JP2000077973A - Transversal saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maximize passing band width and to steepen an attenuation inclination by weighting main lobe and first and second side lobes. SOLUTION: On the main side of a piezoelectric substrate 1, an IDT electrode 3 is arranged at a prescribed interval with an IDT electrode 2 along with the propagating direction of surface waves. One comb-line electrode of the IDT electrode 2 is connected to an input terminal IN-1 and the other comb-line electrode is connected to an input terminal IN-2. Further, one comb-line electrode of the IDT electrode 3 is connected to an output terminal OUT-1 and the other comb-line electrode is connected to an output terminal OUT-2. Concerning the IDT electrode 2, the main lobe and the first and second side lobes are weighted for providing the desired frequency characteristics of a SAW filter. When the number of blocks of the main lobe is defined as L and the number of blocks of inverse SPUDT included in the main lobe is defined as N1, within the range of 0.08L<N1<0.6L, the filter of wide band and steep attenuation characteristics can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a transversal S.
The present invention relates to an AW filter, and more particularly, to a transversal SAW filter having an increased bandwidth and a sharp attenuation gradient.

[0002]

2. Description of the Related Art In recent years, SAW devices have been widely used in the field of communications, and have been used particularly in mobile phones and the like because of their excellent features such as high performance, small size, and mass productivity. One of the SAW devices is a transversal SAW filter. FIG. 16 is a schematic diagram showing an example using a normal electrode, and is shown on the main surface of the piezoelectric substrate 11 along the propagation direction of the surface wave. IDT electrode 12 and I
The DT electrode 13 is arranged and configured. IDT electrodes 12, 1
Reference numeral 3 denotes a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed therebetween. One of the IDT electrodes 12 is connected to the input terminal IN_1, and the other comb-shaped electrode is connected to the input terminal IN_2. Connecting. Further, one of the IDT electrodes 13 is connected to the output terminal OUT_1, and the other IDT electrode 13 is connected to the output terminal OUT_2.

[0003] Transversal SA of regular IDT electrode
As is well known, the transmission characteristics of the W filter are represented by a pass bandwidth B =
0.88 / N (N is the number of electrode finger pairs), and has a rounded characteristic with its center frequency at the top. Further, the suppression level of the side lobe is 26 dB. As a method of flattening the rounded passband characteristic peculiar to the normal type IDT electrode, there is a method of weighting any of the IDT electrodes 12 and 13 so that the excitation intensity of the surface wave becomes a function of the position in the propagation direction. This is a method of weighting. The weighting method is roughly classified into two methods: a method of changing the intersection length W of the IDT electrode to perform weighting (apodizing method), and a weighting method by changing the excitation intensity while keeping the intersection length W constant. The feature of the apodizing method is that it is relatively easy to perform accurate weighting, but there is a drawback in that a portion having a small intersection length W has a large diffraction loss and deteriorates an insertion loss. On the other hand, one of the techniques for changing the excitation intensity is an electrode thinning method, which has the opposite relationship to the advantages and disadvantages of the apodization method.

[0004] Here, the apodizing method and the electrode thinning method, which are weighting methods for IDT electrodes, will be briefly described. The apodization method is a weighting method that determines the crossing length of the IDT electrode from the Fourier coefficient when the transmission characteristic of the filter is inversely Fourier transformed, and includes a main lobe, a first side lobe,
The second side lobe,... recently,
The design is made using the Remez Exchange algorithm with further improved design accuracy. In this case, a method of stopping at the second side lobe is used. FIG. 18A shows an IDT in which the intersection length is weighted in an envelope based on the above method.
It is a schematic structure figure of an electrode.

On the other hand, the electrode thinning method refers to the IDT electrode having the longest crossing length of the main lobe of the IDT electrode designed by the above method, and determines the presence or absence of an electrode finger by a well-known procedure. FIG. 18B shows the characteristics equivalent to those shown in FIG. 18A formed by thinning electrodes. At the position where the value of the envelope is large in FIG.
The arrangement of the electrode fingers in FIG. 3A is dense, and the arrangement of the electrode fingers in FIG. However, in an actual IDT electrode configuration, for example, an excitable I
A DT electrode is provided, and an electrode having no excitation action is provided at a position where there is no electrode finger.

