JP2000295072A - Triple mode piezoelectric filter - Google Patents

Abstract

(57) [Problem] To provide a means for improving a group delay time deviation in a pass band of a triple mode piezoelectric filter. SOLUTION: In a triple mode piezoelectric filter constituted by arranging three pairs of electrodes on a piezoelectric substrate, thin films smaller than the central electrodes are attached near both sides of the center electrode, and Q values of symmetric primary and secondary modes are provided. Is degraded from the Q value of the antisymmetric first-order mode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric filter, and more particularly, to a triple mode piezoelectric filter having improved group delay time characteristics.

[0002]

2. Description of the Related Art Dual mode piezoelectric filters are widely used as first IF filters for mobile terminals because of their small size, high attenuation, and robustness. FIGS. 3A and 3B are diagrams showing a configuration of a conventional dual mode piezoelectric filter (hereinafter, referred to as a dual mode filter). FIG. 3A is a plan view, and FIG. It is sectional drawing in -A. The dual mode filter is
Electrodes 12a and 13a having substantially the same shape are arranged on the main surface of the piezoelectric substrate 11 with a predetermined gap g, and electrodes 12b and 13b having the same shape are arranged on the opposite side to the electrodes. 12a
To 13b, lead electrodes 14a to 15b extend to the ends of the piezoelectric substrate 11, respectively. In this configuration, for example, when a high frequency signal is applied between the electrodes 12a to 13b, the electrodes 12a to 1b
As a result of the acoustic coupling between 3b, a number of thickness-shear vibrations are excited, of which the symmetric primary resonance mode F1 (resonance frequency f ₁ ) and the antisymmetric primary resonance mode F2 (resonance frequency f ₂ ) Is well-excited and functions as a bandpass filter using these two modes.

FIG. 4 shows a configuration in which the center frequency is 45 MHz, the electrodes 12a and 13a are 0.4 mm × 0.8 mm, and the electrode gap g is
It is a figure which shows an example of the filter characteristic at the time of comprising so that it may be set to 0.4 mm and a center frequency may be set to 45 MHz, frequency is set on a horizontal axis, the curve (alpha) in an figure is an amplitude characteristic (scale on the left side in a figure), Indicates the group delay time characteristic (the scale on the right side in the figure). The bandwidth is ± 15 kHz at 3 dB, and the group delay time deviation within the 3 dB bandwidth is 4.6 μs.

In recent years, in response to the rapid increase in demand for wireless communication devices such as mobile phones, the number of communication channels has been increased by increasing the number of channels. There is a demand for improved selectivity. As a solution to this, there are a method of improving the selectivity by cascading dual mode filters in two stages, and a method of using a triple mode piezoelectric filter (hereinafter referred to as a triple mode filter) instead of the dual mode filter. The latter is more advantageous than the former in terms of group delay time characteristics, cost and miniaturization, and recently triple mode filters have been widely used. FIGS. 5A and 5B are diagrams showing a configuration of a triple mode filter, wherein FIG. 5A is a plan view and FIG. 5B is a cross-sectional view taken along AA. The triple-mode filter includes three electrodes 22a, 23a, and 24a having substantially the same shape on the main surface of the piezoelectric substrate 21 with an electrode gap g1 therebetween.
And the electrodes 22a, 23a, 24a
The electrodes 22b, 23b having the same shape on the back side of the substrate 21 opposite to
24b will be provided respectively. Further, lead electrodes 25a to 27b extend from the electrodes 22a to 24b to the ends of the piezoelectric substrate 21, respectively, to constitute a triple mode filter.

As is well known, when a high-frequency voltage is applied between the electrodes 22a and 22b, acoustic coupling occurs between the electrodes 22a and 27b, and as a result, a large number of thickness-slip vibrations are excited. Symmetric primary resonance mode F1 (resonance frequency
f ₁ ), the anti-symmetric primary resonance mode F2 (resonance frequency f ₂ ) and the symmetric secondary resonance mode F 3 (resonance frequency f ₃ ) are strongly excited, and function as a bandpass filter using these three modes. As shown in FIG. 6 (a), the electrical equivalent circuit of the triple mode filter is represented by a lattice type circuit, the series arm of which is an antisymmetric primary mode F2 resonance circuit, and the lattice arm is a symmetric primary mode F1. And a resonance circuit of the symmetrical secondary mode F3. Symmetry 1
The first-order mode, the antisymmetric first-order mode and the symmetric first-order mode are both
This is represented by a known resonance circuit shown in FIG.

