JP2002368579A - Double track type surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means for reducing the number of pairs of IDT electrodes constituting a double track type surface acoustic wave(SAW) filter and improving characteristics. SOLUTION: The double track-type SAW filter is configured by using a basic block IDT electrode β different at 180 deg., basic block IDT electrode γ without excitation, and a basic block IDT electrode δ with the phase of excitation center different from α at 90 deg. with the phase of the excitation center of a basic block IDT electrode α as a reference together with the electrode α.

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 double track surface acoustic wave filter having improved in-band ripple deviation and out-of-band attenuation while maintaining phase linearity in a pass band. Related.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave devices (hereinafter referred to as SAs) have been developed.
W devices) are widely used in the field of communication, and have excellent characteristics such as high performance, small size, and mass productivity, and are therefore often used particularly in mobile phones and the like. Recently, a mobile phone system using a CDMA system (eg, cdmaOne)
, As an IF filter standard for portable terminals, the pass bandwidth is wide and in-band (1.25 MHz).
The phase linearity of z) is required to be 3.0 ° or less. It is extremely difficult to realize such severe characteristics with, for example, a resonator type surface acoustic wave filter (hereinafter, referred to as a resonator type SAW filter), and it is more suitable to use a transversal type SAW filter having a good phase characteristic. I have.

FIG. 10A shows a transversal type SAW.
FIG. 5 is a plan view showing a configuration of a filter, and shows two IDT electrodes 51 and 52 along a propagation direction of a surface wave of a piezoelectric substrate 50;
And a shielding electrode 5 between the electrodes 51 and 52.
3 is arranged. Each of the IDT electrodes 51 and 52 is formed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed between each other. The shape electrode is connected to the output terminals OUT1 and OUT2 using a bonding wire or the like. The shield electrode 53 is grounded and acts to suppress a direct wave between input and output. FIG.
0 (a), the transmission characteristics of a transversal SAW filter using a normal IDT electrode, that is, the center frequency f
0, the bandwidth B (3 dB) is determined by the characteristics of the piezoelectric substrate (for example, the phase velocity v of the surface wave), the electrode period λ, the number of electrode pairs n, etc., as is well known. / N. Since the surface wave excited by the normal type IDT electrode propagates equally to the left and right, the insertion loss of the transversal type SAW filter is essentially 6 dB or more.

Therefore, a converter invented to improve the insertion loss of a transversal type SAW filter is a so-called one-way converter (SPUDT), and FIG. It is a SAW filter. The IDT electrode 54 can strongly excite the surface wave to the right in the drawing, and the IDT electrode 55 has a configuration symmetrical with respect to the center of the SAW filter 54, so that the surface wave arriving from the left can be reduced. It can be efficiently picked up as an electric signal, and the insertion loss of the filter can be greatly reduced. FIG. 11 shows a basic section I which is a SPUDT.
It is a figure for explaining a DT electrode and its operation. The center coordinate d of each electrode finger when the left end in the figure is the origin.
1, d2, d3 and the electrode finger widths w1, w2, w3 are set as shown in the figure, the basic section IDT electrode (SP
The coordinate d1 of the reflection center of (UDT) is (λ / 4) / λ · 2π =
90 °, and the coordinate d2 of the excitation center is (0.625λ / λ)
It is well known that 2π = 225 °, and the surface wave can be strongly excited rightward in the figure.

However, the basic section IDT electrode (SPUD)
T) Transversal type SA constructed by connecting a plurality of
The W filter has a problem that the insertion band cannot be widened to a desired value although the insertion loss can be reduced. As a means to solve this, for example, US Patent 5,703,
No. 427 discloses DART (Distributed Acoustic Reflec
A transversal SAW filter using an internal resonance type single-phase unidirectional electrode (RSPUDT) using an electrode of an internal resonance type is disclosed. D used here
The ART electrode is, as shown in FIG. 12, a plurality of basic sections IDT having different reflection directions in one IDT electrode.
An electrode (SPUDT) is arranged. In other words, 50 basic section IDT electrodes having a reflection direction in the forward direction + R (the left direction in the figure is defined as the forward direction in the aforementioned U.S. Pat.
0λ), 40 basic section IDT electrodes having reflection directions in the reverse direction -R (right direction in the figure) (40λ), and 20 IDT basic sections having forward reflection directions (20λ) are arranged. FIG. 13 shows an example in which the IDT electrode is used, and FIG. 13 shows the weighting of reflection depending on the position of the IDT electrode. The transversal type filter using the DART type IDT electrode configured as described above has a reduction in insertion loss from 7.4 dB to 6.5 dB and a group delay time of 200 ns as compared with a filter using a conventional electrode thinning method. It is described that it is characterized by halving to 100 ns, increasing the attenuation of the second side lobe, and broadening the pass band.

