JP3368087B2 - Surface acoustic wave filter device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter device which obtains bandpass characteristics by using a surface acoustic wave propagating on a piezoelectric substrate. 2. Description of the Related Art Conventionally, for example, Japanese Patent Publication No.
As shown in Japanese Patent Application Laid-Open Publication No. H11-209, a comb-shaped input electrode and an output electrode are arranged in series in a propagation direction of surface acoustic waves on a piezoelectric substrate, and a reflector is arranged outside each of the two electrodes. There is known a surface acoustic wave filter device which obtains a band-pass characteristic by using two vibration modes including two kinds of standing waves of a symmetric mode and an anti-symmetric mode generated inside both electrodes. However, in the above-mentioned conventional apparatus, in order to widen the signal passing bandwidth of the band-pass characteristic, the electrode index of the input electrode and the output electrode is reduced to reduce the resonance of the two vibrations. It is necessary to widen the frequency difference. On the other hand, when the electrode index is reduced, there is a problem that insertion loss increases. Therefore, in the above-described conventional surface acoustic wave filter device, the signal pass bandwidth cannot be sufficiently widened, and for example, the center frequency is 449.945.
The signal pass bandwidth of the surface acoustic wave filter of MHz is 490.
It was about KHz. An object of the present invention is to provide a surface acoustic wave filter device capable of expanding a signal passing bandwidth without increasing insertion loss. [0005] In order to achieve the above object, a structural characteristic of a surface acoustic wave filter device according to the present invention is to propagate a distance between each inner side of an input electrode and an output electrode. The resonance between the two reflectors provided outside each of the two electrodes, the internal resonance of the two electrodes themselves, and the two electrodes between the input electrode and the output electrode are set at 18 times or more the wavelength of the surface acoustic wave. The bandpass characteristic is obtained by using three resonances consisting of the above resonances. According to the present invention constructed as described above, the band index filter characteristics are obtained by utilizing three resonances, so that the electrode index of the input electrode and the output electrode is reduced. Without passing through, the signal pass band could be expanded. Further, by setting the distance between the inner sides of the input electrode and the output electrode to be 18 times or more the half wavelength of the propagating surface acoustic wave, the difference between the peak values of the three resonance points could be reduced. . Accordingly, a bandpass filter having a wide signal pass band and flat frequency characteristics can be realized without increasing insertion loss. FIG. 1 is a plan view schematically showing a surface acoustic wave filter device according to an embodiment of the present invention, and FIG. The generation state of a surface wave is schematically shown. This surface acoustic wave device is made of lithium niobate.
The piezoelectric substrate 10 has a rectangular parallelepiped shape formed of a piezoelectric material such as (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), or quartz. On the piezoelectric substrate 10, an input electrode 11 and an output electrode 12 are arranged in series in the propagation direction of the surface acoustic wave. The input electrode 11 is composed of a large number of electrode fingers 11a and 11b that are alternately crossed and arranged.
Also a large number of electrode fingers 12a, 12
b. The total number of electrode fingers 11a and 11b and the total number of electrode fingers 12a and 12b are 300 to 700, respectively.
It is about. In addition, each electrode finger 11a, 11b, 12a,
The interval of 12b is the wavelength λ of the propagating surface acoustic wave (for example,
(Approximately 6.96 μm).
1a and 12a are located at the center of each of the electrode fingers 11b and 12b. The distance d between the inner sides of the two electrodes 11 and 12 (more specifically, the distance between the centers of the innermost electrode fingers 11b and 12b) is set to nλ / 2. n is a positive integer, and is preferably set to a value of “18” or more, as can be understood from the experimental results described later. On the outside of each of the electrodes 11, 12, a Pλ / 2
Reflectors 13, 1 composed of a plurality (for example, about 200) of gratings 13a, 14a provided at intervals of
4 are arranged respectively. P is 1.002-
The value is about 1.010. Reflector 13 and input electrode 11
And the distance between the reflector 14 and the output electrode 12 are both m
It is set to about λ / 2. m is a positive integer;
A small value such as “1” or “2” is preferable. In the surface acoustic wave filter device configured as described above, as a result of the simulation, it was confirmed that the change of the insertion loss (gain) with respect to the frequency as the frequency transfer characteristic is as shown in the graph of FIG. In addition,
The specifications of the surface acoustic wave filter device in this case are as shown in Table 1 below. [Table 1] According to this graph, it is confirmed that the first to third resonance peaks P1 to P3 in which the resonance frequency increases sequentially occur around 450 MHz. The first resonance peak P1 relates to the resonance between the reflectors 13 and 14, that is, the resonance of the surface acoustic wave forming the standing wave between the equivalent reflection surfaces X1 and X1 of the reflectors 13 and 14. The second resonance peak P2 relates to the resonance of the surface acoustic waves inside each of the input electrode 11 and the output electrode 12 itself. The third resonance peak P3 is formed by two electrodes 1
This relates to the resonance between the surface acoustic waves constituting the standing waves between the equivalent reflection surfaces X2 and X2 of the electrodes 11 and 12, that is, the resonance between the electrodes 1 and 12. In addition, both electrodes 1
By changing the distance d between the first and third resonance peaks P
When the changes in the frequency positions 1 to P3 and the maximum peak level difference PP are confirmed, the frequency positions of the first to third resonance peaks P1 to P3 change as shown in the graph of FIG. It was confirmed that PP changed as shown in the graph of FIG. On the other hand, each electrode finger 11 of both electrodes 11 and 12
a, 11b, 12a, 12b and the number of reflectors 13, 14
Even if the number of the gratings 13a and 14a is changed, the frequency positions of the first to third resonance peaks P1 to P3 and the maximum peak level difference PP do not greatly change so as to affect the design of the bandpass filter. It was also confirmed. From the above simulation results, in order to obtain a wide band and flat bandpass characteristic, it is necessary to increase the frequency difference between the first to third resonance peaks P1 to P3 from the graph of FIG. The integer n that is appropriate and determines the distance d between the two electrodes is several tens (for example, “80”).
