JP3721229B2 - Surface acoustic wave filter - Google Patents

Description

[0001]
[Field of the Invention]
The present invention relates to a surface acoustic wave filter, and more particularly to a ladder-type surface acoustic wave filter in which a plurality of surface acoustic wave resonators are arranged in a series arm and a parallel arm.
[0002]
[Prior art]
Conventionally, surface acoustic wave filters (hereinafter referred to as SAW filters) suitable for miniaturization have been used as RF and IF filters in portable communication devices and the like, but with the recent increase in carrier frequency, cellular phones There is an increasing demand for realizing a SAW filter having a center frequency of 700 to 2000 MHz and a passband width of 20 to 60 MHz.
A ladder-type SAW filter of a type in which a ceramic resonator is replaced with a surface acoustic wave resonator (hereinafter referred to as a SAW resonator) in accordance with a ladder-type circuit that is a circuit configuration of a 455 kHz ceramic filter that has been widely used in radio equipment and the like. However, it has been proposed and put into practical use several years ago. Since the filter is a resonator type filter composed of only a SAW resonator having a high Q, it has characteristics such as low loss and a steep cut-off characteristic in the pass band.
[0003]
FIG. 5 is a block diagram showing the configuration of such a ladder-type SAW filter, in which a SAW resonator 11 is connected to a series arm of a two-terminal pair circuit, and a SAW resonator 12 or 13 is connected to a parallel arm in a ladder shape. It is. The ladder type filter has a low loss because it can form a multistage connection filter circuit only with a resonator having a high Q without using a transformer or the like like other Yarman circuits, and further, on the same piezoelectric substrate. If a plurality of SAW resonators are formed to form a ladder-type SAW filter, it is suitable for downsizing, mass productivity, and the like. FIG. 6 is a diagram showing a basic configuration of each SAW resonator used in the ladder-type SAW filter, in which the IDT 14 is formed on the main surface of the piezoelectric substrate 1 and the reflectors 15a and 15b are formed on both sides thereof. This utilizes the phenomenon in which vibration energy is confined between the reflectors. Here, W is the cross finger width of the IDT, and is an important factor that influences the characteristics of the SAW resonator together with the logarithm N of the IDT, the electrode film thickness H, and the like.
[0004]
Examples of the filtering characteristics of such a conventional ladder-type SAW filter include, for example, 36 ° Y-cut X-propagating lithium tantalate as a piezoelectric substrate, seven SAW resonators, and an IDT logarithm on the parallel arm at the input / output terminals. N = 40, electrode finger cross finger width W = 28λ (λ is the wavelength of the surface acoustic wave excited by the IDT), IDT logarithm N = 80 for other parallel arms, electrode finger cross finger width W = 28λ When SAW resonators having IDT logarithm N = 80 and electrode finger crossing finger width W = 10λ are arranged on the series arm, the filtering characteristics are as shown in FIG.
[0005]
However, as apparent from FIG. 7, the cutoff characteristics lack steepness, the attenuation gradient is not sufficient, and the filter insertion loss is as large as 2.5 dB. The reason for this is presumed to be that the ladder type filter is constructed using a simple image parameter method generally used in designing. Therefore, a simulation was performed while making full use of the operating parameter method that can be expected to provide better characteristics and changing various parameters. As a result, the SAW resonators constituting the ladder-type filter having different capacitance ratios were appropriately used. It was found that it was necessary to arrange in series arms and parallel arms. Depending on the desired filter characteristics, in the design using the operation parameter method, the ratio of the minimum value and the maximum value of the capacitance ratio of the SAW resonator used in the ladder-type circuit can be freely set within a range of approximately 2 It is necessary to select
[0006]
[Problems to be solved by the invention]
However, in a general SAW resonator as shown in FIG. 6, the capacity ratio of each SAW resonator is substantially the same, and the difference is less than 10% even when manufacturing variations are taken into account. The most important factor for determining the capacitance ratio is the piezoelectric substrate, and it is difficult to construct SAW resonators having different capacitance ratios on the same substrate. Other determinants include IDT logarithm N, IDT crossing finger width ratio W / λ, electrode film thickness ratio H / λ, and IDT shape, but changing these parameters will change the capacitance ratio to some extent. However, it was extremely difficult to set freely.
[0007]
For example, when a plurality of SAW resonators are not simultaneously formed on one piezoelectric substrate as shown in FIG. 5 and a ladder-type SAW filter is configured using individual SAW resonators, individual capacitors are used. It is easy to change the apparent capacity ratio by connecting the SAW resonator in parallel to the SAW resonator, and this means has been generally used in the case where an ordinary crystal filter or the like is configured in a ladder type. Also, even if this means is applied to one that forms a plurality of SAW resonators on a single piezoelectric substrate, the SAW filter, which is characterized by miniaturization and mass productivity, has no room to add individual capacitors, even if it is added. Even if it is possible, it is clear that there is a problem in miniaturization.
