JPH1117494A - Multiplex mode saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To flatten a group delay time characteristic while keeping insertion loss and attenuating inclination by arranging reflectors at the both sides of IDT, clogging surface wave energy between the reflectors so as to permit acoustic connection to occur and strongly exciting first, third and fifth modes so as to constitute a filter. SOLUTION: Three IDTs 2-4 are arranged on the main surface of a piezoelectric substrate 1, for example, a 45 deg.-X cut lithium of tetraboric acid substrate along the propagating direction of surface wave, the reflectors 5a and 5b are arranged at the both outer sides of IDT 2-4 and also grating electrodes 6a and 6b are provided among IDTs 2-4. In the filter, the energy of surface wave excited by IDT 2-4 is clogged between the reflectors 5a and 5b so as to be acoustically connected and, in result, the vertical first, third and fifth resonance modes are strongly excited. Then, center IDT 2 is made to be an input terminal, IDTs 3 and 4 are mutually connected to as to be output terminals and, then, a triple mode filter is constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-mode filter, and more particularly to a first-order third-order fifth-order longitudinally-coupled triple-mode SAW having improved delay time characteristics in the tens of MHz to 1 GHz band.
Regarding filters.

[0002]

2. Description of the Related Art Recently, portable telephones in the 800 MHz to 1.9 GHz band have rapidly spread, and with this, the 1st receiver of the receiving section has been developed.
As an IF filter, a demand for a middle band SAW filter (having a relative bandwidth of about 0.5%) is increasing. FIG. 3 (a)
FIG. 3 is a schematic plan view showing an example of an electrode pattern of a conventional first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter, in which the IDT is excited or received on the main surface of the piezoelectric substrate 11 in a propagation direction of a surface wave. The three IDTs 12, 13 and 14 are arranged close to each other, and reflectors 15a and 15b are arranged on both sides of the IDT. IDT12,
The IDTs 12 and 14 each include a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween.
Is connected to the input terminal and the other is grounded. One of the IDT 13 and IDT 14 is connected to an output terminal by being connected to each other.
The other interdigital electrodes of DT13 and IDT14 are connected to each other and grounded. Further, as shown in FIG. 3A, the center-to-center spacing G of the innermost electrode fingers facing the central input IDT 12 and the output IDTs 13 and 14 on both sides thereof is 0.5λ (where λ is the surface to be excited). Wave wavelength), and other parameters are appropriately set, for example, the number of electrode pairs of the input IDT is 57, and the number of electrode pairs of the output IDTs 13 and 14 is 5 respectively.
Set to 7 pairs.

As is well known, the operation of the first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter having such a structure is known as ID.
Surface waves excited by T12, 13, 14 are confined between reflectors 15a, 15b, and the IDT 12,
As a result, acoustic coupling occurs between the first and third order,
The fifth-order three longitudinal resonance modes are strongly excited, and operate as a first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter using these modes. It is well known that the pass band of the triple mode SAW filter is proportional to the frequency difference between the first resonance mode and the fifth resonance mode. FIG.
As shown in (b), first-order third-order fifth-order longitudinally coupled triple mode SA
It is a well-known means that a plurality of W filters are juxtaposed on a piezoelectric substrate and connected in cascade to improve the attenuation gradient and the guaranteed attenuation of the SAW filter.

[0004]

However, when a first-order third-order fifth-order longitudinally coupled triple-mode SAW filter is configured with the electrode pattern configuration of FIG. 3B cascaded in two stages,
There is a disadvantage that the group delay time deviation increases. As an example, lithium tetraborate is applied to the piezoelectric substrate and the center frequency is set to 7
1MHz, bandwidth 370kHz, impedance 1.5
When the requirement of kΩ is calculated based on the conventional design method, the following parameters are obtained. That is, the number of electrode pairs N1 of the input IDT 12 is 57 pairs, the number of electrode pairs N2 of the output IDTs 13 and 14 is 57 each, the logarithmic ratio N1 / N of the input IDT 12 to the total number of electrode pairs of the IDT is 0.33, and ID
The distance between the centers of the innermost electrode fingers facing each other between T12 and IDTs 13 and 14 is 0.40λ, the number of reflectors is 15a,
The number of 15b is 100 each. FIG. 4A shows the attenuation characteristics and the pass band characteristics when the filter characteristics of the first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter are simulated using the parameters, and FIG. This is shown in FIG.

