JPH0312485B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a surface acoustic wave filter, and more particularly to a surface acoustic wave filter with flat passband characteristics and a wide fractional bandwidth.

(Prior art) In general, a surface acoustic wave filter has input side and output side comb-like electrodes formed on a piezoelectric substrate, separated by a predetermined distance, and utilizes surface waves propagating from the input side electrode to the output side electrode. In particular, the frequency characteristics based on the electrode shape are designed to achieve various characteristics required for, for example, a TV VIF filter. In such a surface acoustic wave filter, in addition to the desired surface waves, TTE, edge reflected waves,
Unwanted waves such as bulk waves are also generated. Conventionally,
In order to eliminate the effects of unnecessary waves, each had a split electrode structure, was coated with sound-absorbing material, and had its bottom surface processed.

However, unlike TV VIF filters,
For example, the center frequency is 70MHz and the 3dB passband width is
When configuring a filter with a very wide specific bandwidth such as 23MHz, in addition to the above-mentioned unnecessary waves, spurious waves called surface wave type spurious waves become relatively large _. at f ₀
= 70MHz, 3dB bandwidth = 23MHz, propagation distance =
In the case of 1.6 mm, large undulations with an amplitude of 0.9 dB and a period of about 6 MHz appear. This is because the fractional bandwidth is wide, so the number of logarithms of the weighted electrodes decreases, and the conductance becomes smaller.As a result, the surface wave type spurious generated from the most extreme electrode finger of the weighted electrode becomes smaller than the desired surface wave. become relatively large,
This is because the influence of interference due to a phase shift caused by the difference in propagation distance between the two waves becomes noticeable.

A means for reducing such surface wave type spurious is disclosed in, for example, Japanese Patent Application Laid-Open No. 122215/1982,
A typical example is shown in FIG. Figure 4 shows
Electrode fingers 21, 22, 23, 24 having different lengths are arranged at the ends of the comb-like electrodes at a repetition period of λ/.
4 in the direction of wave propagation.

With this configuration, the electrode finger 21 and the electrode finger 2
The unnecessary surface wave response caused by the shaded area in 3 is
Since they have a difference of λ/2, they have opposite phases and cancel each other out.

Unnecessary surface wave responses caused by the hatched portions of the electrode fingers 22, 24 are also canceled out.

(Problem to be Solved by the Invention) However, in the configuration shown in FIG. 4, the electrode fingers 21, 22, 23, and 24 having different lengths are formed along the wave propagation direction, so that the electrode fingers 21, 22, 23, and 24 are formed at right angles to the wave propagation direction. The problem is that the propagation time of the surface wave is different in the direction of , and as a result, the wavefront is curved and the frequency characteristics are deteriorated.

(Means for solving the problem) The endmost electrode finger on the opposite electrode side of the weighted electrode is electrically connected to a plurality of electrode finger portions separated from each other by a distance of (N+1/2)λ ₀ . The total length of the odd-numbered electrode fingers from the weighted electrode side of the plurality of electrode finger parts and the total length of the even-numbered electrode finger parts from the weighted electrode side. These are surface acoustic wave filters constructed almost identically.

(Effect) (N+1/2)λ ₀ By shifting the outermost electrode finger,
Unwanted waves seen from the opposing electrodes have the same size and opposite phases, and therefore they cancel each other out on the opposing electrode side. Moreover, the ratio of the area with electrodes to the area without electrodes does not change due to this endmost electrode finger.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

