JPH09294046A - Transversal surface acoustic wave band pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the surface acoustic wave filter with a low insert loss and an excellent out-band attenuation characteristic. SOLUTION: Regular electrodes are adopted for input interdigital electrodes 2A, 2B and cross width weighting electrodes are adopted for output interdigital electrodes 3A, 3B formed on a crystal substrate whose temperature stability is enhanced, and a cross width W(n) of the cross width weighting electrodes is decided by a following function; W(n)=W.sin(2nπ/N)(a.cos(nπ/N)+b)/8, where W is a maximum cross width, n is an electrode part toward both ends with respect to a middle of electrode pairs set to zero, N is (N2-1)/2 when an electrode pair number N2 is an odd number and N2/2 when the electrode pair number N2 is an even number, and a, b are constants. Furthermore, the structure is realized, in which a proper number is set to the IDT electrode pair number and the cross width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transversal surface acoustic wave bandpass filter.

[0002]

2. Description of the Related Art Generally, a surface acoustic wave element has an IDT electrode (Inter electrode) formed on a piezoelectric substrate by using an electrode material such as aluminum.
Digital Transducer: Comb-shaped electrodes are formed to excite surface acoustic waves. Further, by using this excited surface acoustic wave, a functional element such as a filter, a resonator, a signal processing device or the like is configured.

FIG. 6 shows an example of the basic structure of a surface acoustic wave filter. This type of filter is generally defined as a transversal type filter.

The operating principle will be briefly described. The electric signal applied from the input terminal is converted into a surface acoustic wave which becomes acoustic energy by the input side IDT electrodes 2A and 2B. The converted surface acoustic wave propagates on the piezoelectric substrate 1, is converted again into an electric signal by the output side IDT electrodes 3A and 3B, and is taken out from the output terminal as an electric signal.

One (or both) IDT electrode pattern
The crossing width of the IDT electrodes 3A and 3B is weighted, and patterning is performed so that attenuation in the frequency region outside the pass frequency band can be sufficiently obtained.

Further, the input side IDT electrode and the output side IDT
The electrodes emit acoustic energy bidirectionally when converting electrical energy and acoustic energy, and the sound absorbing material 4 provided at both ends of the substrate 1 absorbs acoustic energy and eliminates unnecessary reflection.

[0007]

The performances required of a bandpass filter are mainly the following four.

(1) A low insertion loss in the pass frequency range, a low group delay time ripple, and a desired bandwidth can be obtained.

(2) Sufficient attenuation can be obtained outside the pass frequency range.

(3) The characteristics such that the center frequency is stable even when the temperature changes.

(4) Small size and light weight.

It is very difficult to satisfy the above four performances at the same time. For example, for downsizing, the number of IDT electrode pairs should be small and the crossing width should be small. However, reducing the number of IDT electrode pairs is a frequency bandwidth. And the desired amount of out-of-band attenuation cannot be obtained. Also, if the cross width is reduced,
Due to the diffraction of the surface acoustic wave, the effect of increasing the out-of-band attenuation amount due to the weighting of the cross width of the IDT electrodes cannot be obtained, and the desired out-of-band attenuation amount cannot be obtained.

Similarly, in order to improve the temperature characteristics, a quartz substrate is generally used. However, since the quartz substrate has a low electromechanical coupling coefficient, the insertion loss becomes large.

An object of the present invention is to provide a transversal type surface acoustic wave filter which is a bandpass filter having a low insertion loss and an excellent out-of-band attenuation characteristic.

[0015]

In order to achieve the above object, the present invention provides a transversal type surface acoustic wave bandpass filter in which an input side IDT electrode and an output side IDT electrode are provided.
One of the electrodes is a normal electrode and the other is a cross width weighting electrode, and the cross width W (n) of the cross width weighting electrode is the following function: W (n) = W · sin (2nπ / N) (a · cos (N
π / N) + b) / 8 W: Maximum crossing width n: Electrode part facing both ends with the center of the electrode pair set to 0 N: (N2-1) / 2 when the number of electrode pairs N2 is odd, N2 / 2 a when the number is even , B: characterized by a structure determined according to a constant.

The cross weighting electrode has a cross width W.
In the range of (n) ≧ 0, it is characterized by a structure of weighting with gradient type.

Further, the number of pairs of the normal type electrodes is 60 to 1
00 pairs, and the number of pairs of cross width weighting electrodes is 200 to 30.
It is characterized by a structure of 0 pairs.

The crossing width W of the normal type electrode is defined by the wavelength λ of the surface acoustic wave at the center frequency of the pass band of the filter.
Is characterized by the following structure: W = c · λ (40 ≦ c ≦ 60).

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a surface acoustic wave filter structure showing an embodiment of the present invention. In the present embodiment, a quartz substrate having good temperature characteristics is used as the piezoelectric substrate 1, the input side IDT electrodes 2A and 2B have a normal type electrode structure, and the output side IDT electrodes 3A and 3B have a cross width weighting electrode structure. Conversely, the input-side IDT electrodes 2A and 2B may be cross-weighting electrodes, and the output-side IDT electrodes 3A and 3B may be normal electrodes.