[0008]
The present invention has been made to solve the above-described problem, and is to realize a SAW resonator in which the capacitance ratio of the SAW resonator can be freely set. By using this, the filtering characteristics of the ladder-type SAW filter can be improved. The object is to improve and provide a good SAW filter.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a surface acoustic wave filter according to a first aspect of the present invention is characterized in that a plurality of surface acoustic wave resonators of a predetermined pattern are arranged on a piezoelectric substrate with a series arm and a parallel arm of a ladder circuit. In each bandpass filter circuit, at least one of the surface acoustic wave resonators is obtained by dividing an electrode finger portion of the IDT into a plurality of portions along a propagation direction, and an interval between the divided electrode finger portions is set. A surface acoustic wave filter having a wavelength of 0.55 to 0.85 times . The surface acoustic wave filter according to claim 1 is characterized in that the piezoelectric substrate is 36 ° ± 3 ° or 41 ° ± 1 ° Y-cut X-propagating lithium tantalate.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.
FIG. 1 is a diagram showing an embodiment of the configuration of a SAW resonator constituting a ladder-type SAW filter according to the present invention. For example, an elastic surface is formed on the main surface of a 36 ° Y-cut X-propagating lithium tantalate substrate. An IDT 2 with a logarithm of N is arranged along the wave propagation direction, the IDT is divided into two electrode fingers 3 and 4 with a distance D, and reflectors 5a and 5b are arranged on both sides of the IDT 2 This is a SAW resonator configured as described above. Here, the electrode finger pitches of the IDT electrode finger portions 3 and 4 are substantially equal, and the distance D between the electrode finger portions 3 and 4 is 0.55 to 0.85 times the wavelength λ of the surface acoustic wave to be used. Further, the interval between the IDT and the reflector is the half wavelength of the excited surface acoustic wave.
[0011]
When the distance D provided between the electrode fingers 3 and 4 of the SAW resonator is changed, the capacitance ratio of the SAW resonator changes accordingly as shown in FIG. Here, the horizontal axis is D / λ obtained by standardizing the interval D with the wavelength λ of the surface acoustic wave excited by the IDT 2, and the vertical axis is the capacitance ratio magnification (the capacitance ratio magnification is the interval D between the IDT electrode fingers). Is the ratio of the capacitance ratio in the conventional electrode finger arrangement and the capacitance ratio when the interval D is formed). As is apparent from this figure, by changing the standardization interval D / λ, that is, the interval D, the capacitance ratio of the SAW resonator can be changed greatly. Capacitance ratio can be realized.
As described above, when the SAW resonator of FIG. 6 is used in a ladder circuit, the difference between the minimum value and the maximum value of the capacitance ratio of the SAW resonator is less than 10% (capacity ratio magnification is 1. It is considered that the characteristic deterioration was caused. Accordingly, the capacitance ratio magnification must be at least 1.1 or more, and the design using the operation parameter method needs to select the capacitance ratio magnification within a range up to about 2. In order to select the capacitance ratio magnification in such a range, it can be seen from FIG. 2 that the interval D should be set to 0.55 to 0.85 times the wavelength.
[0012]
FIG. 3 is a diagram showing a characteristic example of a surface acoustic wave filter to which the present invention is applied. That is, an appropriate capacity ratio is obtained for each SAW resonator of the conventional ladder-type SAW filter shown in FIG. 7, and the IDT of each SAW resonator is divided at a predetermined interval according to the present invention to realize this. It is a figure which shows the characteristic of a ladder type SAW filter. The piezoelectric substrate is a 36 ° Y-cut X-propagation lithium tantalate substrate, the ladder type circuit has 7 SAW resonators, the parallel arms at the input and output ends have IDT logarithm N = 40, and electrode finger crossing width W = SAW resonators having 28λ, IDT logarithm N = 80 and electrode finger crossing width W = 28λ on the other parallel arms, and IDT logarithm N = 80 and electrode finger crossing width W = 10λ on the serial arms, respectively. As shown in FIG. 3, the center frequency is 836.5 MHz, and as is clear from the conventional example shown in FIG. 7, the cutoff characteristic of the pass band is steep, the attenuation gradient is improved, and the stopband attenuation amount is also obtained. It can be understood that the insertion loss is sufficiently ensured and the insertion loss is reduced (to 2 dB or less).
[0013]
