JP3201017B2 - Ladder type surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ladder type surface acoustic wave filter in which a plurality of surface acoustic wave resonators are connected in a ladder type.

[0002]

2. Description of the Related Art FIG. 2 shows an example of a conventional ladder type surface acoustic wave filter as an equivalent circuit. Ladder type surface acoustic wave filter 1, a plurality of surface acoustic wave resonators a ₁ ~a _n, b ₁
Ｂb _n are connected in a ladder shape as shown in the figure.
That is, during a plurality of surface acoustic wave resonators a ₁ ~a _n are inserted in the series arm, while the series arm, respectively, the surface acoustic wave resonator b ₁ ~b _n ground potential To form a plurality of parallel arms. As the ladder type filter, a filter using another piezoelectric resonator, a quartz resonator, or the like in addition to the surface acoustic wave resonator is known.

FIG. 4 shows an example of the above-mentioned ladder type surface acoustic wave filter in which a series arm is formed by surface acoustic wave resonators a ₁ and a ₂ , and one parallel arm having a surface acoustic wave resonator b ₁ is provided. The ladder-type surface acoustic wave filter 2 is shown.
(FIG. 3 shows this as an equivalent circuit.)

As shown in FIG. 4, a ladder type surface acoustic wave filter 2 has three interdigital transducers (IDTs) 4 to 6 for forming a surface acoustic wave resonator on the surface of a rectangular piezoelectric substrate 3. Is formed. Each I
Each of the DTs 4 to 6 is configured by arranging a pair of comb-shaped electrodes each having a plurality of electrode fingers such that the electrode fingers are mutually inserted. IDT4
6 are provided with grating reflectors 7 respectively.
a, 7b to 9a, 9b are arranged.

The surface acoustic wave resonators constituted by the IDTs ₄ and 6 correspond to the surface acoustic wave resonators a ₁ and a ₂ inserted in the series arm of FIG. 3, respectively. Also, ID
SAW resonator constituted by T5 corresponds to the surface acoustic wave resonator b ₁ which is connected to a parallel arm shown in FIG.

In the surface acoustic wave filter 2, the surface acoustic wave
Wave resonator a₁And surface acoustic wave resonator a _TwoAre the same frequency characteristics
Surface acoustic wave resonator b₁Anti-resonator frequency
Number f _apIs a surface acoustic wave resonator a₁, A_TwoResonance frequency f
_rsIt is configured to match.

The input / output terminals 10a, 10a,
FIG. 6 shows the suppression-frequency characteristics between 10b. Figure
As is evident from FIG. 6, a series arm surface acoustic wave resonator
a₁, A _TwoResonance frequency f_rsA band whose neighborhood is a pass band
A band-pass filter has been realized. Note that FIG.
And f_rpAnd f_apAre surface acoustic wave resonators b, respectively.
₁And the anti-resonance frequency of_rsAnd f_as
Is a surface acoustic wave resonator a₁, A _TwoResonance frequency and
Vibration frequency.

In FIGS. 3 and 4, three surface acoustic wave resonators are connected. However, if a greater amount of suppression is required, as shown in FIG. Are connected in multiple stages.

[0009]

The above-described conventional ladder type surface acoustic wave filter 2 has a problem that a ripple is generated at a position indicated by a point A in FIG. The presence of such ripples does not increase the pass bandwidth. Therefore, the advantage of the ladder-type surface acoustic wave filter having a low loss and a wide band cannot be fully utilized.
An object of the present invention is to provide a structure capable of effectively suppressing a ripple generated in the vicinity of a passband and realizing a ladder-type surface acoustic wave filter having a wider band.

[0010]

The present invention provides a piezoelectric substrate,
The piezoelectric substrate is formed on at least one
Ladder with serial arms and at least one parallel arm
A plurality of surface acoustic wave resonators connected to the mold,
The resonance frequency of the SAW resonator in the series arm and the
Ladder-type elasticity that almost matches the anti-resonance frequency of a surface acoustic wave
In the surface acoustic wave filter, each surface acoustic wave of the
An additional capacitor is connected in parallel with the pendulum, and
Let the anti-resonance frequency of the surface acoustic wave resonator be f_as, Series arm electrostatic
Capacity is C _os, The resonance frequency of the surface acoustic wave resonator in the parallel arm
f_rp, The capacitance of the parallel arm is C_opAnd when

[0011]

(Equation 2)

This is a ladder type surface acoustic wave filter in which the magnitude of the additional capacitance is selected so that the vicinity of the frequency point f _sp represented by the above equation (1) falls within the pass band.

[0013]

In the ladder-type surface acoustic wave filter according to the present invention, the additional capacitance is selected so that spurious resonance occurring near the frequency f _sp represented by the equation (1) falls within the pass band. _Therefore, the impedance at the point f _sp is reduced, thereby effectively suppressing the ripple in the pass band.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. The impedance characteristic of the conventional ladder type surface acoustic wave filter shown in FIG. 4 is shown in the Smith chart of FIG. In FIG. 7, point A corresponds to a frequency point f _sp at which spurious resonance occurs, and A in FIG.
Equivalent to a point. To improve the ripple at point A in Figure 6, lowers the impedance at the point A, near the passband, i.e. it is brought close to the impedance at f _ap.

