JP2001298343A - Transversal surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means for flattening the intra-band characteristics of a transversal surface acoustic wave(SAW) filter. SOLUTION: In the traversal SAW filter in which two IDT electrodes are located on a piezoelectric substrate at a prescribed interval, one of IDT electrodes is formed by using the basic block of an electrode cycle λR having a reflecting function and the basic block of an electrode cycle λS having no reflecting function and the electrode cycles λR and λS are made different from each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a transversal surface acoustic wave filter (hereinafter, referred to as a transversal SAW filter), and more particularly to a transversal SAW filter with improved ripple in a pass band.

[0002]

2. Description of the Related Art Recently, a surface acoustic wave filter (hereinafter referred to as SA) has been developed.
W filters) are widely used in the field of communications and have excellent characteristics such as high performance, small size, and mass productivity, and are therefore particularly frequently used in mobile phones and the like. Due to the spread of mobile phones, which exceeds expectations in recent years, and the demand for high-speed data communication including images, the system using narrow carrier frequency intervals, such as the conventional PDC and AMPS systems, is suitable for high-speed, large-capacity, wide-band. The CDMA system has begun to spread. CDM
An IF circuit used in an A-type terminal needs a wide-band filter for transmitting high-speed data, and also requires a steep attenuation gradient characteristic to block adjacent carriers. Further, in order to maintain high-speed digital data quality, the IF filter is required to have a flat delay time characteristic. A transversal SAW filter that can design the amplitude characteristic and the phase characteristic separately from each other is suitable as a filter satisfying such requirements.

FIG. 4 is a plan view showing the structure of a conventional transversal SAW filter. The IDT electrode 22 is arranged on a main surface of a piezoelectric substrate 21 along a propagation direction of a surface wave at a predetermined interval. The electrode 23 is arranged and configured. I
Each of the DT electrodes 22 and 23 is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed between each other. One of the IDT electrodes 22 is connected to the input terminal IN, and the other of the IDT electrodes 22 is connected to the input terminal IN. Ground. Further, one comb-shaped electrode of the IDT electrode 23 is connected to the output terminal OUT, and the other comb-shaped electrode is grounded and connected to the transversal SA.
Construct a W filter. Note that sound absorbing members 24 for absorbing the reflected waves from the end surfaces are attached to both ends of the piezoelectric substrate.

As shown in FIG. 4, IDT electrodes 22, 23
When a regular IDT electrode is used, the transversal S
As is well known, the transmission characteristics of the AW filter are such that the pass bandwidth B is B = 0.88 / N (N is the number of pairs of electrode fingers), the characteristics are rounded with the center frequency at the top, and the side lobes The suppression level is 26 dB. As a method of flattening the rounded passband characteristic peculiar to the normal type IDT electrode, a method of weighting one of the IDT electrodes 22 and 23, that is, such that the excitation intensity of the surface wave becomes a function of the position in the propagation direction. There is a method of performing weighting. The weighting method is roughly classified into two methods: a method in which the crossing length W of the IDT electrode is changed to perform weighting (apodizing method), and a weighting method in which the crossing length W is fixed and the excitation intensity is changed.
The feature of the apodization method is that it is relatively easy to perform accurate weighting, but the diffraction loss increases at a small portion of the intersection length W,
There is a disadvantage that the insertion loss is deteriorated. 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.

In the electrode thinning method, as is well known, the IDT electrode designed by the apodization method is standardized by the longest intersecting length of the main lobe, and the arrangement of the electrode fingers is converted densely or roughly. In an actual IDT electrode configuration, for example, this is realized by arranging an excitable IDT electrode at a position where an electrode finger is present, and arranging an electrode without excitation at a position where no electrode finger is present. .

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 electrode fingers having a width of λ / 8, and has a reflecting action in the direction of the arrow (forward direction) in the figure. Here, λ corresponds to one wavelength, and the basic section means an IDT electrode configuration for one wavelength. Although the basic section of the IDT electrode configuration shown in FIG. 5B has an exciting action, it is a type without unidirectionality. The IDT electrode in the basic section having the configuration shown in FIG. 5C has no excitation function, but has a reflection function in the direction of the arrow in the figure. FIG. 5 (d) shows a structure in which the λ / 8 electrode is connected only to the electrode of the same polarity, and has neither excitation nor reflection. The IDT electrode in the basic section in FIG. 5E is symmetrical to the electrode in FIG. 5A, and has an excitation action and one-way direction in the direction of the arrow (reverse direction) in the figure. The IDT electrode in the basic section 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.
T, the electrode configuration of FIG.
I will say.

