JPS58223912A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To realize uniformly low loss in a band, by providing a surface acoustic wave reflector where intervals of reflecting grating are changed gradually to a three-electrode filter. CONSTITUTION:An input converter comprising a reed screen electrode 3 to convert a sound signal to an electric signal and an output converter comprising reed screen electrodes to convert the sound signal into the electric signal are provided on a piezoelectric substrate 1. Reflectors 5, 6 reflecting a leakage surface wave irradiated from the input converter 3 and propagated without being converted into the electric signal at the output converters 2, 4, in the direction of the output converters 2, 4 is provided at the outside of the output converts. The intervals of the grating are increased or decreased gradually along the incident propagating direction of the leaked surface wave in the reflectors 5, 6. Further, the interval of grating of the reflectors is determined so that the distance from the center of the reed screen electrode closest to the reflectors between the reed screen electrodes 2, 4 to the reflecting point of the reflectors is N/2(N is an integral number) of the wavelength of the leakage surface wave all times.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter comprising a plurality of interdigital electrodes and reflectors.

Filters using surface acoustic waves are constructed by forming various electrodes on a piezoelectric substrate, and are constructed using conventional coils and coils.

Compared to a W filter combined with a capacitor, it has the advantage of being smaller and lighter, and it is possible to achieve higher frequencies by making full use of various microfabrication techniques developed for LSI. Therefore, in order to take advantage of these characteristics, research and development of IQ)h band surface acoustic wave filters for use in mobile devices such as car telephones and cordless telephones has recently been carried out. Filters for mobile devices are particularly required to have low loss in order to reduce the power consumption of the power supply as much as possible, in addition to being small and lightweight. For example, 90 for smaller size and lighter weight.
Since this is a 0 MHz filter and is housed in a 6111Iφ×2■ package, this is hardly a problem, but in order to reduce the loss, it is necessary to consider the structure of the filter itself.

In a two-electrode filter with input and output interdigital electrodes facing each other on a piezoelectric substrate, a minimum loss of 6 dB occurs even if impedance matching is achieved for each interdigital electrode. In a three-electrode filter in which the interdigitated electrodes on both sides are connected for input and the middle interdigitated electrode is used for output, it is common for a minimum loss of 3 dB to occur even with impedance matching. Are known. As a way to further reduce losses,
A type that has zero loss in principle and uses unidirectional electrodes has been proposed. A three-phase excitation type is generally used as a unidirectional electrode. This involves appropriately arranging three electrode fingers with different excitation phases within one section of the electrode f, j
The idea is to propagate one surface wave in one direction at +=, but in order to form three electrodes with different phases, three-dimensional intersections between electrode fingers are required, which complicates the creation process. There are drawbacks. Therefore, unidirectional electrodes with a small number of logarithms are arranged in a distributed manner as one group using a 90 degree phase shifter, and the signals from each group are added in the same phase to eliminate stereoscopic crossover of the W pole and reduce loss. Group formats aiming for this have also been proposed. However, the group type must also be able to provide a predetermined value only by using a phase shifter, but it is extremely difficult to approximate it well. Therefore, even if the KtN loss is zero in principle, a loss of several dB occurs in an actual device. For example, IG that is likely to be used for mobile devices
An example of a filter with the least loss in the Hz band was the 1979 Ultra Sonics Symposium sponsored by Ili'3F.
B. R. Botta (B, R,, Pot
However, the insertion loss is still relatively large at 3.8 dB.

Therefore, methods have been proposed to reduce the loss as much as possible without using a phase shifter that causes loss. One of them is to place surface acoustic wave reflectors with equal lattice spacing outside the interdigital electrodes on both sides of a three-electrode filter to reflect as much of the surface waves as possible, which would otherwise escape and become a loss, and extract them again as a signal. This is a method of trying to do so. However, in this case, as the number of reflecting elements increases in an attempt to reflect more surface waves, the reflector becomes narrow band, and as a result, the loss drops sharply only near the center frequency, making it difficult to apply to a wide band filter. Met.

Therefore, it is possible to provide a three-electrode filter with a reflection grating that can provide a certain level of reflected wave intensity over a wide band, and the loss can be uniformly reduced within the band. The filter of the present invention includes, on a piezoelectric substrate, an input transducer comprising at least one interdigital electrode for converting an electric signal into an acoustic signal, and at least one transducer for converting an acoustic signal into an electric signal. an output transducer consisting of a plurality of interdigital electrodes; In addition, the configuration includes a plurality of reflectors in which the grating spacing is gradually increased or decreased along the incident propagation direction of the leaky surface wave, and the %KN is set to 2.
The distance from the center of the interdigital electrode closest to the reflector to the reflection point of the reflector is always equal to the wavelength of the leaky surface wave. If the grating spacing of the reflector is determined to be N/2 times larger, the effect will be greater.

