JPH04354412A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To provide the excellent performance of a SAW filter single body on one and same chip by forming an inductor on a piezoelectric substrate to cancel a damping capacitor in existence to an input and an output terminal of the SAW filter. CONSTITUTION:Inductors 41, 42 for cancelling a damping capacitance cancelling made of a strip line utilizing a dielectric constant of a piezoelectric substrate 21 are formed on the piezoelectric substrate 21 on which an input side interdigital electrode 31 and an output side interdigital electrode are formed. The inductor 41 (42) is connected respectively in parallel with input terminals 22-1, 22-2 (output terminals 23-1, 23-2).

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to surface acoustic wave filters used in wireless communication devices, etc.
This relates to a stic wave filter (hereinafter referred to as a SAW filter).

0002

[Prior Art] Conventionally, as microwave band filters used in wireless communication devices, dielectric filters and SA
A W filter or the like is used. With the miniaturization of wireless communication devices and the like, devices such as filters used in these devices are increasingly being made into chips. In particular, SAW filters are small and stable against changes in temperature and aging, and their amplitude and phase characteristics can be designed almost independently. It is thought that it will be used more and more in the future as a filter to meet the demands of An example of its configuration is shown in FIG.

FIG. 2 is a basic structural diagram of a conventional SAW filter.

[0004] This SAW filter has a piezoelectric substrate 1, and on the piezoelectric substrate 1 there is an input side that converts electrical signals inputted from input terminals 2-1 and 2-2 into surface acoustic waves (SAW). A comb-shaped electrode (interdigital transducer) 11 and an output terminal 3-1 that converts the SAW into an electrical signal.
, 3-2 are formed.

[0005] In this type of SAW filter, input terminal 2
When a high frequency voltage Vs is applied to -1 and 2-2, the SAW is excited by the electric field generated between the electrode fingers of the input side comb-shaped electrode 11, and the SAW propagates left and right on the surface of the piezoelectric substrate perpendicular to the electrode fingers. do. The electrode width of this input side comb-shaped electrode 11 is V
It is determined at t/4f0. Vt is the propagation velocity of the SAW propagating on the piezoelectric substrate, and f0 is the center frequency of this SAW filter.

When the SAW that has propagated on the piezoelectric substrate surface reaches the output side comb-shaped electrode 12, it is converted into an electric signal again by the comb-shaped electrode 12, and energy is supplied to the load RL via the output terminals 3-1 and 3-2. be done.

The frequency characteristics of this SAW filter can be expressed by the transfer function H(ω) of the following equation (1).

[0008]

[Math 1]

[0009] However, ω0; filter center frequency wave f0
Angular frequency N; logarithm of electrode fingers. A frequency characteristic diagram of one comb-shaped electrode in FIG. 2 is shown in FIG.

Regarding the frequency characteristics of one comb-shaped electrode, in the case of the electrode structure shown in FIG. 2, the maximum output is obtained at a frequency f0 that corresponds to the surface wave output excited in each gap of the comb-shaped electrode.
Conversely, when there is a mismatch, the output becomes minimum and can be used as a trap. Therefore, in the case of one comb-shaped electrode, the frequency characteristics shown in FIG. 3 are obtained.

[0011]

[Problems to be Solved by the Invention] However, conventional SAW filters have the following problems.

FIGS. 4(a) and 4(b) are structural diagrams of a SAW filter showing the problem of FIG. 2. As shown in FIG. 4(a), in the conventional SAW filter, since comb-shaped electrodes 11 and 12 are provided on the surface of the piezoelectric substrate 1, the input terminal 2-1
, 2-2 and the output terminals 3-1, 3-2 respectively have a braking capacity C0. Therefore, in order to operate the SAW filter with desired characteristics, it is desirable to cancel the braking capacitance C0 inserted into the input/output.

Conventionally, in order to cancel the braking capacity C0, for example, as shown in FIG.
Inductors L are inserted in parallel to the input and output terminals for the AW filter, and a value of the inductor L is realized such that the braking capacitance C0 apparently cancels at the center frequency f0 of the SAW filter. However, in this cancellation method, since the inductor L is externally connected, the SA
There was a problem in that it was difficult to evaluate the performance of the W filter alone.

