JP3890731B2 - Vertically coupled multimode SAW filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a longitudinally coupled multimode SAW filter, and more particularly to a longitudinally coupled multimode SAW filter in which deterioration of the attenuation gradient occurring on the high frequency side near the passband characteristic of the longitudinally coupled multimode SAW filter is improved.
[0002]
[Prior art]
In recent years, SAW filters have been used in many communication fields, and have played a role in widespread use of mobile phones and the like because of their characteristics such as high performance, small size, and mass productivity.
FIG. 7 is a schematic plan view showing a configuration of an electrode pattern of a conventional primary-third-order longitudinally coupled double mode SAW filter (hereinafter referred to as a dual mode SAW filter), on the main surface of the piezoelectric substrate 11. Further, three IDT electrodes 12, 13, and 14 are arranged close to each other along the propagation direction of the surface wave, and reflectors 15a and 15b are arranged on both sides of the IDT electrodes 12, 13, and 14.
The IDT electrodes 12, 13, and 14 are each composed of a pair of comb-shaped electrodes having a plurality of electrode fingers that are interleaved with each other. One comb-shaped electrode of the IDT electrode 12 is connected to the input terminal IN, and the other comb-shaped electrode. The electrode is grounded. One of the comb electrodes of the IDT electrodes 13 and 14 are connected to each other and connected to the output terminal OUT, and the other comb electrodes are grounded.
[0003]
As is well known, the operation of the dual mode SAW filter shown in FIG. 7 is performed by confining a plurality of surface waves excited by the IDT electrodes 12, 13, and 14 between the reflectors 15a and 15b. As a result, the first and third longitudinal resonance modes are vigorously excited. Therefore, a dual mode SAW filter using these two modes by applying appropriate terminations. Works as. 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.
[0004]
For example, 36 ° Y cut X propagation LiTaO ₃ is used as the piezoelectric substrate, the filter center frequency is 836.5 MHz, the number of electrode pairs of the input IDT electrode 12 is 17.5 pairs, the number of electrode pairs of the output IDT electrodes 13 and 14 is 11.5 pairs, The filtering characteristics of the dual mode SAW filter when the number of grating reflectors 15a and 15b is 250 and the thickness of the electrode is 6% λ (λ is the wavelength of the excited surface wave) using an aluminum alloy as an electrode is as follows. As shown in FIG.
[0005]
[Problems to be solved by the invention]
However, in the above-described conventional dual mode filter, as is clear from the filtering characteristics shown in FIG. 8, the stopband attenuation indicated by the arrow A in the high frequency side near the passband is deteriorated, and the low frequency Attenuation equivalent to the side is not obtained. That is, in filtering characteristics of FIG. 8, when the center frequency of 1dB passband width is F _0, while the attenuation amount is obtained about 20dB at the low frequency side of the F ₀ -45MHz, in the high-frequency side of the F ₀ + 45 MHz is The attenuation is about 8 dB. As described above, in the conventional dual mode filter, the attenuation on the high frequency side in the vicinity of the pass band is about 10 dB as compared with the attenuation on the low frequency side in the vicinity of the pass band. When such a dual mode filter is used for an RF filter such as a cellular phone, there is a problem that the amount of attenuation is insufficient on the high frequency side.
The present invention has been made to solve the above-described problems of the dual-mode SAW filter, and provides a filter that has a wide band and greatly improves the amount of blocking attenuation on the high-frequency side near the pass band. With the goal.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 of the longitudinally coupled multimode SAW filter according to the present invention comprises a plurality of IDT electrodes and reflectors on both sides thereof on the piezoelectric substrate along the propagation direction of the surface wave. In the longitudinally coupled multimode SAW filter configured as described above, a reactance element is inserted and connected between the input IDT electrode and the output IDT electrode, and the polarities of the electrode fingers closest to each other adjacent to each other are made different from each other. Is a longitudinally coupled multimode SAW filter characterized in that 0.1 pF ≦ X <1.0 pF.