As is apparent from the filter characteristic of FIG. 4A, when the ratio B / A of the 20 dB attenuation bandwidth B to the 3 dB bandwidth A is defined as the steepness S, the value of the steepness S at this time is as follows. This is extremely good at about 1.48. However, FIG.
As is clear from (b), the group delay time deviation becomes flat near the center frequency in the pass band, but the group delay time deviation becomes extremely 4.8 μs as the frequency moves away from the center frequency to the lower frequency side. There is a disadvantage that it becomes larger. In order to cope with the above-described disadvantage of the group delay time deviation, conventionally, measures such as reducing the number of reflectors and reducing the film thickness have been performed. That is, the group delay time deviation is reduced by intentionally deteriorating the Q of the SAW filter and sacrificing the steepness of the filter. However, although the group delay time deviation can be slightly improved by deteriorating Q, the steepness of the filter is significantly degraded, and the insertion loss of the filter is greatly increased. The present invention has been made to solve the above-described drawbacks, and has as its object to provide a multi-mode SAW filter in which the group delay time deviation is reduced while maintaining the steepness and the insertion loss.

[0006]

According to a first aspect of the present invention, there is provided a multi-mode SAW according to the present invention, wherein three IDTs are provided on a lithium tetraborate substrate along a surface acoustic wave propagation direction. And reflectors on both sides of the IDT, and a grating electrode provided between the IDTs to confine the energy of the surface wave between the reflectors to generate acoustic coupling, thereby achieving the first-order third-fifth-order mode. This is a multi-mode SAW filter characterized in that a filter is constructed by strongly exciting the filter. The invention according to claim 2 is
Three IDs along the propagation direction of the surface acoustic wave on the piezoelectric substrate
T and reflectors on both sides of the IDT,
Primary 3 with grating electrode provided between DT
In the fifth-order longitudinally coupled multi-mode SAW filter, when the number of electrode pairs of the three IDTs is N and the number of electrode pairs of the central IDT is N1, the following relationship is satisfied: 0.30 ≦ N1 / N ≦ 0.54. Is a multi-mode filter. According to a third aspect of the present invention, when the distance between the centers of the innermost electrode fingers of the center IDT electrode and the grating electrodes on both sides thereof facing each other is defined as L1, the relationship of 0.38λ ≦ L1 ≦ 0.44λ is satisfied. A multi-mode SAW filter according to claim 2, wherein: The invention according to claim 4 is the multi-mode SAW filter according to claim 2 or 3, wherein when the number of the grating electrodes is equal to M, the relationship of 18 ≦ M ≦ 48 is satisfied. The invention according to claim 5, wherein the center-to-center distance between the innermost electrode fingers facing each other between the outer IDT electrodes of the three IDTs and the grating electrode adjacent to the IDT electrodes is L2. The multi-mode SAW filter according to claim 2, 3 or 4, wherein the relationship of 44λ ≦ L2 ≦ 0.54λ is satisfied.

[0007]

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 plan view showing an embodiment of an electrode pattern of a first-order third-order fifth-order longitudinally coupled triple-mode SAW filter (hereinafter, referred to as a triple-mode filter) according to the present invention. Piezoelectric substrate 1, for example, 45
Three IDs on the main surface of the X-cut lithium tetraborate substrate
T2, IDT3 and IDT4 are arranged along the propagation direction of the surface wave, reflectors 5a and 5b are arranged on both outer sides of these IDTs, and grating electrodes 6a and 6b are provided between the IDTs 2, 3 and 4. . Each of the IDTs 2, 3, and 4 is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and one of the IDTs 2 is used as an input terminal and the other comb-shaped electrode is grounded. Further, one of the IDT3 and IDT4 interdigital electrodes is connected to each other to form an output terminal, and the other of the IDT3 and IDT4 is grounded.

