JPH08204502A - Longitudinal composite quadruple mode saw filter - Google Patents

Abstract

PURPOSE: To reduce an input output impedance and to decrease the number of matching inductors by adopting the configuration of the filter such that two longitudinal triple mode SAW filters each having three (primary, secondary and ternary) vibration modes are connected in parallel. CONSTITUTION: IDT electrodes 11, 12 each in nT1 pairs are arranged on a major surface of an ST-cut crystal substrate 1 along the propagation direction of a surface acoustic wave, and a center IDT electrode 13 in nC1 pairs is arranged between the electrodes 11, 12, and reflectors 14, 15 having m1 -sets of gratings are arranged aside the electrodes 11, 12 respectively to form a longitudinal triple mode SAW filter 101. IDT electrodes 21, 22 each in nT2 pairs are arranged on the major surface of the ST-cut crystal substrate 1 along the propagation direction of a surface acoustic wave, and a center IDT electrode 23 in nC2 pairs is arranged between the electrodes 21, 22, and reflectors 24, 25 having m2 -sets of gratings are arranged aside the electrodes 21, 22 respectively to form a longitudinal triple mode SAW filter 201. Then a longitudinal composite quadruple mode SAW filter is formed by connecting the electrodes 11, 21 and connecting the electrodes 12, 22 respectively in parallel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vertical composite quadruple-mode SAW utilizing the waves of SAW excited by an IDT electrode.
Regarding filters.

[0002]

2. Description of the Related Art SAW filters suitable for downsizing have been used in portable communication equipment and the like, and a center frequency f ₀ as an IF filter has been increased with the recent increase in carrier frequency and digitalization. Several hundred MHz, pass band width Δf is several hundred kHz (ratio band Δf / f ₀ is 0.1%
The demand for what is called "degree" is increasing.

For example, f ₀ = 240 MHz, Δf = 30
To meet the requirement of ₀ kHz (ratio Δf / f ₀ = 0.125%), ST-cut quartz should be used as the substrate material because the frequency shift due to temperature is small, but the known transversal SAW The filter has a drawback that the number of electrode pairs must be greatly increased to obtain a desired bandwidth, resulting in a large element shape, which is disadvantageous for downsizing. In addition, a direction orthogonal to the SAW propagation direction is also required. To 2
It was difficult to realize a filter having a narrow pass band width and a desired pass band width in the lateral double mode SAW filter in which two IDT electrodes are arranged.

Therefore, as shown in FIG. 6, input / output interdigital transducer (IDT) electrodes 31 and 32 are arranged on the piezoelectric substrate 1 along the SAW propagation direction.
A central IDT electrode 33 and reflectors 34 and 35 on both sides of the central IDT electrode 33 are arranged between the T electrodes, and a pass band is obtained by three vibration modes of primary, secondary and tertiary confined in the three IDT electrodes. Vertical triple mode SA configured as
A W filter 3 has been devised (Japanese Patent Application No. 4-352687).

However, in order to use this filter alone in a situation where a larger amount of attenuation in the stop band is required, it is necessary to connect two filters of the above construction in series as shown in FIG. When the filter having the above-mentioned specifications is configured by using the quartz substrate, the input / output impedance of each vertical triple mode SAW filter becomes capacitive, and therefore, as shown in FIG. A matching inductor is required at each of the dynamic connection points. In general, inductors are easily affected by electrical and mechanical disturbances and also lead to cost increase, so in mobile communication equipment, use should be limited as much as possible. Its use is a major drawback. Further, the above-mentioned f ₀ = 240 MHz, Δf = 300
It has been found that the required filter of kHz has a drawback that the input / output impedance increases to about 1.6 kΩ.

That is, although a filter having a desired pass band width can be realized, there is a problem in that when it is mounted on a communication device, the number of parts and the adjustment man-hour increase, which leads to an increase in cost.

