JPH10242796A - Multimode saw filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attenuate the spurious at a point near the center frequency and to decrease the number of cascaded stages for reduction of the cost and size of a multimode SAW filter by shifting the position of at least one of laterally coupled multimode filters in the transmitting direction of a surface wave. SOLUTION: The positions of IDT 2 and 4 are shifted from the positions of IDT 6 and 8 of a laterally coupled multimode filter in the transmitting direction of a surface wave with a space D set as D=(λ/8+nλ/4) between them (n = 1, 2, 3...). When the IDT of a filter A is shifted from the IDT of a filter B by λ/8 in the transmitting direction of the surface wave, the phase difference is equal to λ/4 between the surface waves which are leaked out of both filters A and B and reflected at both ends of a substrate. Thereby, no standing wave is generated by a spurious component to the filter B to suppress the spurious since the filter B has a phase difference between the reflected and traveling waves that is shifted from the phase difference of the filter A by λ/4 when the filter A has the spurious.

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 multi-mode SAW filter in which spurious components generated near a center frequency are reduced.

[0002]

2. Description of the Related Art Recently, a surface acoustic wave filter (hereinafter referred to as SA) has been developed.
W filter) is excellent in miniaturization, high frequency, mass productivity, etc.
It is widely used as an F filter or an RF filter. In particular, a multi-mode SAW filter is small, but can secure a large amount of attenuation in a stop band, and thus is suitable for a small wireless device. Multimode SAW filters can be broadly classified into two types according to the coupling mode of the vibration modes. The first is a so-called longitudinally-coupled multi-mode SAW filter whose schematic plan view is shown in FIG. 3 (a). The two IDTs 21 and 22 are disposed close to each other, and reflectors 23a and 23b are disposed on both sides of the two IDTs 21 and 22, thereby forming a SAW filter (filter A).
Each of the IDTs 21 and 22 includes a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween.
One of the interdigital electrodes constituting T21 and T22 is connected to a ground potential terminal, and the other interdigital electrode is electrically connected to an input or output terminal.

Another set (IDTs 24, 25 and reflectors 26) is formed on the substrate 20 so as to be spaced apart from the filter A so as not to cause acoustic coupling.
The two-stage cascade-connected vertical-coupling dual-mode SAW filter is constructed by cascade-connecting the vertical-coupling multi-mode SAW filter (filter B) a) and 26b) with the filter A. The function of the filter A is to confine the vibration energy between the reflectors by reflecting the surface waves excited by the IDTs 21 and 22 toward the IDTs 21 and 22 by the reflectors 23a and 23b, so that the vibration energy is acoustically coupled. The two modes are strongly excited and function as a band-pass filter using these two vibration modes. The filter B operates in the same manner as the filter A, and the filter A and the filter B are connected in cascade, so that the attenuation gradient is steeper than that of a single filter, and the SAW in which the stop band attenuation is increased.
A filter can be realized.

The second type of the multi-mode SAW filter is shown in FIG.
A so-called laterally-coupled multi-mode S whose schematic plan view is shown in FIG.
An AW filter, an IDT 27 and reflectors 28a and 28b disposed on both sides of the IDT 27 along a direction in which a surface wave propagates on a main surface of a piezoelectric substrate such as quartz to form a resonator; Another set (IDT29) similarly configured on the substrate
And the resonators of the reflectors 30a and 30b) are juxtaposed and S
An AW filter (filter A) is configured. Filter A
Another set of transversely coupled multimode SAW filters (Filter B) configured in the same manner as Filter A are cascaded with Filter A at such an interval that acoustic coupling with Filter A does not occur on the piezoelectric substrate on which is formed. These are connected to form a two-stage cascade-connected horizontal-coupling dual-mode SAW filter. Each of the IDTs 27 and 29 of the filter A is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween, and one of the comb-shaped electrodes constituting the IDTs 27 and 29 is connected to a ground potential terminal. The other comb electrode is electrically connected to an input or output terminal. IDT27
Reflectors 28a, 28b and 30 arranged on both sides of
a and 30b confine the vibration energy between the reflectors by reflecting the surface waves excited by the IDTs 27 and 29 toward the center of the respective IDTs.
By placing the 7, 7 in close proximity, the confined vibrational energy is acoustically coupled and the resulting 2
It functions as a bandpass filter using two vibration modes. The filter B operates in the same manner as the filter A. By cascade-connecting the filter A and the filter B, it is possible to realize a SAW filter having a steeper attenuation gradient and a larger rejection attenuation.

