JP2001326556A - Dual mode piezoelectric filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a means for improving attenuation on the side of a low frequency from the center frequency of a balanced dual mode piezoelectric filter. SOLUTION: First and second electrodes are arranged closely on a piezoelectric substrate, third and fourth electrodes are arranged opposed to the two electrodes and the fourth electrode is made to be larger than the first, second and third electrodes nearly in the same shape as that of the fourth electrode and is grounded to constitute the dual mode piezoelectric filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dual-mode piezoelectric filter, and more particularly to a dual-mode piezoelectric filter having improved attenuation at a frequency lower than a center frequency.

[0002]

2. Description of the Related Art A dual mode piezoelectric filter (hereinafter, referred to as a dual mode filter) is used in a mobile communication device because of its small size, high attenuation, and robustness. Widely used as IF filters.
In recent years, along with demands for miniaturization of mobile communication devices and improvement of communication quality, there has been a strong demand for downsizing IF circuits and balanced input / output circuits. This is because, in a conventional portable terminal or the like, since an unbalanced filter is used in an IF circuit, noise is apt to be mixed into a transmission / reception signal via a ground, and there is a problem that communication quality is deteriorated. In particular,
In a recent terminal, since a digital circuit and an analog circuit are mixed, the internal noise of the terminal is also increased as compared with before. In order to reduce noise from other circuit blocks and to realize a terminal having good speech quality, it has been strongly desired that the IF circuit be a balanced circuit.

FIG. 4A is a plan view showing the structure of a conventional unbalanced double mode filter, FIG. 4B is a sectional view taken along the line QQ, and FIG. Things. Two electrodes 12a and 12b are arranged close to each other on the piezoelectric substrate 11, and an electrode 13 is provided on the back surface of the piezoelectric substrate 11 so as to face the electrodes 12a and 12b.
Lead electrodes 14a, 14b and 15 extend from a, 12b and 13 toward the ends of the piezoelectric substrate, respectively, to form a dual mode filter element. When the dual mode filter is represented by the symbol shown in FIG.
Is generally used as an input / output terminal, and the terminal T3 is used as a ground terminal. Here, when a high-frequency voltage is applied between the terminals T1 and T3, the symmetric mode S ₀ and the anti-symmetric mode A ₀ are vigorously excited, as is well known, and a proper termination is applied to the dual mode filter. Function as

FIG. 5A is a plan view showing a configuration of a conventional balanced double mode filter, FIG. 5B is a sectional view taken along line QQ, and FIG. It is.
The two electrodes 22a, 23a are arranged close to each other on the piezoelectric substrate 21, and the electrodes 22b, 23b are arranged on the back surface of the piezoelectric substrate 21 so as to face the electrodes 22a, 23a.
a, 22b, 23a and 23b from the piezoelectric substrate 2 respectively.
Lead electrodes 24a, 24b, 25a and 25b are extended toward one end to constitute a balanced double mode filter element. As shown in FIG. 5C, the terminals T1 and T2
Is generally used as an input terminal and the terminals T3 and T4 are used as output terminals. Depending on the circuit configuration before and after the connection of the dual mode filter, in addition to making the input / output terminals balanced circuits, for example, as shown in FIG. 6, the input terminals T1 and T2 are made balanced circuits, the terminal T3 is made the output terminal, and T4 becomes the output terminal. Are grounded, or conversely, the input terminal is an unbalanced circuit and only the output terminal is a balanced circuit.

FIG. 7 shows an AT-cut quartz substrate as a piezoelectric substrate, a center frequency of 109.65 MHz and a pass bandwidth of 10.5 kHz or more at triple harmonics, and an electrode 22 shown in FIG.
a is a filter characteristic of a balanced double mode filter in which the dimension of a (23a) is 0.4 mm × 1.13 mm.

[0006]

However, as shown in FIG. 6, the balanced dual mode filter shown in FIG. 5 has input terminals T1 and T2 connected to balanced circuits and terminals T3 and T3.
When 4 is used as an unbalanced circuit, as shown by β in FIG. 7, the low frequency side of the center frequency, −910 kHz ± 10 kHz, cannot meet the required standard of TDMA mobile phones of 70 dB or more, which is about 67 dB. There was a problem. The present invention has been made to solve the above-described problem, and has as its object to provide a dual mode filter that satisfies the required standard of 70 dB or more at a lower frequency side of the center frequency of -910 kHz.

[0007]

In order to achieve the above object, a dual mode piezoelectric filter according to the present invention has a first mode and a second mode in which a first electrode and a second electrode are arranged close to each other on a piezoelectric substrate. A balanced dual-mode piezoelectric filter in which third and fourth electrodes are disposed opposite to the two electrodes,
The fourth electrode is larger and closer to the third electrode than the first, second and third electrodes having substantially the same shape and is grounded, and extends from the electrode facing the electrode to form an unbalanced terminal. And a balanced terminal pair extending from the other electrode pair. Claim 2
The invention described in claim 1 is the dual mode piezoelectric filter according to claim 1, wherein the first and second electrodes are accommodated so as to face a metal lid of a ceramic package. The invention according to claim 3 is the dual mode piezoelectric filter according to claim 1, wherein a shielding electrode is provided between the first and second electrodes and the electrode is grounded.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1A is a plan view showing the configuration of a balanced double mode filter according to the present invention, FIG. 1B is a cross-sectional view taken along QQ, and FIG. is there. First and second on the piezoelectric substrate 1
Electrodes 2a, 3a are arranged in close proximity to each other, and third and fourth electrodes 2b, 3b are arranged in opposition to the two electrodes. , The lead electrodes 4a, 4b, 5a and 5b are extended to form a balanced double mode filter element.