Next, the present invention will be explained with reference to the drawings. 1st
The figure is a plan view showing a one-cusp embodiment of the surface acoustic wave filter according to the present invention, in which 1 is a piezoelectric substrate, 2, 3, 4 are interdigital electrodes, and as shown in the figure, the interdigital electrode 3 is used for inputting electrical signals. Terminal IN
, connect the interdigital electrodes 2.4 to electrical signal output terminal 0.
(It constitutes a 3-electrode filter with JT. 5.6
is a surface acoustic wave reflector in which the reflection grating spacing gradually changes, and is composed of, for example, a group of grooves cut into a substrate, or a metal strip made of vapor-deposited metal. In a 3-electrode filter, when impedance matching is performed on the input side and the output side, a signal of P o /2 of the electrical signal with a power of 9 Po that enters from the input side is extracted from the output side, but the rest is The interdigital electrodes 2 and 4 are also radiated toward the ends of the piezoelectric substrate 1 as leaky surface waves with a power of Po/4, causing filter loss. Therefore 5
By providing a reflector as shown in , 6, the surface waves can be returned to the interdigital electrode 2.4 again, making it possible to significantly improve the loss. Furthermore, since this reflector has a reflection grating that gradually increases or decreases, it can be made to have a much wider band than a conventional reflector with regular intervals. However, in this case, the leaky surface waves
When it enters the interdigital electrode 4, is reflected by the reflector 6, and propagates through the interdigital electrode 4 again, the central axis 0 of the interdigital electrode 4
．． -0. It is desirable that the phases of the incident wave and reflected wave are always in phase. Therefore, the position of each reflective element must be such that it satisfies the above conditions.

Figure 2 shows how to determine the position of each reflective element in Figure 1.00.
"0." and reflector 6 are shown again, 10
is a reflector, 11 is the first reflective element, and 12 is −i
+ 1st reflective element. Frequency fi of the surface waves reflected by the i-th and i+1-th reflecting elements.

When the wavelength λi is the velocity V, the following relationship holds between them.

The central axis of the surface wave fi is 0. -0. The average value of the round-trip propagation path to the reflective element is χ! It is 11+χi. The surface wave incident on the interdigital electrode is reflected by the reflector 10, and the surface wave that propagates through the interdigital electrode again is aligned with the center axis 0. -0
．． In order to always have the same phase, χ1−H1l+χi =
The expression Nλ; (2) is a necessary and sufficient condition. Here, N is a positive integer.

By combining equations (1) and (2), a recurrence equation is obtained, and by transforming this equation, the following equation is obtained. Here, D is the central axis 0. -0. o of the reflective element closest to
-0, is the distance from. The frequency fi of the surface waves reflected by the i-th and i+1-th reflecting elements is expressed by the formula (
4) into equation (1).

Therefore, if the lower limit frequency of the broadband filter is Fl and the upper limit frequency is F, the relationship between the number m of reflective elements for reflecting waves in these bands and D is given by equation (4),
Once F and Ft are determined, the relationship between D and m is determined. Here υ is the surface wave velocity. As an example of numerical calculation, F, =
910MHz, F2=940PJi11z, Y-Z
Assuming that a LiNbOa substrate is used, υ=3488% is calculated using the formula (
6) to find the relationship between m and D. The solid line shows the value of D, and the broken line shows the sum of D and the length of the reflector.

In this figure, the center axis 0. -0. For example, 3
If 10 pairs of unevenly spaced reflection gratings are placed between 828 μm and 401.5 μm, the center axis will be 0. −〇、910M passing through
A surface wave with a frequency of Hz to 940 MHz is reflected by the reflection grating and returns to the center axis 0. -0. The surface waves in the same band that return to the surface are always in phase regardless of the frequency, and part or all of the surface waves that would normally escape if the reflector is weak can be extracted as an electrical signal from the output converter.

As mentioned above, if the position of the reflective element of the reflector is given as in equation (4), the surface wave with the frequency shown in equation (5) will be
1-0. The phases of the incident surface wave and the reflected surface wave are always in phase regardless of the frequency, and as a result, it becomes possible to improve the loss of the broadband filter uniformly within the band. Furthermore, unlike filters using unidirectional electrodes, a phase shifter is not required, making it easy to realize a filter with extremely low loss.