[0014] The present invention solves the problem of the prior art, which is that it is difficult to evaluate the performance of the SAW filter alone due to the damping capacitance C0 present at the input and output terminals of the SAW filter. It provides a filter.

[0015]

[Means for Solving the Problems] In order to solve the above problems, the present invention provides an input-side comb-shaped electrode that converts an electrical signal input from an input terminal into a surface acoustic wave, and a comb-shaped electrode that converts the surface acoustic wave into an electrical signal. In a SAW filter in which an output side comb-shaped electrode that outputs to an output terminal is formed on a piezoelectric substrate,
A damping capacitance canceling inductor made of a strip line with a line length corresponding to the dielectric constant of the piezoelectric substrate is formed on the piezoelectric substrate and connected in parallel to the input terminal and the output terminal, respectively.

[0016]

[Operation] According to the present invention, since the SAW filter is configured as described above, the inductors connected in parallel to the input terminal and the output terminal respectively reduce the braking capacitance existing at the input terminal and the output terminal, for example, to the SAW filter. It works to apparently cancel at the center frequency at which the filter operates. As a result, performance such as frequency characteristics of a single SAW filter with low insertion loss can be realized on the same chip. Therefore, the above problem can be solved.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a structural diagram of a SAW filter showing an embodiment of the present invention.

This SAW filter has a piezoelectric plate 21 having a predetermined dielectric constant. On the piezoelectric substrate 21,
An input side comb-shaped electrode 31 converts electrical signals input from input terminals 22-1 and 22-2 into surface acoustic waves, and converts the surface acoustic waves into electrical signals and outputs them to output terminals 23-1 and 23-2. An output side comb-shaped electrode 32 is formed. S
A braking capacity C0 exists at the input terminals 22-1, 22-2 and the output terminals 23-1, 23-2 of the AW filter, respectively. In order to cancel this braking capacitance C0, an inductor 41 is provided on the piezoelectric substrate 21, which is a strip line having a line length corresponding to the dielectric constant of the piezoelectric substrate 21.
42 is formed, and its inductors 41 and 42 are connected in parallel to the input terminals 22-1 and 22-2 and the output terminals 23-1 and 23-2, respectively.

Note that Vs in FIG. 1 is the input terminal 22-1,
The high frequency voltage applied to 22-2, RL, is the output terminal 23
-1 and 23-2.

Next, the operation of this SAW filter will be explained.

[0021] The high frequency voltage Vs is applied to the input terminals 22-1, 22
-2, the high frequency voltage Vs is converted into SAW by the input side comb-shaped electrode 31. This comb-shaped electrode 31
The electrode width is Vt/4f0, and the width of the electrode is Vt/4f0.
0MHz band), the piezoelectric substrate 21 is made of, for example, a lithium tantalate substrate (LiTaO3) with excellent temperature characteristics.
When configured using , the width is approximately 1.2 μm.

The SAW converted by the input-side comb-shaped electrode 31 propagates on the surface of the piezoelectric substrate 21 and reaches the output-side comb-shaped electrode 32 . This output-side comb-shaped electrode 32 converts it into an electric signal again, and the load RL
Energy is supplied to. In this SAW filter, the logarithm N of the comb-shaped electrodes 31 and 32 provided on the piezoelectric substrate 21 causes the input terminals 22-1 and 22-2 and the output terminals 23-1 and 23-2 to be ) braking capacity C
0 occurs.

[0023]

[Math 2]

However, ω ; angular frequency K ; complete elliptic integral of the first kind ε0 ; permittivity in vacuum εs ; dielectric constant L0 ; inductance in vacuum Ls ; inductance In order to cancel this damping capacity C0, the following equation is used. (
3) must be satisfied.

[0025]

[Math 3]

[0026] However, ω0; filter center frequency f0
In order to realize this inductance L, inductors 41 and 42 made of strip lines are connected to the input terminals 22-1, 22-2 and the output terminals 23-1, 23-2 on the piezoelectric substrate 21 made of a dielectric material. This is possible by connecting to.

