JP3638434B2 - Surface acoustic wave device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave device such as a surface acoustic wave filter used for mobile communication equipment such as a mobile phone, and more particularly, a plurality of surface acoustic wave resonators are arranged in a ladder shape. The present invention relates to a balanced input-balanced output type surface acoustic wave device.
[0002]
[Prior art and its problems]
The SAW filter for mobile communication is desirably low loss in order to reduce the size and power consumption of the mobile phone terminal. In addition, SAW filters used for reception and transmission each have a suppression band outside the passband and are strongly required to have a high attenuation over a wide range from the vicinity of the end of the passband. Depending on the specifications of the body communication system, a considerably large attenuation amount may be required only in a specific frequency band locally.
[0003]
Conventionally, since a general SAW filter is an unbalanced input-unbalanced output type, a circuit configuration has been adopted in which an unbalanced-balanced converter such as a balun circuit is inserted between the latter stage of the SAW filter and an electronic circuit. On the other hand, when the electronic circuit etc. of the front stage of the SAW filter is a balanced output type, the circuit configuration is such that a balanced-unbalanced converter is inserted between the electronic circuit of the previous stage and the SAW filter.
[0004]
In recent years, in order to remove the unbalanced-balanced converter or balanced-unbalanced converter from the above circuit configuration, the SAW filter itself has the function of a balanced-unbalanced converter or unbalanced-balanced converter. A so-called unbalanced-balanced output type SAW filter or balanced-unbalanced output type SAW filter (hereinafter referred to as a balanced type or balanced type SAW filter) has been put into practical use.
[0005]
Conventionally, the electrode configuration of the SAW filter is roughly classified into four types: a propagation type, a resonator type, a ladder type (ladder type), and a lattice type (lattice type). Each of these is a design method with its own characteristics, and there are advantages and disadvantages in terms of performance. However, among them, the design freedom is high, and the ladder type design is particularly useful.
[0006]
FIG. 1 shows a conventional ladder type circuit configuration diagram. Here, in the SAW filter J, a plurality of surface acoustic wave resonators are connected in series and parallel on a piezoelectric substrate, a series resonator 4a is connected to the series arm, and a parallel resonator 4b is connected to the parallel arm. The resonator 4 is grounded to the ground 3.
[0007]
The feature of this filter is that it is easy to realize a steep shoulder characteristic as compared with other structures. However, the disadvantage of the ladder type cannot be applied to balanced input / output among the above four types (unbalanced input-unbalanced output type, so balanced input / output cannot be realized). Normally, when incorporated in a mobile phone set, as shown in FIG. 6, the balun element 9 is inserted and used after (before) the SAW filter 10 in order to perform a balancing operation (balanced input / output). For this reason, there was a problem that the number of parts increased. The purpose of the balance operation is that it is not directly in contact with the ground, and therefore has a merit that noise components via the ground are difficult to ride.
[0008]
In addition, in order to reduce the size, weight, and cost of mobile phone terminals, it is an urgent task to reduce the number of parts used. For this reason, it is required to add new functions to the SAW filter. For example, for filters used for RF (Radio Frequency) stages and IF (Intermediate Frequency) stages, for example, unbalanced input-balanced output type or balanced input-unbalanced output There is a demand to be able to be configured as a type or a balanced input-balanced output type.
[0009]
Therefore, an object of the present invention is to provide an electrode configuration of a ladder-type SAW filter that can cope with a balanced input / output type (balanced type) without changing characteristics and has excellent power durability. Thereby, the balun elements can be reduced, and the mobile phone can be reduced in size, weight, and cost.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a surface acoustic wave device according to the present invention includes two ladder-type circuits in which a plurality of surface acoustic wave resonators are connected in series and parallel on a piezoelectric substrate, and each ladder-type circuit includes: A parallel resonator connected to the parallel arm is connected to the ground, and an output difference between the two ladder circuits is taken out.
[0011]
In particular, the series resonator connected to the serial arm of the ladder circuit and the parallel resonator connected to the parallel arm are arranged so as to satisfy the following formula.
[0012]
Cp / Cs <1.55
(Where Cp is the sum of the capacities of the parallel resonators, Cs is the sum of the capacities of the series resonators)
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the drawings.
