JPH02172312A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To reduce insertion loss at a broad band by arranging a 1st (2nd) reflector adjacent to an outside of a 1st (3rd) interdigital electrode, connecting the 1st and 3rd interdigital electrodes in parallel electrically to a load or a signal source and connecting the 2nd interdigital electrode to the signal source or the load. CONSTITUTION:The 1st, 2nd and 3rd interdigital electrodes 3, 4, 3' arranged sequentially between the reflectors 2, 2' side by side are arranged nearly in symmetry with both sides of the 2nd interdigital electrode 4 in the center of the electrode 4, the 2nd reflector 2' is arranged at the outside of the 1st reflector 2 and the 3rd interdigital electrode 3', the 1st interdigital electrode 3 and the 3rd interdigital electrode 3' are connected in parallel electrically to the load or the signal source and the 2nd interdigital electrode 4 is connected to the signal source or the load. Thus, ripple in the pass band or the like is reduced by varying slightly the electrode finger width and the electrode finger interval of the reflectors 2, 2' and the interdigital electrodes 3, 3', 4. Thus, the insertion loss at a broad band is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention particularly relates to a surface acoustic wave filter with low insertion loss and a wide band.

[Conventional technology]

Conventionally, the use of interdigital electrodes as surface acoustic wave reflectors has been proposed, for example, at the 4th Symposium on the Fundamentals and Applications of Ultrasonic Electronics, sponsored by the Japan Society of Applied Physics.
As discussed in paragraphs 81 and 82 of Lecture Preliminary Lecture Collection (December 1983) by Eiji et al., it was considered as a resonator.

[Problem to be solved by the invention]

Therefore, the conventional technology described in Section 2 does not take into account widening the band, and has the problem that its application is limited to oscillators or narrow band filters.

An object of the present invention is to provide a surface acoustic wave filter with a wide band and low insertion loss.

[Means to solve the problem]

In order to achieve the above object, in the present invention, the piezoelectric substrate' secondly, the surface acoustic wave wavelength on the piezoelectric substrate at the center frequency of the filter passband is set to λ. When the width of the electrode fingers and the gap between the electrode fingers are approximately λ0/4, respectively, and the number of pairs of electrode fingers] -5 to 40 pairs of interdigital electrodes, opposing electrode finger groups are connected by a coil. The electrode finger width and electrode finger spacing are approximately λ.
0/4, the first, second, and third interdigital electrodes arranged adjacent to each other between both reflectors are arranged approximately symmetrically on both sides of the second interdigital electrode with the electrode center of the second interdigital electrode as the center. A first reflector is disposed adjacent to the outside of the first interdigital electrode, a second reflector is disposed adjacent to the outside of the third interdigital electrode, and the first interdigital electrode and the third interdigital electrode are electrically connected. The second interdigital electrode was connected in parallel to the load or signal source, and the second interdigital electrode was connected to the signal source or load.

The center frequency of this filter is f. , if the velocity of the surface acoustic wave of the piezoelectric substrate 2 is , then of course .

, 3.

In addition, the distance between the end of the reflector and the end of the adjacent interdigital electrode is kept small, and the time required for the surface acoustic wave to travel back and forth between them is determined by the group delay time of the reflector's reflection coefficient (the phase of the reflection coefficient). (expressed by the frequency differential of the frequency characteristic) is sufficiently small, for example, 1/i-0 or less, that is, the following formula holds true.

V 10 [Operation] As will be explained again in the Example section, the number of pairs of electrode fingers of the reflector is changed to find the bandwidth where the reflection coefficient is approximately 1, and the relative The relationship between the bandwidth and the number of electrode finger pairs of the reflector is illustrated in FIG. 2. If the number of electrode finger pairs is set to -5 to 40, the reflection coefficient will be approximately 1 and the relative bandwidth of reflection will be 0.02 or more. In addition, when examining the reflection characteristics, we connected the opposing electrode fingers of the interdigital electrodes that form the reflector with coils that have the optimal inductance for each pair of electrode fingers. In case of short circuit without coil (,
) is shown in Figure 3.

In this way, terminating with a coil allows the reflection coefficient to be approximately 1 over a wide band. Furthermore, since the distance between the end of the reflector and the end of the adjacent interdigital electrode is short, the delay time required for the surface acoustic wave to travel back and forth between them can be ignored, and the interdigital electrode The phase difference between the surface wave generated by the surface wave and the surface wave reflected by the reflector and returned is almost determined by the group delay time τR of reflection from the reflector, so extra interference may occur within the passband of the filter. There's nothing to do. Note that the value of 1/10 in equation (2) was arbitrarily determined by the inventors, but the inventors have determined through experiments that good results can be obtained in practice by doing so. We have confirmed that.

Furthermore, since surface waves are incident on the second interdigital electrode from both sides of the electrode, if the impedance of the load connected to the second interdigital electrode is appropriately set, the white root on both sides can be ignored.
It is known that the reflected wave and the transmitted wave at the electrode end cancel each other out, so the reflection at the second interdigital electrode end is ignored5.

