JP3137081B2 - Surface acoustic wave 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.

[0002]

2. Description of the Related Art Generally, as shown in FIG. 7, a one-port type surface acoustic wave resonator is composed of an interdigital electrode IDT and reflectors GR disposed on both sides thereof.

The input signal terminal 11 and the output signal terminal 12 are 1
Interdigital electrode I of port type surface acoustic wave (SAW) resonator
The transmission characteristic is connected to la and lb of the DT, and the transmission characteristic in this case is the resonance point fr at which the loss is minimized as shown in FIG.
And one anti-resonance point fa at which the loss is maximized.
The frequency difference Δf1 (= fa−fr) between r and the anti-resonance point fa is almost uniquely determined by the piezoelectric substrate used.

[0004]

The width of the attenuation band .DELTA.f2 of the transmission characteristic shown in FIG. 8 depends on the logarithm and the cross width of the interdigital electrode IDT, and the logarithm is reduced in order to widen the attenuation band .DELTA.f2. Or the crossover width must be reduced.

[0005] However, when the logarithm is reduced or the crossover width is reduced, the frequency region (pass band) Δf3 where the loss is small near the resonance point fr where the loss is minimized becomes narrow. Further, there is a problem that spurious noise occurs near the resonance point fr of the transmission characteristics.

In a ladder-type filter (a configuration example is shown in FIG. 10) in which one-port surface acoustic wave (SAW) resonators are connected in series in parallel and in series, the frequency domain Δf3 is a pass band of the filter, and the frequency domain Δf2 is an attenuation band. Used as In this ladder-type filter, it is necessary that the pass band Δf3 is wide and the attenuation band Δf2 is also wide.

The one-port surface acoustic wave (SAW) resonator is also used as a notch filter for removing a signal of a specific frequency. In this case, the attenuation band Δf2
Is the width of the notch, so that the attenuation band Δf2 is naturally desired to be wide.

[0008] The present invention has been made in view of the above-mentioned conventional circumstances, and an object of the present invention is to provide a novel surface acoustic wave filter capable of achieving the above-mentioned demands inherent in the prior art. Is to do.

[0009]

In order to achieve the above object, a surface acoustic wave filter according to the present invention comprises a cross finger-like electrode and a first reflection electrode provided on both sides of the cross finger-like electrode. And a second reflector disposed on the opposite side of each of the first reflectors from the interdigital electrode.

[0010] The distance between the reflector electrodes of the first reflector and the second reflector is equal.

The distance between the reflector electrodes of the first reflector and the distance between the reflector electrodes of the second reflector are different from each other.

The distance between the interdigital electrode and the first reflector is set within a range of 0.4 to 0.5λ.

[0013] The distance between the first reflector and the second reflector is set within a range of 0.3 to 0.4λ.

[0014]

Next, a preferred embodiment of the present invention will be described in detail with reference to the drawings.

[Structure of Embodiment] FIG. 1 is a schematic diagram (front view) showing an embodiment of the present invention.

The conventional one-port type surface acoustic wave (S)
AW) As shown in FIG. 7 in the section of the prior art, the structure of the resonator has a structure in which interdigital electrodes IDT formed of an Al thin film or the like on a piezoelectric substrate and reflectors GR on both sides thereof. ing.

In the present invention, as shown in FIG. 1, reflectors GR1 are arranged on both sides of the interdigital electrode IDT. Further, a reflector GR2 is arranged outside the reflector GR1.

The reflector GR1 is provided with reflector electrodes, which are constituent elements thereof, arranged at equal intervals P. Similarly to the reflector GR1, the reflector GR2 is also provided with reflector electrodes, which are constituent elements thereof, arranged at equal intervals P. The equal interval P of the reflector GR1 may be different from the equal interval P of the reflector GR2. For example, the equal interval P of the reflector GR1 may be P1, and the equal interval P of the reflector GR2 may be P2, or vice versa.

Further, in the present invention, the interdigital electrode ID
The interval between T and the reflector GR1 is 0.4λ to 0.5λ, and the interval between the reflector GR1 and the reflector GR2 is 0.3λ to 0.4λ.
It is characterized by being adjusted to be.

Here, λ represents the wavelength of one period of the surface acoustic wave (SAW).

[Operation of Embodiment] Next, the operation of the embodiment according to the present invention will be described.

Generally, one-port surface acoustic wave (SAW)
A signal input to the resonator is converted into a surface acoustic wave (SAW) by an interdigital electrode (IDT) and converted to an interdigital electrode (ID).
T) is incident on reflectors (GR) provided on both sides. 1
In a reflector (GR) composed of an electrode group having a reflectance of at most about several percent, a large number of electrodes are arranged so that surface acoustic waves (SAW) are almost completely reflected. As a result,
An equivalent reflecting surface is formed in the reflector, and a standing wave is generated in the cavity. The standing wave is converted into an electric signal by the interdigital electrode (IDT) and extracted.

As shown in FIG. 8, the transmission characteristics of this conventional configuration are represented by the resonance frequency (fr) and the anti-resonance frequency (fa).
, And the resonance frequency difference Δf1 (= fa−fr) is substantially determined by the piezoelectric substrate used.

Another resonance fs on the transmission characteristic of FIG. 9 is a longitudinal mode higher-order resonance spurious generated when the distance between the interdigital electrode IDT and the reflector GR is set to an appropriate value. Called resonance.

In general, in a ladder type filter in which one-port surface acoustic wave (SAW) resonators are connected in multiple stages in series and parallel,
The vicinity of the resonance point fr of the one-port surface acoustic wave resonator is used as a pass band, and the vicinity of the anti-resonance point fa is used as an attenuation band. As a condition of the filter, it is indispensable that the pass band and the attenuation band are wide.