Here, the excitable IDT electrode and the non-exciting IDT electrode will be described with reference to several examples. The basic section of the IDT electrode shown in FIG. This is a unidirectional transducer (EWC-SPUDT) composed of two 1 / 8λ-wide electrode fingers, and has a reflection function in the direction of the arrow (forward direction) in the figure. Note that λ is 1
The basic section corresponds to the wavelength, and the IDT electrode configuration for one wavelength. The basic section of the electrode configuration shown in FIG. 3B has an excitation function but does not have one-way reflection. Figure (c)
Although the basic section electrode having the configuration shown in FIG. 1 has no excitation action, it has a reflection action in the direction of the arrow in the figure. FIG. 1D shows a structure in which the ８λ electrode is connected only to an electrode having the same polarity, and has neither excitation nor reflection. The basic section electrode having the configuration shown in FIG. 3E is symmetrical to the electrode shown in FIG. 3A, and has an excitation function and a unidirectionality in the direction of the arrow (reverse direction) in the figure. Further, the basic section electrode having the configuration shown in FIG. 3F has no excitation function, but has a unidirectional (reverse) reflection function in the direction of the arrow. The electrode configuration shown in FIGS.
DT, the electrode configuration of FIG.
It will be referred to as T.

Hereinafter, FIG. 18B will be described with reference to the configuration using the above-described basic section. Since the IDT electrode in FIG. 18A is symmetric with respect to the center, it is sufficient to describe only the left half from the center. The main lobe of the IDT electrode shown in FIG.
b, c, and d are arranged, and 12, 27, 2, 32, respectively.
It consists of basic sections. Further, 13, 57 basic sections b and d in FIG. 17 are arranged for the first side lobe, and 2 and 1 basic sections b and d are arranged for the second side lobe. FIG. 18 (b) and normal type I
The transmission characteristics of the transversal SAW filter composed of the DT electrode 126 pair have filtering characteristics as shown in FIG.
A is an enlarged view of the pass band (vertical axis at the right end), and B is the transmission characteristic of the entire filter.

In designing a transversal SAW filter using, for example, the six basic sections shown in FIG. 17, a complete design algorithm has not yet been established. For example, the type and number of basic sections of the excitation IDT electrode are selected from empirical values, and cut-and-try is repeated until a desired transmission characteristic is obtained while confirming by simulation.

On the other hand, some proposals have been made in which the transmission characteristics of a transversal SAW filter are improved by a method different from the above. For example, in US Pat. No. 5,703,427, DART (Distributed Acoustic Reflection Transd
Ucer) Transversal SAW filters using electrodes are disclosed. The DART electrode is an IDT electrode that can be excited in one direction, and the transversal SAW filter according to the present invention includes three IDT electrode fingers having different reflection directions in one IDT electrode as shown in FIG. The basic section of the DART type IDT having a reflection direction in the forward direction + R (the left direction in the figure is the forward direction in the U.S. Pat. DART type IDT with reflection direction (right direction)
An example in which a transversal SAW filter is configured using an IDT electrode having a basic section of 40λ and a DART type IDT basic section in the forward direction arranged at 20λ, and its transmission characteristics are disclosed. This filter can reduce the insertion loss from 7.4 dB to 6.5 dB, reduce the group delay time from 200 ns to 100 ns in half compared to the filter using the electrode thinning method which has been reported so far, and the second side. It is characterized in that the attenuation of the lobe is increased and the pass band is further widened.

[0010]

However, in the conventional transversal SAW filter using the apodizing method or the electrode thinning method, the bandwidth is insufficient for use in the IF filter of a new digital portable telephone such as the CDMA system. However, there is a problem that the attenuation gradient is insufficient. In the above-mentioned US patent invention, how the IDT basic section for forward reflection (forward SPUDT) and the IDT basic section for reverse reflection (reverse SPUDT) are arranged to increase the passband width. + R = 50λ, −R = 40, not clarified and shown in the examples
In order to construct a filter under conditions other than λ and + R = 20λ, cut-and-try must be repeated, and there is a problem that a wideband filter cannot be realized by this method. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and is directed to a transversal S which maximizes a pass band width and has a steep attenuation gradient.
It is an object to provide an AW filter.