FIG. 7 is a diagram showing a reactance characteristic of the triple mode filter shown in FIG. 6, that is, a frequency-reactance curve, wherein the horizontal axis represents frequency and the vertical axis represents reactance. As shown in the figure, changing the frequency on the high frequency side, the resonance frequency f ₁ of the symmetrical primary mode F1 grid arms indicated by the solid line appears at the beginning, the antiresonance frequency f _1a appears at a distance. Furthermore, when the frequency is increased, the anti-symmetric 1
The resonance frequency f ₂ and the anti-resonance frequency f _2a of the mode F2, the anti-resonance frequency f _3a and the resonance frequency f ₃ of the symmetrical secondary mode F3 indicated by the solid line
Appear in order. According to the filter theory, the anti-resonant frequency f _1a of the symmetric first-order mode and the resonance frequency f ₂ of the anti-symmetric first-order mode
It is well known that when the resonance frequency f _2a of the anti-symmetric first-order mode and the resonance frequency f ₃ of the symmetric second-order mode are almost equal to each other, a filter with less ripple in the band can be obtained. It is.

FIG. 8 shows electrodes 22a, 23a and 24a each having a size of 0.4 mm × 0.8 m.
m, the electrode gap g1 is 0.3 mm, and shows an example of filter characteristics when the center frequency is set to 45 MHz. The horizontal axis represents the frequency, and the curve α in the figure represents the amplitude characteristic (FIG. The curve β shows the group delay time characteristic (the scale on the right side in the figure). Bandwidth is ± 15kHz at 3dB,
The group delay time deviation within the 3 dB bandwidth is 9.9 μs. In addition,
The Q value of each vibration mode is about 40 × 10 ³ .

[0008]

However, for example, in an IF filter for a terminal of a cellular system (Amps) in North America, (1) 3 dB band is ± 15 kHz, and (2) 3 dB.
The standard that the group delay time deviation within the bandwidth is 10 μm or less is defined, and the above-described conventional triple mode filter has a problem that the group delay time deviation has no margin relative to the standard and the product yield is extremely poor. The present invention has been made to solve the above-described problem, and has as its object to provide a triple mode filter in which the group delay time deviation is significantly improved.

[0009]

According to a first aspect of the present invention, there is provided a triple mode piezoelectric filter having three pairs of electrodes disposed on a piezoelectric substrate. Characterized in that a thin film having a shape smaller than that of the electrode is attached to the vicinity of the electrode disposed in the piezoelectric device. According to a second aspect of the present invention, in the triple mode piezoelectric filter in which three pairs of electrodes are arranged on a piezoelectric substrate, when the dimension of the three centrally arranged electrodes in the three electrode arrangement direction is L, the arrangement direction of the electrodes is perpendicular to, and, wherein the width W ₁ at a position distance between electrodes W2 leaves L / 2 or more from the center electrode end a straight line passing through the center of the center electrode is attached to the following film 0.8L Is a triple mode piezoelectric filter. The invention according to claim 3 is the triple mode piezoelectric filter, wherein the thin film is a viscous substance. According to a fourth aspect of the present invention, there is provided the triple mode piezoelectric filter according to the first or second aspect, wherein a group delay time deviation in a pass band is reduced as compared with before the additional thin film is attached.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIGS. 1A and 1B are diagrams showing a configuration of a triple mode filter according to the present invention in which the group delay time deviation is improved, wherein FIG.
(B) is sectional drawing in AA. The triple mode filter arranges three electrodes 2a, 3a, 4a having substantially the same shape on the main surface of the piezoelectric substrate 1 in parallel with the long side of the substrate with an electrode gap g1, respectively, and the electrodes 2a, 3a, Electrodes 2b, 3b, 4b are arranged on the back side of substrate 1 facing 4a. Then, the lead electrodes 5a are respectively connected to the ends of the piezoelectric substrate 1 from the electrodes 2a to 4b.
Extends ~ 7b. Further, a strip-shaped thin film 8 having a width W1 is formed at a position W2 away from an end side parallel to the electrode arrangement direction of the center electrodes 3a, 3b and overlapping with or opposite to the lead electrodes 6a, 6b. . Considering the shortening of the process, it is preferable to form the substrate 1 on either one of the front and back surfaces. Referring to the embodiment shown in FIG. 1, the center electrode 3a is substantially at the center of the lead electrode 6a, and the center of the lead electrode 6b and on the substrate on the opposite side, that is, by using means such as vapor deposition. 3a
A strip W-shaped silver thin film 8 having a width W ₁ is formed at a distance W ₂ away from the end of the substrate. In this example, the strip-shaped thin film extends to the end of the substrate 1.