[0007] Using the six types of basic sections shown in FIG.
Algorithms for designing transversal SAW filters with desired characteristics have not been established so far. Therefore, the type and number of basic sections of the IDT electrode for excitation and the type and number of basic sections of the IDT electrode having no excitation action are set from experience, and the desired transmission is performed while confirming the filter characteristics by simulation. At present, it is designed by repeating so-called cut-and-try to approach characteristics.

FIGS. 6A and 6B are disclosed in Japanese Patent Application Laid-Open No. 2000-077973 filed by the present applicant, and the IDT electrode 32 shown in FIG. 6A is symmetric with respect to the center. Therefore, only the left half is shown. That is, FIG.
Numbers (1, 2,...) Indicating positions in order from the left end in FIG.
.. N) and FIG. 6B shows the type and number of basic sections (IDT patterns) of the IDT electrodes arranged at the positions. ID to af are shown in FIG.
It corresponds to the pattern of the IDT electrode shown in (a) to (f). Here, the main lobes 70 to 28, the first side lobes 27 to 3, the second side lobes 2 to 1, and a pattern (f) that is a backward SPUDT with respect to the number 73 of sections of the main lobe. ), 35 sections,
A forward SPUDT having 23 sections in the first side lobe;
In the second side lobe, a backward SPUDT having three sections is arranged.

FIG. 7 shows an example of the filter characteristics of a transversal SAW filter designed based on the above-described method, and is a prototype produced for CDMA-one.
Use a quartz ST cut substrate for the piezoelectric substrate and set the center frequency to 11
This is a filter characteristic when 1.85 MHz, the bandwidth is 1.25 MHz, and the electrode film thickness is 2.3% λ.

[0010]

However, the filter characteristics of the transversal SAW filter as described above have a large ripple as is clear from FIG. 7, and the pass band characteristic is symmetrical with respect to the center frequency. However, there is a problem that the transmission quality of the IF circuit is deteriorated. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a filter in which the passband characteristics of a transversal SAW filter are made substantially symmetrical and ripples are improved.

[0011]

According to a first aspect of the present invention, there is provided an IDT electrode having two IDT electrodes formed on a piezoelectric substrate at a predetermined interval along a surface wave propagation direction. In the transversal surface acoustic wave filter, the one of the IDT electrodes is formed using a basic section of an electrode cycle λR having a reflection function and a basic section of an electrode cycle λS without a reflection function. , The electrode periods λR and λS are different from each other. Claim 2
The described invention is characterized in that the respective electrode periods are set such that the frequency of the surface wave excited by the electrode period λR is substantially the same as the frequency of the surface wave excited or resonated by the electrode period λS. Claim 1 or 2
It is a transversal surface acoustic wave filter of the description. According to the third aspect of the present invention, the electrode period deviation ratio d is set to d = ((λR
2. The transversal surface acoustic wave according to claim 1, wherein d is in a range of 0.1% to 0.15% when a quartz substrate is used as the piezoelectric substrate when -λS) / λR). 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 a configuration of a transversal SAW filter according to the present invention. The transversal SAW filter is spaced apart from an IDT electrode 2 on a main surface of a piezoelectric substrate 1 along a propagation direction of a surface wave by a predetermined distance. The IDT electrodes 3 and 3 are each composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and one of the IDT electrodes 2 is connected to the input terminal IN-1. And the other comb-shaped electrode 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 to form a transversal SAW filter. In the embodiment shown in FIG. 1A, the IDT electrode 3 is constituted by a regular electrode, and the IDT electrode 2 can obtain desired characteristics in the basic section of the IDT electrode shown in FIGS. 1B and 1C. Are arranged as follows.

The method of realizing desired characteristics while combining the IDT electrodes of the basic section shown in FIG. 5 based on experience and confirming the transmission characteristics by simulation is the same as the conventional method. A transversal SAW filter in which an electrode period λR in a basic section having a reflection function as shown in FIG. 1B and an electrode period λS in a basic section without a reflection function as shown in FIG. Is where it is composed. For example, CDMA-o
As the piezoelectric substrate of a transversal SAW filter for an IF filter used for ne, a crystal ST cut is often used. When a quartz crystal substrate is used, one electrode finger as shown in FIG. 1B is set to 3λ / 8, and the other two are set to λ / 8.
The phase velocity in the region having the reflection function as V
R, assuming that the phase velocity in a region in which the electrode fingers are all λ / 8 as shown in FIG.
Are the same, the phase velocities VR and VS are different from each other.