In the transmission characteristics of the conventional one-port surface acoustic wave resonator (see FIG. 8), the attenuation band is narrow, and in FIG. 9 utilizing the sub-resonance, an unnecessary wave exists at the center of the attenuation band. No properties were obtained.

According to the structure of the present invention shown in FIG. 1, the surface acoustic wave SAW generated by the interdigital electrode IDT is reflected by the reflector GR.
1, the number of the reflectors GR1 is adjusted so that 30 to 70% of the surface acoustic wave is reflected, and the number of the reflectors GR2 is set so that the remaining surface acoustic wave SAW is almost completely reflected by the reflector GR2. Is adjusted.

Next, the occurrence of sub-resonance due to the distance between the interdigital electrode IDT and the reflector GR will be described with reference to the drawings.

FIG. 3 shows the configuration of the reflector GR2 shown in FIG.
FIG. 7 is a front view showing a configuration in which is removed.

Referring to FIG. 3, as shown in the configuration of the one-port surface acoustic wave resonator in FIG. 3, the interval between the interdigital electrode IDT and the reflector GR1 is set to 0.4λ to 0.5λ. Thereby, the resonance mode can be adjusted, and fs can be arranged slightly higher than the anti-resonance point fa as shown in the transmission characteristics of FIG. This interdigital electrode IDT and the reflector GR1
When the interval between them is 0.5λ or more, fs is the resonance point f
In the vicinity of r, or lower than the resonance point fr, the frequency fs moves to a higher frequency side farther than the anti-resonance point fa when the frequency is 0.4λ or less.

The one-port surface acoustic wave (SA) shown in FIG.
W) The resonator has a configuration in which the interval between the reflector GR1 and the reflector GR2 is set to 0.3λ to 0.4λ and the reflector GR1 is eliminated.

As shown in FIG. 6, the transmission characteristic in the case of the structure shown in FIG. 5 can be arranged such that fs is slightly lower than the anti-resonance point fa. When the distance between the reflector GR1 and the reflector GR2 is 0.4λ or more, f
s moves farther away from the anti-resonance point fa to the lower frequency side,
If it becomes 0.3λ or less, fs moves to a higher frequency side than the anti-resonance point fa, and the level of spurious between fa and fs becomes a problem.

If the transmission characteristics of FIGS. 4 and 6 are combined, FIG.
The transmission characteristics of a one-port surface acoustic wave resonator having a wide attenuation band Δf2 as shown in FIG.

Therefore, the ladder-type filter using the one-port surface acoustic wave resonator according to the present invention can realize a characteristic with a wide attenuation band as a filter (see FIG. 11).

[0035]

As described above, according to the structure of the present invention, the attenuation band of the one-port type surface acoustic wave resonator is widened for the reason described in the above embodiment. Can obtain good transmission characteristics with a wide attenuation band.

That is, according to the present invention, as shown in FIG. 1, reflectors GR1 arranged at equal intervals P are provided on both sides of the interdigital electrode, and the equal pitches P are provided outside the reflector GR1.
Are provided, and the intersecting finger electrodes I
The interval between DT and the reflector GR1 is set to 0.4λ to 0.5λ, and the interval between the reflector GR1 and the reflector GR2 is set to 0.3λ to 0.5λ.
By setting the value to 0.4λ, two sub-resonance points (fs1, fs2) are generated near the anti-resonance point fa as seen in the transmission characteristic example shown in FIG.

As a result, one-port surface acoustic wave (SAW)
As shown in FIG. 2, the transmission characteristic of the resonator has a pass band Δf3
Can be widened without narrowing the attenuation band Δf2.

Therefore, the one-port surface acoustic wave (SA)
W) A ladder-type filter using a resonator can realize a filter having a wide attenuation band while having a wide pass band.

The one-port surface acoustic wave (SAW)
Even in the case of a notch filter using a resonator, a notch filter capable of removing a wide range of frequency signals can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram (front view) showing an embodiment according to the present invention.

FIG. 2 is a diagram showing an example of transmission characteristics of a one-port SAW resonator according to the present invention.

FIG. 3 is a front view showing a configuration in a case where a reflector GR2 is removed from the configuration shown in FIG. 1;

FIG. 4 is a diagram illustrating a characteristic example when the reflector GR2 is removed in the configuration of FIG. 1;

FIG. 5 is a front view showing a configuration in a case where a reflector GR1 is removed from the configuration shown in FIG. 1;

FIG. 6 is a diagram illustrating a characteristic example when the reflector GR1 is removed in the configuration of FIG. 1;

FIG. 7 is a schematic view of a conventional one-port SAW resonator.

FIG. 8 is a diagram illustrating a first characteristic example of the one-port SAW resonator according to the related art.

FIG. 9 is a diagram illustrating a characteristic example 2 of the one-port SAW resonator according to the related art.

FIG. 10 is a diagram illustrating a configuration example of a ladder-type filter.

FIG. 11 is a diagram illustrating a characteristic example of a ladder-type filter using a one-port SAW resonator according to the present invention.

[Explanation of symbols]

 IDT: interdigital electrode GR1, GR2: reflector SAW: surface acoustic wave

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 9/25 H03H 9/145

Claims (1)

(57) [Claims] 1. A cross-finger electrode, first reflectors disposed on both sides of the cross-finger electrode, respectively, and a first reflector on each side of the first reflector opposite to the cross-finger electrode. A second reflector disposed therein, wherein the first reflector and the second reflector are provided.
Set the distance between the reflectors within the range of 0.3λ to 0.4λ
A surface acoustic wave filter characterized by the following.