[0011]

According to a first aspect of the present invention, there is provided a transversal SAW filter according to the present invention, wherein two transversal SAW filters are provided on a main surface of a piezoelectric substrate along a propagation direction of a surface wave. In a transversal SAW filter configured by arranging IDT electrodes, a main lobe is formed on at least one of the IDT electrodes using an electrode thinning method.
The first side lobe and the second side lobe are weighted, the number of basic sections of the main lobe is set to L, and the number of sections of the backward SPUDT for replacing the main lobe is set to N1.
In this case, the transversal SAW filter is characterized by satisfying 0.08 L <N1 <0.6 L. According to a second aspect of the present invention, when the number N2 of forward SPUDT sections arranged in the first side lobe is 0 <N2 <
2. The transversal SAW filter according to claim 1, wherein N1 is set. The invention according to claim 3 is
3. The transversal S according to claim 1, wherein the second side lobe is replaced with a backward SPUDT.
An AW 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. 1A is a plan view showing an electrode configuration of a transversal SAW filter according to the present invention, in which a predetermined interval is provided on the main surface of a piezoelectric substrate 1 along with a direction of surface wave propagation from an IDT electrode 2. The IDT electrode 3 is arranged. One of the IDT electrodes 12 is connected to the input terminal IN_1, and the other is connected to the input terminal IN_2. Further, one comb-shaped electrode of the IDT electrode 3 is connected to the output terminal OUT_1, and the other comb-shaped electrode is connected to the output terminal OUT_2. IDT electrode 2
In this example, the main lobe, the first side lobe, and the second side lobe are weighted using an electrode thinning method in order to obtain a desired frequency characteristic of the transversal SAW filter. Also, in FIG.
A normal IDT composed of pairs is used. FIG. 1B shows a detailed configuration example of the IDT electrode 2 according to the present invention. Since the IDT electrode 2 is symmetric with respect to the center, only the left half is shown. A number indicating the position is assigned in order from the left end in FIG.
It is a figure showing the kind and the number of sections of the basic section (IDT pattern) of the IDT electrode arranged in the position. IDT patterns a to f correspond to the patterns shown in FIGS. Here, 70-28 is the main lobe, 27
To 3 are the first side lobes, 2 to 1 are the second side lobes, and the number of sections of the pattern (f) which is the reverse SPUDT is 35 for the number of sections 73 of the main lobe.
And the forward direction SPUDT having 23 sections in the first side lobe.
And the backward SPUDT of section number 3 is arranged in the second side lobe. With this configuration, FIG.
It is possible to realize a filter having a steep attenuation amount in a wide bandwidth of 1.23 MHz or more as shown in FIG. As will be described later, the number of sections of the main lobe is set to L by various simulations and experiments, and the backward direction S included in the main lobe is set.
When the number of sections of the PUDT is N1, 0.08L <N1
It was found that a filter having a wide band and steep attenuation characteristics was realized when the range was <0.6 L.

The process leading to the present invention will be described step by step. Although the simulation according to the present invention is performed by using the electrode thinning method, the description will be made while comparing the envelope of the equivalent apodizing method for easy understanding of the contents. First, an IDT electrode formed by the electrode thinning method shown in FIG. 18B is formed by a conventional method using a basic section not including the backward SPUDT. Next, at the left end of the figure (second
17 for every four basic sections in order from side lobe)
(E) Replaced by the reverse SPUDT, and its transmission characteristics were determined by simulation. FIG. 2 shows the replacement position, the 5 dB bandwidth of the transmission characteristic at that time, and the bandwidth increase rates B5 and B33 of the 33 dB bandwidth. The 5 dB bandwidth obtained by the IDT electrode configuration before the replacement, The difference between the replacement value and the reference value is divided by the reference value with the 33 dB bandwidth as the reference value. The position is 0 at the left end in FIG. FIG. 2 shows a weighting curve α.
(The envelope of FIG. 18A) is also shown. FIG.
As is clear from B5, the bandwidth is increased by 5 dB in the main lobe and the second side lobe by replacing the main lobe and the second side lobe with the backward SPUDT, but the bandwidth is reduced in the first side lobe. Further, I in FIG.
In the DT electrode configuration, the reverse direction SP is sequentially applied from the left end of the electrode.
Substitution was performed with 20 UDT basic sections, and the transmission characteristics of the transversal SAW filter at that time were calculated.
FIG. 3 shows bandwidth increase rates B5 and B33 at 5 dB and 33 dB. As can be seen from FIG. 2, B5 increases when the electrode finger of the second side lobe is replaced and when the electrode finger of the main lobe is replaced. Also, B
It can be seen that 33 has relatively little change in bandwidth.
5 when replacing with 4 basic sections of backward SPUDT
While the dB bandwidth increase rate B5 is about 2% at the maximum, when replaced by 20 basic sections, the maximum is about 8% at the maximum.
Was found to reach.

