JP3321443B2 - Primary-tertiary longitudinally coupled dual-mode SAW filter - Google Patents

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 first-order / third-order longitudinally-coupled dual-mode surface acoustic wave filter having an improved low-side attenuation gradient near a pass band.

[0002]

2. Description of the Related Art In recent years, cellular cellular telephone systems have increased carrier frequencies or expanded carrier frequency bands due to a rapid increase in subscribers, so that the boundaries between transmission and reception bands have become close to each other. Characteristics that are steeper than those of the above are required. FIG. 8 shows a conventional primary-tertiary-longitudinal coupled dual-mode SAW using first- and third-order longitudinal modes.
FIG. 3 is a plan view illustrating a configuration of a filter (hereinafter, referred to as a dual mode SAW filter), in which three IDT electrodes 12, 13, and 13 are arranged on a main surface of a piezoelectric substrate 11 along a propagation direction of a surface wave.
14 are arranged close to each other, and grating reflectors 15a and 15b (hereinafter, referred to as reflectors) are arranged on both sides thereof. IDT electrode 12,
Reference numerals 13 and 14 each include a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed therebetween. One of the IDT electrodes 12 at the center in the figure is connected to the input terminal IN, and the other is connected to the input terminal IN. The electrodes are grounded. Further, one of the IDT electrodes 13 and 14 on both sides is connected to each other and connected to the output terminal OUT, and the other IDT electrodes 13 and 14 are grounded. Here, the electrode finger widths of the IDT electrodes 12, 13, and 14 are T1, the space width is T2, the electrode finger widths of the reflectors 15a and 15b are R1, and the space width is R2.
Let D be the innermost electrode finger spacing between the reflector 14 and the adjacent reflectors 15a and 15b (hereinafter referred to as IR spacing). When the parameters are set as described above, the IDT electrodes 12, 1
3,14 and the reflectors 15a, 15b of the electrode period L _T, L
_R and the line occupancy η ₁ and η ₂ are respectively L _T = 2 (T1 +
T2), L _R = 2 (R1 + R2), η ₁ = T1 / (T1
+ T2), η ₂ = R1 / (R1 + R2).

As is well known, the operation of the dual mode SAW filter shown in FIG. 8 is that a plurality of surface waves excited by the IDT electrodes 12, 13, 14 are confined between the reflectors 15a, 15b and acoustically coupled. And 1 depending on the electrode pattern
Since the two longitudinal resonance modes of the second and third orders are strongly excited, the device operates as a dual mode SAW filter using these two modes by applying an appropriate termination.
It is well known that the pass bandwidth of the dual mode SAW filter is determined by the frequency difference between the primary resonance mode and the tertiary resonance mode.

FIG. 9 shows a configuration in which the dual mode SAW filters A and A 'shown in FIG. 8 are arranged on the same substrate 11 at a distance that does not acoustically couple each other, and their inputs and outputs are cascaded.
FIG. 4 is a plan view showing a configuration of a two-stage cascade connection type dual mode SAW filter. As is well known, the inductance L connected in parallel between the stages is inserted to compensate for the capacitance ratio of the dual mode SAW filter and to flatten the ripple in the pass band. The reason for using a cascade filter like this is
It is used when the attenuation gradient near the pass band is steep or when a large amount of guaranteed attenuation is secured.

FIG. 10 shows a transmission RF filter (pass band P of 887 MHz to 925 MHz, attenuation range A) of a domestic N-CDMA mobile phone.
Is a diagram illustrating the filter characteristics of a two-stage cascaded double-mode SAW filter prototyped for 851 MHz ± 19 MHz), on a 36 ° Y-cut X-propagation LiTaO3 substrate 11.
15.5 pairs of center IDT electrodes 12 and IDT electrodes 1 on both sides
9.5 pairs of 3 and 14 each, 200 reflectors 15a and 15b each, intersection length 60λ, electrode thickness H 8.0%
lambda, the IDT electrode and the reflectors line occupancy η _1, 0.68,0.43 η ₂ respectively, 0.5 I-R interval D, the electrode period ratio of the IDT electrode and the reflector (hereinafter referred to as the electrode period ratio) L _T / L _R
Is 0.9715. The horizontal axis is frequency (MH
z), and the vertical axis indicates insertion loss (Loss). P indicated by hatching in the figure is a passband standard (F ₀ = 906 MHz ±
19MHz), A is the standard of attenuation range (40d at 851MHz ± 19MHz)
B). As is clear from the figure, the ripples of the small periods exhibited by the reflectors 15a and 15b are superimposed on the side lobes formed by the IDT electrodes 12, 13 or 14 particularly on the low frequency side near the pass band, and Degrades the damping characteristics of