As basic IDT electrodes constituting a DART electrode, nine types of basic sections I as shown in FIGS.
DT electrodes (E1 to E9) are well known. For example,
There are three types of basic section IDT electrodes having excitation centers at 90 ° and 270 ° where the phases of the excitation centers are different by 180 °, and two types having different reflection centers, respectively. There are a total of nine types (E1 to E9 shown) of the basic section IDT electrodes with a total of three types with no reflection. Using the nine types of basic section IDT electrodes, the above-described internal resonance type single-phase unidirectional electrode (RSPUDT) can be configured. Here, 315 ° with respect to the reflection direction
± 180 ° · n (n is an integer) is “+”, 225 ° ± 18
0 ° · n is defined as “−”. Then E1, E4, E
7 is "-", E3, E6, E9 are "+", E2,
E5 and E8 can be classified as "0" without reflection. Regarding the excitation direction, 270 ° ± 360 ° · n is “+”, 9
0 ° ± 360 ° · n is defined as “−”. Then E
1, E2, E3 are "-", E7, E8, E9 are "+", E4, E5, E6 have no excitation, "0"
Can be classified.

A SAW filter in which the filter characteristics shown in the above-mentioned US patent are further improved is disclosed in Japanese Patent Application Laid-Open No. 2000-77974. That is, as shown in FIG. 16A, two sets of input / output IDT electrodes 1, 2 are formed so as to form two propagation paths (hereinafter referred to as tracks) on the piezoelectric substrate along the propagation direction of the surface wave. This is a double track type SAW filter in which 1 ′ and 2 ′ are juxtaposed. Each of the input IDT electrodes 1, 1 ′ of the tracks 1 and 2, the output IDT electrode 2,
2 'and the input / output IDT electrodes 1 and 1' of the tracks 1 and 2 as shown in FIG.
And 2 and 2 ′ are connected in parallel. Each IDT electrode 1, 2, 1 ', 2' is configured by connecting a plurality of nine types of basic section IDT electrodes shown in FIGS. 14 and 15, for example, as shown in FIG. In order to make the characteristics of the double track type SAW filter close to the desired characteristics, the basic section I
A method of optimizing the combination of DT electrodes has been used. The above publication discloses the excitation IDT electrode of each track, reflection I
An example of weighting for a DT electrode is disclosed.

[0008] The double track type S thus optimized
Although the filter characteristics of the tracks 1 and 2 of the AW filter are different, two tracks 1 and 2 near the center frequency are used.
Are in phase, and are added to each other so as to reinforce the amplitude at the pass band level. On the other hand, in the attenuation range, the phases of the transfer functions of the two tracks 1 and 2 are opposite to each other, and the amplitude is subtracted to act to increase the amount of attenuation. It is described that by optimizing the transfer function of each track in this manner, the number of IDT electrodes in each track can be significantly reduced, and thus the size of the SAW filter can be significantly reduced.

[0009]

However, in the above-described double track type SAW filter, the number of basic section IDT electrodes to be used is as small as nine, for example, as described above. In order to realize a desired transmission characteristic by using the above, there is a problem that the number of IDT electrode pairs increases and the shape of the double track SAW filter becomes large. The present invention has been made to solve the above problems, and has as its object to provide a double-track SAW filter in which desired transmission characteristics are configured with a small number of IDT electrode pairs and the shape of the filter is reduced. .

[0010]

According to a first aspect of the present invention, there is provided a double-track surface acoustic wave filter according to the present invention, wherein a phase of an excitation center of a basic section IDT electrode .alpha. The phase of this and the excitation center is 180
° Different basic section IDT electrode β and basic section I without excitation
In a double track surface acoustic wave filter configured by using a DT electrode γ and the basic section IDT electrode α in a mixed manner, the basic section IDT electrode whose excitation center phase differs from the basic section IDT electrode α by 90 °. δ is used together with the basic section IDT electrodes α, β, and γ to provide an internal resonance single-phase unidirectional double track surface acoustic wave filter.

[0011]

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 plan view showing a configuration of a double track type SAW filter according to the present invention, in which two IDT electrodes 2 and 3 are formed on a main surface of a piezoelectric substrate 1 along a propagation direction of a surface wave with a predetermined gap. The first SAW filter is formed at a distance, and the propagation path of the first SAW filter is referred to as track 1. Two IDT electrodes 4 and 5 are juxtaposed at a predetermined distance from the first SAW filter to form a second SAW filter. The propagation path of the filter is referred to as track 2. I of tracks 1 and 2
One of the DT electrodes 2 and 4 is connected to one of the comb electrodes, and the input terminal IN is connected to one of the IDT electrodes 3 and 5, and one of the IDT electrodes 3 and 5 is connected to the output terminal OUT using a bonding wire or the like. Other comb electrodes are grounded. That is, a double track SAW filter in which the respective SAW filters of the tracks 1 and 2 are connected in parallel is configured.