Therefore, it was presumed that the filter characteristics could be improved by setting VR / λR = VS / λS so that the frequencies of the surface waves in the basic sections in FIGS. 1B and 1C would be the same. Here, a transversal SAW filter in the case where the electrode period deviation ratio d with respect to the electrode period λR in the basic section having the reflection function is d = ((λR−λS) / λR), and the electrode period deviation ratio d is gradually changed. Was measured. FIG. 2 shows filter characteristics when a quartz crystal ST cut substrate is used as the piezoelectric substrate, the center frequency is 111.85 MHz, the bandwidth is 1.25 MHz, and the electrode film thickness is 2.3% λ. FIGS. 2A, 2B and 2C show that the electrode period deviation ratio d is 0.0.
It is a filter characteristic of a pass band in case it is set to 6%, 0.11%, and 0.16%.

FIG. 3 is a diagram in which the deviation in the pass band is measured by changing the electrode period deviation ratio d in more detail, and the horizontal axis is plotted as the electrode period deviation ratio d and the vertical axis is plotted as the band deviation (dB). is there. As is clear from this figure, when the electrode period deviation ratio d is set to about 0.13%, the in-band deviation is minimized, and the in-band deviation of 0.6 dB in the conventional case is 0.3 dB or less. And was found to be improved.

Although the present invention has been described with reference to a transversal SAW filter using a quartz crystal ST-cut for a piezoelectric substrate, the present invention is not limited to this.
The present invention can also be applied to a case where lithium tantalate, lithium niobate, lithium tetraborate, langasite, or the like is used for the piezoelectric substrate. In this case, a basic section having a reflection function and a basic section having no reflection function are provided on each piezoelectric substrate, and the phase velocity at this time is obtained by simulation or experiment, and corresponds to FIG. A diagram may be created and the electrode period deviation ratio d may be set so that the in-band ripple of the transversal SAW filter becomes a minimum value. Further, the basic sections constituting the IDT electrode are not limited to those shown in FIG. 1 or FIG.
It goes without saying that the present invention can be applied to any combination of a member having a reflection function and a member having no reflection function.

[0017]

According to the present invention, as described above, the passband characteristic of a transversal SAW filter used in a CDMA-one IF circuit can be flattened, and the transmission quality of the IF circuit can be reduced. It shows an excellent effect that it can be improved.

[Brief description of the drawings]

FIG. 1 (a) is a transversal SAW according to the present invention.
FIG. 3B is a plan view illustrating the configuration of the filter. FIG. 3B is a diagram illustrating a basic section having a reflection function of an IDT electrode, and FIG. 3C is a view illustrating a basic section having no reflection function of an IDT electrode.

FIGS. 2A, 2B, and 2C show filter characteristics when an electrode period deviation ratio d is changed.

FIG. 3 is a view showing a relationship between an electrode period deviation ratio d and an in-band deviation when a quartz crystal ST cut is used for a piezoelectric substrate.

FIG. 4 is a diagram showing a configuration of a conventional transversal SAW filter.

FIG. 5 (a), (b), (c), (d), (e),
(F) is a basic section of an IDT electrode forming a conventional transversal SAW filter.

6A is a plan view of a transversal SAW filter configured using a conventional basic section, and FIG. 6B is a plan view of FIG.
FIG. 4 is a diagram showing a position, an IDT pattern, and the number of basic sections in a detailed view of FIG.

FIG. 7 is a diagram showing filter characteristics of a conventional transversal SAW filter.

[Explanation of symbols]

1. Piezoelectric substrate 2, 3 IDT electrode λR Electrode period of basic section of IDT electrode having reflection function λS Electrode period of basic section of IDT electrode without reflection function

Claims (3)

[Claims] 1. A transversal surface acoustic wave filter comprising two IDT electrodes arranged on a piezoelectric substrate at a predetermined interval along a propagation direction of a surface acoustic wave.
One of the T electrodes is formed using a basic section of an electrode cycle λR having a reflection function and a basic section of an electrode cycle λS without a reflection function, and the electrode cycles λR and λS are different from each other. Transversal surface acoustic wave filter. 2. The method according to claim 1, wherein each of the electrode periods is set such that the frequency of the surface wave excited by the electrode period λR is substantially equal to the frequency of the surface wave excited or resonated by the electrode period λS. Claim 1 or 2
The transversal surface acoustic wave filter according to any of the preceding claims. 3. The electrode period deviation ratio d is defined as d = ((λR−λ
The transversal elastic surface according to claim 1 or 2, wherein when S) / λR), when a quartz substrate is used as the piezoelectric substrate, d is in a range of 0.1% to 0.15%. Wave filter.