According to a second aspect of the present invention, there is provided a longitudinally coupled multimode SAW filter configured by arranging a plurality of IDT electrodes along a propagation direction of a surface wave on a piezoelectric substrate and reflectors on both sides thereof, and an input IDT electrode and an output IDT. A reactance element is inserted and connected between the electrodes, the polarities of closest electrode fingers of adjacent IDT electrodes are the same, and the value of the reactance element is 10 nH ≦ X ≦ 240 nH It is a mode SAW filter.
According to a third aspect of the present invention, the reactance element is a conductive pattern formed on the same substrate as the IDT electrode by the same process as the IDT electrode. It is a combined multimode SAW filter.
The invention according to claim 4 is the longitudinally coupled multi-mode SAW filter according to any one of claims 1 to 3, wherein the reactance element is formed in a package holding the filter element.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.
In the dual mode SAW filter, the attenuation amount on the high frequency side in the vicinity of the pass band is significantly inferior to that on the low frequency side due to the transversal characteristics between a plurality of IDT electrodes arranged on the piezoelectric substrate. However, no means for fundamentally eliminating this characteristic has been found yet.
[0008]
FIG. 1 is a plan view showing the configuration of an electrode pattern of a dual mode SAW filter with improved attenuation on the high frequency side near the passband according to the present invention. As shown in the figure, three IDT electrodes 2, 3, 4 are arranged close to each other along the surface wave propagation direction on the main surface of the piezoelectric substrate 1, and reflectors 5 a are provided on both sides of the IDT electrodes 2, 3, 4. 5b, and the IDT electrodes 2, 3 and 4 are each composed of a pair of comb electrodes having a plurality of electrode fingers interleaved with each other, and one comb electrode of the IDT electrode 2 is connected to the input terminal IN. And the other comb electrode is grounded. On the other hand, one of the comb electrodes of the IDT electrodes 3 and 4 is connected to the output terminal OUT and connected to the output terminal OUT, and the other comb electrode is grounded in the same manner as in the prior art.
[0009]
The characteristic configuration of the present invention is that lead electrodes 6a and 6b extend from a comb electrode connected to the input terminal IN side of the IDT electrode 2 and a comb electrode connected to the output terminal OUT of the IDT electrodes 3 and 4, respectively. The lead electrode is connected to the reactance element 7 formed on the piezoelectric substrate. Here, the reactance element 7 is a conductive pattern composed of the same material as the IDT electrode and in the same process. If the capacitance is required, it may be configured as an IDT type electrode or the like. A well-known planar coil structure such as a shape is appropriately adopted.
[0010]
As is well known, the operation of the dual mode SAW filter shown in FIG. 1 is performed by confining a plurality of surface waves excited by the IDT electrodes 2, 3, 4 between the reflectors 5 a, 5 b. As a result of the acoustic coupling between the first and second longitudinal resonance modes, the first and third longitudinal resonance modes are vigorously excited. Therefore, a dual mode SAW filter using these two modes by applying appropriate terminations. Works as.
[0011]
In an ordinary low-frequency filter such as a monolithic crystal filter, it is known to form an attenuation pole by inserting a capacitor in parallel between input and output terminals. Based on this knowledge, the inventor of the present application can also filter the filtering characteristics of the dual mode SAW filter by inserting the reactance element 7 in parallel between the input and output terminals (IN-OUT) in the dual mode SAW filter having the configuration shown in FIG. Various experiments were conducted based on the assumption that an attenuation pole was formed on the top, and that the attenuation in the stop band higher than the pass band unique to the dual mode SAW filter could be improved by this attenuation pole.
[0012]
For example, 36 ° Y cut X propagation LiTaO ₃ is used for the piezoelectric substrate, the filter center frequency is 900 MHz, the input IDT electrode 2 has 17.5 electrode pairs, and the output IDT electrodes 3 and 4 have 11.5 electrode pairs. The experiment was performed by setting the number of the grating reflectors 5a and 5b to 250, using the aluminum alloy for the electrode and the film thickness of 6% λ, and setting the reactance element 7 inserted between the input and output IDT electrodes to various values. As a result, for example, the filtering characteristics of the dual-mode SAW filter when a capacitance of 0.3 pF is inserted and connected as the reactance element are as shown in FIG.