Here, the ID of the multi-mode SAW filter is
The characteristics of T and the reflector will be described in some detail. Figure 4 shows the IDT
FIG. 5 is a diagram showing a relationship between normalized radiative conductance Ga / GN of the reflector and reflection coefficient | Γ | of the reflector. Generally, IDT
When the electrode periods of the reflector and the reflector are made equal, fT <f is between the center frequencies fT and fR at which the respective values become maximum.
It is known that a relationship of R occurs and the center frequency of the IDT becomes lower than the frequency of the reflector. This indicates that sufficient reflection cannot be obtained at the center frequency fT of the IDT. Therefore, the two frequencies are matched and the electrode period of the IDT or the reflector is adjusted so that the reflection coefficient | Γ | becomes maximum at the frequency fT. The normalized radiated conductance Ga / GN and the reflection of the reflector shown in FIG. FIG. 6 is a characteristic diagram of a coefficient | Γ |. In addition, the electrode thickness and the normalized radiation conductance G
The relationship between a / GN and the reflection coefficient | Γ | of the reflector is such that the normalized radiated conductance Ga /
GN increases, the reflection coefficient | Γ | approaches 1, and the width of the flat portion increases. As is apparent from FIG. 4, the reflection coefficient | Γ | of the reflector takes a value close to 1 in a range of about ± 1% of the center frequency, and the function of the reflector is sufficient at frequencies outside this frequency range. Will not be obtained.

The equivalent circuit of the multi-mode SAW filter of either the vertical coupling type or the horizontal coupling type is shown in FIG. 5A, and by applying the bisecting theorem to this, FIG.
Can be converted into a lattice type circuit. As is clear from the figure, at a frequency distant from the resonance frequency, the equivalent circuit is a circuit composed of the capacitance C0. Normally, two capacitances C0 are formed to be equal. The amount of attenuation in the stop band can be secured.

[0007]

However, in the above-described longitudinally-coupled multi-mode filter or laterally-coupled multi-mode filter, the surface wave leaking from the reflector to the side opposite to the IDT is as shown by the solid line in FIG. The light travels on the piezoelectric substrate and is reflected at the end face of the piezoelectric substrate, and the reflected wave (broken line) interferes with the traveling wave between the two reflectors, causing a defect, for example, a large amount of spurious components, resulting in a sufficient stopband attenuation. It is difficult to secure. Further, when the number of electrode fingers of the reflector is not sufficient and the reflection characteristics are incomplete, surface waves relating to main resonance leak from the reflector, and the number of reflected waves reflected at both end faces of the piezoelectric substrate increases. The reflected wave travels between the reflectors,
For example, in the case of interference in opposite phases, it acts to cancel the standing wave between the reflectors, lowers the Q value of the SAW resonator,
There is also a disadvantage that the insertion loss of the filter formed by the resonator is increased. Further, even if the number of reflectors is relatively large and the reflection of the surface wave relating to the main resonance is sufficient, the reflection characteristics cannot be sufficiently obtained as described with reference to FIG. The surface waves leaking from the reflector are reflected at both end surfaces of the piezoelectric substrate, and when the standing waves are generated by the reflected waves, the waves become spurious.

As a measure for reducing the spurious caused by the reflected waves from both ends of the piezoelectric substrate, conventionally, as shown in FIG. 7, a sound absorbing agent 31 is applied to both ends of the piezoelectric substrate to reduce the surface acoustic waves leaking from the reflector. Means have been taken to suppress the spurious by absorbing it into the sound absorbing agent 31. However, sound absorber 3
The number of steps for applying 1 to each chip is enormous, and there is a problem that it is extremely difficult to apply an appropriate amount to an appropriate place for absorbing a reflected wave. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a filter that has reduced spurious in the vicinity of the center frequency of a multimode SAW filter.