[0007]

SUMMARY OF THE INVENTION The present invention is directed to the vertical triple mode S as described above.
The vertical composite quadruple-mode SAW was made in order to eliminate the drawbacks of the AW filter, and aims to reduce the input / output impedance and the number of matching inductors.
Intended to provide a filter.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, a vertical composite quadruple mode SAW filter according to the present invention comprises an input / output interdigital transducer (IDT) electrode on a piezoelectric substrate along a SAW propagation direction. A third IDT (central IDT) electrode and reflectors on both sides thereof are arranged between the output IDT electrodes, and have three vibration modes of primary, secondary and tertiary confined in the three IDT electrodes. Vertical type 3
Two multi-mode SAW filters are connected in parallel. In this configuration, the ratio L _T / L _G of the electrode finger pitch L _T of the input / output IDT electrode and the electrode finger pitch L _G of the central IDT electrode
Is set to be larger than 1, the first-order mode of the first triple-mode filter, the second-order mode of the second triple-mode filter, and the second-order mode and the second triple-mode of the first triple-mode filter. The frequency and phase of the third-order mode of the filter are made to coincide with each other so that a pass band is obtained by four vibration modes.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to the drawings showing the embodiments and the experimental results. The conventional vertical triple mode SAW filter has the above-mentioned drawbacks, and as a means for improving this kind of drawback, for example, two vertical double mode SAW filters are connected in parallel to form a vertical composite triple mode SAW filter. A filter has been proposed and realized. As shown in FIG. 8B, this idea is applied to a vertical triple mode SAW filter to apply two vertical triple mode SAs.
If W type filters are connected in parallel to form a vertical composite quadruple mode SAW filter, the number of inductors can be reduced to two. A more specific structure is shown in FIG. As shown in the figure, two input / output IDT electrodes 11 and 12 are arranged between the reflectors 14 and 15 on the same substrate, and the input / output IDT electrodes are arranged.
The vertical triple mode SAW filter 106 in which the central IDT electrode 17 is arranged between the electrodes and two input / output IDT electrodes 21 and 22 are arranged between the reflectors 24 and 25.
A vertical triple mode SAW filter 107 in which a central IDT electrode 27 is arranged between the electrodes is connected in parallel. Here, the electrode finger pitches of the IDT electrodes 11, 12, 17 are set to be equal to each other, and the IDT electrodes 21, 22, 2,
The electrode finger pitches of 7 are also set to be equal to each other. FIG. 4B is a graph showing the attenuation characteristic when the above filter is measured under the condition without an impedance matching circuit to observe the resonance state of the vibration mode in order to show the frequency arrangement of the filter. The characteristics of the filter 106 are shown on the upper side, and the characteristics of the filter 107 are shown on the lower side. Generally, in a vertical triple mode SAW filter, since the frequency difference between the first-order mode and the second-order mode and the frequency difference between the second-order mode and the third-order mode are not equal, for example, as shown in FIG.
It is clear that when the frequency of the first-order mode of (1) and the frequency of the second-order mode of the filter 107 are made to coincide with each other, a difference occurs between the frequency of the second-order mode of the filter 106 and the frequency of the third-order mode of the filter 107. FIG. 4C shows the overall filter characteristic of this filter. Due to the above-mentioned cause, a defect that a dip occurs near the frequency of the secondary mode of the filter 106 in the pass band occurs.

