JPS60264112A - Surface wave filter - Google Patents

Abstract

PURPOSE:To reduce spurious radiation by constituting an input electrode with a normal interdigital electrode and constituting an output electrode with an interdigital electrode applied with crossing width weighting so as to eliminate unbalance of direct waves. CONSTITUTION:Since the output electrode 24 is subjected to crossing width weight, the area of the electrode is large. Thus, a capacitance with a shield electrode at the back face is large. A direct wave output Vs01 extracted from a bus bar 32 placed between an input electrode 23 and the output electrode 24 is a voltage division of the level of the direct wave from the electrode 23 to the 24 through a capacitance Cs1 between both the electrodes and a capacitance C01 between the bus bar 32 and the 2nd shield electrode at the back face. This is similar as to a bus bar 33. In this case, the relation of the capacitances is Cs1<<C01 and since the C01 is nearly 5 times the capacitance of the normal output electrode in using a PZT substrate, the direct wave output is decreased to nearly 1/5. Since the direct wave amount propagated to the bus bars 32 and 33 is reduced largely, the amount of unbalance of the propagated direct wave is reduced largely.

Description

DETAILED DESCRIPTION OF THE INVENTION OBJECT OF THE INVENTION Technical Field The present invention relates to improvements in the structure of surface wave filters, particularly in the structure of surface wave filters in which the electrodes are constituted by cross-width weighted interdigital transducers. Regarding.

BACKGROUND OF THE INVENTION FIG. 4 is a plan view showing an example of a conventional surface wave filter. 4th rj! In the surface wave filter 1 shown in J, an interdigital input electrode 3 and an interdigital output electrode 4 are formed on the surface of a piezoelectric substrate 2, and each constitutes an interdigital transducer together with the piezoelectric substrate 2. In order to obtain a desired frequency characteristic, the input electrode 3 is subjected to cross-width weighting as shown, while the output electrode 4 has a regular configuration. As shown in FIG. 4, in the input electrode 3, the hot-side and ground-side electrode fingers are alternately drawn out to the hot-side busper 5 and the ground-side buspar 6. Note that the hot-side buspar 5 and the ground-side buspar 6 are connected to the hot-side extraction electrode 7 and the earth-side extraction electrode 8 at the negative end portions, respectively. Also,
Dummy electrodes 9 and 10 consisting of a group of dummy electrode fingers placed outside the cross-width weighted portion are also connected to the hot-side busper 5 and the ground-side busper 6, respectively.
The ground busper 6 is further connected to a first shield electrode 11 disposed between the input electrode 3 and the output electrode 4.

Since the first shield electrode 11 is configured in this way, it is possible to reduce the number of parts that need to be soldered.

On the other hand, also in the output electrode 4, each electrode finger is alternately connected to the buspars 12.13.
3 are not grounded.

Although not particularly shown, a second shield electrode is formed on the back surface of the piezoelectric substrate 2 over almost the entire surface.

By the way, when the conventional surface wave filter 1 shown in FIG. 4 is actually operated, the direct waves that cause spurious waves are more likely to reach the upper buspar 12 than the lower buspar 13 of the output electrode 4. The disadvantage is that the spurious level becomes high due to the imbalance in the level of the direct wave.

Problems to be Solved by the Invention Therefore, an object of the present invention is to provide a surface wave filter that can effectively eliminate the imbalance of direct waves and reduce spurious waves based on direct waves.

Means for Solving the Constituent Problems of the Invention To summarize, the present invention includes an interdigital input/output electrode disposed on one side of a piezoelectric substrate, and an input electrode disposed between the input electrode and the output electrode. A first shield electrode connected to the ground side buspar of the
A surface wave filter comprising a shield electrode, characterized in that the input electrode is constituted by a regular interdigital electrode, and the output electrode is constituted by an interdigital electrode subjected to cross-width weighting. It is a wave filter.

Embodiment FIG. 1 is a plan view showing an embodiment of the present invention. In the surface wave filter 21 of this embodiment, the input electrode 23 is constituted by a regular interdigital electrode, and the output electrode 24 is constituted by an interdigital electrode which is weighted and numbered as shown. . Regarding other points, the conventional surface wave filter 1 shown in FIG.
Since it is the same as that, the description thereof will be omitted by assigning the corresponding reference numbers to the corresponding parts. Here, the output electrode 24 is subjected to cross-width weighting as shown. Therefore, it can be seen that since the weighted output electrode 24 has a considerably large area, the capacitance between it and the shield electrode on the back side becomes considerably large. It has been confirmed that, for example, when a PZT substrate is used, this is about five times as large as that of a regular output electrode.

Now, if we consider the direct wave output that is taken out from one busper 32 of the output electrode 24, the output of the direct wave propagating from the input type tii 23 to the output electrode 24 is as shown in FIG. Interelectrode capacitance C11 and output tU[i24
Direct wave output V
so+ is Vso + −Vt x[cs +/(Cs + +G
o, + )]. The lower busper 33 can also be expressed by a similar formula by changing the subscript 1 to 2.

As mentioned above, the output type 1! with cross-width weighting is applied. !
! 24, the capacitance between it and the second shield electrode on the back surface is extremely large, so in the above equation, Cs + <<Q
o+, and for example, when using a PZT substrate, Go
It can be seen that the direct wave output is about 115, since + is about 5 times that of the normal type output electrode. In this manner, in the embodiment shown in FIG. 1, the output electrode 24 is constructed of an interdigital type with cross-width weighting, so that the amount of direct waves propagated is greatly reduced.