[0006]

As means for steepening the attenuation gradient on the low frequency side near the pass band, which is required for recent transmission RF filters of N-CDMA mobile phones, 1) input / output I / O
The distance between the DT electrodes, that is, in FIG.
To increase the distance between the innermost electrode fingers facing the IDT electrodes 12 and 14, or 2) dual mode SAW
There is a method of forming an attenuation pole by connecting a SAW resonator to a filter in parallel. However, in the means of 1), there is a problem that the interval between the two resonance modes to be excited is narrowed and the pass bandwidth is narrowed. In the means of 2), the attenuation pole is formed by the anti-resonance frequency of the SAW resonator. However, the effect of improving the required attenuation gradient near the pass band as expected cannot be expected, and there is a problem that the pass band may be deteriorated due to the impedance relationship between the SAW resonator and the SAW filter.

The inventor of the present application has proposed a prior patent application (Japanese Patent Application No. Hei. 370540) proposed a dual-mode SAW filter having a normal-wedge reflector as shown in FIG. That is, three IDs are formed on the main surface of the piezoelectric substrate (not shown) along the propagation direction of the surface wave.
T electrodes 22, 23 and 24 are arranged close to each other,
The reflectors 25a and 25b are provided on both sides thereof.
The feature of the present invention lies in the shape of the reflectors 25a and 25b arranged on both sides of the IDT electrodes 22, 23 and 24. As shown in the figure, the shape of the reflectors 25a and 25b on the side close to the IDT electrodes 23 and 24 has a regular structure having electrode fingers having a length substantially equal to the opening length of the IDT electrodes, and The wedge-shaped structure has a wedge-shaped structure in which the length of the electrode finger decreases as the distance from the IDT electrode increases. FIG. 12 shows attenuation characteristics when the dual-mode SAW filters shown in FIG. 11 are connected in cascade in two stages, and the shapes of the reflectors 25a and 25b (normal number: 100, wedge type: 100) The other characteristics are characteristics when the same parameters as those of the dual mode SAW filter shown in FIG. 10 are used. As is clear from FIG. 12, it can be seen that small ripples generated on the low frequency side near the pass band are suppressed. However, the above dual mode SAW filter has a problem that, although the pass band P is satisfied among the standards defined for the transmission RF filter of the N-CDMA mobile phone, the standard of the attenuation region A cannot be satisfied. there were. The present invention has been made to solve the above problem, and provides a dual-mode SAW filter having an improved low-frequency side attenuation gradient so as to satisfy a passband characteristic P and a low-frequency side attenuation characteristic A. The purpose is to do.

[0008]

According to a first aspect of the present invention, there is provided a first- / third-order longitudinally-coupled dual-mode SAW filter according to the present invention. In the first-third-order longitudinally-coupled dual-mode SAW filter in which three IDT electrodes are arranged close to each other along the propagation direction and grating reflectors are arranged on both sides of the IDT electrodes, The distance between the center of the innermost electrode finger and the grating reflector is set to 0.45λ to 0.49λ (λ is the wavelength of the surface wave to be excited), and the shape of the reflector is set closer to the IDT electrode. The length of the electrode finger is substantially equal to the length of the opening of the IDT electrode, and the length of the electrode finger is reduced as the distance from the IDT electrode is increased. SA It is a filter.

[0009]

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 dual mode SAW filter according to the present invention,
Three IDT electrodes 2, 3, and 4 are arranged close to each other on the main surface of the piezoelectric substrate 1 along the propagation direction of the surface acoustic wave. A reflector having a wedge shape near the end of the piezoelectric substrate 1 (hereinafter, referred to as a regular-wedge reflector)
5a and 5b are arranged. Each of the IDT electrodes 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 IDT electrodes 2 at the center in the drawing is connected to the input terminal IN. The other comb electrode is grounded. Further, one of the IDT electrodes 3 and 4 is connected to one another and connected to the output terminal OUT, and the other IDT electrodes 3 and 4 are grounded. Where I
The electrode finger width of the DT electrodes 2, 3, and 4 is T1, and the space width is T.
2. The electrode finger width of the reflectors 5a and 5b is R1, the space width is R2, and the innermost electrode finger distance between the IDT electrode 3 or 4 and the reflector 5a or 5b is G. And I
And DT electrode 2, the center-to-center spacing between the innermost electrode fingers of the 3 or 4, L _TT. When the parameters are set as described above, the electrode periods L _T and L _{R of} the IDT electrodes 2, 3 and 4 and the reflectors 5 a and 5 b and the line occupancy η ₁ and η ₂ are respectively L _T =
2 (T1 + T2), L _R = 2 (R1 + R2), η ₁ = T
1 / (T1 + T2), η ₂ = R1 / (R1 + R2).