[0009]

According to a first aspect of the present invention, there is provided a multi-mode SAW filter according to the present invention, comprising an IDT disposed on a piezoelectric substrate along a propagation direction of a surface wave. In a filter in which a plurality of transversely coupled multimode filters in which a plurality of surface acoustic wave resonators each including a reflector are juxtaposed and arranged in parallel with each other are cascaded in multiple stages, at least one position of the laterally coupled multimode filter is shifted in a propagation direction of the surface wave. And a spurious near the center frequency is reduced. According to a second aspect of the present invention, there is provided a multi-stage transversely coupled multimode filter in which a plurality of surface acoustic wave resonators each composed of an IDT and a reflector arranged in the direction of propagation of a surface acoustic wave are arranged in parallel on a piezoelectric substrate. In cascaded filters,
At least one ID of the transversely coupled multimode filter
This is a multi-mode SAW filter characterized in that the position of T is shifted in the direction of propagation of a surface acoustic wave and spurious near the center frequency is reduced. According to a third aspect of the present invention, there is provided a filter comprising a vertically coupled multi-mode SAW filter comprising a plurality of IDTs and reflectors arranged in a surface acoustic wave propagation direction along a propagation direction on a piezoelectric substrate. A multi-mode SAW characterized in that at least one position of a longitudinally-coupled multi-mode filter is displaced in a propagation direction of a surface wave to reduce spurious near a center frequency.
Filter. According to a fourth aspect of the present invention, in the filter, a longitudinally coupled multimode SAW filter including a plurality of IDTs and reflectors arranged along a propagation direction of a surface acoustic wave on a piezoelectric substrate is configured by cascading multiple stages. A multimode SAW in which at least one IDT of a longitudinally coupled multimode filter is arranged so as to be shifted in a propagation direction of a surface wave to reduce spurious near a center frequency.
Filter. According to a fifth aspect of the present invention, in the multimode SAW filter according to the first to fourth aspects, the IDT
Λ / 16 + nλ / 4 to 3λ / 16 + nλ
/ 4 is a multi-mode SAW filter. According to a sixth aspect of the present invention, there is provided the multi-mode SAW filter according to any one of the first to fourth aspects, wherein an interval of shifting the IDT is set to 3λ / 16 + nλ / 2 to 5λ / 16 + nλ / 2. It is.

[0010]

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 schematic diagram showing a plan view of a SAW filter constructed by cascade-connecting two transversely coupled dual-mode SAW filters according to the present invention. The propagation of a surface wave on the main surface of a piezoelectric substrate 1 such as a quartz crystal is shown. The IDT 2 and the reflectors 3a and 3b on both sides thereof are arranged along the direction in which the resonator is formed, and another pair of resonators (IDT 4 and reflectors 5a and 5b) similarly configured on the substrate 1. Is a SAW filter (filter A) configured by juxtaposing and juxtaposed. Each of the IDTs 2 and 4 is composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. One of the IDTs 2 and 4 is connected to a ground potential terminal, and the other comb-shaped electrode is connected to the ground potential terminal. The electrodes are electrically connected to input or output terminals. Further, the piezoelectric substrate 1
Another set of transversely coupled multimode SAW filters (filters B) configured in the same manner as the filter A are cascade-connected to the filter A with an interval on the main surface of the filter A such that acoustic coupling does not occur with the filter A. To form a two-stage cascade-connected horizontal-coupling dual-mode SAW filter. The difference between the SAW filter according to the present invention and the conventional cascade connection type laterally coupled dual mode SAW filter is that the IDTs 2, 4 and
The arrangement positions of 6 and 8 are shifted in the propagation direction of the surface wave, and the interval D is set to D = (λ / 8 + nλ / 4) (where n = 1, 2, 2
3).

The operation of the filter A shown in FIG.
The reflectors 3a, 3b and 5a, 5b arranged on both sides of the IDTs 2 and 4 respectively convert the surface waves excited by the IDTs 2 and 4 into IDs.
Vibration energy is confined between the reflectors by reflecting toward the center of T, and the two IDTs 2, 4 are arranged in close proximity to acoustically couple the confined vibration energy, thereby resulting in two vibration modes. It functions as a bandpass filter using. The operation of the filter B shown in FIG. 1 is the same as that of the filter A. By cascade-connecting the filters A and B, it is possible to realize a SAW filter having a steeper attenuation gradient and a larger stopband attenuation. It becomes possible.

