JP2003347896A - Surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a means for improving deterioration in attenuation, to solve the problem there is a frequency region, where the attenuation is deteriorated in a low pass side of the neighborhood of the passband in a longitudinally- coupled multi-mode SAW (surface acoustic wave) filter using reflection inverted type IDT (interdigital transducer) electrodes. <P>SOLUTION: A longitudinally-coupled multi-mode SAW filter provided with a plurality of reflection inverted type IDT electrodes on a piezoelectric substrate and reflectors on both the sides thereof, and an SAW rosonator provided with IDT electrodes on the same substrate and reflectors on both the sides of the IDT electrodes are connected in series. A resonance frequency fs and an antiresonance frequency fa of a composite resonator in which an electric element is connected to the SAW resonator in series are made to meet the relation of fa<fs, and the antiresonance frequency fa is set in the frequency region of the deterioration in attenuation occuring in the low pass side of the neighborhood of the passband in a longitudinally-coupled multi-mode SAW filter. <P>COPYRIGHT: (C)2004,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter (hereinafter, referred to as a SAW filter), and more particularly, to a longitudinally coupled multiple mode S using a reflection inversion type IDT electrode as an IDT electrode.
The present invention relates to a surface acoustic wave filter in which attenuation deterioration that occurs on a low frequency side near the pass band of an AW filter is improved.

[0002]

2. Description of the Related Art In recent years, SAW filters have been widely used in the field of communications, and have been used particularly in portable telephones and the like because of their excellent characteristics such as high performance, small size, and mass productivity. Among them, resonator-type SAW filters are often used as RF filters because of their low insertion loss. FIG.
(A) is a kind of resonator type SAW filter, which is a primary-tertiary third-order longitudinally coupled double mode SA using first- and third-order longitudinal modes.
W filter (hereinafter referred to as dual mode SAW filter)
It is a top view which shows the structure of. That is, three IDT electrodes 2 are formed on the main surface of the piezoelectric substrate 21 along the propagation direction of the surface wave.
2, 23, 24 in close proximity and their I
Grating reflectors (hereinafter, referred to as reflectors) 25a, 25b are provided on both sides of the DT electrodes 22, 23, 24.
Each of the IDT electrodes 22, 23, and 24 is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. One of the comb-shaped electrodes of the central IDT electrode 22 is connected to the input terminal IN by means such as wire bonding. Connected and the other comb electrode is grounded. Furthermore, both outer IDT electrodes 2
The comb electrodes 3, 24 at the bottom in the figure are short-circuited to each other and connected to the output terminal OUT, and the other comb electrodes are grounded to form a dual mode SAW filter.

As is well known, the center frequency of a dual mode SAW filter is proportional to the propagation velocity V of a surface wave, and
It is inversely proportional to the electrode period λ of the electrodes 22, 23, 24. The pass band width of the dual mode SAW filter has a resonance frequency f1 of the vertical first mode and a resonance frequency f of the vertical third mode.
The impedance is proportional to the difference (f1−f3) from 3 and its impedance depends on the number of electrode finger pairs, cross length, and the like. Also, as a means for expanding the pass band, it is common practice to widen the outermost electrode finger of the central IDT electrode 22.

FIG. 6 (b) shows an I
FIG. 4 is a schematic diagram showing filter characteristics in the vicinity of a pass band of a dual mode SAW filter using normal type IDT electrodes as DT electrodes 22, 23, and 24. Deterioration occurs essentially. As shown in the plan view of FIG. 7, the dual mode SAW filter F shown in FIG.
And an IDT electrode 2 on the same piezoelectric substrate 21.
6 and reflectors 27a, 28b on both sides
A SAW filter in which W resonators R are formed and these are connected in series has been put to practical use.

FIG. 8 is a diagram for explaining the reason why the attenuation in the frequency region Q is improved. The passband characteristic of the dual mode SAW filter F and the reactance characteristic of the SAW resonator R are plotted on the frequency axis. It is a figure which was made to be common and overwritten. S
By setting the resonance frequency fs of the AW resonator R within the pass band of the dual mode SAW filter F and setting the anti-resonance frequency fa in the frequency region Q, the high-frequency current is greatly reduced at the anti-resonance frequency fa. The attenuation of Q can be compensated for and the attenuation can be improved. On the other hand, since the resonance frequency fs is set in the pass band, the insertion loss in the pass band is hardly affected.