A feature of the present invention is that a normal-wedge type reflector is used as the reflector of the dual mode SAW filter, and the IDT electrodes 3 or 4 on both sides in FIG.
center distance G of the innermost electrode finger facing a or 5b
(Hereinafter, referred to as an IR interval G) is usually used.
That is, the dual mode SAW filter is configured to be narrower than 0.5λ (λ is the wavelength of the surface wave to be excited).

In order to improve the attenuation gradient on the low frequency side near the pass band exhibited by the dual mode SAW filter, the parameters of each part of the dual mode SAW filter were changed. Among them, it has been found that when the IR interval G is changed, the attenuation gradient on the low frequency side near the pass band becomes steep. That is, the normally used IR interval G is shown in FIGS.
As shown in FIG. It has been experimentally found that when this value is reduced, the attenuation gradient on the low frequency side can be improved. The dual mode SAW filter described below is a filter in which the filter having the configuration shown in FIG. 1 is cascaded in two stages as shown in FIG. 9, and the inductance L is connected in parallel between the stages. A 36 ° Y-cut X-propagation LiTaO 3 substrate was used for the piezoelectric substrate 1, and the center IDT electrode 2 was 15.5
9.5 pairs of IDT electrodes 3 and 4 on both sides and 100 reflectors 5a and 5b, 100 regular wedges and 100 wedges, respectively.
Electrode finger crossing length 60Ramuda, the electrode thickness 8.0% lambda, IDT electrodes and the reflectors line occupancy η _1, η _2, respectively 0.68,0.43, L _T
/ L _R of 0.9715, a case of a 0.25λ the L _TT. In the following, only the parameter values changed from the above values will be described. FIG. 2A shows IRs used for comparison.
The filter characteristics of the conventional filter when the interval G is 0.5λ, (b) and (c) show that the IR interval G is 0.47λ,
This is a filter characteristic when 0.53λ is set. As apparent from FIG. 2B, the IR interval G is set to be smaller than 0.5λ and 0.4.
When it was changed to 7λ, it was found that an attenuation pole was generated on the low frequency side near the passband, and the attenuation gradient became steep. Also, I-
When the R interval G is changed from 0.5λ to 0.53λ, the attenuation gradient on the high frequency side near the pass band is slightly improved as shown in FIG. 2C, but the attenuation gradient on the low frequency side is improved. Was not seen.

FIG. 3 is an enlarged view of the passband of the filter characteristic shown in FIG. 2, and (a), (b) and (c) show the IR intervals G as 0.5λ, 0.47λ and 0.53λ, respectively. Is the case. It can be seen from the ripples in the passband that the impedance of the load matches that of the filter when G is set to 0.47λ than when set to another value.

FIG. 4 shows the line occupancy η ₁ and η ₂ respectively.
These are filter characteristics when the number of reflectors is slightly changed to 0.7 and 0.45, the number of reflectors is 80 for normal type, 100 for wedge type, and the intersection length of the electrode fingers is 55λ. Electrode period ratio L _T / L _R 0.972
And set the value of the IR interval G from 0.47λ to 0.015 before and after.
FIG. 9 is a diagram illustrating a state of a change in an attenuation characteristic on a low frequency side near a pass band when λ is changed. It can be seen that the attenuation gradient and the first side lobe on the low frequency side are slightly changed.

FIG. 5 uses the parameters shown in FIG.
The value of the IR interval G is fixed at 0.47λ, and the electrode period ratio L _T /
FIG. 9 is a diagram illustrating the attenuation gradient on the low frequency side near the pass band and the attenuation characteristics of the first side lobe on the low frequency side when _LR is changed. FIGS. 5A, 5B and 5C show the electrode period ratio L _T
These are filter characteristics when / _LR is 0.972, 0.969, and 0.975, respectively. In the case of FIG. 5B, not only the pass band P but also the attenuation region A (the attenuation gradient and the first side lobe) are N-
It is understood that the CDMA standard is satisfied.

