number of two reflectors is different, and FIG. 9B is a diagram illustrating a stop band attenuation amount near a lower side of a pass band. is there.

FIG. 10A is a diagram showing the filtering characteristics of a dual mode SAW filter in which the wavelengths of the reflectors are different, and FIG.
FIG. 7 is a diagram showing a stop band attenuation amount near a low band side of a pass band.

[Explanation of symbols]

1. Piezoelectric substrate 2, 3, 4 IDT electrode 5a, 5b Reflector α, α 'Normal part of reflector β, β' ... Wedge part of reflector R N A hatched portion of the CDMA standard is shown. A. Attenuation characteristics of the conventional dual mode SAW filter b. Attenuation characteristics of the dual mode SAW filter according to the present invention γ Cut-out knife-shaped portion δ Semi-elliptical portion

Claims (6)

[Claims] 1. A longitudinally coupled multi-mode SAW filter comprising a plurality of IDT electrodes arranged on a piezoelectric substrate along a propagation direction of a surface acoustic wave and reflectors on both sides thereof, wherein the reflectors are arranged at IDT electrodes at both ends. A surface acoustic wave filter configured to shorten the electrode fingers of the reflector as the distance increases. 2. A method according to claim 1, wherein the surface acoustic wave is propagated on the piezoelectric substrate.
In a longitudinally coupled dual mode SAW filter configured by arranging three IDT electrodes and reflectors on both sides of the IDT electrodes, the electrode fingers of the reflector are shortened as the distance from the IDT electrodes at both ends increases. A surface acoustic wave filter characterized in that: 3. A surface acoustic wave resonator in which an IDT electrode and reflectors are arranged on a piezoelectric substrate along a propagation direction of a surface acoustic wave and reflectors are disposed on both sides of the IDT electrode. A surface acoustic wave resonator characterized in that the electrode fingers of the reflector are configured to be short. 4. The surface acoustic wave filter according to claim 1, wherein said reflector comprises a regular portion and a wedge-shaped portion. 5. A surface acoustic wave filter according to claim 1, wherein said reflector comprises a normal part and a semi-elliptical part. 6. The surface acoustic wave filter according to claim 1, wherein said reflector comprises a regular portion and a semicircular portion.