[0013]
As F ₀ the center frequency of 1dB passband width, the relationship between the value of the reactance element 7 to be inserted between input and output F ₀ + 45 MHz blocking attenuation in the vicinity was as shown in FIG 2. The horizontal axis is the capacitance value (pF), and the vertical axis is the blocking attenuation (dB). As is clear from FIG. 2, the attenuation in the frequency region near F ₀ +45 MHz is greatly improved when the reactance value becomes a capacitance component while maintaining a good passband. However, it was found that particularly good attenuation characteristics can be obtained when the value is 0.1 pF ≦ C <1.0 pF.
In addition, as a result of performing the same examination at the frequencies of 300 MHz band and 1.5 GHz band, it was confirmed that the value of the capacitance C was within the above range regardless of the frequency band.
[0014]
As is apparent from FIG. 3, it can be seen that the attenuation and steepness on the high frequency side in the vicinity of the pass band are greatly improved as compared with the conventional dual mode SAW filter. Thus, a filter with improved attenuation and steepness on the high frequency side in the vicinity of the pass band can satisfy the requirements of an RF filter such as a mobile phone that requires high attenuation in the vicinity of the pass band.
[0015]
Next, FIG. 4 shows another embodiment of the present invention, and parts having the same functions as those in FIG. The configuration of the IDT electrodes 8 and 9 on both sides of the input IDT electrode 2 is changed by 180 degrees from the polarity of the IDT electrodes 3 and 4 shown in FIG.
With respect to the dual mode SAW filter having the electrode pattern shown in FIG. 4, it was experimentally examined whether or not an attenuation pole is formed on the filtering characteristics of the dual mode SAW filter when the reactance element 7 has a capacitance or inductance.
As a result, in this case, it was found that if the reactance element 7 is an inductance, an attenuation pole is formed on the characteristics of the filter.
[0016]
For example, 36 ° Y cut X propagation LiTaO ₃ is used for the piezoelectric substrate, the filter center frequency is 836 MHz, the input IDT electrode 2 has 18.5 electrode pairs, the output IDT electrodes 3 and 4 have 11.5 electrode pairs, and a grating reflector. Double mode when the number of 5a, 5b is 250, the electrode is made of aluminum alloy, the film thickness is 6% λ, and the inductance is inserted between the input / output IDT electrodes, for example, the value of the reactance element is 140 nH The filtering characteristics of the SAW filter are as shown in FIG.
[0017]
FIG. 5 shows an experimental relationship between the value of the reactance element 7 of the dual mode SAW filter and the blocking attenuation amount of the filter. In the figure, the horizontal axis represents inductance (nH), and the vertical axis represents the attenuation at F ₀ +45 MHz.
While keeping the pass band of the double mode SAW filters in good condition, the inductance L of blocking attenuation in the range becomes large with respect to the center frequency F ₀ from F ₀ -45MHz to F ₀ + 45 MHz is, 10 nH ≦ L It was found that ≦ 240 nH.
[0018]
As can be seen from FIG. 6, the attenuation and steepness on the high frequency side in the vicinity of the pass band are greatly improved as compared with the conventional dual mode SAW filter. Thus, if the RF filter has a large attenuation amount and attenuation gradient on the high frequency side in the vicinity of the pass band, the required standard can be satisfied as an RF filter for a mobile phone or the like.
[0019]
Further, the reactance element 7 formed between the input IDT electrode 2 and the output IDT electrodes 3 and 4 may be formed not in the same piezoelectric substrate on which the dual mode SAW filter is formed but in the package holding the filter. It is clear that a similar effect can be obtained.