As described above, the feature of the SAW filter according to the present invention is that the arrangement positions of the IDTs 2 and 4 and the IDTs 6 and 8 are set at intervals D = (λ / 8 + nλ / 4) (where n = 1, 2, 3).
..) in the direction of propagation of the surface wave. Focusing on the fact that the spurious generated in the conventional multimode SAW filter is excited by the interference between the traveling wave and the reflected wave, it is easy to understand that this spurious has a periodicity of λ due to the phase difference between the traveling wave and the reflected wave. Let's do it. Therefore, the IDT of the filter A and the IDT of the filter B in FIG.
Are displaced by λ / 8 in the propagation direction of the surface wave, the surface waves leaking from the filter A and reflected at both ends of the substrate and the reflected waves leaking from the filter B and reflected at the end surface of the substrate Is λ / 4. This is because when the filter A has a frequency near the center frequency and the reflected wave due to the end face reflection of the piezoelectric substrate and the traveling wave form a standing wave and spurious occurs, the filter B has a phase difference between the reflected wave and the traveling wave. Since it is shifted by λ / 4 compared to the filter A, a standing wave due to the spurious component is not formed, and the spurious can be suppressed.

In another embodiment, the arrangement intervals D of the IDTs 2 and 4 of the filter A and the IDTs 6 and 8 of the filter B shown in FIG. 1 are set to (λ / 4 + nλ / 2) (where n = 1,
2), the phases of the reflected waves reflected at both ends of the piezoelectric substrate are shifted by λ / 2 between the filter A and the filter B. IDTs 2, 4 and 6, 8
The sign of the electric charge picked up by the counter is reversed, and the spurious response becomes extremely small.

In the above embodiment, the IDT arrangement interval, that is, I
The case where the center-to-center distance D of the DT is set to (λ / 8 + nλ / 4) or (λ / 4 + nλ / 2) has been described. It is clear that it is effective in suppressing the above. FIG. 2 is a diagram showing the arrangement interval D between the IDTs 2 and 4 and the IDTs 6 and 8 and the phase difference between the respective reflected waves leaking from the respective reflectors and reflected at the end faces. D = λ
The phase differences of the reflected waves in the case of / 8, λ / 4, and 3λ / 8 are λ / 4, λ / 2, and 3λ / 8, respectively. In the lower part, the phase difference of the reflected wave is shown in the upper part of the two-step display. Another important value is D
1 = λ / 16, D2 = 3λ / 16, D3 = 5λ / 16,
The value of D4 = 7λ / 16 is displayed.

The above description is directed to a filter in which a transversely coupled dual mode filter in which surface acoustic wave resonators each composed of an IDT and a reflector disposed along the propagation direction are arranged in parallel with each other is connected in multiple stages. The case where one of the filters, ie, the IDT and the two reflectors, are shifted in the propagation direction has been described. However, although there are various intervals between the center of the electrode finger at the end of the IDT 2 shown in FIG. 1 and the center of the electrode finger at the end of the reflectors 3a and 3b, for example, (λ / 2 + nλ) (n =
0, 1, 2,...). In this case, for example, when n = 1, an effect equivalent to the above-described spurious suppression effect can be obtained even if only one of the laterally coupled dual mode filters is shifted in the propagation direction, instead of being shifted in the propagation direction. become.

When two or more dual-mode SAW filters are cascaded to form a higher-order SAW filter, spurious becomes a problem because each dual-mode SAW filter has
This is the case where the spurious frequencies are almost the same in the filter and their respective responses are added. If at least one of the cascade-connected dual mode SAW filters has its spurious suppressed, the spurious in the overall transmission characteristics of the cascade connection is It will be greatly improved.

In the above description, the laterally coupled dual mode SAW filter has been described in detail as an example.
Obviously, the present invention can be applied to a filter, for example, a filter in which laterally coupled triple mode SAW filters are cascaded. Also, a longitudinally-coupled dual-mode SAW filter using first-order / second-order modes, a vertically-coupled dual-mode SAW filter using first-order / third-order modes, and a vertically-coupled triple-mode SAW using first-order to third-order modes. It goes without saying that the present invention can be applied to a filter in which filters and the like are connected in cascade. It is also apparent that the present invention can be applied to a case where the cascade connection stages of the dual mode SAW filter are three or more.

Also, an example has been described in which a piezoelectric crystal is used for the substrate as the piezoelectric material. However, the piezoelectric substrate is not limited to quartz, and other piezoelectric materials such as LiTaO3 and LiNbO
3. Needless to say, LBO, langasite, etc. may be used.