Based on the above results, the main lobe, the first side lobe, and the second side lobe will be discussed in more detail. From the above results, it is presumed that the main lobe contributes to widening the bandwidth, so the main lobe will be examined first. A predetermined number of sections from the left end of the main lobe of the electrode configuration shown in FIG.
, The number of basic sections to be replaced and the 5 dB / 33 dB bandwidth increase rate B of the transversal SAW filter
5, B33 was determined by simulation. FIG. 4 is a graph showing the results. It is found that the 5 dB bandwidth increase rate B5 increases almost in proportion to the number of replaced basic sections, but B33 temporarily decreases and increases with the number of sections 30 as a peak. . FIG. 5 shows the electrode configuration of FIG. 18B, in which 30 sections from the left end of the main lobe toward the center in the figure are arranged in the reverse direction S.
In the transmission characteristics in the case of replacing with the PUDT, A is an enlarged view of the pass band, and the right vertical axis shows the attenuation, and B is the whole attenuation characteristic, which is shown on the left vertical axis. As can be seen from the figure, although the bandwidth is wide and a steep attenuation characteristic is obtained, a large ripple of about 2 dB is generated almost at the center of the pass band. P in FIG. 4 shows the magnitude of the ripple with respect to the number of replaced basic sections, and it can be seen that the ripple increases as the number of replaced sections increases. Then, in order to suppress this ripple, attention was paid to the first side lobe. FIG. 6 shows that in the IDT electrode in which the backward SPUDT is replaced by 30 sections, when the predetermined number of sections from the right end in the drawing of the first side lobe is replaced by the forward SPUDT, the increase in the number of basic sections and the ripple P, 5 dB, It is the figure which showed the relationship with 33dB bandwidth increase rates B5 and B33.

As is apparent from FIG. 5, the IDT in which the electrode finger of the main lobe is replaced with 30 sections of backward SPUDT.
The electrode had a ripple P of about 2 dB, and the interval between its peaks (maximum values) was about 660 kHz. In contrast, the first
When the basic section of the forward SPUDT to be replaced from the right end of the side lobe is sequentially increased, a new ripple is generated between the ripples, and the peak interval is from 250 kHz to 300 kHz.
kHz. It has been found that the new ripple generated in the center of the pass band corrects the dent of the ripple generated outside and acts to flatten the pass band. For example, 20 sections from the right end of the first side lobe in the forward direction S
When the PUDT was replaced, the transmission characteristics were as shown in FIG. 7, and the ripple could be significantly reduced to approximately 0.2 dB. At this time, the ripple interval due to the forward SPUDT is about 240 kHz, and is inside the ripple interval due to the backward SPUDT, which is about 660 kHz. The two ripples overlap each other so as to cancel each other, and as a result, the filter ripple is reduced. Works.
That is, the interval between the maximum values of the ripple P is 210 kHz, 24
The frequencies are 0 kHz and 210 kHz, and the maximum points are arranged at substantially equal intervals. As is clear from FIG.
When the number of basic sections of the forward SPUDT to be replaced by side lobes is increased, ripples gradually decrease, and the basic section becomes 2
It has been confirmed that when the number becomes zero, the value becomes minimum, and when the number further increases, the ripple of the filter increases.

