JPH10209808A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resonance synthesis type SAW filter in which a group delay time of a pass band is improved. SOLUTION: The resonance synthesis SAW filter is configured by placing two longitudinal connection triple mode SAW filters A, B on a piezoelectric substrate and connecting them in parallel electrically. In this case, let four resonance peak frequencies cased by mis-matching the termination impedance of the filter be F1, F2, F3, F4 in the descending order, then relations of (F3-F4)≈(F1-F2) and 2.0<=F2-F3)/(F1-F2) are in existence.

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, and more particularly, to a filter having a flattened group delay time of a resonance combined type surface acoustic wave filter formed by electrically connecting longitudinally coupled multimode filters in parallel.

[0002]

2. Description of the Related Art Recently, a surface acoustic wave filter (hereinafter referred to as SA) has been developed.
W filters) are excellent in miniaturization, higher frequency, mass productivity, and the like, and are therefore often used as RF and IF stage filters for wireless devices such as mobile phones. In particular, in addition to the conditions of the wideband filter, the first IF filter of a digital communication radio such as recent GSM and CDMA is used.
There is a demand for a SAW filter having a flat group delay time.

A so-called resonance-combined SAW filter in which two coupled multi-mode SAW filters are electrically connected in parallel has been proposed as means for constructing a wide band SAW filter. FIG. 4A shows such a conventional resonance combined type SAW.
It is a plan view showing a schematic electrode pattern of the filter,
Two IDTs 21a and 21c are arranged on the main surface of the piezoelectric substrate 20 along the propagation direction of the surface wave, and reflectors 22a and
2b, and the input / output IDTs 21a and 21c
The middle grating 21b is arranged between the two. A longitudinally-coupled triple-mode filter A (filter A) is constructed using three modes generated as a result of acoustic coupling of surface waves between the reflectors 22a and 22b. Each of the IDTs 21a and 21c includes a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween.
One interdigital electrode constituting 1c is connected to the ground potential terminal, and the other interdigital electrode is electrically connected to the input or output terminal. Also, the middle grating 21
b is grounded. As is well known, the acoustic coupling results in 3
A1 (primary), A2 (2)
Next) and A3 (Third order), and each resonance frequency is F
If a1, Fa2, and Fa3, Fa3 <Fa2 <
The magnitude relationship is Fa1. That is, in the longitudinally-coupled multimode filter, as the mode becomes higher, the resonance point appears at a lower frequency.

Further, two IDTs 23a and 23c and reflectors 24a and 24b disposed on both sides of the IDTs 23a and 23c on the main surface of the piezoelectric substrate 20 along the propagation direction of the surface wave and the input / output IDT
Middle grating 23 arranged between 4a and 24c
b. A vertically coupled triple mode filter B (filter B) is arranged in parallel with filter A. At this time, the three modes of the filter B are changed from the low order mode to B1 (first order), B1
2 (secondary) and B3 (tertiary), and their respective resonance frequencies are Fb1, Fb2, and Fb3. The magnitude relationship is Fb3 <Fb2 <Fb1. Then, the comb-shaped electrodes of the IDTs 21a and 21c of the filter A from the filter B and the filters A of the IDTs 23a and 23c of the filter B
Are connected to one another as an input terminal (IN) and the other as an output terminal (OUT).
The comb electrodes outside the DT are grounded.

The phase relationship between the filters A and B is opposite,
That is, corresponding modes, for example, the modes A1 and B1
Of the filter A so that the phases of
The electrode of the IDT 23c of the filter B is set to λ /
It is configured to be shifted by two in the direction of the reflector 24b. FIG.
(B) shows a transmission characteristic α (frequency-Loss) and a phase characteristic β (frequency-Phase) in which only the filter A is taken out and the terminating condition is changed from an appropriate terminating impedance of the filter A to an extremely small impedance (generally, terminating is mismatched). Thus, by making the mismatch as described above, the graph of the transmission characteristic shows A1 (1
It can be observed that three resonance peaks corresponding to the resonance frequencies of the (second), A2 (secondary) and A3 (third) modes appear. The phase relationship between the above modes is as follows: if the phase of the A1 (primary) mode is 0 degree, the phase of the A2 (secondary) mode is 180 degrees;
The third (tertiary) mode is at 0 degrees.

FIG. 4C shows only the filter B,
The transmission characteristics and the phase characteristics with their terminal impedances mismatched are shown in the graph of the transmission characteristics in the same manner as in (b), in which the longitudinal modes B1 (primary), B2 (secondary) and B3 (3
It can be observed that three resonance peaks corresponding to the resonance frequency of the (second) mode appear. The phase of these modes is B
The phase of the first (first-order) mode is 180 degrees, and B2 (2
Next) has a phase of 0 degrees, and B3 (third order) has a phase of 180 degrees. FIG. 2 shows an arrangement relationship between the resonance frequencies of the filters A and B.
As shown in (b) and (c), Fa2 = Fb3, Fa1 =
The so-called resonance-combined SAW filter C is configured by setting the period of the electrode fingers of the IDT so as to be Fb2 and electrically connecting the filters A and B in parallel.