FIG. 9 shows regular IDT electrodes 22, 23 and 24.
In place of the above, a dual mode SAW filter F2 formed using reflection inversion type IDT electrodes 32, 33 and 34, a regular type IDT electrode 36 on the same piezoelectric substrate and reflectors 37a and 37b on both sides thereof are arranged. FIG. 7 is a schematic plan view showing a SAW filter configured by connecting a SAW resonator R2 formed in series with the SAW resonator. The filter characteristic in the vicinity of the pass band of the dual mode SAW filter F2 using the reflection inversion type IDT electrode has a region P in which the attenuation is deteriorated in the lower band near the pass band as shown by F2 in FIG. Occurs.

Here, the reflection inversion type IDT electrode will be described. The reflection inversion type electrode has been described in detail in the preceding patent application (Japanese Patent Application No. 10-023918).
Here, a brief description will be given. As shown in FIG. 11A, the reflection inversion type IDT electrode has a first electrode finger 18 having a width W1.
And the second electrode finger 19 having a width W2 with a gap g1 to the right in the figure, and a third electrode finger W3 having a gap g2 with a gap g2 to the right in the figure.
Of the electrode finger 20 and a unit section composed of a space of (g3) / 2 on both sides of the electrode fingers 18 and 20, that is, a unit section composed of three electrode fingers per wavelength λ is repeatedly arranged on the piezoelectric substrate. Things. Further, the width W1 of the first electrode finger 18 and the width W3 of the third electrode finger 20 are set to W1 = W3, the gap g1 between the first electrode finger 18 and the second electrode finger 19, and the second The gap g2 between the electrode finger 19 and the third electrode finger 20 is defined as g1 = g2. Then, the electrode fingers 18 and 20 are driven in the opposite phase to the electrode finger 19.

FIG. 11B is a cross-sectional view taken along line AA of FIG. 11A.
This shows the surface potential at a certain moment when the DT electrode is driven. It can be seen that the potential is symmetric with respect to the second electrode finger 19, and the center of excitation is at the center of the second electrode finger 19. As shown below, I using three electrode fingers for one wavelength λ
A reflection coefficient Γ1 (reflection vector) per unit section of the DT electrode is obtained. As shown in FIG. 12A, the reflection vectors E1 to E6 (Ei (i = i = i) in an arbitrary section of the IDT electrode, that is, short-circuit conditions from the six edge surfaces E1 to E6 at both ends of the electrode fingers 18 to 20 1 to 6) indicate an edge surface and also indicate a reflection vector from the edge), and six reflection vectors E1 to E6 are obtained as shown in FIG. In this case, the center of the second electrode finger 19 shown in FIG. 12A is used as a reference for reflection so that the normal type IDT electrode uses the center of the electrode finger as the excitation center as a reference, and the reflection at that position is used. The coefficient is illustrated. The composite vector of these reflection vectors E1 to E6 is shown in FIG.
As shown in (b), the reflection vector becomes Г1.

The reflection vector Г1 with respect to the excitation center
Represents a phase of π / 2, which is different from the reflection vector of the regular IDT electrode (−π / 2 at the center of the electrode finger). Therefore, the phases of the two differ by π. Since the phase rotation of the reflection coefficient contributes to the reciprocation of the wave, it corresponds to a movement of the reflection surface by half of the phase. That is, the reflection center differs from the normal type by λ / 4.

[0010]

However, the dual mode SAW filter F2 is formed using the reflection inversion type IDT electrode as shown in FIG. 9, and the SAW filter using the normal type IDT electrode on the same piezoelectric substrate. The SAW filter formed by forming the resonators R2 and connecting them in series has the following problems in the filter characteristics near the pass band.
That is, in order to improve the deterioration of the attenuation in the frequency domain P, S
When the anti-resonance frequency fa of the AW resonator R2 is set in the region P, the resonance frequency fs is located on the lower frequency side. In the pass band of the dual mode SAW filter F2, the reactance of the SAW resonator R2 becomes capacitive, so that the attenuation loss in the region P is improved, but the insertion loss in the pass band of the SAW filter is reversed. However, there was a problem of deterioration. SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and it is an object of the present invention to provide a SAW filter that compensates for deterioration of the attenuation of a dual mode SAW filter using a reflection inversion type IDT electrode and has a small insertion loss. With the goal.