Here, the electrode portion n of the cross width weighting electrode
The intersection width W (n) of is determined according to the following function.

[0021]

[Formula 1] W (n) = W · sin (2nπ / N) (a · c
os (nπ / N) + b) / 8 where W is the maximum crossing width and electrode N is an odd number when N2 is an odd number when the number of IDT electrode pairs of crossing weighting electrodes is N2.

[0022]

## EQU2 ## N = (N2-1) / 2, and when N2 is an even number,

[0023]

## EQU3 ## Let N = N2 / 2.

When the number N2 of IDT electrode pairs is an odd number, n is the N + 1th pair of the number of IDT electrode pairs of the cross width weighting electrodes from the end, that is, as shown in FIG.
With the center of the electrode pair n = 0, n = 1 toward both ends,
2, 3, ..., N. When N2 is an even number, as shown in FIG. 3, the IDT electrode pairs of the cross-weighting electrodes are counted from the ends, the N-th pair is set as n = 1, and N = 1, 2,
3, ..., N.

Regarding the constants a and b, for example, a + b =
8 and a value satisfying 0.8 ≦ a ≦ 5.

According to the bandpass filter having the IDT electrode structure satisfying the above conditions, the specific bandwidth is 1.0 to 1.2%.
A transversal-type surface acoustic wave filter with the filter characteristics of and a large out-of-band attenuation was realized.

In addition to the above conditions, the weighting shape is W
In the range of (n) ≧ 0 (see FIG. 2 and FIG. 3), a preferable result was obtained in the structure with the gradient type weighting.

Also, the normal IDT electrode and the cross weighting I
Preferable results were obtained with a structure in which the number of pairs of DT electrodes is 60 to 100 pairs of normal type IDT electrodes and 200 to 300 pairs of cross width weighting IDT electrodes.

Further, the cross width W of the normal type IDT electrode is
When the wavelength of the surface acoustic wave at the center frequency of the pass band of the filter is λ,

[0030]

## EQU4 ## A preferable result was obtained in the structure in which W = c.λ (40≤c≤60).

As an experiment based on the present invention, the characteristics of the bandpass filter are shown in FIGS. FIG. 4 shows the frequency characteristic of the center frequency 112.32 MH _Z , and the characteristic that the out-of-band attenuation amount is sufficiently large with low insertion loss was obtained. FIG. 5 shows frequency characteristics and delay time ripples in which the vicinity of the center frequency is enlarged.

[0032]

As described above, according to the transversal type surface acoustic wave bandpass filter of the present invention, an appropriate I
Bandwidth characteristics of the bandpass filter with low insertion loss and high out-of-band attenuation are realized by weighting the width of the DT electrode intersection and the number of pairs of IDT electrodes.

Further, by using a quartz substrate as the piezoelectric substrate, a bandpass filter having a small frequency characteristic variation due to temperature can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a transversal surface acoustic wave filter showing an embodiment of the present invention.

FIG. 2 is a cross width weighted IDT part structure in the case where the logarithm N2 is an odd number in the embodiment.

FIG. 3 is a cross width weighted IDT part structure in the case where the logarithm N2 is an even number in the embodiment.

FIG. 4 is a frequency characteristic diagram of a filter showing an experimental result based on the present invention.

FIG. 5 is a frequency characteristic and delay time characteristic diagram of a filter showing an experimental result based on the present invention.

FIG. 6 is a configuration diagram of a conventional transversal surface acoustic wave filter.

[Explanation of symbols]

 1 ... Piezoelectric substrate 2A, 2B ... Input side IDT electrode 3A, 3B ... Output side IDT electrode 4 ... Sound absorbing plate

Claims (4)

[Claims] 1. In a transversal surface acoustic wave bandpass filter, one of an input side IDT electrode and an output side IDT electrode is a normal type electrode and the other is a cross width weighting electrode, and a cross width W ( n) is the following function: W (n) = W · sin (2nπ / N) (a · cos (n
π / N) + b) / 8 W: Maximum crossing width n: Electrode site facing both ends with the center of the electrode pair set to 0 N: (N2-1) / 2 when the number N2 of electrode pairs is odd, N2 / 2 a when the number is even , B: a transversal surface acoustic wave bandpass filter characterized by a structure determined according to a constant. 2. The crossing weight electrode has a crossing width W.
2. The transversal surface acoustic wave bandpass filter according to claim 1, wherein the weighting is performed in a tilted manner within a range of (n) ≧ 0. 3. The number of pairs of the normal electrodes is 60 to 100.
The transversal surface acoustic wave bandpass filter according to claim 1, wherein the transversal surface acoustic wave bandpass filter has a structure in which the number of pairs of cross width weighting electrodes is 200 to 300. 4. The crossing width W of the normal electrodes is W = c · λ (40 ≦ c ≦ 60), where λ is the wavelength of the surface acoustic wave at the center frequency of the pass band of the filter. The structure according to any one of claims 1 to 3, which is characterized by a structure.
The transversal-type surface acoustic wave bandpass filter described in 1.