In FIG. 1, on a piezoelectric substrate 1, for example, a weighted comb-shaped electrode 2, which serves as an input electrode, is formed, and a regular comb-shaped electrode 3, which serves as an output electrode, is formed at a predetermined distance from this electrode 2. . Weighted electrode 2
has a given pass characteristic, e.g. center frequency is 70M
In order to obtain a pass characteristic with a 3 dB passband width of 23 MHz, the cross width is weighted, and a split electrode consisting of a pair of electrode fingers is used to eliminate TTE. The most end pair of electrode fingers 6 and 7 on the opposing electrode 3 side of the hot side comb-like electrode 4 of this weighted electrode 2
Of these, the electrode fingers 7 on the counter electrode 3 side include an electrode finger portion 7a arranged at a predetermined pitch on the electrode 2 and exciting a desired surface wave, and an electrode finger portion 7a arranged at a predetermined pitch on the electrode 2, and an electrode finger portion 7a that is arranged at a predetermined pitch on the electrode 2, and an electrode finger portion 7a that is arranged at a predetermined pitch on the electrode 2, and an electrode finger portion 7a that is arranged at a predetermined pitch on the electrode 2 and excite a desired surface wave by λ ₀ /2 from this finger portion 7a in the wave propagation direction. The electrode finger portion 7b is shifted toward the counter electrode 3 and does not contribute to the excitation of surface waves. Moreover, these electrode finger portions 7a and 7b have approximately equal lengths a/2, and are configured such that the total length a is approximately equal to one electrode finger, and are electrically connected. . In this embodiment, also in the earth-side comb-shaped electrode 5 of the weighted electrode 2, the outer electrode 9 of the outermost pair of electrode fingers 8, 9 is connected to the electrode finger portions 7a, 7b.
Similarly, it is composed of bent electrode fingers consisting of two electrode finger portions 9a and 9b separated by a distance of λ ₀ /2. In the surface acoustic wave filter configured as described above, in addition to the surface acoustic waves excited or received from the split electrodes constituting the weighted electrode 2, there is a There is a surface acoustic wave component that is excited or received by the endmost electrode finger depending on the length of the electrode finger, and this becomes spurious. The cause of this is the end effect of the capacitance between the entire hot-side comb-teeth electrode 4 and the ground (for example, the base part of the grounded hermetic case (not shown)) (charge is concentrated in the periphery of the electrode). It is believed that this is due to the

In the surface acoustic wave filter of the above embodiment,
The surface waveform spurious generated from the farthest electrode finger 7 cancels each other because the spurious based on the electrode finger portion 7a and the spurious based on the electrode finger portion 7b appear on the output side electrode 3 with a time difference corresponding to λ ₀ /2. will be done. Furthermore, although the spurious level is relatively small at the endmost electrode finger 9 of the ground-side comb-shaped electrode 5, it is canceled out in the same way. For example, in the past, the amplitude width within the passband
The waviness of 0.9 dB and a period of about 6 MHz was generated, but according to this embodiment, the waviness is about 0.5 dB, and a practically sufficient flatness can be obtained. Moreover,
Since the electrode finger portion 7a arranged on the center side opposite to the electrode bus bar also contributes to excitation of the original surface wave as a split electrode, it is possible to flatten the passband without adversely affecting the frequency characteristics. can.

FIG. 2 shows a second embodiment, which differs from the above embodiment in that a dummy electrode finger is used as the endmost electrode finger. That is, dummy end electrode fingers 10 and 11 are added to the weighted electrode 2 at the end on the counter electrode 3 side, that is, further outside of the electrode fingers 7 and 9 closest to the counter electrode 3. . These endmost dummy electrode fingers 10 and 11 have approximately the same length as the adjacent electrode fingers 7 and 9, and are separated by a distance of λ ₀ /2 in the wave propagation direction at approximately the center thereof, as in the above embodiment. It is constructed by shifting and bending.

FIG. 3 shows a third embodiment, in which the outermost electrode finger 1
2 and 13 are three electrode finger parts 12a and 1, respectively.
2b, 12c, and 13a, 13b, 13c. Electrode finger portions 12a, 12
b, 12c are finger portions 12b, 1 at intervals of λ ₀ /2 sequentially from the finger portion 12a toward the counter electrode 3 side.
2c is placed. Earth side electrode finger part 13
Similarly, a, 13b, and 13c are arranged with an interval of λ ₀ /2.

(Effects) According to the present invention, even in a filter with a wide fractional bandwidth, surface waveform spurious can be suppressed without adversely affecting the excitation of a predetermined surface wave, and a flatter passband characteristic can be obtained. In particular, it does not disturb the wavefront of surface waves.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, and FIG. 3 are diagrams showing an embodiment of a surface acoustic wave filter according to the present invention, respectively;
FIG. 4 is a diagram showing a conventional example. 1 is a piezoelectric substrate, 2 is a weighted electrode, 3 is a regular type electrode, 7, 10, 12 are extreme electrode fingers, 7a, 7b,
10a, 10b, 12a, 12b, and 12c are electrode finger portions.

Claims (1)

[Scope of Claims] 1. In a surface acoustic wave filter in which at least one of the input and output comb-shaped electrodes is a weighted electrode, at least the hot side of the weighted electrode and the most opposite electrode side of the comb-shaped electrode The end electrode finger is constituted by an electrode finger formed by electrically connecting a plurality of electrode finger parts separated from each other by a distance of (N + 1/2) λ ₀ , and the weighting of these plurality of electrode finger parts is A surface acoustic wave filter characterized in that the total length of the odd-numbered electrode finger portions from the electrode side is approximately equal to the total length of the even-numbered electrode finger portions. 2. The weighted electrode is comprised of a split electrode consisting of a pair of electrode fingers.
The surface acoustic wave filter described in .