FIG. 4 is a diagram of an electrode finger configuration example of a SAW resonator according to another embodiment of the present invention. In this example, a surface acoustic wave wave is formed on the main surface of the 36 ° Y-cut X-propagating lithium tantalate substrate 1. An IDT 6 with N pairs of IDTs is arranged along the propagation direction, only the electrode finger part of the IDT 6 is divided into IDT electrode finger parts 7, 8 and 9, and reflectors 10a and 10b are arranged on both sides of the IDT 6. The SAW resonator is configured as described above. Here, the interval between the IDT electrode finger portions 7 and 8 and the interval between the IDT electrode finger portions 8 and 9 are also D, and the interval D is 0.55 to 0.85 times the wavelength λ excited by the IDT. 1, the capacitance ratio of the SAW resonator can be greatly varied by the same effect as the SAW resonator according to the present invention of FIG.
Although the example in which the IDT is divided into two and three as the configuration of the SAW resonator has been described above, the same effect can be obtained even if a gap is provided in a place other than the illustrated one, and the number of divisions can be further increased. Also good. In addition, an arbitrary capacity ratio can be realized by appropriately selecting and combining the IDT division position, number, and width.
[0014]
Further, although the present invention has been described by taking as an example a mobile phone RF filter having a center frequency of 836.5 MHz using 36 ° Y-cut X-propagation lithium tantalate as a substrate, the present invention is not limited to this, and is suitable. It is clear that a ladder-type SAW filter can be realized with the same configuration by setting the appropriate substrate, electrode material, electrode film thickness, IDT logarithm, cross width, and number of reflectors, and surface acoustic waves can be excited at the center frequency. Any range is acceptable. In addition, by improving the characteristics by applying well-known techniques in SAW resonators such as correction of the IDT electrode finger pitch and reflector electrode finger pitch ratio, correction of the distance between the IDT and reflector electrodes, and weighting of the IDT electrodes. It is also possible to plan.
[0015]
Furthermore, it goes without saying that the SAW resonator according to the present invention can be applied to various filters, for example, a Chebyshev filter, an amplitude flat filter (Butterworth or Wagner characteristic), and a delay flat characteristic filter. As a configuration example of the ladder-type SAW filter, FIG. 5 shows a case where both ends are constituted by parallel arm resonators, but it is possible that a configuration in which both ends end with an arbitrary combination of a series arm resonator and a parallel arm resonator is possible. It is self-explanatory.
[0016]
【The invention's effect】
Since the present invention is configured as described above, the capacitance ratio of the SAW resonator can be set to an arbitrary value by a mask pattern, and the operation parameter method can be adopted as a design method. A ladder-type SAW filter with a good balance of band characteristics can be realized. As a result, compared with the characteristics by the image parameter method, the cutoff characteristic of the pass band is steep, the attenuation gradient is improved, the stop band attenuation is increased, and the insertion loss can be reduced. A remarkable effect is exhibited in improving the characteristics of the type SAW filter.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an IDT configuration of a resonator used as a constituent element of a ladder-type SAW filter in an embodiment according to the present invention.
FIG. 2 is a diagram showing a relationship between a distance between IDT electrode finger portions of a SAW resonator and a capacitance ratio magnification.
FIG. 3 is a diagram showing the filtering characteristics of a ladder-type SAW filter configured using the SAW resonator according to the present invention.
FIG. 4 is a schematic diagram showing a configuration of an IDT electrode finger portion applied to a SAW resonator in another embodiment according to the present invention.
FIG. 5 is a diagram showing a configuration of a ladder-type SAW filter used conventionally.
FIG. 6 is a diagram illustrating a configuration of an IDT and a reflector of a SAW resonator having a conventional configuration.
FIG. 7 is a diagram illustrating a filtering characteristic of a ladder-type SAW filter having a conventional configuration.
[Explanation of symbols]
1 Piezoelectric substrate 2, 6 IDT
3, 4, 7, 8, 9 IDT electrode fingers 5a, 5b, 10a, 10b Reflector D Distance between IDT electrode fingers

Claims (2)

In a bandpass filter circuit in which a plurality of surface acoustic wave resonators having a predetermined pattern are arranged on each of a serial arm and a parallel arm of a ladder circuit on a piezoelectric substrate,
At least one of the surface acoustic wave resonators is obtained by dividing the electrode finger portion of the IDT into a plurality of portions along the propagation direction, and the interval between the divided electrode finger portions is 0.55 to 0.85 times the wavelength. A surface acoustic wave filter characterized by that. 2. The surface acoustic wave filter according to claim 1, wherein the piezoelectric substrate is a 36 ° ± 3 ° or 41 ° ± 1 ° Y-cut X-propagating lithium tantalate.

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