JPH10224179A - Double-mode saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the capacity conversion value of the interstage impedance of a filter to nearly zero by differentiating the logarithms of IDT on both sides of inter digital transducers, so as to maintain filtering characteristic. SOLUTION: Three IDT 2 to 4 and reflectors 5 and 5 are arranged along the propagating direction of surface waves, by arranging IDT 2 to 4 on the main surface of the piezoelectric substrate 1 of a double-mode surface acoustic wave(SAW) filter for energizing acoustic waves of first and third modes along the propagating direction of a surface wave to constitute a first to third vertically connecting double-mode SAW(DMS) filter utilizing the two modes. In this case IDT 3 and 4 on both sides of center IDT 2 are made asymmetrical, that is, electrode finger logarithms are differentiated. In this way, the capacitor conversion value of inter-stage impedance is made nearly zero, one of intervals between the tip part of IDT 2, positioned in the center and the tip parts of respective IDT 3 and 4 on both sides, is made nearly 5λ/4, and the other is made different from this to reduce ripples within a passing band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonator type surface acoustic wave filter (hereinafter referred to as "SAW filter"), and more particularly, to eliminating the need for coupling capacitance between stages which is required when connecting resonator type SAW filters in multiple stages. And a miniaturized first- and third-order longitudinally-coupled dual-mode SAW filter.

[0002]

2. Description of the Related Art In recent years, SAW filters have been widely used in wireless devices such as mobile phones, because of their excellent miniaturization, high frequency, mass productivity, and the like. Particularly in recent PHS, cordless telephones, etc., the first IF filter is required to have a higher frequency and a wider band.
An AW filter is optimal. As a means for widening the band of a resonator type SAW filter, a so-called first-order / third-order longitudinally-coupled double-mode SAW filter (hereinafter, referred to as a first-order-third-order DMS filter) using first-order and third-order modes is known. Have been. FIG. 5 (a) is a schematic plan view showing an example of this, in which three interdigital transducers (hereinafter referred to as IDTs) 12, 13, 1
4 and grating-type reflectors 15 on both sides thereof are arranged along the propagation direction of the surface wave. IDT12
14 to 14 are constituted by a pair of comb-shaped electrodes each having a plurality of electrode fingers interposed therebetween.
The number of electrode finger pairs of DT13 and DT14 is the same as each other and the center ID
It is arranged symmetrically with respect to the center line of T12. Also, IDT
One of the electrodes 12 to 14 is connected to a ground potential, and the other electrode is electrically connected to an input or an output.

[0003] Reflectors 15, 15 shown in FIG.
It has a function of reflecting surface acoustic waves leaking from T12 and T14, and confine energy of surface acoustic waves excited by the IDTs 12 to 14 between the reflectors 15 and 15 to acoustically couple the primary to third. Forces the next mode. At this time, the secondary mode is naturally excited. However, since the displacement distribution of the mode is distributed anti-symmetrically with respect to the center of the IDT 12 in the propagation direction of the surface wave, the generated charges are similarly generated in the IDT 1.
In the arrangement of the IDTs as shown in FIG. 5 (a), electric charges of different signs are generated at the center of the second and the input and output IDTs are offset and substantially not excited. Can be used.

[0004] Each IDT shown in FIG.
DT, and the width of each electrode finger and the space between the electrode fingers are generally set to 波長 of the wavelength λ of a desired center frequency. In the first-third-order DMS filter, the maximum bandwidth is obtained as is well known when the electrode finger interval L shown in FIG. 5B is L = λ / 4. However, FIG.
The electrode fingers 17, 1 at the ends of adjacent IDTs shown in FIG.
When the center distance L between the electrodes 8 is set to λ / 4, the width of the electrode fingers 17 and 18 is λ / 4 as described above, so that the adjacent electrode fingers 17 and 18 come into contact with each other to input and output. And ground will be short-circuited. Therefore, it is general that the electrode finger is configured as one electrode finger having a width of λ / 2 like the electrode finger 16 shown in FIG. Therefore, in the present specification, when describing the electrode pattern that provides the maximum bandwidth, the center distance L between the electrode fingers is not expressed as λ / 4, but as shown in FIG. The distance between the centers of the electrode fingers adjacent to each other is set as 5 / 4λ.