It is contemplated that the following is appropriate. Also,
To obtain a flat bandpass characteristic, the first to third
It is necessary that the maximum peak level difference PP of the resonance peaks P1 to P3 is equal to or less than a predetermined value (for example, 3.0 dB or less). From the graph of FIG.
It is understood that it is necessary to set more than two. If the surface acoustic wave filter device is constructed under the above conditions, the first to third resonance peaks P1 to P1 can be obtained.
Since three resonances respectively corresponding to P3 can be used,
A bandpass filter having a flat frequency transfer characteristic and a wide band can be realized. In this case, both electrodes 1
Since it is not necessary to reduce the number of the electrode fingers 11a, 11b, 12a, 12b of 1 and 12, the insertion loss can be reduced. When using the surface acoustic wave filter device 10A configured as described above, as shown in FIG. 5, an input impedance adjustment circuit 21 including an inductance and an output impedance adjustment circuit 22 including an inductance are connected to the device 10A. Circuit matching so that the signal passband characteristic is as flat as possible. FIG. 6 is a graph showing the frequency transfer characteristic of the output voltage Eout with respect to the input voltage Ein in terms of the insertion loss (gain) with respect to the frequency. As a result, the signal pass band has a very flat frequency transfer characteristic and the center frequency 44
This means that a bandpass filter having a 9.885 MHz and a signal pass bandwidth of 630 KHz has been realized. The surface acoustic wave filter device 10A used in the experiment of FIG. 6 has the same specifications as those in Table 1 above.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a surface acoustic wave filter device according to an embodiment of the present invention in a plan view and simulating a surface acoustic wave propagating through the device. FIG. 2 is a graph showing a change in insertion loss (gain) with respect to a frequency of the surface acoustic wave filter device of FIG. 1; FIG. 3 is a graph showing a change in a frequency position of a resonance peak when a distance between an input electrode and an output electrode is changed. FIG. 4 is a graph showing a change in a maximum peak level difference of a resonance peak when a distance between an input electrode and an output electrode is changed. FIG. 5 is a circuit diagram showing an example of use of the surface acoustic wave filter device. FIG. 6 shows an insertion loss (gain) with respect to a frequency in the example of use.
6 is a graph showing a change in the graph. [Description of Signs] 10: piezoelectric substrate, 11: input electrode, 11a, 11b: electrode finger, 12: output electrode, 12a, 12b: electrode finger, 1
3, 14 ... reflector, 13a, 14a ... grating.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-283309 (JP, A) JP-A-62-12206 (JP, A) JP-A-62-30413 (JP, A) Yasushi Yamamoto, Composite vertical Mode resonator type SAW filter, IEICE Transactions, February 25, 1993, Vol. J76-A, No. 2 (No. 302, pp. 219-226 (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 9/64 H03H 9/145 H03H 9/25

Claims (1)

(57) [Claim 1] A comb-shaped input electrode and an output electrode are arranged on a piezoelectric substrate in series in a propagation direction of a surface acoustic wave, and a reflector is provided outside each of the two electrodes. In the surface acoustic wave filter device arranged respectively, the distance between each inner side of the input electrode and the output electrode is set to be 18 times or more of 1/2 wavelength of the propagating surface acoustic wave, and the distance between the two reflectors is set. A surface acoustic wave filter device wherein bandpass characteristics are obtained by using three resonances including resonance, internal resonance of the electrodes themselves, and resonance between the electrodes.