[0014]
As shown in FIG. 2, the surface acoustic wave device S of the present invention is formed on a piezoelectric substrate made of a lithium niobate single crystal, a lithium tantalate single crystal, a lithium tetraborate single crystal, a langasite type single crystal, etc. In addition, two ladder type circuits in which a plurality of surface acoustic wave resonators are connected in series and parallel are arranged. And the parallel resonator 4a connected to the parallel arm of each ladder type circuit is connected to the ground terminal 3, and the output difference of two ladder type circuits is taken out. Furthermore, the ratio of Cp (total capacitance of parallel resonator 4a) and Cs (total capacitance of series resonator 4b), that is, Cp / Cs is set to less than 1.55. In the figure, 5 is an input terminal and 7 is an output terminal in the upper ladder circuit. In the lower ladder circuit, 6 is an input terminal and 8 is an output terminal. Here, the ground terminal 3 is common to the upper SAW filter and the lower SAW filter, and two circuits are symmetrically arranged with respect to the ground terminal 3.
[0015]
According to the above circuit configuration, when a balanced input signal is applied between terminals 5-6 indicated by "IN" in the figure, a balanced output signal that has passed through the filter is output between terminals 7-8 indicated by "OUT" in the figure. The Here, since the applied input signal is floating from the ground terminal, the phase difference between the terminals 5 and 6 is 180 °. For this reason, since the configurations of the upper SAW filter and the lower SAW filter are exactly the same, the phase difference between the terminals 7 and 8 on the output side is 180 ° because the phase difference at the time of input is preserved, and a complete balanced output is obtained. It is done.
[0016]
5A to 5D show amplitude characteristics and phase characteristics of the terminal 7-3 and the terminal 8-3. As is apparent from the figure, the same amplitude characteristics appear at terminals 7 and 8 as the amplitude characteristics. This is because, as described above, the constituent elements of the ladder-type SAW filter are exactly the same in the upper filter and the lower filter. Further, the phase characteristic is also completely inverted by 180 ° within the pass band. This is because the phase difference at the time of input is completely preserved because the upper filter and the lower filter are exactly the same.
[0017]
In addition, since the ladder-type SAW filter having the structure of the present invention is configured in two circuits in parallel, the power is divided into two equal parts, and the power durability (life) of the entire SAW filter is doubled.
[0018]
It has also been found that the electrical characteristics of the ladder filter of the present invention are related to the capacitance ratio of the series resonator 4a and the parallel resonator 4b that constitute the ladder circuit. FIG. 7 illustrates the relationship between the capacitance ratio (Cp / Cs) of the resonator and the standing wave ratio (VSWR). From this figure, the impedance matching condition in the passband of the filter differs depending on the capacitance ratio of the resonator, and the capacitance ratio is less than 1.55 that satisfies the general matching condition VSWR <2. found. Further, as shown in FIG. 7, it has been found that a stable insertion loss can be obtained in the capacity ratio range from the relationship between the capacity ratio and the maximum insertion loss in a desired pass bandwidth.
[0019]
In the present invention, the substrate material is 42 ° Y-cut LiTaO 3, but it may be 36 ° to 42 ° Y-cut LiTaO 3, 64 ° Y-cut LiNbO 3, etc. .
[0020]
【Example】
Next, an example in which a ladder type SAW filter according to the present invention is specifically manufactured will be described.
[0021]
A fine electrode pattern (standardized electrode film thickness: 9.6%) containing Al as a main component was formed on a 42 ° Y-cut substrate of lithium tantalate single crystal. For pattern production, photolithography was performed with a contact exposure machine using Deep-UV light.
[0022]
First, the substrate material was washed with acetone, IPA or the like while applying ultrasonic waves to remove organic components. Next, after sufficiently drying the substrate with a clean oven, a photoresist was spin-coated on the substrate by a spin coater with a thickness of about 1 μm.
[0023]
The photomask had a thickness of 0.09 inch with quartz glass as the main component and an outer size of 5 inches. For electrode film formation, an electron beam evaporation machine was used, and an Al or Al-Cu (2%) material was formed by evaporation. The electrode film thickness was about 4200 mm.
[0024]
Next, the unnecessary Al pattern was lifted off by applying ultrasonic waves to the substrate in a resist stripping solution. In order to lift off a material such as Al by lift-off, it is necessary to set conditions for making the cross-sectional shape of the resist reversely tapered (or a device such as a two-layer resist is necessary).