[0005]

However, FIG.
In a conventional primary-tertiary DMS filter as shown in (a), the impedance of the filter is inevitably determined in accordance with the configuration of the logarithm of each IDT, the electrode finger width, and the like. When a plurality of DMS filters are connected in cascade in order to improve the characteristics and increase the amount of blocking attenuation, it is necessary to add a capacitance or an inductance having substantially the same value according to the capacitance or the inductance generated between the stages. Since this is a well-known matter, the description is omitted.
For this reason, conventionally, it is necessary to form the coupling capacitance or inductance on the same substrate as the filter or to add it as an individual component to the outside, so that the shape of the filter becomes large and the primary-tertiary DMS filter is reduced in size. This has been a problem when it is mounted on a portable wireless device or the like that requires the above. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems and problems, and it is an object of the present invention to provide a first-to-third-order DMS filter which can be downsized by eliminating the above-mentioned coupling capacitance or inductance between stages. With the goal.

[0006]

According to a first aspect of the present invention, there is provided a piezoelectric device comprising three IDTs on a piezoelectric substrate and two reflectors disposed on both sides thereof.
In a multi-stage cascade-connected vertical-coupling double-mode SAW filter in which a next-third-order vertical-coupling double-mode SAW filter is cascaded in multiple stages, the logarithms of the IDTs on both sides of the three IDTs are made different from each other, and the impedance between the stages is changed. The capacitance conversion value is set to approximately 0, and one of the intervals between the center end of the IDT and the ends of the IDTs on both sides is set to approximately 5λ / 4.
A multi-stage cascaded first-third-order longitudinally-coupled dual-mode SAW filter characterized in that ripples in the pass band are reduced by making the other different from this.

[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 schematic electrode pattern showing one embodiment of a dual mode SAW filter according to the present invention.
By disposing the reflectors 5, 5 on both sides of the DTs 2, 3, and 4 along the propagation direction of the surface wave, the first-order and third-order modes of the elastic waves along the propagation direction of the surface wave are excited. And said 2
A first-third-order DMS filter using two modes is constructed, and the feature of the present invention is that the I-D filters on both sides of the central IDT 2 are used.
DT3 and DT4 are asymmetric, that is, the number of electrode finger pairs is made different.

FIG. 2 is a diagram showing a measurement example of characteristics in one embodiment of the present invention. That is, in this example, 36 ° YX LiTaO3 is used as the piezoelectric substrate, and the frequency F0 = 11.
0.592 MHz, passband B = primary-3 with 2 MHz
In the next DMS filter, 42.5 pairs of central IDT2, ID
23.5 pairs of T3 and 30 reflectors 5 and 5
The distance G between the end of the book, the center IDT2 and the end of the IDT3
1 is fixed to 5λ / 4, and the distance G2 between the end of the central IDT 2 and the end of the IDT 4 is fixed to 9λ / 10. Further, using the logarithm of the IDT 4 as a parameter, a two-stage cascaded primary-tertiary DMS
When the resistance R0 and the reactance X0 of the terminal impedance of the filter and the interstage impedance Zc when cascaded in multiple stages are obtained by simulation, the result shown in FIG. 2 is obtained. Note that Zc is a value obtained by converting the impedance into a capacitance value [pF]. As is clear from FIG. 2, the resistance value R0 [Ω] of the termination impedance and the capacitance value [pF] of the reactance component X0 both decrease as the logarithm of the IDT 4 decreases. on the other hand,
It can be seen that the impedance Zc between the stages decreases to zero as the logarithm of the IDT 4 decreases, and further changes to a negative capacitance, that is, an inductance.

That is, the interstage impedance Z shown in FIG.
If the logarithm of the IDT 4 in which the capacitance conversion value of c becomes zero is used, the interstage capacitance of the two-stage cascaded primary-tertiary DMS filter can be eliminated. Conventionally, the inter-stage capacitance formed by arranging various electrodes on the same piezoelectric substrate becomes unnecessary, and the size of the piezoelectric substrate can be significantly reduced. Generally, if the number of electrode fingers of the IDTs 3 and 4 is different from each other, a ripple is generated in a high-frequency portion in the passband, and the amplitude characteristics and the phase characteristics are distorted. Had been.

