JPH08288783A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE: To obtain a surface acoustic wave filter having an optional filter characteristic corresponding to high frequency by regulating the width size of each electrode digit, the combination of opposed electrode digits and the directivity of a weighting area on a cross part so as to satisfy specific conditions. CONSTITUTION: As to interdigit electrode pairs 24, 25 in a wave transmitter 22 or a wave receiver, the width size of each of electrode digits 24a, 25a is set up to 1/6 of wavelength corresponding to center frequency and one of the opposed electrode digits 24a, 25a is formed so that digits are alternately intersected in each two digits. The directivity of cross areas on both the side parts S1, S2 of a center part M is set up so as to be inclined from the propagation direction of surface acoustic waves. Even if surface acoustic waves are reflected on the end parts of the electrode digits 24a, 25a on the surface acoustic wave filter formed by the method, phases are not matched and the adverse effect of reflected waves can be prevented. In addition, the transmission characteristic of a pass band can be optionally set up and a filter characteristic corresponding to a higher frequency band can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention comprises a wave transmitter and a wave receiver in which comb-shaped interdigital electrodes are arranged opposite to each other on a substrate.
The present invention relates to a surface acoustic wave filter whose pass band is set to a center frequency fo.

[0002]

2. Description of the Related Art Generally, a surface acoustic wave filter has a wave transmitter 2 and a wave receiver 3 on a piezoelectric substrate 1 as shown in FIG.
The wave transmitter 2 and the wave receiver 3 are composed of interdigitated interdigital electrodes 4 and 5 and interdigital electrodes 6 and 7, respectively. The interdigital electrodes 4 and 5 of the wave transmitter 2 or the wave receivers 6 and 7 of the wave receiver 3 respectively convert an electric signal applied between them into a surface acoustic wave and output the surface acoustic wave to the piezoelectric substrate 1. The reversible operation of converting the surface acoustic wave into the electric signal and outputting the electric signal is possible.

In this case, for example, the interdigital electrodes 4 and 5 of the wave transmitter 2 are arranged so that the opposing electrode fingers 4a and 5a are staggered, and the electrode fingers 4a, 4a or 5 are arranged.
The pitch between a and 5a is set to λo. This pitch λo is a dimension corresponding to the reciprocal of the center frequency fo when operating as a filter.

When a voltage is applied between the interdigital electrodes 4 and 5 of the wave transmitter 2, a surface acoustic wave is generated, which is approximated by positive and negative impulses as shown in FIG. At this time, the surface acoustic waves propagate to the left and right, but if they match the period of the electric signal, they will strengthen each other, and if they are in the opposite phase, they will weaken each other. As a result, they are transmitted from the transmitter 2 side to the receiver 3 side. The frequency of the generated signal is as shown in FIG. Further, in the actual filter, since the wave transmitter 2 and the wave receiver 3 are present, the product of the transfer characteristics f1 (Ω) and f2 (Ω) of each is obtained.
Therefore, the characteristic F (Ω) of the entire filter can be expressed by the following equation (1). Here, Ω is a frequency obtained by dividing an arbitrary frequency f by the center frequency fo and standardizing it, as shown in Expression (2).

[0005]

[Equation 1]

[Equation 2]

In addition, in an actual filter, in order to flatten the characteristics in the pass band and make the overall characteristics rectangular, one of the wave transmitter 2 and the wave receiver 3 is shown in FIG. As shown in the figure, the length dimension of the portion where the electrode fingers 4a and 5a of the crossed finger electrodes 4 and 5 intersect is changed to perform weighting, whereby the required characteristics are designed. And such weighting is called an apodize method.

By the way, in the above-mentioned configuration,
There is a problem in the following points. That is, since the widths and intervals of the electrode fingers 4a and 5a are set to ¼ of λo, the surface acoustic waves reflected at the electrode end portions are overlapped with each other to become a large reflected wave. . That is, in FIG. 11 showing a cross section of the interdigital electrodes 4 and 5, the velocity of the surface acoustic wave changes depending on the presence or absence of the metal electrode on the surface, and therefore is reflected by the end portions of the electrode fingers 4a and 5a. In this case, the electrode finger 4
Reflection when going from a portion without a and 5a to a certain portion has the same phase, and reflection when going from a portion with no electrode fingers 4a and 5a to a non-having portion has a reverse phase shift.