Further, the main lobe extends from the left end to the center.
The number of basic sections of the backward SPUDT to be arranged in succession,
Forward direction S arranged from right end to left end of side lobe
Various combinations of the number of PUDT basic sections and
Simulate the transmission characteristics of a versal SAW filter
I asked more. Figure 8 summarizes some of the results.
Therefore, the vertical axis indicates the forward direction SP to be replaced with the first side lobe.
Along with the number of UDT sections and the 5 dB bandwidth increase rate (%),
Ripple (dB) is displayed on the vertical axis at the right end. this
Keep the filter ripple small as shown
Includes the basis of the forward SPUDT in the first side lobe.
The number N2 of the sections is determined by the backward SPU arranged in the main lobe.
Should be 60% to 70% of the basic section number N1 of DT
It was confirmed that. Figure 9 shows the main robe
SPUDT for the basic section number L of the entire probe
Of the basic section number N1 replaced by
FIG. 4 is a diagram showing a relationship between the data and the ripple. From this figure
Reverse SPUD installed on the main lobe for easy understanding
When the number of basic sections of T increases, the resources
The recess of the ripple is located in the first side lobe in the forward direction S.
Compensate for inner ripple caused by PUDT
It is so big that it is not. For example, main row
Of backward SPUDT out of 73
If the number of sections is 40, the ripple
Order of the first side lobe required to correct the dent
Although the number of basic sections in the direction SPUDT is 26 sections, FIG.
As shown in the figure, the ripple exceeds the ripple correction limit.
Cannot be reduced. Therefore, the ripple is 1d
In order to keep it lower than B, the above ratio is set to 60% or less.
Should be set, preferably below 45%
U.

Next, consider a case where the electrode finger is replaced with the backward SPUDT from the second side lobe to the first side lobe of the IDT electrode in FIG. FIG. 11 is a diagram showing the relationship between the ripple P and the bandwidth increase rates B5 and B33 when the number of basic sections to be replaced with the backward SPUDT from the left end of the second sidelobe in the figure is increased. From the figure, it was confirmed that the pass band width was about 1% at the maximum but increased. FIG. 12 shows a reverse SPUD to be replaced.
It is a figure which shows the relationship between the basic interval number of T and the frequency interval of the peak of the ripple which arises by this reverse SPUDT. From this figure, it was found that by arranging the backward SPUDT from the second side lobe to the first side lobe, the peak interval of the ripple occurs in the range of 300 kHz to 400 kHz. The ripple generated by replacing the electrode finger with the backward SPUDT from the second sidelobe to the first sidelobe is used to correct the ripple dent generated when replacing the electrode finger of the main lobe with the backward SPUDT. It is possible.

FIG. 13 shows that the 35 sections from the left end of the main lobe of the electrode configuration shown in FIG.
This is a transmission characteristic when three sections are replaced with forward SPUDTs, respectively. FIG. 14 shows the transmission characteristics in the case where three sections from the left end of the second side lobe are replaced with the backward SPUDT in addition to the above. It can be seen that the ripple is more flat.

In order to experimentally confirm the simulation results shown in FIG. 14, a transversal SAW filter was experimentally manufactured. FIG. 15 shows the characteristics of the prototyped filter. FIG. 15A shows the pass band characteristics A and the group delay time τ, and FIG. 15B shows the transmission characteristics including the attenuation characteristics. , Its center frequency is 111.85 MHz, 5
The dB bandwidth is 1.23 MHz and the 33 dB bandwidth is 1.8
MHz or less, ripple 0.2dB, insertion loss 12.5d
B, The group delay time was 0.6 μs. As can be seen from the figure, it was confirmed that the prototype also had a wide band and steep attenuation characteristics, and could realize flat characteristics with little ripple in the passband.

In the above description, the electrode-thinning method is used for the input-side IDT electrodes, the normal type is used for the output-side IDT electrodes, and the input-side IDT electrodes are replaced with various basic sections shown in FIG. Although various characteristics in the case have been described, the present invention is not limited to this, and the output side IDT electrode may be replaced with a unidirectional electrode such as a SPUDT electrode by using an electrode thinning method.

[0021]

According to the present invention, as described above, the pass band width (5 dB) of the transversal SAW filter is enlarged and the attenuation bandwidth (33 dB) is almost maintained, so that the attenuation gradient is greatly reduced. The steepness can be achieved, and an excellent effect that the performance required for a digital mobile phone or the like can be realized is achieved.