You may do so. Therefore, most of the energy generated in the interdigital electrodes on both sides is incident on the second interdigital electrode. Therefore, low loss can be achieved.

〔Example〕

FIG. 1 is a schematic plan view of a first embodiment of the present invention. This surface acoustic wave filter has a piezoelectric substrate 1 with a Y angle of 36 degrees.
Using rotary-cut lithium tantalate, an aluminum film with a thickness of about 1000 mm was formed on this piezoelectric substrate 1,
The reflector 2.2' and the interdigital electrodes 3.3', 4 are formed using well-known photolithography techniques. Reflector 2.
The electrode finger widths and electrode finger gaps of 2' and the interdigital electrodes 3, 3', and 4 were determined so as to substantially satisfy equation (1).

The number of logarithms of the electrode fingers constituting the reflector 2.2' is 1.
The number of pairs may be in the range of 5 to 40, but in this example, it is set to 25 pairs. The opposing electrode fingers of the reflector 2.2' are each connected by a coil 5.5'. When the number of pairs of electrode fingers is 25, good reflection characteristics can be obtained when the inductance of the coil is approximately 25 nH.

The interdigital electrodes 3.3' are electrodes connected to a load or a signal source, and the picture electrodes are electrically connected in parallel. Reflector 2
The distance L□ between the interdigital electrode 3 and the distance L2 between the reflector 2' and the interdigital electrode 3' are determined so as to satisfy equation (2). An interdigital electrode 4 is provided between the interdigital electrodes 3.3' and is connected to a signal source or a load. With the configuration as described above, for example, by connecting the interdigital electrodes 3 and 3' to a signal source and the interdigital electrode 4 to a load, it can be operated as a surface acoustic wave filter.

Next, the reflection characteristics of reflector 2.2' will be explained. By changing the number N of electrode pairs of the reflector 2, a bandwidth where the reflection coefficient becomes approximately 1 was determined. FIG. 2 shows the relative bandwidth obtained by normalizing this bandwidth with the center frequency. From this result, in order to satisfy the conditions that the reflection coefficient is approximately 0.02 and the relative bandwidth of reflection is 0.02 or more, the number N of electrode pairs must be 15 or more and 240 or less. The study of this reflection characteristic was carried out using a coil with an optimal inductance for each N, and a soaring electrode.
7.

This was done by connecting opposing electrode finger groups. In the case of N-25, the reflection characteristics are shown in FIG. 3 when it is terminated with a coil having an inductance of 25 nH (line b) and when it is short-circuited without a coil (line a). From this it can be seen that terminating with a coil allows the reflection coefficient to be approximately 1 over a wide band.

Next, the operation when the interdigital electrodes 3 and 3' are connected to a signal source and the interdigital electrode 4 is connected to a load will be described. The phase difference between the surface wave generated by the interdigital electrode 3 and the surface wave reflected by the reflector 2 is such that the distance L1 is sufficiently small, so that the distance L1 required for the surface wave to travel back and forth is The delay time can be ignored and is almost determined by the group delay time of the reflection from the reflector 2, so that no extra interference occurs within the passband of the filter. In addition, since surface waves are incident on the interdigital electrode 4 from both sides, if the impedance of the load is set appropriately, the reflected wave at the electrode end and the transmitted wave from the other electrode end can be canceled out. It can be assumed that there is no reflection at the electrode end.

Therefore, most of the energy of the plane waves generated from the interdigital electrodes 3 and 3' is converted into an electrical signal by the interdigital electrodes and supplied to the load, so that the insertion loss is small.

In the invention, the reflector 2.2'' and the interdigital electrode 3.3'
, 4, by slightly changing the electrode finger width and the electrode finger gap, it is possible to reduce ripples within one pass band. In this embodiment, from the average value of the electrode finger width of the reflector 2.2'' and the distance between the electrode fingers, the interdigital electrode 3
．． By reducing the average value of the 3' electrode finger width and the distance between the electrode fingers, the shoulder characteristics within the passband could be improved.

As described above, by implementing the present invention, a surface acoustic wave filter with a wide band and low insertion loss can be realized. Furthermore, since the number of pairs of interdigital electrodes used in the reflector is small, there is an advantage that the required area of the piezoelectric substrate is small.

Although this embodiment does not specifically refer to the weighting of the electrodes, it goes without saying that well-known weighting techniques such as the apodization method, thinning method, phase weighting, etc. can be applied. In addition, there are piezoelectric substrates 1 that can be made of the same lithium tantalate and have similar results with other combinations of cut angles and propagation directions. Further, other materials such as lithium niobate can also be used as the piezoelectric substrate 1-.