The surface acoustic wave A1 shown in FIG. 11 is reflected at the ends of the electrode fingers 4a in the same phase. Further, the surface acoustic wave A2 that passes through the electrode finger 4a portion and is reflected at the end portion in the opposite phase is twice λo / 4 of the surface acoustic wave A1, that is, a half wavelength λo.
Since they are deviated by / 2 and are in the opposite phase, they eventually have the same phase as the surface acoustic wave A1. Hereinafter, similarly, the electrode fingers 5a
The surface acoustic waves A3 and A4 respectively reflected at both end portions of the same eventually have the same phase as the surface acoustic wave A1, and as a whole, the reflected waves of the same phase are superposed, and the interdigital electrodes 4, Therefore, a large reflected wave is generated from No. 5.

As described above, when the interdigital electrodes 4 and 5 are reflected, the surface acoustic wave output from the wave transmitter 2 is reflected by the wave receiver 3 and returns to the wave transmitter 2 side. The transmitter 2
It becomes a surface acoustic wave that is further reflected by the portion and proceeds to the side of the wave receiver 3. However, the surface acoustic wave caused by the reflection is transmitted by the transmitter 2
Since the transmission time is different from the surface acoustic wave initially output from, it will appear as ripples on the transfer characteristic,
This is not desirable as the transmission characteristic of the filter.

Therefore, a structure for eliminating such a problem has been considered. For example, FIG.
There is a configuration shown in (a). That is, in this device, the width dimension of the electrode fingers 8a and 9a of the interdigital electrodes 8 and 9 is set to ⅛ of the wavelength λo corresponding to the center frequency fo, and two electrodes are arranged to intersect each other. It is a thing.

According to this structure, a surface acoustic wave as indicated by an arrow in FIG. 2B is generated and propagates through the substrate. However, the elastic surface propagated from the transmitter side to the receiver side. Even when the waves are reflected at the electrode finger ends, their phases cancel each other in the opposite manner to the above, so that the interdigitated electrodes 8 and 9 do not generate reflected waves as a whole and the transfer characteristics are improved. Will be able to. Further, in this configuration, since the impulse trains generated between the electrode fingers are at equal intervals, the frequency characteristic can be easily designed as a whole, and an arbitrary characteristic can be obtained.

[0012]

However, in the case of using the λo / 8 electrode as described above, in addition to the characteristic advantage as described above, there are the following drawbacks.

That is, as the pass frequency of the filter becomes higher, the width dimension of the electrode fingers becomes smaller, which makes it difficult for the manufacturing method using an ordinary apparatus. At present, at most 43
Up to 0 MHz. On the other hand, the intermediate frequency used in satellite broadcast receivers is set to a very high frequency, and the filter used in the processing circuit that handles the intermediate frequency is required to be able to cover the corresponding frequency. To be done. This is, for example, 4 in Japan.
About 03MHz is required, 480M in Europe
Although Hz is required, in order to cover this European one, if the width dimension of the electrode finger is λo / 8, it exceeds the manufacturing limit described above, and it is difficult to achieve it.

Therefore, in order to obtain a filter that can be realized even in the European system, an apparatus having a highly accurate manufacturing technique is required, and conversely, as long as the conventional apparatus is used, the processing accuracy is high. Since the limit is exceeded, this leads to the problem that the yield is greatly reduced.

Therefore, what can be considered is the electrode fingers 10a, 1 of the interdigital electrodes 10, 11 as shown in FIG. 13 (a).
The width dimension of 1a is λo / 6, and only one of them is arranged so as to intersect every two. In this case, a surface acoustic wave indicated by an arrow in FIG. 7B is generated and propagates through the substrate. However, the surface acoustic wave propagated from the wave transmitter side to the wave receiver side. Is reflected at the electrode finger end,
Since the phases are different from each other, they do not cancel each other, but the phases are not aligned as compared with the case where the width dimension of the electrode fingers 10a and 11a is λo / 4 described above. As a result, the reflected wave, which is a problem in practical use, does not occur.

In this structure, the width of the electrode finger can be made wider for the same center frequency fo than that of λo / 8. In other words, the value of the center frequency fo is high. To be able to set
The above-mentioned problems can be solved. Therefore, for example, the interdigital electrode 1 having a configuration having a large number of electrode fingers 12a and 13a by the apodization method as shown in FIG.
By forming 2 and 13, it is possible to obtain a surface acoustic wave filter having a pass band characteristic as shown in FIG.

However, in such a structure, as shown in FIG. 15, although the transfer characteristic of the region corresponding to the frequency fo in the pass band can be made flat, it is arbitrary. It is difficult to design the shape. That is, in this configuration, as shown in FIG. 13B, since the impulse does not occur every two electrode fingers, the restriction on freely designing the characteristics is increased. .

The present invention has been made in view of the above circumstances, and it is an object of the present invention to be manufactured within the range of conventional manufacturing techniques and to obtain an arbitrary filter characteristic corresponding to a high frequency. The object is to provide a surface acoustic wave filter as described above.