When a high-frequency voltage is applied between the electrodes 2a and 2b shown in FIG. 1 (a), acoustic coupling occurs between the electrodes 2a and 7b, and as a result, a number of thickness-slip vibrations are excited. Symmetric primary resonance mode F1 (resonance frequency f ₁ ),
Is excited in the stress antisymmetric first resonance mode F2 (resonance frequency f ₂₎ and symmetrical secondary resonance mode F3 (resonance frequency f _3), functions as a band filter using these three modes. Here, before describing the operation of the present invention, the vibration displacement will be described in a little more detail. The displacement distribution of the symmetric primary resonance mode F1 is represented by a curve shown in FIG.
The displacement of the symmetric primary resonance mode as shown in 101 is exhibited, and the antisymmetric primary resonance mode F2 is shown in FIG.
A displacement as shown by a curve 102 in FIG. The symmetric secondary resonance mode F3 corresponds to the curve 10 in FIG.
It exhibits a displacement as shown in FIG. At this time, in the direction orthogonal to this, in any of the symmetric primary resonance mode F1, the antisymmetric primary resonance mode F2 and the symmetric secondary resonance mode F3, FIG.
As shown by the curve 104 in FIG. 7A, the symmetric primary resonance mode is displaced.

As is well known, the electrical equivalent circuit of the triple mode filter shown in FIG. 1A can be represented by a lattice type circuit shown in FIG. However, since the small strip-shaped metal film 8 is added to almost the center of the lead electrodes 6a and 6b as shown in FIG. 1A, the mass of the metal film 8 causes vibration at the center of the piezoelectric substrate 1 in the longitudinal direction. Symmetry 1 for maximum displacement
In the next (F1) and second (F3) modes, the vibration energy leaks to the strip-shaped metal film 8 near the center, and the Q value of each resonance mode is deteriorated. On the other hand, in the anti-symmetric primary resonance mode F2, the vibration displacement is minimized at the longitudinal center of the piezoelectric substrate 1, so that the metal film 8 is hardly affected. If this is explained by the electric equivalent circuit shown in FIG. 6A, the Q values of the symmetric first and second modes of the lattice arm are degraded, that is, the resonance circuit shown in FIG. Corresponds to an increase in the resistance value r _i (i = 1, 3).

Therefore, the triple mode filter shown in FIG.
The center frequency is 45MHz, and the size of the electrodes 2a, 3a, 4a is 0.4mm ×
0.8 mm (L x H), electrode gap g1 is 0.3 mm, strip-shaped metal film 8
Width W ₁0.3mm (0.75L), end of electrode 3a and strip-shaped metal film
Gap W with 8_TwoWas 0.2 mm (0.5 L) and the silver film thickness was 1000 mm.
Fig. 2 shows the results of trial production and measurement of a triple mode filter.
Such a filter characteristic α and group delay time characteristic β
It was confirmed. Comparing this with FIG. 8, the insertion loss is
Although it increased by about 0.6 dB, as is apparent from FIG.
It is clear that the delay time deviation has been halved to 4.6 μs.
You.

Based on this finding, as shown in FIG. 1A, the dimension of the central electrode 3a in the arrangement direction of the three pairs of electrodes is represented by L, and the strip-shaped metal in the direction orthogonal to the arrangement direction of the three pairs of electrodes. the gap W ₂ of the membrane 8 0.2mm~1mm (0.5L~2.5L),
The width W1 of the strip-shaped metal film 8 is 0.2 mm to 0.4 mm (0.5 L to ₁ L)
(The length of the strip is up to the edge of the substrate.) The thickness of the metal film 8 (in this experiment, silver was used) was 1000 to 3000 mm (0.1 to 0.3 μm).
m) The resonance characteristics were measured while changing. As a result, the width W ₁ ,
Symmetric primary by the value of the gap W _2, the second-order resonance mode equivalent resistance
While it was found that r ₁ and r ₃ fluctuated, the change in the equivalent resistance r ₂ of the antisymmetric primary resonance mode was extremely small. That is,
When the gap W ₂ is reduced, the equivalent resistances r ₁ and r ₃ increase, and the width
W ₁ a largely equivalent resistance r ₁ also, r ₃ is increased. When the filter characteristics and group delay time characteristics of the triple mode filter at this time are measured, as the equivalent resistances r ₁ and r ₃ increase, the insertion loss of the filter increases, and the cutoff characteristics of the filter characteristics are rounded. come. At the same time, the protrusions of the group delay time characteristics at both ends of the passband are rounded,
It has been found that the protrusion itself becomes smaller, that is, the group delay time characteristics can be improved.