FIG. 4 is a plan view showing another embodiment of the surface acoustic wave filter according to the present invention, and is an output converter formed by connecting an input transducer consisting of an interdigital electrode 23 and interdigital electrodes 22 and 24 on a piezoelectric substrate 21. A container is provided. Further, reference numerals 25 to 30 indicate six reflectors in which the spacing between reflective elements changes along the surface wave propagation direction. In the embodiment shown in Fig. 1, there is only one reflector on one side of the interdigital electrode for reflecting leaky surface waves, and if the number of reflecting elements is small, the intensity of the reflected surface waves may not be very strong. I can do it. Therefore, if we take advantage of the fact that the location of the reflector will change depending on the number of reflective elements as shown in FIG.
It is possible to provide a plurality of such electrodes so that the interdigital electrodes do not cause damage, and the intensity of the surface waves reflected to the interdigital electrodes can be significantly increased. In the embodiment shown in FIG. 4, the number of reflective elements of the reflectors 25 and 28 is flJ, and the number of reflection elements of the reflectors 26 and 29 is m2, and the number of reflection elements is 27°3.
When 0 is m3, I
The condition TI < m2 < ml is satisfied. In this case, the leaky surface waves radiated from the input transducer and not converted into electrical signals by the output transducer are reflected by the three reflectors and reach the output transducer again, making it possible to realize a filter with extremely low loss. I can do it. In this example, three reflectors are provided on one side of the interdigital electrode, but if the number of reflectors is increased by two or more, the loss of the filter can be further reduced. I can do it. Furthermore, Figures 1 and 4
In the example shown in the figure, a three-electrode filter is described, but
It goes without saying that even in the case of a pole-shaped filter (2), if the leakage surface waves are reflected by the reflector shown in the embodiment, the loss of the filter can be further reduced.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing an embodiment of the surface acoustic wave filter according to the present invention, showing 1 piezoelectric substrate, 23, 4 interdigital electrodes, and 5.6 reflector, and Fig. 2 shows the surface acoustic wave reflector. In the model diagram for determining the position of the reflective element, 10 surface acoustic wave reflector, 11 i-th reflective element, 1
The 2i+1th reflective element is shown. Also, Figure 3 shows, as a calculation example, the number of reflective elements in the reflector and the number of reflections from the interdigital electrodes of the reflector when using a Y-Z LiNbC)s substrate with a lower limit frequency of 910 h and an upper limit frequency of 950 M)Iz. It shows the distance. FIG. 4 is a plan view showing another embodiment of the surface acoustic wave filter according to the present invention.21 Piezoelectric substrate, 22.23.24 Interdigital interdigital electrode,
25-30 reflectors are shown. Raku 3D 70 20 30
4-Dman-Shikonban logarithm 几■ Procedural amendment Commissioner of the Patent Office 1, Indication of case 1981 Patent application No. 1078
No. 19 No. 2, Title of the invention: Surface acoustic wave filter 3, Relationship with the person making the amendment case: Applicant: 7, Shiba, Minato-ku, Tokyo
i, l' ■No. 33 12'1 (423) NEC Corporation Representative Tadahiro Sekimoto 4, Agent 5, Column 6 for detailed explanation of the invention in the specification subject to amendment, Contents of amendment (1) Description On page 4, line 19, the phrase "Be Arbotta" is corrected to "Be R Botta." (2) r x t+t (-4F
4-) ' D J [X = (+P-+-)
ld”. (3) On page 10, line 5 of the specification, it says, "The solid line shows the value of D, and the broken line shows the sum of D and the length of the reflector."
The sum of D and D and the length of the reflector is shown.'' (4) - 12-electrode filter on page 12, line 12 of the specification
The statement "2-electrode filter, and 4 is a multi-electrode filter with more than one type" is corrected. 1-

Claims (2)

[Claims] (1) On a piezoelectric substrate, there is an input transducer consisting of at least one interdigital electrode for converting an electric signal into an acoustic signal, and an input transducer consisting of at least one interdigital electrode for converting an acoustic signal into an electric signal. and an output converter consisting of interdigital electrodes,
Further, the leaky surface waves radiated from the input transducer and propagating without being converted into electrical signals by the output transducer are reflected toward the output transducer, and the grating spacing is arranged along the incident propagation direction of the leaky surface waves. A surface acoustic wave filter comprising: a plurality of reflectors that gradually increase or decrease . (2) N is an integer of 2 or more, and the distance from the center of the interdigital electrode closest to the reflector among the interdigital electrodes constituting the output converter to the reflection point of the reflector is always 2. The surface acoustic wave filter according to claim 1, wherein the lattice spacing of the reflector is determined to be N/2 times the wavelength of the leaky surface wave.