FIG. 2 shows a double track type SAW of the present invention.
The above nine types of basic section IDT are applied to the IDT electrode of the filter.
It is an electrode pattern figure (E10-E12) which shows the structure of three types of basic section IDT electrodes used with an electrode. Conventionally, IF
Since the filter characteristics are required to be symmetrical with respect to the center frequency with respect to the center frequency, the phase of the excitation center is changed to the basic section IDT electrode E shown in FIGS.
It has been considered that if a basic section IDT electrode different from 1 to E9 by 90 ° is used, it cannot be used because the symmetry of the filter is greatly deteriorated. In the present invention, a basic section IDT electrode not conventionally used as shown in FIG. 2 is actively used in a double track type SAW filter. As shown in FIG. 2, when the origin is the left end of each basic section IDT electrode pattern and the coordinates d1, d2, and d3 are set as shown in FIG. 180 °, 4 reflection centers each
5 °, none, 315 °. If the state of reflection is represented by “+” and “−” defined earlier, E10 is 45 ° = 225 ° −
“−” At 180 ° · n (n = 1), E0 is “0” without reflection, and E12 is 315 ° = 315 ° −180 ° · n
(N = 0) becomes “+”. Regarding the excitation state, the excitation centers of E10, E11, and E12 are near 180 °, are in the middle between “+” and “−”, and are expressed as “N”.

The feature of the present invention is that the IDT electrodes 2, 3, constituting the tracks 1, 2 of the double track type SAW filter, respectively.
4 and 5. That is, in addition to the conventional nine types of basic section IDT electrodes E1 to E9 shown in FIGS. 14 and 15, three types of basic section IDT electrodes E10, E11, and E12 shown in FIG. That is, each IDT electrode of the track type SAW filter is formed. As described above, by using the twelve types of basic section IDT electrodes, the degree of freedom in designing the IDT electrodes is increased as compared with the conventional case, and it is possible to realize desired filter characteristics with a small number of electrode pairs.

The design of a double-track SAW filter is performed by using an optimization method such as a so-called GA (genetic) algorithm as shown in FIG. In the optimization method using this method, for example, 1000 sets of double track type SAW filters are first set, and their transmission characteristics are evaluated. truncate. 200 sets are newly set or copied from 800 sets to 1000 sets, and the GA operation, that is, the exchange of the basic section IDT electrode, is performed on these sets to evaluate the transmission characteristics of the double track type SAW filter obtained. Will do. Each set of IDT electrodes is an ID using a plurality of the above-described 12 types of basic section IDT electrodes.
It is composed of a T electrode. The cycle of FIG. 3A is repeated until the transmission characteristics fall within a predetermined error range. FIG. 3B shows that the input / output IDT electrodes 2 and 3 of the tracks 1 and 2 are formed by using the above-mentioned method so that the IF filter for cdmaOne is constituted by a double track SAW filter.
The basic section IDT electrodes forming the fourth and fifth electrodes are obtained by simulation. IDs that make up each track
The number sequence of T electrodes 2, 3, 4, 5 is the basic section IDT electrode E
The letter E of Ei representing 1 to E12 is omitted.
For example, if the excitation state of the IDT 2 of the track 1 is represented using conventional “+”, “−”, and “0”,
--- NNNNN0NNNNNNNN0N ++++++++
It can be expressed as +｝. Here, "N" is the basic section ID shown in FIG.
Refers to the T electrode.

The filter characteristics shown in FIGS. 4A and 4B are obtained by simulating tracks 1 and 2 using the IDTs 2, 3, 4, and 5 shown in FIG. This is the result of obtaining the filter characteristics. Although the termination is 50Ω, the insertion loss is large because no matching circuit is added. It is also a feature of the present invention that the SAW filters of tracks 1 and 2 have different center frequencies, bandwidths, and the like.

FIG. 5A shows the filter characteristics when the SAW filters of tracks 1 and 2 are connected in parallel to form a double track type SAW filter, terminated at 50Ω, and added with an LC matching circuit. FIG. 3B is a diagram showing a passband characteristic (solid line) and a phase characteristic (dashed line) of the double track SAW filter. From this figure, it can be seen that the insertion loss is 14.7 dB, the in-band deviation is 0.36 dB, and the phase linearity in the pass band is 2.9 degrees, which satisfies the required standard. FIG. 6A shows the phase difference between the input and output of tracks 1 and 2. JP 2000-77
It can be seen that the phase difference is significantly different from the phase difference disclosed in FIG. FIG. 6B shows the required standard, the filter characteristics in the case of designing using nine types of conventional basic section IDT electrodes, and the 12 basic sections I according to the present invention.
Filter characteristics when designed using DT electrodes, ie,
The insertion loss, in-band deviation, attenuation, phase linearity, and the like are shown in comparison.