FIG. 4D shows a resonance combined type SAW filter C.
Are transmission characteristics and phase characteristics in the case where the terminal impedances of the above are mismatched. Four resonance peaks appear in the transmission characteristics, and it is understood that the filter is a fourth-order filter. By properly terminating the filter, a bandpass filter having a wide pass bandwidth including the above four resonance peaks can be obtained. FIG. 5 shows a passband characteristic D of a resonance-combined SAW filter prototyped to satisfy a center frequency of 250 MHz, a bandwidth of 260 kHz or more, and a group delay deviation of 1 μs or less for a first IF filter of a digital cellular phone PCS in the United States.
And the group delay time characteristic E. At this time, the prototype conditions were such that quartz was used for the piezoelectric substrate, and the IDT logarithm of the filter A was 411.
The middle grating pitch is 1.006, the IDT logarithm of the filter B is 333 pairs, the middle grating pitch is 1.006, and the number of middle gratings is 20.
The number of the reflectors is 60. Here, the middle grating pitch refers to the ratio of the pitch of the IDT to the pitch of the middle grating.

[0008]

However, in the above-mentioned conventional resonance combined type SAW filter, the curve E in FIG.
As shown in (1), there is a disadvantage that the group delay time characteristic of the pass band is greatly curved at both ends of the pass band, and the group delay deviation exceeds 1 μs, which does not satisfy the requirement of the first IF of the digital communication system. The present invention has been made in order to solve the above-mentioned drawbacks, and an object of the present invention is to provide a resonance combined type SAW filter having an improved group delay time of a pass band.

[0009]

According to a first aspect of the present invention, there is provided a resonance-combination type SAW filter according to the present invention.
IDTs and reflectors on both sides thereof,
In a resonance-combined SAW filter in which two longitudinally coupled triple-mode SAW filters each having a middle grating arranged between them and electrically connected in parallel, the frequencies of the four resonance points of the filter are set to be higher. From F1, F
2, F3 and F4, (F3-F4) and (F1
−F2) and 2.0 ≦ (F2−F3)
/ (F1-F2) is a surface acoustic wave filter.

[0010]

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 schematic plan view showing a first embodiment of a resonance-combination type SAW filter according to the present invention. IDTs 2a and 2c and reflectors 3a and 3b on both sides thereof are further provided.
The middle grating 2b is disposed between the two gratings 2b to form a longitudinally coupled triple mode SAW filter (filter A).
Each of the IDTs 2a and 2c is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. One of the IDTs 2a and 2c is connected to a ground potential terminal, and the other is shaped as a comb. The electrodes are electrically connected to input or output terminals. The middle grating 2b is grounded.

As in the case of the filter A, three IDTs 4a and 4c and reflectors 5a and 5b on both sides thereof are arranged on the piezoelectric substrate 1 along the propagation direction of the surface acoustic wave.
A middle grating 4b is arranged between 4a and 4c to form a longitudinally coupled triple mode SAW filter (filter B).

The IDT of the filter B and the IDT of the filter B are set so that the phase relationship between the filters A and B is opposite.
The electrode finger of DT is arranged in consideration. FIG. 1B shows a transmission characteristic α (frequency-Loss) and a phase characteristic β (frequency-Pha) in which the terminal impedance of the filter A is mismatched.
se) and A1 (primary), A2 (secondary) in the vertical mode
And three resonance peaks corresponding to the resonance frequency of the A3 (third order) mode. The phase relation of the above modes is A
If the phase of the 1 (primary) mode is 0 degree, A2 (secondary)
The mode is 180 degrees, and the A3 (tertiary) mode is 0 degrees. FIG. 2C shows a transmission characteristic α and a phase characteristic β in which the terminal impedance of the filter B is mismatched, and similarly to FIG. 2B, B1 (primary) and B2 (secondary) in the longitudinal mode.
And three resonance peaks corresponding to the resonance frequencies of the B3 (third order) mode. Since the phases of these modes are arranged in consideration of the polarity of the electrode fingers so as to be opposite to the phase of the corresponding mode of the filter A, B1 (primary)
The phase of the mode is 180 degrees, B2 (second order) is 0 degrees,
B3 (3rd order) has a phase of 180 degrees. Furthermore, as shown in FIGS. 2B and 2C, the arrangement of the electrode fingers of the IDT is set so that the resonance frequencies of the filters A and B are substantially equal to Fa2 = Fb3 and Fa1 = Fb2 as shown in FIGS. And B
Are electrically connected in parallel to form a resonance synthesis type SAW filter.

