JPH06303090A - Piezoelectric filter - Google Patents

Abstract

PURPOSE:To provide a double mode filter with high precision in which a wide area can be obtained, and the temperature change of a frequency can be small by realizing a double mode structure using a thickness-shear vibration mode substrate constituted of LiTaO3 single crystal as a piezoelectric substrate. CONSTITUTION:This device is equipped with a filter element 1, case 20 which houses the filter element 1, and a cover 30 which closes the opening of the case 20. In the filter element 1, two double mode resonators of the thickness- shear vibration are formed on one piezoelectric substrate, and the piezoelectric substrate is obtained by segmenting the X plate of the LiTaO3 single crystal from an Y-axis at -57 deg.+ or -0.5 deg. angle. The temperature change of the frequency of the LiTaO3 single crystal being the piezoelectric single crystal is smaller than that of a piezoelectric ceramics, and the electrical equipment coupling coefficient is larger than a crystal. Thus, the temperature characteristic of a piezoelectric filter using the LiTaO3 single crystal can be more excellent than that of the filter using the piezoelectric ceramics, and the band can be wider than that of the filter using the crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric filter having a dual mode structure.

[0002]

2. Description of the Related Art In recent years, in the market of communication devices and the like, there is an increasing demand for a filter in the MHz band which has a wide band and a small frequency change with temperature. Conventionally, as a filter used in the MHz band, a dual mode filter having a good spurious characteristic and easy to manufacture is known. As a material of this filter, there is one using piezoelectric ceramics or crystal.

[0003]

However, in the dual mode filter using the piezoelectric ceramics, the temperature change of the frequency in the practical temperature range (-20 ° C to 80 ° C) is large, and it is difficult to obtain a highly accurate filter. There is.
On the other hand, a dual-mode filter using a crystal can suppress the temperature change in frequency very small, but on the other hand, its bandwidth is too narrow to meet the market demand. Therefore, an object of the present invention is to provide a high-precision dual-mode filter that has a wide band and a small frequency temperature change.

[0004]

In order to achieve the above object, the piezoelectric filter of the present invention uses a thickness shear vibration mode piezoelectric substrate made of a LiTaO ₃ single crystal, and the piezoelectric substrate has a dual mode structure vibration electrode. Is provided.

[0005]

The single crystal of LiTaO ₃ which is a piezoelectric single crystal has a smaller frequency change in frequency than the piezoelectric ceramic and has a larger electromechanical coupling coefficient than that of quartz. Therefore, LiTa
The piezoelectric filter using the O ₃ single crystal has an advantage that it has better temperature characteristics than a filter using piezoelectric ceramics and has a wider band than a filter using quartz. Moreover, in LiTaO ₃ single crystal, unnecessary vibration is easily excited,
Spurious tends to grow. Therefore, as the vibration mode, the thickness-shear vibration mode having smaller spurious than other vibration modes is used.

The LiTaO ₃ single crystal X plate has a −
Since it has a cleavage plane at 57 °, it has good workability during cutting. Further, since the temperature coefficient is good and the characteristics such as roll-off characteristics and group delay characteristics are also good, it is desirable to cut out the X plate at an angle of −57 ° ± 0.5 ° from the Y axis.

[0007]

FIG. 1 shows an example of a piezoelectric filter according to the present invention. This piezoelectric filter includes a filter element 1, a case 10 that houses the filter element 1, and a cover 30 that closes an opening of the case 20.

As shown in FIG. 2, the filter element 1 is formed by forming two thickness-shear vibration dual mode resonators on one piezoelectric substrate 2. As shown in FIG. The LiTaO ₃ single crystal X plate was placed at -57 from the Y axis.
The one cut out at an angle of ± 0.5 ° is used. An input (or output) terminal electrode 3 and an output (or input) terminal electrode 4 are formed at both ends of the surface of the piezoelectric substrate 2, and the terminal electrode 3 is provided at a position near the center of the surface. , 4
Vibrating electrodes 5 and 6 are formed so as to be electrically connected to each other. Further, in the vicinity of the vibrating electrodes 5, 6, paired vibrating electrodes 7, 8 are formed adjacent to each other, and these vibrating electrodes 7, 8 are electrically connected to each other. On the back surface of the piezoelectric substrate 2, a wide vibrating electrode 9 facing the vibrating electrodes 5, 7 and vibrating electrodes 6, 8 are provided.
A wide vibrating electrode 10 is formed so as to face with the vibrating electrodes 9 and 10 and are electrically connected to each other through a grounding terminal electrode 11.