FIG. 4 is a schematic plan view of a second embodiment of the present invention. In this example, the width of the electrode fingers and the gap between the electrode fingers of the interdigital electrodes used in the reflector]-2 were patterned so that they gradually became smaller as they approached the interdigital electrodes 13. Similarly, the electrode finger width and electrode finger gap of the interdigital electrode used in one reflector 12' are patterned so that they gradually become smaller as they approach the interdigital electrode 13''. Chirp-shaped electrode reflector”-2,12
', it is particularly effective for controlling the phase frequency characteristics of the reflection coefficient. For example, the reflector 12 creates a frequency band in which the group delay time of the reflection coefficient is small in the high frequency range for the surface waves incident from the interdigital electrode 13 side, and therefore the phase delay of the reflected surface waves is gentle. I can do it. For this reason, when the surface wave converted from an electrical signal by the interdigital electrode 13 interferes with the surface wave reflected and returned by the reflector 1z, the overlapping band in approximately the same phase is extended toward the high frequency region. It is possible to achieve wide bandwidth.

FIG. 5 is a schematic plan view of a third embodiment of the present invention. In this example, the coil 5J) + 55 connected to the reflector 2
' is provided on the piezoelectric substrate]'2. The line width of the pattern forming the coil 55 is wider than the electrode finger width of the reflector 2. Therefore, the Q of the coil itself can be increased, and even if the coil is provided on the piezoelectric substrate 1, the performance will not deteriorate compared to when the coil is attached externally. As described above, in this embodiment, since the coil is provided on the piezoelectric substrate, the number of components constituting the surface acoustic wave filter can be reduced, which is economical. Also, the coil on the piezoelectric substrate is economical because it can be formed in the same process as forming the reflector and the interdigital electrode.

FIG. 6 is a schematic plan view of a fourth embodiment of the present invention. In this embodiment, the filters of the third embodiment are arranged in parallel in three stages on the same piezoelectric substrate. Interdigital electrodes 3, . 3-2
．． 3-3. a',,3''l,:3'-3 are electrically connected in parallel, and the interdigital electrodes 4, . 4-2.
71I-3 are also connected in parallel. With such a configuration, the influence on the electrical resistance or insertion loss of each electrode finger can be minimized.

FIG. 7 is a stylistic plan view of a fifth embodiment of the present invention. This embodiment also uses the same piezoelectric substrate as the filter of the first embodiment:
(Although they are arranged parallel to each other, reflectors 21.2-2.2-
3 are connected in parallel and terminated with a coil 75. The reflectors 2', 2'-2, 2'-3 were similarly terminated with coils 75'. This configuration has the advantage of reducing the number of parts.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to realize a surface acoustic wave filter with a wide band, small ripples, and low insertion loss.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the first embodiment of the present invention, FIG. 2 is a diagram showing the relationship between the number of electrode finger pairs and the relative bandwidth of the filter when the reflection coefficient of the reflector is approximately 1, and FIG. The relationship between the relative reflectance and the relative frequency when the opposing electrode fingers of the interdigital electrodes forming the reflector are connected with a coil with the optimum inductance value (b line) and when they are short-circuited without a coil (a line). 4 is a diagram of the second embodiment, FIG. 5 is a diagram of the third embodiment, FIG. 6 is a diagram of the fourth embodiment, and FIG. 7 is a diagram of the fifth embodiment. ]・Piezoelectric substrate, 2.Reflector; 3.4.Interdigital electrode, 5.5′・Coil.

Claims (6)

[Claims] 1. When the elastic surface wavelength on the piezoelectric substrate at the center frequency of the filter passband is λ_0, the electrode finger width and the electrode finger gap are each approximately λ_0/4, and the number of electrode finger pairs is 15 to 4.
A first and second reflector formed by connecting a group of opposing electrode fingers of 0 pairs of interdigital electrodes with a coil, and an electrode finger width and an electrode finger gap of approximately λ_0/4, respectively, are arranged between the two reflectors. The first, second, and third interdigital electrodes arranged adjacent to each other are arranged symmetrically on both sides of the electrode center of the second interdigital electrode, with the three interdigital electrodes on the inside, and the first interdigital electrode A first reflector is arranged adjacent to the outside, a second reflector is arranged adjacent to the outside of the third interdigital electrode, and the first interdigital electrode and the third interdigital electrode are electrically connected in parallel. load or signal source,
A surface acoustic wave filter having a second interdigital electrode connected to a signal source or a load. 2. 2. The surface acoustic wave filter according to claim 1, wherein the electrode finger width and electrode finger gap of each reflector are made larger than the electrode finger width and electrode finger gap of each interdigital electrode, respectively. . 3. 2. The surface acoustic wave filter according to claim 1, wherein the electrode finger width and the electrode finger gap of each reflector are made to gradually become smaller as they approach adjacent interdigital electrodes. . 4. 2. The surface acoustic wave filter according to claim 1, wherein groups of opposing electrode fingers of the reflector are connected by a coil pattern having a pattern line width wider than the electrode finger width and arranged on the same piezoelectric substrate. Surface acoustic wave filter. 5. A surface acoustic wave filter comprising a plurality of surface acoustic wave filters according to claim 1 arranged in parallel and connected in parallel on the same piezoelectric substrate. 6. A claim characterized in that the time required for a surface acoustic wave to travel back and forth between the end of the reflector and the end of an adjacent interdigital electrode is 1/(10) or less of the group delay time of the reflection coefficient of the reflector. Item 1. The surface acoustic wave filter according to item 1.