From the experimental results shown in FIG. 5, the IR interval G
Is constant at 0.47λ, the electrode period ratio L _T / L _{R is set} to 0.
Since it was found that good characteristics were obtained when it was set to 969, further experiments in the vicinity of the value were conducted in detail, and the results are shown in FIG. Line occupancy η ₁ , η ₂ is 0.76, respectively
0.5, the electrode thickness H is 8.5% λ, and the electrode finger intersection length w is 60λ.
This is a case where the electrode period ratio L _T / L _R is slightly changed from 0.969. FIGS. 6B and 6C show the case where the electrode period ratio L _T / L _R is 0.967 and 0.971, respectively. As is clear from FIG. 6, it can be seen that any of the parameters sufficiently satisfies the N-CDMA standard.

FIG. 7 shows passband characteristics corresponding to the attenuation characteristics shown in FIG. 6, and the state of impedance matching between the filter and the load can be estimated from the ripples in the passband. In each case, the ripple was as small as less than 1 dB, and it was found that the impedance matching was good.

In the above description, the case where a 36-degree lithium tantalate substrate is used as the piezoelectric substrate has been exemplified. However, the present invention is not limited to this, and other cutting angles may be used. For example lithium niobate,
It goes without saying that the present invention can be applied to quartz, langasite, lithium tetraoxide and the like. Also, an example has been described in which the configuration of the reflector is composed of a regular type and a wedge type, but instead of the wedge shape,
Any structure may be used as long as the length of the electrode finger of the reflector decreases as the distance from the IDT electrode increases.

[0018]

According to the present invention, as described above, the attenuation slope on the low frequency side of the dual mode SAW filter can be greatly improved.
When used for transmission RF of a mobile phone, it exhibits an excellent effect of not only improving the signal quality but also reducing the size.

[Brief description of the drawings]

FIG. 1 is a primary-tertiary longitudinally coupled dual mode SA according to the present invention.
It is a top view showing the composition of a W filter.

FIGS. 2A, 2B, and 2C are filter characteristic diagrams for explaining an effect of the dual mode SAW filter according to the present invention, in which an IR interval G is changed.

FIGS. 3A, 3B, and 3C are diagrams illustrating passband characteristics when an IR interval G is changed.

[4] (a), a change in filter characteristics when changing the (b), (c) is the electrode period ratio L _T / L _R of the IDT electrode reflectors is constant, I-R interval G FIG.

FIGS. 5A, 5B, and 5C are diagrams showing changes in filter characteristics when G is constant and the electrode period ratio L _T / L _R is changed.

FIGS. 6A, 6B, and 6C are diagrams showing changes in filter characteristics when G is constant and the electrode period ratio L _T / L _R is changed in more detail.

FIGS. 7 (a), (b) and (c) are diagrams showing changes in passband characteristics when G is constant and the electrode period ratio L _T / L _R is changed in more detail.

FIG. 8 is a plan view showing a configuration of a conventional primary-tertiary-order longitudinally coupled dual-mode SAW filter.

FIG. 9 is a plan view showing a configuration of a conventional two-stage cascade connection type dual mode SAW filter.

FIG. 10 is a diagram showing the filter characteristics of a conventional two-stage cascade connection type dual mode SAW filter.

FIG. 11 shows a prior patent application (Japanese Patent Application No. 10-37054).
FIG. 2 is a plan view showing a configuration of a dual mode SAW filter proposed in (0).

12 is a diagram illustrating filter characteristics of a filter in which the dual mode SAW filter illustrated in FIG. 11 is cascaded in two stages.

[Explanation of symbols]

1. Piezoelectric substrate 2, 3, 4 ... IDT electrode 5a, 5b Grating G ... Center-to-center spacing of innermost electrode finger between IDT electrode and reflector adjacent to it T1 ... Electrode finger width of IDT electrode T2: space width of IDT electrode R1: electrode finger width of reflector R2: space width of reflector L _TT : center distance between innermost electrode fingers of adjacent IDT electrodes P: pass band A: Lower attenuation band λ ・ ・ Wavelength of the excited surface wave L _T / L _R・ ・ Electrode period ratio between IDT electrode and reflector

Claims (1)

(57) [Claims] 1. Three IDT electrodes are arranged close to each other along a propagation direction of a surface acoustic wave on a main surface of a piezoelectric substrate.
Primary with grating reflectors on both sides of T electrode-
In the third-order longitudinally-coupled dual-mode SAW filter, the center-to-center distance between the innermost electrode fingers between the adjacent IDT electrodes and the grating reflector is set to 0.45λ to 0.49.
λ (λ is the wavelength of the surface acoustic wave to be excited), and the shape of the reflector near the IDT electrode is such that the length of the electrode finger is substantially equal to the opening length of the IDT electrode.
A primary-tertiary longitudinally-coupled dual-mode SAW filter, wherein a length of an electrode finger becomes shorter as a distance from a DT electrode increases.