The fifteen basic sections I shown in FIGS.
The DT electrode is an IDT electrode that has not been used in a conventional transversal type SAW filter. When these basic section IDT electrodes are used in a double track type SAW filter, it not only helps to improve the characteristics of the filter,
The degree of freedom of design is increased, and the number of logs constituting the IDT electrode can be further reduced, and the size of the double track SAW filter can be reduced. By the way, "+", "-" which defined the reflection state earlier
E1 ', E4', E7 ', E10', E1
3 ′ is “−”, E3 ′, E6 ′, E9 ′, E12 ′, E
15 ′ is “+”, E2 ′, E5 ′, E8 ′, E11 ′,
E14 'is "0" without reflection. Regarding the excitation state, E1 ', E2', E3 'are "-", E4', E4 '.
5 ′, E6 ′ are −90 degrees and “−N ′”, E7 ′, E
8 'and E9' are "+", E10 ', E11' E12 '
Is +90 degrees, "N '", E13', E14 'E15'
Is a basic section IDT electrode that can be expressed as "0" without excitation.

[0018]

Since the present invention is constructed as described above, the invention according to claim 1 increases the degree of freedom in designing a double track SAW filter, so that the constructed filter has good filter characteristics. In addition, it shows an excellent effect that miniaturization is possible.

[Brief description of the drawings]

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

FIG. 2 is a plan view showing a configuration of a basic section IDT electrodes E10, E11, E12.

3A is a diagram showing an algorithm for performing evaluation, selection and GA operation of a double track SAW filter, and FIG. 3B is a diagram showing a configuration of each IDT electrode of the double track SAW filter obtained by convergence.

FIGS. 4A and 4B are diagrams showing filter characteristics (without a matching circuit) of each of the SAW filters of tracks 1 and 2. FIG.

5A is a diagram illustrating filter characteristics of a double-track SAW filter in which SAW filters of tracks 1 and 2 are connected in parallel, and FIG. 5B is a diagram illustrating passband characteristics and phase characteristics.

FIG. 6 (a) shows a phase difference characteristic between tracks 1 and 2, (b)
It is a comparison between a standard and filter characteristics configured using 9 types and 12 types of basic section IDTs.

FIG. 7 is a plan view showing a configuration of basic section IDT electrodes E1 ′ to E6 ′.

FIG. 8 is a plan view showing a configuration of basic section IDT electrodes E7 ′ to E12 ′.

FIG. 9 is a plan view showing a configuration of basic section IDT electrodes E13 ′ to E15 ′.

10A is a plan view showing a configuration of a conventional transversal SAW filter, and FIG. 10B is a plan view showing a configuration of a transversal SAW filter using a SPUDT converter.

FIG. 11 is a diagram illustrating dimensions of each electrode finger of a basic section IDT electrode, an origin, a reflection center, and an excitation center.

FIG. 12: Internal resonance type single layer unidirectional electrode (RSPDT)
FIG. 2 is a diagram for explaining one configuration.

FIG. 13 is a diagram illustrating reflection weighting of RPDT.

FIG. 14 is a diagram showing a configuration of a conventional basic section IDT electrodes E1 to E6.

FIG. 15 is a diagram showing a configuration of a conventional basic section IDT electrodes E7 to E9.

16A is a plan view showing a configuration of a double track SAW filter, and FIG. 16B is a plan view showing an example of an IDT electrode used in the double track SAW filter.

[Explanation of symbols]

1. Piezoelectric substrate 2, 3, 4, 5 IDT electrodes E10, E11, E12 Basic section IDT electrodes E1 ', E2', E3 ', E4', E5 ', E6', E
7 ', E8', E9 ', E10', E11 ', E1
2 ', E13', E14 ', E15' ... basic section ID
T electrode W1, W2, W3 ... electrode finger width d1, d2, d3 ... distance from origin

Claims (1)

[Claims] 1. With reference to the phase of the excitation center of the basic section IDT electrode α, the basic section IDT electrode β and the basic section IDT electrode γ having the phase of the excitation center differing by 180 ° from the basic section IDT electrode α are referred to as the basic section IDT electrode. In the double track surface acoustic wave filter configured to be used in combination with α, the phase of the excitation center is 9 with respect to the basic section IDT electrode α.
0 ° different basic section IDT electrode δ is referred to as the basic section ID.
An internal resonance single-phase unidirectional double track surface acoustic wave filter characterized by being used together with T electrodes α, β and γ.