[Brief description of the drawings]

FIG. 1A is a plan view illustrating an electrode configuration of a transversal SAW filter according to the present invention, and FIG. 1B is a diagram illustrating a detailed configuration of an IDT electrode.

FIG. 2 is a diagram showing the relationship between the position and the 5 dB and 33 dB bandwidth increase rates B5 and B33 when the input-side IDT electrode is replaced with four backward SPUDT sections.

FIG. 3 is a diagram showing the relationship between the position and the 5 dB and 33 dB bandwidth increase rates B5 and B33 when the input-side IDT electrode is replaced with a backward SPUDT20 section.

FIG. 4 shows the number of replacement sections, ripple P, and bandwidth increase rate B when an input IDT electrode is replaced with a reverse SPUDT.
5 is a diagram showing the relationship with B33.

FIG. 5 shows transmission characteristics when the input-side IDT electrode is replaced with a backward SPUDT30 section.

FIG. 6 shows the case where the first side lobe is replaced with a forward SPUDT, the number of replacement sections, ripple P, bandwidth increase rate B5,
It is a figure showing the relation with B33.

FIG. 7 is a diagram illustrating transmission characteristics when a main lobe is replaced with a backward SPUDT and a first side lobe is replaced with a forward SPUDT.

FIG. 8 is a diagram showing the relationship between the number of sections, ripple P, and bandwidth increase rates B5 and B33 when the main lobe is replaced with a backward SPUDT and the first side lobe is replaced with a forward SPUDT.

FIG. 9 is a diagram showing the relationship between the ratio of the number of basic sections of the main lobe to the number of basic sections of the backward SPUDT replaced with the main lobe and the ripple.

FIG. 10 shows a main lobe in a backward SPUDT section 40;
This is a transmission characteristic when the first side lobe is replaced by a forward SPUDT 26 section.

FIG. 11 is a diagram illustrating a relationship between a ripple P and bandwidth increase rates B5 and B33 when a second side lobe is replaced with a backward SPUDT.

FIG. 12 is a diagram illustrating a relationship between the number of sections and peak intervals of generated ripples when a second side lobe is replaced with a backward SPUDT.

FIG. 13 shows a main lobe in a reverse SPUDT35 section,
This is a transmission characteristic when the first side lobe is replaced with a forward SPUDT23 section.

FIG. 14 shows transmission characteristics when the second side lobe is replaced with a backward SPUDT3 section in addition to the various constants shown in FIG.

FIG. 15A shows a passband characteristic and a group delay time characteristic of a prototype transversal SAW filter, and FIG. 15B shows an attenuation characteristic.

FIG. 16 is a plan view showing a configuration of a conventional transversal SAW filter.

FIGS. 17A to 17F are plan views showing the configuration of a basic section (1λ) of an IDT electrode.

FIG. 18 is a diagram illustrating (a) an apodizing method and (b) an electrode thinning method.

FIG. 19 is a diagram illustrating transmission characteristics of a conventional transversal SAW filter.

FIG. 20 is a diagram showing the configuration of FIG. 10 of US Pat. No. 5,703,427.

[Explanation of symbols]

 1. Piezoelectric substrate 2, 3 ... IDT electrode IN_1, IN_2 ... Input terminal OUT_1, OUT_2 ... Output terminal n ... Position where IDT pattern is arranged

Claims (3)

[Claims] 1. A transversal SAW filter comprising two IDT electrodes arranged on a main surface of a piezoelectric substrate along a propagation direction of a surface wave, wherein at least one of the IDT electrodes has a main lobe and a first lobe.
When weighting of the side lobe and the second side lobe is performed, the number of basic sections of the main lobe is L, and the number of sections of the backward SPUDT replacing the main lobe is N1, 0.08L <N1 <0. A transversal SAW filter having 6L. 2. The transversal SAW filter according to claim 1, wherein 0 <N2 <N1 when the number N2 of forward SPUDT sections arranged in the first side lobe is satisfied. 3. The transversal SAW filter according to claim 1, wherein the second side lobe is replaced with a backward SPUDT.