[0015] The frequency f _sp is be determined from the resonance condition of the surface acoustic wave resonator a ₁ or a surface acoustic wave resonator a ₂ series arm in FIG. 3, the surface acoustic wave resonator b ₁ parallel arm it can. Assuming that the impedances of the surface acoustic wave resonators of the series arm and the parallel arm are Z ₁ and Z ₂ , respectively, Z
The spurious frequency at the frequency point f _sp is obtained from the resonance condition of ₁ + Z ₂ = 0, and is approximately given by the following equation (1).

[0016]

(Equation 3)

In equation (1), f_rpIs the elasticity table of the parallel arms
Surface wave resonator b₁The resonance frequency of f _asIs the elasticity table of the serial arm
Let the anti-resonance frequency of the surface wave resonator be f_rpIs the elastic surface of the parallel arms
Let the resonance frequency of the wave resonator be C_opAre the SAWs of the parallel arms
Let the pendulum capacitance be C_osIs the series arm surface acoustic wave resonator
Indicates the capacitance.

Paying attention to the impedance at the frequency f _sp , FIG. 1 shows the structure of the first embodiment of the present invention in which an additional capacitor is connected to the parallel arm in parallel with the surface acoustic wave resonator b ₁ .

In FIG. 1, a surface acoustic wave filter 11 of the present embodiment has an IDT 13 on an upper surface of a rectangular piezoelectric substrate 12.
15 to 15 are arranged such that the propagation directions of the respective surface waves become parallel. Grating reflectors 16a, 16b-
18a and 18b are arranged.

The IDTs 13 and 15 form the surface arm resonators a ₁ and a ₂ of the series arm, respectively.
Also, the IDT 14 allows the surface acoustic wave resonator b of the parallel arm to be used.
₁ is configured. That is, the surface acoustic wave resonators a ₁ and a ₂ forming a series arm are connected in series between the input terminal 20a and the output terminal 20b. Also, the between the surface acoustic wave resonator a ₁ and the surface acoustic wave resonator a ₂ series arm surface acoustic wave resonator b ₁ constituting the parallel arm between the ground potential is connected .

In the present embodiment, one of the comb electrodes 14 of the IDT 14 of the surface acoustic wave resonator b ₁ forming the parallel arm is used.
a is electrically connected to the ground potential as described above, and the additional capacitance 19 is connected between the other comb electrode 14b and the ground potential. The additional capacitance 19 is configured by arranging a pair of comb-shaped electrodes having a plurality of electrode fingers such that their electrode indices are interposed at a predetermined distance.

[0022] Then, the capacitance is taken out by this additional capacitor 19, the frequency f _sp is because they are selected to a size that satisfies the equation (1), by the additional capacitance 19, the impedance at the frequency f _sp Be lowered. Therefore, as shown in FIG. 8, in the ladder type surface acoustic wave filter 11 of the present embodiment, the ripple at the point A is effectively suppressed.

FIG. 9 is a Smith chart showing the impedance characteristics of the surface acoustic wave filter of the above embodiment. The point A in FIG. 9 corresponds to the point A shown in FIG. 8, that is, the frequency f _sp , but almost coincides with the characteristic impedance of the surface acoustic wave filter 11, and therefore, in addition to flattening the pass band, The pass band width of the filter having the suppression amount of 1 dB can be increased by about 20% or more as compared with the conventional surface acoustic wave filter shown in FIG.

Next, an experimental result on the optimum value of the additional capacitance 19 will be shown, and how the magnitude of the additional capacitance should be selected in order to satisfy the above equation (1) will be described. FIG.
Is the width W of the electrode finger of the IDT ₁₄ of the surface acoustic wave resonator b1 in the parallel arm, the logarithm N, and the wavelength λ _{R of the} surface acoustic wave.
-Indicates the amount of suppression outside the pass band when N / λ _R is changed. As is apparent from FIG. 10, _Cop = 0.5 _Cos ,
In both cases of 1.0 C _os and 2.0 C _os , W ·
It can be seen that as N / λ _R decreases, the amount of suppression outside the passband increases.

Further, in FIG. 10, the capacitance C _op of the surface acoustic wave resonator b ₁ parallel arm is generally from 1.0 times the electrostatic capacitance C _os of the surface acoustic wave resonators of the series arm 2. 0 times has been configured to have a large side lobe suppression amount set during, C _op = even _{2.0C os W · N / λ R} > 14
Since it is constant above 00, it is desirable that W · N / λ _R ≦ 1400 in consideration of the electrical resistance loss in the IDT.

FIG. 11 shows the result of _obtaining the impedance at the frequency point _fsp . As is clear from FIG. 10, when W · N / λ _R ≦ 1400, the impedance is at least 100Ω or more.