The operation of the triple mode filter according to the present invention is based on the fact that the energy of the surface waves excited by the IDTs 2, 3, and 4 is confined between the reflectors 5a and 5b and acoustically coupled. Third and fifth order resonance modes are strongly excited. The center IDT2 is used as an input terminal, and I
By connecting the DTs 3 and 4 to each other to form an output terminal, it is possible to configure a triple mode filter using first-, third-, and fifth-order resonance modes. The bandwidth of the filter is substantially determined by the frequencies of the first and fifth longitudinal resonance modes. At this time, even second-order, fourth-order, and other even-order modes are acoustically excited, but the charges generated by the even-order mode are canceled out and are not excited.

The inventor of the present application has studied various parameters in order to make the group delay time deviation in the pass band as flat as possible in the triple mode filter. As a result, the number of electrode pairs of the input IDT2 is N1, and the output IDT3,
When the total number of electrodes of N2 and three IDTs 2, 3, and 4 is N (N = N1 + 2 × N2), the ratio N1 / N of the number of electrode pairs N1 of the input IDT2 to the total number N of electrodes is N1 / N.
Satisfies the relationship of 0.30 ≦ N1 / N ≦ 0.54 (1), and the input IDT2 and the grating 6a,
6b, the distance between the centers of the innermost electrode fingers facing each other is L
When the distance is set to 1, the center-to-center interval L1 is represented by the wavelength λ of the surface wave to be excited, satisfying the following relationship: 0.38λ ≦ L1 ≦ 0.44λ (2), and the number of gratings 6a and 6b is M In the case of a book, the M satisfies the relationship of 18 ≦ M ≦ 48 (3), and the grating 6a and the IDT3
When the distance between the centers of the innermost electrode fingers of the grating 6b and the IDT 4 facing each other is L2, if L2 is set to a value satisfying the relationship of 0.44λ ≦ L2 ≦ 0.54λ (4), the triple mode It has been found that the group delay time deviation in the pass band of the filter can be greatly improved.

For example, if a lithium tetraborate substrate having a large electromechanical coupling coefficient is used as a piezoelectric substrate and the same requirements as those used in FIG. As input IDT2
The number of electrode pairs is 52, and the number of electrode pairs for IDT3 and IDT4 is 36.
Input I for the total number N of electrodes of IDT2, 3, 4
The ratio N1 / N of the number of electrode pairs N1 of DT2 is 0.42, and the input I
The center-to-center spacing L1 between the innermost electrode fingers of the DT2 and the gratings 6a and 6b facing each other is 0.42λ, the number M of the gratings 6a and 6b is 33 each, and the gratings 6a and IDT3 and the gratings 6b and IDT4 face each other. The distance L2 between the centers of the innermost electrode fingers is 0.4
A parameter of 8λ and 100 reflectors 5a and 5b is obtained. Using these parameters, Figure 1
1 in which the electrode patterns shown in FIG.
When the filter characteristics are simulated based on the electrode pattern of (b), the attenuation characteristics and the passband characteristics are as shown in FIG.
In addition, the passband characteristics and the group delay time deviation characteristics obtained the filter characteristics shown in FIG. As is clear from FIG. 2B, the group delay time deviation has a substantially symmetrical characteristic with respect to the center frequency, the group delay time deviation is 1.8 μS, and the conventional group delay time deviation shown in FIG. About 3μS from the value
You can see that it has been greatly improved. In this case, 3 dB
The steepness S defined as the ratio of the 20 dB attenuation bandwidth to the bandwidth is about 1.48, and the group delay time deviation can be significantly improved without deteriorating the steepness. Since the Q value of the triple mode filter does not deteriorate, a filter having a small insertion loss can be realized.