[0025]
Then, the protective film process was performed. Si was adopted as a protective film material. Si was deposited by an electron beam deposition machine, and this was also patterned by lift-off by making the resist reversely tapered. Note that the Si film thickness is 250 mm and short-circuiting due to metal powder can be prevented. Next, the substrate was diced along a scribe line and divided into chips.
[0026]
Each chip was put on a die bonder, picked up, and bonded to the cavity in the SMD package with a die bond resin containing Si resin as a main component. Thereafter, it was heated, dried and cured. The SMD package had a 3 mm square laminated structure, and the chip size was 1.6 × 1.0 mm.
[0027]
Next, a 30 μφ Au wire was ball bonded onto the pad portion of the SMD package and the Al pad on the chip, and then the package lid was placed on the package and sealed with a seam sealer to complete. The ground on the chip was wired separately and bonded to the ground pad on the package by Au wire bonding. Since this electrode is wired on the same chip, it is housed in the same package.
[0028]
The SAW resonator constituting the ladder-type SAW filter has an IDT logarithm of 40 to 120 pairs, an intersection width of 10 to 30λ, and the wavelength of the surface acoustic wave is different between series and parallel, but is approximately 4.4 μm. . The number of reflective electrodes was 20 on the series resonator side and 20 on the parallel resonator side.
[0029]
For comparison, a filter having a conventional configuration shown in FIG. 1 was also produced.
[0030]
What is important here is that in the present invention, the electrode design of the ladder type filter composed of two circuits in parallel is made the same. As a result, responses with the same output waveform amplitude and a phase difference of 180 ° were obtained.
[0031]
For the characteristic measurement, a network analyzer was used, and the frequency characteristic results shown in FIG. 3 (present invention) and FIG. 4 (conventional configuration) were obtained. As is clear from these figures, the change in the frequency characteristic was not so different from the conventional characteristic in the passband.
[0032]
【The invention's effect】
As described above, if manufactured by the electrode configuration of the present invention, it is possible to provide a ladder-type SAW filter that is compatible with a balanced input / output type (balanced type) and has excellent power durability without changing characteristics. Furthermore, the balun elements that have been necessary in the past can be reduced, and the mobile phone can be reduced in size, weight, and cost.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram of a conventional ladder-type surface acoustic wave filter.
FIG. 2 is a circuit configuration diagram of a ladder-type surface acoustic wave filter compatible with balanced input / output according to the present invention.
FIG. 3 is a frequency characteristic diagram of a surface acoustic wave filter according to the present invention.
FIG. 4 is a frequency characteristic diagram of a conventional surface acoustic wave filter.
FIGS. 5A to 5D are diagrams for explaining the confirmation results of the balancing operation of the balanced input / output compatible surface acoustic wave filter of the present invention. FIG.
FIG. 6 is a diagram showing a configuration for obtaining balanced input / output by inserting balun elements before and after a conventional unbalanced input / output ladder type surface acoustic wave filter.
FIG. 7 is a diagram showing the relationship between VSWR and resonator capacitance ratio of the ladder filter of the present invention.
FIG. 8 is a diagram showing the relationship between the maximum in-band insertion loss and the resonator capacitance ratio of the ladder filter of the present invention.
[Explanation of symbols]
1: input terminal 2: output terminal 3: ground terminal 4a: series resonator 4b: parallel resonator 5, 6: input terminal (balanced)
7, 8: Output terminal (balanced type)
9: Balun element 10: Conventional ladder type SAW filter

Claims (2)

Two ladder-type circuits in which a plurality of surface acoustic wave resonators are connected in series and parallel are arranged on a piezoelectric substrate, and parallel resonators connected to parallel arms of each ladder-type circuit are connected to the ground. A surface acoustic wave device designed to extract the output difference between two ladder-type circuits. The elastic surface according to claim 1, wherein a series resonator connected to a series arm of the ladder circuit and a parallel resonator connected to the parallel arm are arranged so as to satisfy the following formula. Wave equipment.
Cp / Cs <1.55
(Where Cp is the sum of the capacities of the parallel resonators, Cs is the sum of the capacities of the series resonators)

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