Therefore, the present invention realizes a vertical composite quadruple mode SAW filter in which vertical triple mode SAW filters which eliminate dips in the pass band are connected in parallel, and FIG. 1 shows an embodiment thereof. It is a figure which shows a structure. That is,
On the main surface of the ST-cut quartz crystal substrate 1, input / output IDT electrodes 11 and ₁ each having a logarithm of n _T1 pairs along the SAW propagation direction.
2 and the central IDT electrode 13 having a pair of n _G1 pairs between the input and output IDT electrodes 11 and 12, and reflectors 14 and 15 each having m ₁ gratings on both sides of these three IDT electrodes. The vertical triple mode SAW filter 101 and the input / output IDT electrodes 21 and 22 each having n _T2 pairs are arranged, and the input / output IDT electrode 2 is arranged.
A vertical triple mode SAW filter 102 in which central IDT electrodes 23 having n _G2 pairs of logarithms and reflectors 24 and 25 each having m ₂ gratings are arranged on both sides of these three IDT electrodes between 1 and 22 are configured. The IDT electrodes 11 and 2
1 is a vertical composite quadruple mode SAW filter in which 1 and IDT electrodes 12 and 22 are connected in parallel. Where ID
The electrode finger pitch of the T electrodes 11 and 12 is L _T1 , the electrode finger pitch of the central IDT electrode 13 is L _G1 , the electrode finger pitch of the IDT electrodes 21 and 22 is L _T2 , and the electrode finger pitch of the central IDT electrode 23 is L _G2 . In that case, L _T1 / L _G1 and L _T2 / L _G2 were set to be slightly larger than 1. Further, in order to improve the reflection efficiency and realize a low-loss filter, when the electrode finger pitch of the reflectors 14 and 15 is L _R1 and the electrode finger pitch of the reflectors 24 and 25 is L _R2 , L _T1 / L _R1 and L _T2 / L _R2 was set to be slightly smaller than 1. In addition, the primary mode of the filter 101 and the filter 1
L _T1 / L _T2 is set so that the frequency of the secondary mode of 02 and the frequency of the secondary mode of the filter 101 and the frequency of the tertiary mode of the filter 102 match. The distance between the IDT electrodes was an integer multiple of the IDT electrode finger pitch, and the distance between the IDT electrodes and the reflector was the half wavelength of the excited SAW.

By constructing as described above, the vertical type 3
It becomes possible to construct a vertical composite quadruple mode SAW filter in which multiple mode SAW filters are connected in parallel. For example, the IDT electrode finger pitch is about 13 μm (240 MH).
z), n _T1 = n _T2 = 170 pairs, n _G1 = n _G2 = 60 pairs, m
₁ = m ₂ = 100 _{present, L T1 / L G1 = L} T2 / L G2 = 1.0
006, L _T1 / L _R1 = L _T2 / L _R2 = 0.993, L _T1
If / L _T2 = 1.00067, then as shown in FIG.
The maximum ripple amount in the pass band can be limited to 0.2 dB or less. Further, the input / output impedance is 400Ω, which is 1/4 of that of the conventional vertical triple mode SAW filter, and the number of matching inductors can be reduced from 3 to 2 as shown in FIG. 8B. It was

FIG. 2B is a diagram showing the attenuation characteristic when the above filter is measured under the condition without an impedance matching circuit in order to observe the state of resonance of the vibration mode. Since four resonances having almost the same resonance level appear at the position corresponding to the pass band of a), it can be easily understood that the present invention is a quadruple-mode SAW filter using all of the four vibration modes. See.

FIG. 5 shows an example of an experiment in which the resonant frequencies of the three vibration modes of the vertical triple mode SAW filter 101 are changed when L _T1 / L _G1 is changed. According to the figure, the frequencies of the first, second and third modes are f
₁ , f ₂ and f ₃ When L _T1 / L _G1 = 1 f ₁ −f ₂
<F ₂ -f ₃ is which was the the L _T1 / L _G1 ≒ 1.0006 at _{f 1 -f 2 ≒ f 2 -f} 3 , and the further L _T1 / L when _G1 a larger f ₁ -f _2> f a ₂ -f _3. As a result of studying various conditions, the result is 1.0002 ≦ L _T1 / L _G1 ≦ 1.015.
It is desirable to be within the range. Although the filter 101 has been described, a similar relationship can be obtained for the filter 102 as well, and it is desirable that the range is within the range of 1.0002 ≦ L _T2 / L _G2 ≦ 1.0015.

In the above, the present invention uses the ST-cut crystal as the substrate, the center frequency is 240 MHz, and the pass band width is 300 kHz.
Although an IF filter that satisfies the specifications of is described as an example,
The present invention is not limited to this, and other piezoelectric materials such as LiNbO ₃ or L that can excite the SAW are used.
Even with iTaO ₃ , Li ₂ B ₄ O _7, etc., it is obvious that a vertical composite quadruple mode SAW filter can be realized with the same configuration by selecting an appropriate IDT logarithm and L _T / L _G. It goes without saying that the center frequency may be arbitrarily selected within the range in which the SAW can be excited.

In the above example, the central IDT electrode is arranged between the input / output IDT electrodes, but as shown in FIG. 3, the central IDT electrode is replaced by 2n _G gratings 16 and 2.
It is confirmed that the same filter characteristics can be obtained although data may be omitted because the arrangement may be such that 6 is arranged and it becomes complicated.