[0019]

SUMMARY OF THE INVENTION The present invention includes a wave transmitter and a wave receiver in which comb-shaped interdigital electrodes are arranged on a substrate so as to face each other, and elasticity set in a pass band of a center frequency fo. It is intended for a surface wave filter, and one of the cross finger electrode pairs of the wave transmitter or the wave receiver is (1) the width dimension of the electrode fingers is 6 at a wavelength λo corresponding to the center frequency fo. It is set to 1 / (2) Two comb-shaped electrode fingers facing each other are arranged side by side on two sides only. (3) Frequency characteristics that are asymmetrical about the center frequency fo. So that the pointing direction of the weighting region at the intersection of the electrode fingers of the interdigital electrode is formed in a direction inclined with respect to the propagation direction of the surface acoustic wave, However, it has a feature.

[0020]

According to the surface acoustic wave filter of the present invention, when an electric signal is applied to the transmitter, the surface acoustic wave corresponding to the width dimension of the electrode finger set corresponding to the frequency is generated. It is output on the substrate surface. When this surface acoustic wave propagates on the surface of the substrate and reaches the wave receiver, it is converted into an electric signal and output. At this time, the width dimension of the electrode fingers is set to 1/6 of the wavelength λo corresponding to the center frequency fo, and the opposing comb-teeth-shaped electrode fingers are combined so that only two electrodes are arranged side by side. Therefore, even if the surface acoustic waves are reflected by the end portions of the electrode fingers, their phases do not coincide with each other, and the adverse effect of the reflected waves as a whole can be prevented.

In addition, by tilting the pointing direction of the weighting region at the intersection of the electrode fingers of the interdigital electrode with respect to the propagation direction of the surface acoustic wave, a frequency characteristic that is asymmetrical about the center frequency fo is provided. Therefore, the transfer characteristic of the pass band can be arbitrarily set. In this case, a pass band is generated even when the frequency is near zero,
Since this is a region that can be almost ignored with respect to the frequency band to be handled, the occurrence of a pass band in the vicinity of zero is not a problem, and therefore, it becomes possible to obtain a pass band characteristic having an arbitrary shape. .

Further, by setting the width dimension of the electrode finger to one sixth of the wavelength λo corresponding to the center frequency fo in this way, within the range of the restriction of the processing accuracy received when forming the electrode finger. It will be possible to manufacture one corresponding to a high frequency band with respect to one eighth.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 2 shows the external appearance of the entire structure. The piezoelectric substrate 2 made of single crystal such as quartz crystal is shown in FIG.
A wave transmitter 22 and a wave receiver 23 are formed on the upper surface of 1. The wave transmitter 22 and the wave receiver 23 are each composed of a pair of two crossing finger electrodes 24, 25 and 26, 27 facing each other. The dimensions of each part of these cross-finger electrodes 24 to 27 are designed based on a design method described later.

FIG. 1 shows, for example, the cross finger electrodes 2 of the transmitter 22.
4 and 25 are plan views schematically showing a width dimension of 1/6 of the wavelength λo corresponding to the center frequency fo, and an interval dimension between adjacent electrode fingers is also a dimension of 1/6 of the wavelength λo. It is set. Then, in the portions where the interdigital electrodes 24 and 25 face each other, two electrode fingers are continuously arranged from one side,
One electrode finger is arranged from the other,
Further, the intersection lengths thereof are set so as to have dimensions calculated based on an arithmetic expression described later.

In this case, the interdigital electrodes 24 and 25 are composed of an intersecting region M formed at the central portion and intersecting regions S1 and S2 formed at both ends thereof, and the intersecting regions S1 and S2 are The direction in which the intersecting portion is oriented is set in a direction orthogonal to the electrode fingers, that is, in a direction inclined with respect to the direction in which the surface acoustic wave propagates.

Next, a method of designing the electrode fingers of the above-mentioned interdigital finger electrodes 24, 25 will be described. In order to arbitrarily set the band characteristic as a filter, the cross finger electrodes 24, 25
, The intersection regions M and S1 and S2 shown in FIG.
As shown in, the design is made by the apodization method. In this case, originally, the wave equation must be solved in order to obtain the apodized interdigital electrodes, but the solution method is complicated and difficult, and in practice it does not require such accuracy. Has obtained practically enough practical value by performing the following approximate calculation.