FIG. 12 shows the result of _calculating the value Cc of the additional capacitance 19 necessary for bringing the impedance at the frequency f _sp close to the vicinity of 50Ω which is the characteristic impedance of the surface acoustic wave filter of this embodiment. FIG. FIG.
2, C _c / C _op when W · N / λ _R ≦ 1400 or less
It is understood that the additional capacitance 19 having a size satisfying ≧ 0.2 may be formed. Note that the range of W · N / λ _R ≦ 1400 does not _{depend on} the value of the capacitance C _c of the additional capacitor 19, as shown in FIG.
This is apparent from the characteristics of the prior art shown in FIG. Therefore,
In the present invention, the range of W · N / λ _R ≦ 1400 is not particularly limited.

In the above embodiment, the additional capacitance 19 is formed by forming a pair of comb-shaped electrodes. However, as shown in FIG. 5, between the surface acoustic wave resonators a ₁ , a ₂ , b ₁ A part of a wiring electrode for electrically connecting the electrodes is formed to have a large area, and electrodes 19a and 19b for taking out capacitance are formed;
Thereby, the additional capacity may be configured. Normally, the piezoelectric substrate 12 is adhered to a grounded metal container or a metal thin film, so that the electrodes 19 for extracting the capacitance are used.
a and 19b have a capacitance to ground. Therefore, as in the embodiment shown in FIG. 1, an additional capacitor having a desired capacitance value can be formed without forming comb-shaped electrodes. The structure shown in FIG.
May be small, for example, can be suitably used for a high frequency filter.

The surface acoustic wave filter according to the embodiment shown in FIGS. 1 and 5 is constituted by using three surface acoustic wave resonators, but a ladder using four or more surface acoustic wave resonators. The present invention can be similarly applied to a surface acoustic wave filter by providing an additional capacitor in parallel with the surface acoustic wave resonator of each parallel arm. Further, in the embodiment shown in FIG. 1, a lithium tantalate substrate is used as the piezoelectric substrate 12, but another piezoelectric substrate or a piezoelectric substrate formed by forming a piezoelectric thin film on an insulating substrate may be used.

[0030]

As described above, according to the present invention, since the additional capacitance having a size satisfying the expression (1) is connected to the surface acoustic wave resonator constituting the parallel arm, the frequency f _sp , The unnecessary ripple in the pass band can be effectively suppressed. Therefore, since the pass band can be widened by reducing the ripple, it is possible to provide a ladder type surface acoustic wave filter having a wide band. Therefore, it is possible to promote the practical use of a ladder-type surface acoustic wave filter, which has been considered to be difficult to achieve a wide band while having a low loss.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating a configuration of a ladder-type surface acoustic wave filter according to an embodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit of a conventional ladder-type surface acoustic wave filter.

FIG. 3 is a diagram showing an equivalent circuit of a conventional ladder-type surface acoustic wave filter using three surface acoustic wave filters.

FIG. 4 is a plan view for explaining a configuration of a conventional ladder-type surface acoustic wave filter.

FIG. 5 is a plan view illustrating a configuration of a ladder-type surface acoustic wave filter according to another embodiment of the present invention.

FIG. 6 is a diagram showing a suppression amount-frequency characteristic of a conventional ladder type surface acoustic wave filter.

FIG. 7 is a Smith chart showing impedance characteristics of a conventional ladder type surface acoustic wave filter.

FIG. 8 shows the suppression amount of the ladder type surface acoustic wave filter according to the embodiment.
The figure which shows a frequency characteristic.

FIG. 9 is a Smith chart for explaining impedance characteristics of the ladder type surface acoustic wave filter according to the embodiment.

FIG. 10 is a diagram showing the relationship between the out-of-band suppression amount of the ladder-type surface acoustic wave filter and W · N / λ _R.

FIG. 11 is a diagram showing the relationship between the impedance at the point f _{sp of the} ladder type surface acoustic wave filter and W · N / λ _R.

FIG. 12 is a diagram illustrating a relationship between a capacitance ratio C _c / C _op and W · N / λ _R.

[Explanation of symbols]

11 ... surface acoustic wave filter 12 ... piezoelectric substrate 13 to 15 ... IDT 19 ... additional capacitance a _1, a ₂ ... series arm surface acoustic wave resonator b ₁ ... parallel arm surface acoustic wave resonator

Claims (1)

(57) [Claims] 1. A piezoelectric substrate, and a plurality of surface acoustic wave resonators formed on the piezoelectric substrate and connected in a ladder shape so as to have at least one series arm and at least one parallel arm. A ladder-type surface acoustic wave filter in which the resonance frequency of the surface arm of the serial arm and the anti-resonance frequency of the surface arm of the parallel arm are substantially matched. An additional capacitance is connected in parallel to the surface acoustic wave resonator, the anti-resonance frequency of the surface arm of the serial arm is f _as , the capacitance of the arm is _Cos , and the surface arm of the parallel arm is _Where f _{rp is} the resonance frequency of the child and C _op is the capacitance of the parallel arm. A ladder type surface acoustic wave filter characterized in that the magnitude of the additional capacitance is selected so that spurious resonance occurring near the frequency f _sp represented by the above formula (1) falls within the pass band. .