FIG. 1D shows the transmission characteristic α and the phase characteristic β when the terminal impedance of the resonance combined type SAW filter is mismatched. The transmission characteristic includes four resonance peaks Fa3, Fa2 (= Fb3), and Fa1. (= Fb2) and Fb1 appear. The interval between each resonance peak (Fa2-Fa
3) When (Fa1-Fa2) and (Fb1-Fa2) are a, b, and c, respectively, as shown in FIG. 1D, a and c are made substantially equal to change the value of b / a. The simulation was performed with emphasis on transmission characteristics and group delay time characteristics, especially on the latter. Elements that control the three resonance frequencies of the longitudinally coupled triple mode filter include the IDT pitch, the number of electrode fingers of the IDT, the pitch of the middle grating, and the like. In the present invention, these elements are variously changed, and as a result of enormous calculations, in the above-described resonance-combined SAW filter, the values of a and c are made substantially equal, and then the value of the resonance interval ratio b / a is changed. It was found that the group delay deviation fluctuated. FIGS. 2A, 2B, and 2C show passband characteristics and group delay time characteristics obtained by the above simulation. If only parameters changed for the prototype example shown in FIG. 5 are described, in FIG.
T logarithm 387 pairs, middle grating pitch 0.
998, and the resonance interval ratio b / a may be 1.7. In FIG. 2B, the filter A has a middle grating pitch of 1.0075, the filter B has 361 IDT logarithms, and a middle grating pitch. 0.997 and the resonance interval ratio b / a is 2.1,
In FIG. 2C, the filter A middle grating pitch is 1.008, the filter B IDT logarithm is 333 pairs,
This is the case where the middle grating pitch is 0.993 and the resonance interval ratio b / a is 2.3. As is apparent from FIG. 2, the group delay deviation decreases as the value of b / a increases.

FIG. 3 summarizes the relationship between the resonance interval ratio b / a and the group delay time (μs) based on the simulation results. It can also be seen from the figure that when a and c are substantially equal, the group delay time in the pass band decreases as the resonance interval ratio b / a increases. When the value of b / a is approximately 2, the group delay time is less than 1 μs. In the above description, the case where the quartz substrate is used as the piezoelectric substrate has been described. However, the present invention is not limited to the quartz substrate, and LiTaO 3, LiNbO 3, L
It is needless to say that the present invention can be applied to any piezoelectric material that can excite BO or other surface waves.

[0015]

Since the present invention is configured as described above, the group value delay time can be incorporated into the design by specifying the resonance interval according to the present invention, as compared with the conventional resonance combined type SAW filter. And a SAW filter having an excellent group delay time can be realized. Since the resonance synthesis type SAW filter according to the present invention can sufficiently satisfy the requirements of recent digital communication type filters, the effect is extremely remarkable if the filter according to the present invention is used for a radio device adopting the digital communication method. It is.

[Brief description of the drawings]

FIG. 1A is a schematic plan view of an electrode pattern according to an embodiment of a resonance synthesis type SAW filter according to the present invention.
(B) and (c) are diagrams showing the respective resonance frequency arrays and phase relationships of filters A and B, and (d) is a resonance-combined SAW
It is a transmission characteristic and phase characteristic figure of a filter.

FIGS. 2A, 2B, and 2C are diagrams showing a resonance frequency interval ratio b / a, a filtering characteristic, and a group delay time characteristic of a resonance combined type SAW filter.

FIG. 3 is a diagram illustrating a relationship between a resonance frequency interval ratio b / a and a group delay time characteristic.

4A and 4B are schematic plan views of an electrode pattern of a conventional resonance-combination type SAW filter, and FIGS. 4B and 4C are diagrams showing resonance frequency arrays and phase relationships of filters A and B, respectively. (D) is a diagram showing the transmission characteristics and phase characteristics of the resonance-combined SAW filter.

FIG. 5 is a diagram showing passband characteristics and group delay time characteristics of a conventional resonance-combined SAW filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate 2a, 2c, 4a, 4c ... IDT 2b, 4b ... Middle grating 3a, 3b, 5a, 5b ... Reflector Fa1 ... Primary mode resonance frequency of filter A Fa2: resonance frequency of the secondary mode of filter A Fa3: resonance frequency of the third mode of filter A Fb1: resonance frequency of the first mode of filter B Fb2: resonance frequency of the second mode of filter B Resonance frequency Fb3: Resonance frequency of 3rd mode of filter B 0 °, 180 °: Mode phase C: Resonance synthesis type SAW filter a, b, c: Resonance frequency interval

Claims (1)

[Claims] 1. An IDT having two IDTs arranged on a piezoelectric substrate along a propagation direction of a surface acoustic wave and reflectors on both sides of the IDTs.
In a resonance combined type SAW filter, two longitudinally coupled triple mode SAW filters each having a middle grating disposed between T and juxtaposed and electrically connected in parallel,
The frequencies of the four resonance points of the filter are set to F1,
When F2, F3 and F4 are set, (F3-F4) and (F
1-F2) and 2.0 ≦ (F2-F
3) / A surface acoustic wave filter characterized by (F1-F2).