First, as a premise, consider an electrode shape model as shown in FIG. The electrode fingers of the interdigitated electrodes 24, 25 are designated as 24a, 24b, ... And 25a, 25b,
Then, when an electric signal is applied between the opposing electrode fingers, an impulse having an amplitude as shown in FIG. 6B is generated, and this impulse moves to the left and right at the surface acoustic wave propagation velocity and propagates. Suppose. That is, the magnitude of the impulse shown in this figure also indicates the time course of the surface acoustic waves output from the interdigital electrodes 24 and 25.

In this figure, when the time reference is set at the center of the interdigital electrodes 24 and 25 and the Fourier transform is performed to transform the state in the time domain into the transfer characteristic in the frequency domain, it is expressed as in equation (3). You can Where ai, bi
Indicates the magnitude of the impulse, n is a value indicating the number of cycles on one side of the interdigital electrode, and 6n + 1 is the number of electrode fingers. Further, the index of ε indicates the frequency characteristic over time.

[0029]

(Equation 3)

In the equation (3), the first term and the second term have the same coefficient and differ only in the sign of the exponent of ε. Therefore, by summing these, the following equation (4) is obtained.

[0031]

[Equation 4]

Looking at this equation (4), it can be seen that since the value of the equation is a pure imaginary number, there is no phase change and the group delay characteristic is flat. This is because the electrode fingers are arranged so as to be symmetric with respect to the reference point described above, and the phase of the surface acoustic wave output before and after the reference point is the same at all frequencies, as if Interdigital electrodes 24, 25
This is because it seems that surface acoustic waves are generated only from the center of. In other words, in order to make the group delay characteristic of the transfer characteristic flat, the interdigital electrodes should be arranged symmetrically.

The values of the coefficients ai and bi in the equation (4) are determined by the required characteristics of the filter. There are various calculation methods such as Fourier series expansion, least squares method, and mathematical programming.

Next, the design of the pass band characteristic will be described. For example, it is assumed that the pass band characteristic of the frequency as shown in FIG. 4A is the required characteristic. Generally, the function shown in the graph in this way can be divided into an axially symmetric component and a point symmetric component centered on an arbitrary point. Therefore, if the pass band characteristic of the filter shown in the same figure is divided into an axially symmetric component and a point symmetric component with a reference frequency Ω = 1, which is a standardized center frequency fo, as the center, become.
In this case, it is easier to understand when the origin is the center. Therefore, if the standardized frequency Ω is replaced by the following equation (5), the scale on the horizontal axis is as shown in the lower side of the figure. .

[0035]

(Equation 5)

As a result, the value of ΩΔ takes a positive or negative value and indicates the distance from the symmetrical center frequency.
Then, by substituting the equation (5) into the equation (4) and dividing the equation into two terms and rewriting the equation, the following equation (6) is obtained. Then, each term in Expression (6) is as shown in Expressions (7) and (8).

[0037]

(Equation 6)

(Equation 7)

(Equation 8)

Of these, in the equation (7), since it is composed only of the term of cos, that is, the even function, g (ΩΔ)
Is an even function and has an axially symmetric component. Further, since the expression (8) is composed only of the term of sin, that is, the odd function, h (ΩΔ) becomes an odd function and has a point symmetric component.

Therefore, in order to obtain the above-mentioned arbitrary frequency characteristic, the following design is performed. In the configuration of FIG. 14 described in the item of the conventional example, a passband characteristic consisting of only even function components which are symmetrical (see also FIG. 15)
Therefore, it was not possible to obtain an arbitrary frequency characteristic.

Therefore, in the present invention, in order to add the point symmetric component, that is, in order to prevent the component shown in the above equation (8) from becoming "0", the value near the 0 frequency of the filter is set. Must be set. Here, the point-symmetrical component h '(ΩΔ) of the required characteristic is calculated by the following equation (9) from the required characteristic k' (ΩΔ) of the frequency above the reference frequency and the required characteristic k '(-ΩΔ) of the lower frequency. Can be obtained as follows. Since the value must match h (Ω), the following expression (10) can be obtained by comparing with the above expression (4).

[0041]

[Equation 9]

[Equation 10]

The equation (10) thus obtained has the required characteristics near zero frequency. As described above, if a value corresponding to the point symmetry component is given in the vicinity of the 0 frequency in advance and designed, an arbitrary band characteristic can be obtained. In this case, for example, the electrode shape of the interdigitated electrode obtained by finding ai and bi in the equation using the least squares method when n = 20 and substituting these values into the equation (4) is As shown in FIG. 1, the frequency characteristic is as shown in FIG.

In this case, the electrode shape of the interdigital electrode is formed such that the direction in which the intersection region of the side portions S1 and S2 is oriented with respect to the central portion M is inclined with respect to the direction in which the surface acoustic wave propagates. become. This is because the component corresponding to the point symmetry component is set to be added in the vicinity of the 0 frequency described above.