[0011]

According to a first aspect of the present invention, there is provided a surface acoustic wave filter according to the present invention, wherein a plurality of surface acoustic wave filters are provided on a main surface of a piezoelectric substrate along a propagation direction of a surface acoustic wave. A longitudinally coupled multi-mode SAW filter having a reflection inversion type IDT electrode and grating reflectors on both sides thereof, and a surface acoustic wave in which a regular type IDT electrode and grating reflectors are arranged on both sides of the IDT electrode on the same piezoelectric substrate. A surface acoustic wave filter in which a composite resonator in which a negative capacitance element X is connected in parallel to a resonator is connected in series, wherein the resonance frequency fs and the antiresonance frequency fa of the composite resonator are fa <fs.
And the anti-resonance frequency fa is equal to the longitudinal coupling multiple mode SA.
The surface acoustic wave filter is characterized in that the resonance frequency fs is set in the pass band within the frequency range of attenuation deterioration occurring on the low band side near the pass band of the W filter. A longitudinally coupled multi-mode SAW filter comprising a plurality of reflection inversion type IDT electrodes on a main surface of a piezoelectric substrate along a propagation direction of a surface acoustic wave and grating reflectors on both sides thereof, A surface acoustic wave filter in which a regular type IDT electrode and a composite resonator in which a negative reflector X is connected in series to a surface acoustic wave resonator in which grating reflectors are arranged on the same piezoelectric substrate on both sides of the IDT electrode are connected in series. The resonance frequency fs and the anti-resonance frequency fa of the composite resonator are fa <fs, and the anti-resonance frequency fa is a frequency of attenuation deterioration occurring on a low frequency side near the pass band of the longitudinally coupled multimode SAW filter. A surface acoustic wave filter characterized in that a resonance frequency fs is set in a pass band in a region. The invention according to claim 3 is characterized in that the negative capacitance element X is an inductance.
Alternatively, the surface acoustic wave filter according to item 2. The invention according to claim 4 is the surface acoustic wave filter according to claim 1 or 2, wherein the negative capacitance element X is a circuit constituted by an operational amplifier, a resistor, and a capacitor. The invention according to claim 5 is characterized in that the longitudinally coupled multimode SA
The surface acoustic wave filter according to any one of claims 1 to 4, wherein the W filter is a primary-secondary longitudinally coupled dual mode SAW filter. The surface acoustic wave according to any one of claims 1 to 4, wherein the longitudinally coupled multimode SAW filter is a first-order to third-order longitudinally coupled dual-mode SAW filter. Filter.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 is a schematic plan view showing a configuration of a SAW filter according to an embodiment of the present invention, in which three reflection inversion type IDT electrodes are provided on a main surface of a piezoelectric substrate (not shown) along a propagation direction of a surface wave. 1, 2, 3
Are arranged close to each other, and these reflection inversion type IDTs
Reflectors 4a, 4b are arranged on both sides of the electrodes 1, 2, 3,
The reflection inversion type dual mode SAW filter F is formed.
Further, a normal type IDT electrode 5 and reflectors 6a and 6b on both sides thereof are arranged in parallel with the dual mode SAW filter F to form a SAW resonator R. And the dual mode S
One comb electrode of the center IDT electrode 1 in the AW filter F is connected to the input terminal IN, and the other comb electrode is grounded. Further, the IDT electrodes 2 and 3 on both outer sides of the dual mode SAW filter F were grounded at the upper comb electrodes in the figure, and the lower comb electrodes were formed on the same piezoelectric substrate. Short-circuit with the lead electrode,
IDT above the lead electrode and SAW resonator R in the figure
The comb electrode of the electrode is connected, and the other comb electrode of the SAW resonator R is connected to the output terminal OUT. And S
An electric element (negative capacitance element) X exhibiting a negative capacitance is connected in parallel to the bus bar of each comb-shaped electrode of the AW resonator R to form a SAW filter.

A feature of the present invention is that a SAW resonator R is connected in series to a dual mode SAW filter F having three reflection inversion type IDT electrodes, and a negative capacitance element X is connected in parallel to the SAW resonator. It is configured. Where SAW
Consider an electrical equivalent circuit in which a negative capacitance element X is connected in parallel to a resonator R. The electric equivalent circuit of the SAW resonator R is represented by an electric equivalent circuit in which a capacitance Cd is connected in parallel to a series resonance circuit composed of L1, C1, and R1 as indicated by α in FIG. When a negative capacitance element X is connected in parallel to this electrical equivalent circuit, it is represented as shown in FIG.