Therefore, it can be seen that since the amount of direct waves propagated to the buspers 32 and 33 of the output electrode 24 is greatly reduced, the amount of imbalance in the amount of direct waves propagated is also greatly reduced.

Note that the surface wave output does not depend on the relationship between the back surface and the shield electrode. Therefore, if the output electrode weighted as described above is used and the capacitance with the shield electrode on the back side becomes five times, the degree of direct wave suppression will be -2010 (1 + a (115 ) -14dB improvement.

In addition, when none of the buspars 32 and 33 are grounded in the output electrode 24 as in the embodiment shown in FIG. ,. 2, but when mounted on a printed circuit board, ■. 1. ■,.

2 varies, so in order to stably suppress direct waves, V
It is desirable that the value of $OT+Vso2 itself be small. Therefore, in the embodiment shown in FIG. 1, Vso
++Vs. It can be seen that since 2 itself becomes dramatically smaller, it is very preferable when mounting it on a printed circuit board.

FIG. 2 is a plan view showing another embodiment of the invention.

Here too, the input electrode 43 is similar to the embodiment shown in FIG.
is constituted by a regular type interdigital electrode, and the output electrode 44 is constituted by an interdigital electrode subjected to cross-width weighting. Therefore, as in the embodiment shown in FIG. 1, the capacitance Cs + + between the input and output electrodes
C wealth 2 and output electrode 44 shield electrode capacitance C81
, C02, the amount of direct waves propagated to each bus spar 52, 53 of the output electrode 44 can be dramatically reduced.

Furthermore, in the embodiment shown in FIG.
Of the pair of buspars 52 and 53, the buspar 52 that is closer to the hot side buspar 45 of the input electrode 43 is connected to the input electrode 43.
The busbar 52 is arranged so as to be close to the envelope of the weighted portion of the output electrode 44 (i.e., the buspar 52 is formed by bending the portion indicated by 52a toward the center along the envelope line). In the embodiment shown in FIG. 2, the busper 53 on the other side of the output electrode 44 is also
As shown, it is bent inward along envelope I5IIA.

Further, a bent portion 52a of each bus spar.

53a, forming dummy electrodes 58 and 59.
An electrode finger group is attached, and the group of electrode fingers is a busper 60.
．． 61 , and each buspar 60 , 61 is connected to the first shield electrode 51 .

In the embodiment shown in No. 25g, the output electrode 4 is
The portion 52a bent to the buspar 52 and 53 of No. 4,
53a is formed, and a dummy electrode 58.53a is formed on the outside thereof.
59 is provided, and each dummy electrode 58,59 is connected to the shield electrode 51. Therefore, compared to the case of the actual Im example shown in FIG.
2.53 is further away from the hot side busper 45 of the input electrode 43. Therefore, by increasing the distance of the input electrode 43 from the hot bus spar 45, the amount of direct wave propagation is reduced by m.

Therefore, it can be seen that the imbalance in the amount of direct waves mtx at the output electrode 44 is further improved compared to the actual example shown in No. IS. Note that the cross-width weighting in this invention is not limited to the cross-width weighting shown in FIGS. 1 and 2, but may be configured in any shape.

Effects of the Invention As described above, in this invention, an input electrode made of a regular interdigital electrode and an output electrode made of an interdigital electrode subjected to cross-width weighting are formed on one side of a piezoelectric substrate. Since the second shield electrode is provided on the other side, direct and fast waves are The output power is reduced dramatically, so the problem of imbalance at the output electrode of the direct wave can be effectively eliminated, and therefore based on the propagation of the direct wave (
It becomes possible to reduce spurious components.

This invention can be applied to surface wave filters in general using substrates made of appropriate materials, but in particular PZT,
It should be pointed out that this method is particularly effective when used for substrate materials with high dielectric constants such as LtNb am and LiTa0@, since the amount of direct wave propagation tends to be large in these types of materials.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing an embodiment of the present invention. FIG. 2 is a plan view showing another embodiment of the invention. Figure 3 shows the capacitance between the input and output electrodes and the output electrode and the second
FIG. 3 is a diagram for explaining voltage division between the shield electrode and the capacitance of the capacitor. FIG. 4 is a plan view showing an example of a conventional surface wave filter. In the figure, 21.41 is a surface wave filter, 22.42
) is the piezoelectric substrate, 23.43 is the input electrode, 24.44 is the output electrode, 25.45 is the hot side bus bar of the input electrode, 2
6.46 is the ground side bus bar of the input electrode, 31.51 is the first shield electrode, 52.53 is the bus bar of the output electrode, and 52a and 53a are bent so as to be close to the envelope on the input electrode side of the output electrode. 58.59 indicates the dummy electrode, and 60.61 indicates the bus bar connecting the dummy electrode finger group: Fig. 1 Fig. 3 SI Fig. 4

Claims (2)

[Claims] (1) An input/output electrode consisting of an interdigital electrode on one side of the piezoelectric substrate, and a first shield electrode distributed between the input electrode and the output electrode and connected to the ground side bus bar of the input electrode. and a second shield electrode on the other surface side, wherein the input electrode is a regular interdigital electrode, and the output electrode is an interdigital electrode subjected to cross-width weighting. A surface wave filter comprising: (2) Of the pair of busbars of the output electrode, at least the busbar that is closer to the hot side busbar of the input electrode,
a group of electrode fingers arranged on the input electrode side so as to be close to the envelope of the weighted portion of the output electrode, and forming a dummy electrode outside the input electrode side portion of the output electrode of the bus bar; is arranged, and the dummy electrode finger group is connected to I! 2. The surface acoustic wave filter according to claim 1, wherein a bus bar is connected to the first shield electrode.