[0019]

According to the present invention, as described above, a mask pattern for a surface acoustic wave filter can be manufactured without applying a sound absorbing agent or the like to both end surfaces of a piezoelectric substrate unlike conventional spurious suppression means. Since only the center position of the IDT of each of the SAW filters connected in cascade is shifted by a predetermined interval, the cost of manufacturing the mask is not increased, and the size of the SAW filter is not increased. By simply shifting the position of the IDT of the mask pattern by the above-mentioned predetermined interval, it is possible to attenuate spurious components near the center frequency of the cascade-connected multimode SAW filter. Therefore, for example, 455 is added to the second IF.
70d at ± 910 kHz away from the center frequency required for the first IF SAW filter in a double super system using a filter such as a ceramic filter of kHz.
What has been realized by the cascade connection of three or more stages so far can be realized by the two-stage cascade connection, and an excellent effect of reducing the number of cascade stages and reducing costs and miniaturization is achieved.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing an embodiment of a two-stage cascade-connected laterally coupled dual mode SAW filter according to the present invention.

FIG. 2 is a diagram illustrating an arrangement interval D between an IDT of a filter A and an IDT of a filter B, and a phase difference between respective reflected waves.

3A is a plan view of a conventional two-stage cascade-connected vertical-coupled double-mode SAW filter, and FIG. 3B is a plan view of a conventional two-stage cascade-connected horizontal-coupled double-mode SAW filter.

FIG. 4 is a normalized radiation conductance Ga / GN of an IDT.
FIG. 4 is a diagram illustrating a reflection coefficient | Γ | of a reflector.

5A is an electrical equivalent circuit of a dual mode SAW filter, and FIG. 5B is an electrical equivalent circuit obtained by converting FIG.

FIG. 6 is a diagram illustrating a state of reflection at an end face of a surface wave leaked from a reflector.

FIG. 7 is a diagram illustrating a state in which a surface wave leaked from a reflector is absorbed by a sound absorbing agent.

[Explanation of symbols]

1 Piezoelectric substrate 4, 6, 8 ... IDT 3a, 3b, 5a, 5b, 7a, 7b, 9a, 9b ...
· Reflector λ: Wavelength of surface wave D, D1, D2, D3, D4: IDT2, 4, and ID
Space between T6 and T8

Claims (6)

[Claims] 1. A transversely coupled multimode filter in which a plurality of surface acoustic wave resonators each composed of an IDT and a reflector arranged along a propagation direction of a surface acoustic wave are arranged in parallel and closely adjacent to each other on a piezoelectric substrate. A multi-mode SAW filter, wherein at least one position of the transversely coupled multi-mode filter is shifted in a propagation direction of a surface acoustic wave to reduce spurious near a center frequency. 2. A transversely coupled multimode filter in which a plurality of surface acoustic wave resonators each composed of an IDT and a reflector arranged along the propagation direction of a surface acoustic wave are arranged in parallel and closely adjacent to each other on a piezoelectric substrate. A multi-mode SAW filter, characterized in that at least one IDT of the transversely coupled multi-mode filter is arranged in a direction shifted in a propagation direction of a surface wave to reduce spurious near a center frequency. 3. A filter comprising a plurality of cascaded longitudinally coupled multimode SAW filters each comprising a plurality of IDTs and reflectors arranged along a propagation direction of a surface wave on a piezoelectric substrate, wherein the longitudinally coupled multiplexed filters are connected in multiple stages. A multi-mode SAW filter, wherein at least one position of a mode filter is displaced in a propagation direction of a surface wave to reduce spurious near a center frequency. 4. A filter comprising a plurality of vertically coupled multiplex mode SAW filters each composed of a plurality of IDTs and reflectors arranged along a propagation direction of a surface wave on a piezoelectric substrate, wherein said longitudinally coupled multiplex mode is connected. A multi-mode SAW filter, wherein at least one IDT of a mode filter is shifted in a propagation direction of a surface wave to reduce spurious near a center frequency. 5. The multi-mode SA according to claim 1, wherein:
In the W filter, the interval of shifting the IDT is λ / 16 +
A multi-mode SAW filter, wherein nλ / 4 to 3λ / 16 + nλ / 4. 6. The multi-mode SA according to claim 1, wherein:
In the W filter, the interval at which the IDT is shifted is 3λ / 16.
+ Nλ / 2 to 5λ / 16 + nλ / 2.