[0014] With the inductance _{L 0} as shown in FIG. 2 (b) as a negative capacitance element X, the inductance _{L 0} negative capacitance _-C near frequency omega is 0 = -1 / (ω ²
L ₀ ), and an electrical equivalent circuit of the SAW resonator R in which the inductance L ₀ is connected in parallel is represented as shown in FIG. Where L ₀ of the inductance
Is set so as to satisfy C ₀ > Cd, the reactance curve becomes the anti-resonance frequency f as shown in FIG.
a can be set to a frequency lower than the resonance frequency fs.

FIG. 3 is a diagram for explaining the filter characteristics of the SAW filter according to the present invention. In the reflection inversion type double mode SAW filter, the attenuation deterioration occurring in the lower frequency region P near the pass band is improved. It is a figure explaining the reason why it is done. That is, the filter characteristic F near the pass band of the reflection inversion type double mode SAW filter F and the SAW resonator R
FIG. 7 is a diagram in which the reactance characteristics of a composite resonator in which a negative capacitance element X is connected in parallel to the above are superimposed with the same frequency axis. When the anti-resonance frequency fa of the composite resonator is set in the region P and the resonance frequency fs is set in the pass band, the high-frequency current is blocked near the anti-resonance frequency of the composite resonator, so that the attenuation in the frequency region P increases. , The deterioration of the attenuation is compensated. On the other hand, since the resonance frequency fs is set in the pass band, the influence on the insertion loss in the pass band can be extremely suppressed.

FIG. 4A shows a SAW resonator R and a negative capacitance element.
In the case where a composite resonator is formed by connecting the
You. The inductance L 'is added to the negative capacitance element X.₀Use
And the inductance L '₀Is negative near frequency ω
Quantity-C '₀= -1 / (ω²L ' ₀) Is equivalent to SA
The inductance L 'is added to the W resonator R.₀Compound connected in series
The reactance curve of the resonator is represented as shown in FIG.
It is. In this case, C '₀To 0 <C ′₀<Cd range
By setting so that the rear shown in FIG.
The resonance frequency fs and the anti-resonance frequency fa of the
a <fs. The characteristics of this composite resonator
C '₀I.e., L '₀Change only the resonance frequency fs
And the anti-resonance frequency fa does not change.

It is well known that means for forming a negative capacitance can be constituted by an operational amplifier, a resistor and a capacitor as shown in FIG. 5, for example, in addition to the inductance. That is, the input and output of the operational amplifier (-) and the directly connected, in terms of connecting the series circuit of the resistor R1 and the capacitor C _R between the output and the input (+), one of the resistors R2 to the connection point Connect the terminal and ground the other terminal. This configuration with the input (+) viewed from the electric equivalent circuit as shown in FIG. 5 (b), the inductance _{L E} appears in a series circuit of a resistor and _{L E = R1R2C R (R1 +} R2). A better inductance conversion circuit using two operational amplifiers is well known.

In the description of FIG. 1, the description has been given of the primary-tertiary longitudinally-coupled dual mode SAW filter using three reflection inversion type IDT electrodes. However, the present invention is not limited to this. The first and second longitudinal modes are excited using two reflection inversion type IDT electrodes, and the longitudinal mode using another resonance mode such as a primary-secondary longitudinally coupled double mode SAW filter using these two resonance modes. Combined multimode SAW
It goes without saying that the same can be applied to the filter.

[0019]

As described above, the present invention is constructed on the lower band near the pass band which essentially occurs in the longitudinally coupled multimode SAW filter without increasing the insertion loss in the pass band. This shows an excellent effect that the attenuation deterioration can be improved.

[Brief description of the drawings]

FIG. 1 is a plan view showing a configuration of a SAW filter according to the present invention.

2A is an electric equivalent circuit of a SAW resonator and an electric element X connected in parallel, FIG. 2B is an inductance,
(C) is an electrical equivalent circuit when an inductance is used for the electric element X, and (d) is a reactance curve of (c).

FIG. 3 is a diagram illustrating the reason why the attenuation deterioration on the low frequency side near the passband is improved in the SAW filter according to the present invention.