FIGS. 5(a) and 5(b) show inductors 41, 4
2, in which (a) is a diagram showing a strip line with a line length D, and (b) is a lumped constant circuit diagram thereof. In the figure, Zo is characteristic impedance, Yo is admittance, B/2 is capacitive reactance, and X is inductive reactance.

From this lumped constant circuit, the inductance L of the strip line can be obtained from the following equation (4).

[0029]

[Math 4]

However, when the line length D in equation (4) is V: voltage, when the piezoelectric substrate 21 made of a dielectric material has a high dielectric constant, inductors 41 and 42 having a large value can be realized with a short line length. Microwave band (800M
Inductance L realized by SAW filter (Hz band)
is approximately 10 nH, which is a value that can be fully realized on a lithium tantalate substrate.

As described above, in this embodiment, as a means for canceling the damping capacitance C0 that occurs in realizing the SAW filter, a Since the inductors 41 and 42 made of strip lines are provided, the SA
The performance of the frequency characteristics of a single W filter on the same chip,
In other words, it can be realized with high precision within the SAW filter package. Therefore, there is no need to provide an inductor for canceling the braking capacity on the main board of a wireless communication device or the like that uses the SAW filter. Therefore, it becomes easy to evaluate the performance of a single SAW filter, and complex problems such as matching of frequency characteristics when using the SAW filter can be easily solved.

Note that the present invention is not limited to the above embodiments,
Various modifications are possible. Examples of such modifications include the following.

(1) The piezoelectric substrate 21 is made of lithium
A substrate other than a tantalate substrate may be used. For example, if a piezoelectric substrate with a multilayer structure in which a ZnO thin film piezoelectric material is attached to a glass substrate is used, a SAW filter with excellent temperature characteristics and low cost due to mass production can be provided. (2) Inductor 4 formed by strip line
1 and 42 are not limited to the arrangement and shape shown in FIG.
, and the comb-shaped electrodes 31, 32, etc., can be changed into various shapes depending on the formation positions and shapes. (3) In order to reduce the loss of the SAW filter shown in FIG. 1, it is possible to make the bidirectional comb-shaped electrodes 31 and 32 have a multi-electrode structure, or to make the comb-shaped electrodes 31 and 32 have a unidirectional structure. It is also possible to change the structure or add other components.

[0034]

As described in detail above, according to the present invention, in order to cancel the damping capacitance that occurs when realizing a SAW filter, strip lines are formed on the same plane as the surface of the piezoelectric substrate on which the comb-shaped electrodes are formed. Since the inductor is provided, the performance such as frequency characteristics of a single SAW filter can be achieved with low insertion loss and high precision on the same chip, that is, within the package of the SAW filter. Therefore, there is no need to provide an inductor for canceling the braking capacity on the main board of a wireless communication device or the like that uses the SAW filter. Therefore, it becomes easy to evaluate the performance of a single SAW filter, and complex problems such as matching of frequency characteristics when using the SAW filter can be easily solved.

[Brief explanation of drawings]

FIG. 1 is a structural diagram of a SAW filter showing an embodiment of the present invention.

FIG. 2 is a basic structural diagram showing a conventional SAW filter.

FIG. 3 is a frequency characteristic diagram of the comb-shaped electrode in FIG. 2;

FIG. 4 is a diagram showing a problem in FIG. 2;

FIG. 5 is an explanatory diagram of an inductor formed of strip lines in FIG. 1;

[Explanation of symbols]

21 Piezoelectric substrate 2
2-1, 22-2 Input terminal 23-1, 23-2 Output terminal

Claims (1)

[Claims] 1. An input-side comb-shaped electrode that converts an electrical signal input from an input terminal into a surface acoustic wave, and an output-side comb-shaped electrode that converts the surface acoustic wave into an electrical signal and outputs it to an output terminal are piezoelectric. In the surface acoustic wave filter formed on the substrate, an inductor for canceling braking capacity consisting of a strip line with a line length corresponding to the dielectric constant of the piezoelectric substrate,
A surface acoustic wave filter, characterized in that it is formed on the piezoelectric substrate and connected in parallel to the input terminal and the output terminal, respectively.