The case 20 is made of a ceramic material such as alumina porcelain, and a recess 21 for accommodating the filter element 1 is formed on the upper surface thereof. Lead electrodes 22, 23, 24 are formed in the width direction on the outer peripheral surfaces of both ends and the central portion of the case 20 and on the inner surface of the recess 21. The filter element 1 is housed in the recess 21 of the case 20 and fixed by a conductive adhesive (not shown). At this time,
Input (or output) terminal electrode 3 of the filter element 1,
The output (or input) terminal electrode 4 and the grounding terminal electrode 11 are respectively the extraction electrodes 22, 23, 2 of the case 20.
4 is connected. FIG. 4 is an equivalent circuit of the piezoelectric filter. In addition, at least the vibrating portion of the filter element 1,
Reduces spurious, top waveform and G.I. D. T. A damping material (not shown) such as silicone gel is applied to improve the characteristics.

The following table shows a 3 dB bandwidth and a frequency deviation (-20 ° C to 60 ° C) of a dual mode filter using a piezoelectric ceramics (PZT), quartz and a LiTaO ₃ single crystal X plate (Y-57 °). ) And is shown.

[Table 1] As is clear from the above table, the LiTaO ₃ X plate has 3 dB.
It has the widest bandwidth and is slightly inferior to quartz in the temperature change of frequency, but is four times more accurate than piezoelectric ceramics.

FIG. 5 shows the filter characteristics of the dual mode filter shown in FIG. 1, and FIG. 6 shows its top waveform and G.I. D. The T characteristic is shown. As is clear from the figure, the spurious response is extremely small, the top waveform is round, and good roll-off characteristics are obtained. D. T. It can be seen that a good filter characteristic with a small characteristic deviation can be obtained.

In the above embodiment, LiTa is used as the piezoelectric substrate.
An X plate of an O ₃ single crystal cut out at an angle of −57 ° ± 0.5 ° from the Y axis was used, but the present invention is not limited to this, and for example, an angle of −50 ° or −58 ° from the Y axis. Similar filter characteristics can be obtained by using other cut angles such as those cut out in. Further, the electrode shape of the dual mode filter of the present invention is not limited to that shown in FIG. 2, and may be a shape as shown in FIG. 7 or 8.
Furthermore, the present invention is not limited to the two-element type dual-mode filter as described above, and may be a one-element type or three-element or more dual-mode filter.

[0013]

As is apparent from the above description, since the piezoelectric filter of the present invention has the dual mode structure by using the thickness shear vibration mode substrate made of LiTaO ₃ single crystal as the piezoelectric substrate, the piezoelectric ceramic or the crystal is used. A band width is wider than that of the filter using, and the temperature change of frequency is smaller than that of the piezoelectric ceramics, and a highly accurate filter can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view of an example of a piezoelectric filter according to the present invention.

FIG. 2 is a front view and a rear view of a filter element.

FIG. 3 is a diagram showing a cut angle of a piezoelectric substrate.

FIG. 4 is an equivalent circuit diagram of a piezoelectric filter.

FIG. 5 is a filter characteristic diagram of a piezoelectric filter.

6 is a top waveform diagram of a piezoelectric filter and G.I. D. It is a T characteristic diagram.

FIG. 7 is a front view and a rear view of another example of the filter element.

FIG. 8 is a front view and a rear view of still another example of the filter element.

[Explanation of symbols]

 1 Filter element 2 Piezoelectric substrate

Claims (2)

[Claims] 1. A piezoelectric filter comprising a piezoelectric substrate of a thickness shear vibration mode made of a LiTaO ₃ single crystal, and a vibration electrode having a dual mode structure provided on the piezoelectric substrate. 2. The piezoelectric filter according to claim 1, wherein the piezoelectric substrate is from the Y axis of an X plate of LiTaO ₃ single crystal.
A piezoelectric filter characterized by being cut out at an angle of 57 ° ± 0.5 °.