FIG. 5 is an example showing the design data of the required characteristic k '(ΩΔ). In this case, for simplicity, n is shown.
= 5, the respective design values f'1 (Ω) obtained for the axially symmetric component and the point symmetric component for each r of ΩΔ = r / n are shown.

In FIG. 6, it can be seen that the pass band characteristic does not have axial symmetry but includes point symmetry components.
In other words, by adding the point-symmetrical component, the frequency characteristic corresponding to the desired characteristic could be obtained. Note that there is a region with a small amount of attenuation near the 0 frequency, but this is a characteristic that was set in advance at the design stage, and since it is not a target for practical use with respect to the frequency in this region, it is practical. No problem,
Only the area corresponding to the center frequency fo can be effectively used.

According to this embodiment as described above, the transmitter 22
In the interdigitated electrodes 24 and 25, the width dimension of the electrode fingers is set to 1/6 of the wavelength λo corresponding to the center frequency fo,
Only the two facing sides are arranged in a combination of two, and the pointing direction of the weighting area at the intersection of the electrode fingers of the interdigital electrode is elastic so as to have a frequency characteristic that is asymmetrical about the center frequency fo. Since it is formed in a direction inclined with respect to the propagation direction of the wave, even if the surface acoustic wave is reflected at the end of the electrode finger, the phases do not match and the adverse effect of the reflected wave as a whole can be prevented. In addition, the transfer characteristic of the pass band can be arbitrarily set by providing the frequency characteristic that is asymmetric with respect to the center frequency fo as the axis. Then, by setting the width dimension of the electrode fingers to a dimension of 1/6 of the wavelength λo corresponding to the center frequency fo, it is possible to set the width of the electrode fingers to 8 minutes within the restriction of the processing accuracy received when forming the interdigital electrode. It becomes possible to manufacture a device corresponding to a higher frequency band than the above-mentioned one.

The present invention is not limited to the above embodiment, but can be modified or expanded as follows. The configuration described above may be used not only on the side of the wave transmitter 22 but also on the side of the wave receiver 23. The value of n can be arbitrarily set as needed.

[Brief description of drawings]

FIG. 1 is a plan view of an interdigital electrode showing an embodiment of the present invention.

FIG. 2 is a schematic external perspective view of the overall configuration.

FIG. 3 is an explanatory diagram of the principle of the electrode fingers of the interdigital electrode.

FIG. 4 is a band characteristic diagram showing in principle the required characteristics of the pass band and the characteristics divided into components.

[Fig. 5] Design data of the interdigital electrodes

[Fig. 6] Passband characteristic diagram

FIG. 7 is an explanatory diagram of a principle showing a conventional example.

FIG. 8 is a conceptual explanatory view of surface acoustic wave output.

FIG. 9 is a frequency characteristic diagram of a filter.

FIG. 10 is an external view of a filter by the apodization method.

FIG. 11 is an explanatory diagram of a reflected wave when the width dimension of the electrode finger is λo / 4.

FIG. 12 is an explanatory view of the principle of the electrode finger having a width dimension of λo / 8.

FIG. 13 is an explanatory view of the principle of the electrode finger having a width dimension of λo / 6.

FIG. 14 is a plan view showing an example of an interdigital electrode in which the width dimension of the electrode fingers is λo / 6.

FIG. 15 is a frequency characteristic diagram of the filter.

[Explanation of symbols]

Reference numeral 21 is a piezoelectric substrate, 22 is a wave transmitter, 23 is a wave receiver, and 24 to
27 is an interdigital finger electrode, 24a and 25a are electrode fingers, and M and S
1, S2 are intersection regions.

Claims (1)

[Claims] 1. A surface acoustic wave filter having a wave transmitter and a wave receiver in which comb-teeth-shaped interdigital electrodes are arranged on a substrate so as to face each other, wherein the surface acoustic wave filter is set in a pass band of a center frequency fo. A surface acoustic wave filter, characterized in that one of the interdigital electrode pairs of the wave receiver or the wave receiver is formed so as to satisfy the following three conditions. (1) The width dimension of the electrode fingers is set to 1/6 of the wavelength λo corresponding to the center frequency fo. (2) In a combination in which two opposing comb-teeth-shaped electrode fingers are arranged on only one side. Arrangement (3) The pointing direction of the weighting area at the intersection of the electrode fingers of the interdigital electrode is set with respect to the propagation direction of the surface acoustic wave so that the frequency characteristics are asymmetrical about the center frequency fo. Being formed in an inclined direction