That is, in the electrode pattern configuration shown in FIG.
It is clear that if a triple mode filter is created using parameters satisfying the relations of the equations (1) to (4), the group delay time deviation can be significantly improved without deteriorating the steepness and insertion loss of the filter. Was. It goes without saying that the triple mode filter according to the present invention may be applied to a filter in which three or more stages are cascaded. Also, a case has been described in which a lithium tetraborate substrate is taken as an example of the piezoelectric substrate, but it goes without saying that quartz, LiTaO3, LiNbO3, ceramic, or the like may be used.

[0012]

Since the present invention is constructed as described above, it is possible to greatly improve the group delay time deviation of the triple mode filter without deteriorating the attenuation gradient and the insertion loss. A steep triple mode filter with low loss was realized. The triple mode filter can sufficiently satisfy the severe group delay time deviation required for a wireless device such as a recent mobile phone, and has a remarkable effect in improving the performance of the wireless device.

[Brief description of the drawings]

FIG. 1A is a plan view of an electrode pattern showing an embodiment of a first-order third-order fifth-order longitudinally coupled triple-mode SAW filter according to the present invention, and FIG. 1B is a two-stage cascade-connected first-order third-order fifth-order. Vertically coupled triple mode SAW filter.

FIG. 2 shows a first-order third-order fifth-order longitudinally coupled triple mode S according to the present invention.
FIG. 7A is a diagram illustrating a two-stage cascade connection filter characteristic of the AW filter, in which FIG. 7A illustrates a pass band and an attenuation band characteristic, and FIG. 7B illustrates a pass band characteristic and a group delay time deviation characteristic.

FIGS. 3A and 3B are plan views showing electrode patterns of a conventional first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter; FIGS.
Is an electrode pattern of a first-stage and two-stage cascade-connected first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter.

FIG. 4 shows two-stage cascade connection characteristics of a conventional first-order third-order fifth-order longitudinally-coupled triple-mode SAW filter, where (a) is a passband and attenuation band characteristic, and (b) is a passband characteristic and a group delay time deviation characteristic. FIG.

[Explanation of symbols]

 1. Piezoelectric substrate 2, 3, 4 IDT 5a, 5b Reflector 6a, 6b Grating

Claims (5)

[Claims] An IDT is provided on a lithium tetraborate substrate along a propagation direction of a surface acoustic wave, and reflectors are provided on both sides of the IDT. A grating electrode is provided between the IDTs. By confining the energy of the surface wave to generate acoustic coupling,
A multi-mode SAW filter characterized in that a filter is constructed by forcibly exciting a next mode. 2. A primary and tertiary arrangement in which three IDTs are arranged on a piezoelectric substrate along a propagation direction of a surface acoustic wave and reflectors are arranged on both sides of the IDTs, and a grating electrode is provided between the IDTs. In the fifth-order longitudinally coupled multimode SAW filter, the number of electrode pairs of the three IDTs is N,
A multimode filter characterized by satisfying a relationship of 0.30 ≦ N1 / N ≦ 0.54 when the number of electrode pairs of the DT is N1. 3. The relationship of 0.38λ ≦ L1 ≦ 0.44λ, where L1 is the distance between the centers of the innermost electrode fingers of the center IDT electrode and the grating electrodes on both sides of the center IDT electrode that face each other. The multi-mode SAW filter according to claim 2. (Where λ is the wavelength of the excited surface wave) 4. The multi-mode SAW filter according to claim 2, wherein a relationship of 18 ≦ M ≦ 48 is satisfied when the number of the grating electrodes is equal to M. 5. The outer IDT of the three IDTs
3. The relationship of 0.44.lamda..ltoreq.L2.ltoreq.0.54.lamda. When the center-to-center spacing of the innermost electrode fingers of the electrode and the IDT electrode and the adjacent grating electrode facing each other is L2. 5. The multi-mode SAW filter according to claim 3, or claim 4.