The feature of the present invention is that the electrodes 2a, 2b, 3a, 3
b, one of the electrodes is made larger than the other electrodes of substantially the same size, the enlarged electrode is grounded, the lead electrode extending from the electrode facing the electrode is output, and the adjacent counter electrode is That is, a dual mode filter is configured as a balanced input terminal. In the embodiment shown in FIG. 1 (c), the width of the electrode 3b in the Z direction is
a, 2b, and 3a, the center of the drawing is made larger, that is, closer to the electrode 2b, and the electrode 3b is grounded, and the terminals T1, T2
2 is an input balanced-output unbalanced dual mode filter having a balanced input terminal 2 and an unbalanced output terminal T3.

FIG. 2 uses an AT-cut quartz substrate of three times higher harmonics as a piezoelectric substrate, a center frequency of 109.65 MHz, a pass bandwidth of 10.5 kHz or more, and electrodes 2a and 2b shown in FIG.
The dimension of 3a is 0.4 mm x 1.13 mm, and the dimension of electrode 3b is 0.5
9 shows filter characteristics of a balanced type dual mode filter with a size of mm × 1.13 mm. As is clear from the drawing, it has an attenuation of 70 dB or more in a frequency range (shown by β) of −910 kHz ± 10 kHz from the center frequency, and it has been found that the standard sufficiently satisfies the standard. It is assumed that the reason why the amount of attenuation is improved is that the electromagnetic shielding between the input terminals T1 and T2 and the output terminal T3 is made effective by increasing the size of the grounded electrode 3b. Further, it has been experimentally revealed that when the electrodes are accommodated in a ceramic package, the attenuation can be increased by mounting the electrodes 2a and 2b shown in FIG. 1A so as to face the metal lid of the package.

3A and 3B show another embodiment according to the present invention. FIG. 3A is a plan view showing the structure of the embodiment, FIG. 3B is a sectional view taken along the line Q--Q, and FIG. It is represented. The difference from the embodiment shown in FIG. 1 is that the shielding electrode 6 is provided between the first electrode 2a and the second electrode 3a, and the electrode 6 is grounded. That is, the shielding electrode 6 is provided between the first electrode 2a and the second electrode 2b, the electrodes 2a, 2b, and 3a have substantially the same shape, and only the width of the electrode 3b is increased toward the center in the figure. And grounded, terminal T
1, T2 is a balanced input terminal, terminal T3 is an unbalanced output terminal, and terminal T4 is grounded to form a dual mode filter.

As a method of adjusting the frequency and the bandwidth, FIG.
(B), as shown in the embodiment of FIG. 3 (b), the coupling between the two modes S ₀ and A ₀ can be strengthened by adding mass to the region of α of the electrode 3b, and conversely, the electrode 2a , 3
By adding mass to the region a or 2b, 3b away from the center, the coupling between the two modes can be weakened. Also, the adjustment may be performed by shaving the electrode film thinly with an electron beam or the like. In this case, when the electrode film in the region α is shaved, the coupling is weakened and the electrodes 2a, 3a or 2b, 3b
If the thin film in a region away from the center of b is scraped off, the bonding will be strengthened.

FIG. 1 or FIG. 3 shows a configuration in which the input terminals T1 and T2 are balanced circuits and the outputs T3 and T4 are unbalanced circuits. Conversely, the terminals T1 and T2 are output terminals and the terminal T
3, T4 may be used as an input terminal.

[0014]

According to the present invention, as described above, the low frequency side of the center frequency, for example, -910 MHz ± 10 kHz
Can be greatly improved, and the use of the present filter in an IF circuit of a wireless device shows an excellent effect that the speech quality can be improved.

[Brief description of the drawings]

1A is a plan view showing the configuration of a balanced dual mode filter according to the present invention, FIG. 1B is a cross-sectional view, and FIG.

FIG. 2 is a diagram showing filter characteristics of a balanced dual mode filter according to the present invention.

3A is a plan view showing a configuration of a balanced dual mode filter according to another embodiment of the present invention, FIG.
(C) is the figure which represented it with the symbol.

4A is a plan view showing a configuration of a conventional unbalanced double mode filter, FIG. 4B is a cross-sectional view, and FIG.

5A is a plan view showing the configuration of a conventional balanced dual mode filter, FIG. 5B is a cross-sectional view, and FIG. 5C is a diagram represented by a symbol.

FIG. 6 is a diagram illustrating a filter in which input terminals T1 and T2 are balanced circuits, and output terminals T3 and T4 are unbalanced circuits.

FIG. 7 is a diagram showing filter characteristics of a conventional balanced dual mode filter.

[Explanation of symbols]

 1 ·· Piezoelectric substrate 2a, 2b, 3a, 3b ··· Electrode 4a, 4b, 5a, 5b ··· Lead electrode 6 ··· Shielding electrode T1, T2, T3, T4 ··· Terminal

Claims (3)

[Claims] A first and a second surface on one main surface of the piezoelectric substrate;
A second mode piezoelectric filter in which a third electrode and a third electrode are respectively disposed on the other main surface of the double mode piezoelectric filter so as to face the two electrodes. While the fourth electrode has substantially the same shape, the shape of the fourth electrode is increased toward the third electrode, and the fourth electrode is grounded, and the terminal extending from the second electrode facing the fourth electrode is not connected. A dual mode piezoelectric filter, wherein the balanced mode terminals are used, and the terminals extending from the first and third electrodes are a balanced type terminal pair. 2. The dual mode piezoelectric filter according to claim 1, wherein said first and second electrodes are accommodated so as to face a metal lid of a ceramic package. 3. The dual mode piezoelectric filter according to claim 1, wherein a shielding electrode is provided between said first and second electrodes and said electrode is grounded.