FIG. 4A is an electrical equivalent circuit of a composite resonator formed by connecting an electric element X to a SAW resonator in series,
(B) is the reactance circuit.

5A is an equivalent inductor including an operational amplifier, a resistor and a capacitor, and FIG. 5B is an electrical equivalent circuit thereof.

FIG. 6A is a plan view showing a configuration of a conventional dual mode SAW filter, and FIG. 6B is a diagram showing filter characteristics in the vicinity of a pass band.

FIG. 7 is a diagram showing a configuration of a conventional SAW filter in which a dual mode SAW filter and a SAW resonator are connected in series.

FIG. 8 is a diagram illustrating the reason why deterioration of attenuation caused on the high frequency side near the passband is improved in the conventional SAW filter.

FIG. 9 shows a double mode S using a reflection inversion type IDT electrode.
FIG. 3 is a diagram illustrating a configuration of a SAW filter in which an AW filter and a SAW resonator using a regular IDT electrode are connected in series.

FIG. 10 is a diagram for explaining the reason why the deterioration of attenuation caused on the low frequency side near the passband is improved.

11A is a diagram illustrating the configuration of a reflection-reversal type IDT electrode (part), and FIG. 11B is a cross-sectional view illustrating a surface potential on the electrode.

12A is a cross-sectional view showing six edge surfaces (E1 to E6) of a unit section of the reflection inversion type IDT electrode, and FIG. 12B is a view showing reflection vectors E1 to E6 and the reflection vectors E1 to E6 in the six edge surfaces. The composite vector # 1 is shown.

[Explanation of symbols]

1, 2, 3... Reflection inversion type IDT electrodes 4a, 4b, 6a, 6b... Grating reflector 5... IDT electrode X... Electric element F... Reflection inversion type double mode SAW filter R. The constants L ₀ , L ′ ₀ ... Inductance values (−C ₀ , −−) of the elements L1, C1, R1, Cd.
_C'0 )

Claims (6)

[Claims] 1. A longitudinally coupled multimode SAW having a plurality of reflection inversion type IDT electrodes on a main surface of a piezoelectric substrate along a propagation direction of a surface wave and grating reflectors on both sides thereof.
An elastic surface in which a filter, a normal type IDT electrode, and a composite resonator in which a negative capacitance element X is connected in parallel to a surface acoustic wave resonator in which grating reflectors are arranged on the same piezoelectric substrate on both sides of the IDT electrode are connected in series. A wave filter, comprising: a resonance frequency fs and an anti-resonance frequency fa of the composite resonator.
Satisfies fa <fs, and the anti-resonance frequency fa is set in the frequency range of attenuation deterioration occurring on the low frequency side near the pass band of the longitudinally coupled multimode SAW filter, and the resonance frequency fs is set in the pass band. A surface acoustic wave filter. 2. A longitudinally coupled multimode SAW having a plurality of reflection inversion type IDT electrodes on a main surface of a piezoelectric substrate along a propagation direction of a surface wave and grating reflectors on both sides of the IDT electrodes.
An elastic surface in which a filter, a regular type IDT electrode, and a composite resonator in which a negative capacitance element X is connected in series to a surface acoustic wave resonator in which grating reflectors are arranged on the same piezoelectric substrate on both sides of the IDT electrode are connected in series. A wave filter, comprising: a resonance frequency fs and an anti-resonance frequency fa of the composite resonator.
Satisfies fa <fs, and the anti-resonance frequency fa is set in the frequency range of attenuation deterioration occurring on the low frequency side near the pass band of the longitudinally coupled multimode SAW filter, and the resonance frequency fs is set in the pass band. A surface acoustic wave filter. 3. The surface acoustic wave filter according to claim 1, wherein the negative capacitance element X is an inductance. 4. The surface acoustic wave filter according to claim 1, wherein the negative capacitance element X is a circuit including an operational amplifier, a resistor, and a capacitor. 5. The surface acoustic wave filter according to claim 1, wherein said longitudinally coupled multi-mode SAW filter is a primary-secondary longitudinally coupled dual-mode SAW filter. 6. The surface acoustic wave filter according to claim 1, wherein the longitudinally-coupled multi-mode SAW filter is a first-order to third-order longitudinally-coupled dual-mode SAW filter.