Further, in order to confirm this by a simulation, in the lattice-type circuit of FIG. 6A, the Q value of the antisymmetric primary resonance mode (F2) of the series arm is fixed, and the symmetric primary (F1) We simulated how the filter characteristics change when the Q value of the secondary (F3) resonance mode is degraded. While keeping the Q value of the antisymmetric primary resonance mode (F2) constant at 40 × 10 ³ close to the next value, the Q values of the symmetric primary (F1) and secondary (F3) resonance modes are almost equal to 30. × 1
0 ³ , 25 × 10 ³ , 20 × 10 ³ , 15 × 10 ³ , 10 × 10 ³ . According to the result of the simulation, it is understood that the characteristic shown in FIG. 2 corresponds to the case where the value of Q in the symmetric first-order (F1) and second-order (F3) resonance modes is approximately 10 × 10 ³ .

In the above description, a method of deteriorating the Q value of the symmetric first-order and second-order modes by attaching a strip-shaped metal film near the center electrode among the three pairs of electrodes of the triple mode filter. However, the shape of the metal to be attached may be elliptical or circular, and the material to be attached is not limited to metal, but may be a dielectric material, an insulating material, or an adhesive material. In short,
Any substance can be used as long as it is a substance that degrades the Q value of the symmetric primary and secondary modes to a predetermined value and is stable under environmental conditions.

[0017]

Since the present invention is configured as described above, the group delay time deviation is halved while maintaining the amplitude characteristics of the triple mode filter at the same level. Intermediate frequency filter (IF) of terminal
When used in the system, an excellent effect of greatly improving the communication quality can be obtained.

[Brief description of the drawings]

FIG. 1A is a plan view showing a configuration of a triple mode filter according to the present invention, FIG. 1B is a cross-sectional view thereof, and FIGS.
FIG. 4 is a diagram showing a displacement distribution.

FIG. 2 is a diagram showing an amplitude characteristic α and a group delay time characteristic β in the filter characteristics of the triple mode filter according to the present invention.

3A is a plan view showing a configuration of a conventional dual mode filter, and FIG. 3B is a cross-sectional view thereof.

FIG. 4 is a diagram showing an amplitude characteristic α and a group delay time characteristic β of a conventional dual mode filter.

5A is a plan view showing a configuration of a conventional triple mode filter, and FIG. 5B is a cross-sectional view thereof.

6A is a diagram showing an electrical equivalent circuit of a triple mode filter, and FIG. 6B is a diagram showing an equivalent circuit of each resonance mode.

FIG. 7 is a frequency-reactance characteristic curve of a triple mode filter.

FIG. 8 is a diagram showing filter characteristics of a triple mode filter, showing an amplitude characteristic α and a group delay time characteristic β.

[Explanation of symbols]

 1. Piezoelectric substrate 2a, 2b, 3a, 3b, 4a, 4b Electrode 5a, 5b, 6a, 6b, 7a, 7b Lead electrode 8 Metal film g1 Electrode gap

Claims (4)

[Claims] 1. A triple mode piezoelectric filter in which three pairs of electrodes are arranged on a piezoelectric substrate, wherein a thin film having a shape smaller than the electrodes is attached near an electrode arranged in the center. 2. In a triple mode piezoelectric filter in which three pairs of electrodes are arranged on a piezoelectric substrate, when the dimension of the centrally arranged electrode in the direction of arrangement of the three pairs of electrodes is L, it is orthogonal to the direction of arrangement of the electrodes. and, triple, wherein the width W ₁ at a position distance between electrodes W2 leaves L / 2 or more from the center electrode end a straight line passing through the center of the center electrode is attached to the following film 0.8L Mode piezoelectric filter. 3. The triple mode piezoelectric filter according to claim 3, wherein said thin film is a viscous substance. 4. The triple mode piezoelectric filter according to claim 1, wherein a group delay time deviation in a pass band is reduced from before the additional thin film is attached.