[0020]
In the above description, a primary-third-order longitudinally coupled double mode SAW filter constituted by three IDT electrodes has been described as an example. However, the present invention is not limited to this, and a plurality of IDT electrodes are provided. Needless to say, the present invention can be applied to a longitudinally coupled multimode SAW filter configured by using the same. However, as described above, depending on the polarity generated at the input / output terminal, the reactance inserted into the input / output terminal is determined to be a capacitance or an inductance.
[0021]
In the above description, 36 ° Y-cut X-propagating LiTaO ₃ has been described as an example of the piezoelectric substrate, but it goes without saying that it can be applied to other cutting angles such as 42 ° Y-cut X-propagating LiTaO ₃ . Also, the piezoelectric substrate is not limited to lithium tantalate, but any of 64 ° Y-cut X-propagating LiNbO ₃ , 45 ° X-cut Z-propagating lithium tetraborate (Li ₂ B ₄ O ₇ ), ST-cut quartz, langasite, etc. Needless to say, the present invention can also be applied to piezoelectric materials.
Further, it is possible to further increase the attenuation amount by cascading the first-order to third-order longitudinally coupled double mode SAW filters.
[0022]
【The invention's effect】
Since the present invention is configured as described above, in the longitudinally coupled multimode SAW filter, it becomes possible to greatly improve the amount of stop attenuation and the attenuation gradient on the high frequency side near the passband. If it is used as an RF filter for a wireless communication device, a remarkable effect is exhibited in improving the characteristics of the wireless device.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an electrode pattern of a longitudinally coupled double mode SAW filter according to the present invention.
FIG. 2 is a diagram showing a relationship between a reactance value inserted between input and output electrodes and a blocking attenuation amount on a high frequency side near a pass band.
FIG. 3 is a filtering characteristic diagram of a longitudinally coupled double mode SAW filter experimentally manufactured using the electrode configuration of the present invention.
FIG. 4 is a diagram showing an electrode configuration of another longitudinally coupled double mode SAW filter according to the present invention.
FIG. 5 is a diagram showing the relationship between the value of reactance inserted between input and output electrodes and the amount of blocking attenuation on the high frequency side near the passband.
6 is a filtering characteristic of a longitudinally coupled double mode SAW filter configured using the electrode pattern shown in FIG.
FIG. 7 is a diagram illustrating an electrode configuration of a conventional primary-cubic longitudinally coupled double mode SAW filter.
FIG. 8 is an example of filtering characteristics of a conventional primary-third longitudinally coupled double mode SAW filter.
[Explanation of symbols]
1 .. Piezoelectric substrates 2, 3, 4.. IDT electrodes 5 a and 5 b... (Grating) reflectors 6 a and 6 b... Lead electrode 7.

Claims (4)

In a longitudinally coupled multimode SAW filter configured by arranging a plurality of IDT electrodes and reflectors on both sides thereof along the propagation direction of surface waves on a piezoelectric substrate,
A reactance element is inserted and connected between the input IDT electrode and the output IDT electrode,
A longitudinally coupled multimode SAW filter characterized in that the nearest electrode fingers of adjacent IDT electrodes have different polarities, and the value of the reactance element is 0.1 pF ≦ X <1.0 pF. In a longitudinally coupled multimode SAW filter configured by arranging a plurality of IDT electrodes and reflectors on both sides thereof along the propagation direction of surface waves on a piezoelectric substrate,
A reactance element is inserted and connected between the input IDT electrode and the output IDT electrode,
A longitudinally coupled multimode SAW filter characterized in that the closest electrode fingers of adjacent IDT electrodes have the same polarity, and the value of the reactance element is 10 nH ≦ X ≦ 240 nH.   3. The longitudinally coupled multimode SAW filter according to claim 1, wherein the reactance element is a conductive pattern formed on the same substrate as the IDT electrode by the same process as the IDT electrode.   The longitudinally coupled multimode SAW filter according to any one of claims 1 to 3, wherein the reactance element is formed in a package holding the filter element.

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