Further, as an example, the pitch of the reflectors is I
The IDT electrode pitch is set to be slightly larger than that of the DT electrode, and the interval between the IDT electrodes is set to an integral multiple of the IDT electrode finger pitch.
Although the basic configuration has been shown in which the distance between the electrode and the reflector is set to half the wavelength of the SAW that excites, the present invention is not limited to this, and the parameter can be passed if the parameters relating to these electrode dimensions are changed appropriately. It is also possible to improve the characteristics by applying a well-known technique to the vertical double mode SAW filter in which the characteristics such as bandwidth, spurious level or insertion loss can be changed. Also, the IDT electrodes 11 and 12
And 21, 22 electrode pairs, reflectors 14, 15 and 2
Although the case where the number of reflectors of 4 and 25 is equal is shown,
It is not limited to this. L _T1 / L _G1 = L _T2 /
The conditions for L _G2 and L _T1 / L _R1 = L _T2 / L _R2 are not limited to these.

[0017]

Since the present invention is configured as described above, a filter having a passband width equivalent to that of a conventional vertical triple mode SAW filter, which reduces the input / output impedance and the number of matching inductors. Demonstrate a remarkable effect in realizing.

[0018]

[Brief description of drawings]

FIG. 1A is a vertical composite quadruple mode S according to the present invention.
The figure which shows the structure of one Example of an AW filter. (B) is a figure which shows the frequency arrangement | positioning of a filter.

2A and 2B are diagrams showing the filter characteristics of FIG.

FIG. 3 is a diagram showing the configuration of another embodiment.

FIG. 4A is a diagram showing a configuration of a vertical quadruple mode SAW filter using a conventional vertical triplet mode SAW filter. FIG. 6B is a diagram showing a frequency arrangement of filters. (C)
FIG. 4 is a diagram showing a filter characteristic.

FIG. 5 is a diagram showing a change in each vibration mode frequency according to the central IDT pitch of the vertical triple mode SAW filter.

FIG. 6 is a diagram showing a basic configuration of a vertical triple mode SAW filter.

FIG. 7 is a diagram showing a configuration of a conventional vertical triple mode SAW filter.

FIG. 8A is a diagram showing an arrangement of matching inductors in a vertical triple mode SAW filter having a conventional configuration. (B) is a figure which shows arrangement | positioning of the matching inductor of a vertical quadruple mode SAW filter.

[Explanation of symbols]

1 ... ST cut crystal substrate 11, 12, 21, 22, 31, 32 ... Input / output ID
T electrode 13, 23, 17, 27, 33 ... Central IDT electrode 14, 15, 24, 25, 34, 35 ... Reflector 16, 26 ... Central grating 3, 101, 102, 103, 104 , 106, 107
... Vertical triple mode SAW filter 2 ... Vertical quad mode SAW filter

Claims (4)

[Claims] 1. An input / output interdigital transducer (IDT) electrode along a SAW propagation direction on a piezoelectric substrate, and a third IDT (central ID) between the input / output IDT electrodes.
T) electrodes and reflectors on both sides thereof are arranged in parallel, and two vertical triple mode SAW filters having three vibration modes of primary, secondary and tertiary confined in the three IDT electrodes are arranged in parallel. Vertical composite quadruple-mode SAW filter connected to. 2. The vertical composite quadruple mode SAW filter according to claim 1, wherein a grating (central grating) is arranged in place of the central IDT electrode. 3. In the above configuration, the first-order mode of the first vertical triple mode SAW filter and the second vertical triple mode S of the SAW filter.
Second mode of AW filter and first vertical triple mode S
Second-order mode of AW filter and second vertical triple mode SA
The vertical composite quadruple-mode SAW filter according to claim 1 or 2, wherein the frequencies and phases of the third-order modes of the W filter are matched. 4. The piezoelectric substrate is an ST cut quartz crystal,
When the electrode finger pitch of the central IDT electrode or the central grating is L _G and the electrode finger pitch of the input / output IDT electrode is L _T , L _T / L _G is larger than 1. The vertical composite quadruple mode SAW filter according to claim 3.