However, the ripple of the high-frequency portion in the above-mentioned pass band appropriately sets the interval G1 between the end of the central IDT2 and the end of the IDT3 and the interval G2 between the end of the central IDT2 and the end of the IDT4 in FIG. As a result, it has been found that it is possible to shift from the pass band to the attenuation band. FIG.
FIG. 3 is a graph showing characteristics of various filters embodying the present invention, in which 36 ° YX LiTaO3 is used for a piezoelectric substrate, a frequency F0 = 1110.592 MHz, and a pass bandwidth B = 2M.
FIG. 6 is a diagram illustrating characteristics of a pass band and an attenuation band when a first-order / third-order DMS filter of Hz is realized. That is, FIG. 3 (a) shows 42.5 pairs of central IDT2 and 23.5 pairs of IDTs 3 and 4 on both sides.
Pairs, 30 reflectors 5, 5 each, G1 5λ / 4,
FIG. 6 is a diagram illustrating filtering characteristics when G2 is set to 9λ / 10. Hereinafter, only parameters different from the data of FIG. 3A will be described for FIGS. 3B and 3C. FIG. 3B is a filtering characteristic diagram when the logarithm of the IDT 4 is reduced to 21.5 pairs. Yes,
FIG. 3C shows that the logarithm of IDT4 is further reduced to 19.5.
It is a filtering characteristic figure at the time of making a pair. 3 (a) to 3 (c)
As can be seen from FIG.
It will be apparent that varying the logarithm of reduces the ripple in the passband and has little adverse effect on the attenuation region.

FIG. 4 is a characteristic diagram showing another embodiment of the present invention, wherein 36 ° Y-XLiTaO3 is used for the piezoelectric substrate, and FIG.
Frequency F0 = 110.592 MHz, pass bandwidth B = 2
MHz, 42.5 pairs of log of central IDT, 23.5 pairs of log of IDT3, 21.5 pairs of log of IDT4, G1 =
5 / 4λ, G2 = 9λ / 10, the number of each reflector is 30
This is a filtering characteristic when a book is used. Also in this example, desired filter characteristics could be obtained without adding an interstage capacitance, and the size could be significantly reduced.

Although the case where LiTaO3 is used for the piezoelectric substrate has been described above, other piezoelectric materials such as quartz, LiN
It goes without saying that bO3, LBO, langasite and the like may be used.

[0013]

As described above, according to the present invention, in the multistage cascade-connected double mode filter primary-third-order DMS filter, the logarithms of the IDTs on both sides of the three IDTs are made different from each other to reduce the number of electrode finger pairs. By setting an appropriate logarithm, the capacitance-converted value of the interstage impedance of the filter can be made substantially zero while maintaining the filtering characteristics. Further, one of the distances between the end of the IDT located at the center and the end of each of the IDTs on both sides is set to approximately 5λ / 4, and the other is made different from this, thereby reducing the ripple in the pass band. It is possible to realize a multi-stage cascade-connected double mode filter that does not require a coupling capacitor, which is remarkable in downsizing.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an electrode pattern of a primary-tertiary DMS filter according to the present invention.

FIG. 2 is a diagram showing a relationship between a termination impedance and a coupling capacitance between stages when the IDT 3 is fixed and the logarithm of the IDT 4 is changed.

FIGS. 3A to 3C are diagrams showing filtering characteristics when the number of electrode finger pairs of the IDT 4 is changed.

FIG. 4 shows a primary-3 using an IDT pattern according to the present invention.
It is a filtering characteristic figure of the next DMS filter.

FIG. 5 (a) is a diagram showing I of a conventional first-order / third-order DMS filter.
FIGS. 3B and 3C are diagrams showing DT patterns, and FIGS.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate 3, 4 ... IDT 3a, 4a ... Electrode finger 5 ... Reflector G1, G2 ... Electrode finger center distance

Claims (1)

[Claims] 1. A cascaded multi-stage cascaded primary-tertiary-coupling double-mode SAW filter composed of three IDTs and two reflectors arranged on both sides of a piezoelectric substrate. In the dual-mode SAW filter,
Of the IDTs, the logarithms of the IDTs on both sides are made different from each other, the capacitance-converted value of the interstage impedance is made substantially zero, and one of the intervals between the end of the IDT located at the center and the end of each of the IDTs on both sides is made substantially equal. A multi-stage cascaded first- and third-order longitudinally-coupled dual-mode SAW filter, characterized in that ripples in a pass band are reduced by setting the wavelength to 5λ / 4 and making the other different from the above.