JPH05251982A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To provide a surface acoustic wave filter whose pass band characteristic is improved by employing a 36 Y-cut X direction propagation LiTaO3 substrate. CONSTITUTION:In a multi-electrode system surface acoustic wave filter employing a 36 deg. Y-cut X direction propagation LiTaO3 substrate for its substrate and consisting of input electrodes 11a-11e and an output electrode on the substrate, a ratio (Lm/Lg) of a line width Lm of each electrode finger (E1,..., E5,...) for the input electrodes 11a-11e and the output electrode to a width Lg of each gap (G1,..., G5,...) among the electrode fingers is selected in a range of 1.2<(Lm/Lg)<3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter, and more particularly to 36 ° Y-cut X-direction propagation LiTa.
The present invention relates to a surface acoustic wave filter using an O ₃ substrate.

[0002]

2. Description of the Related Art With the miniaturization of mobile communication devices, there is a demand for small and lightweight surface acoustic wave filters. In particular, as a filter in the RF band, it is difficult to miniaturize a dielectric filter that has been conventionally used, and development of a surface acoustic wave filter is being advanced. This RF band filter is required to have low loss, high selectivity and high out-of-band attenuation characteristics, and a multi-electrode construction method is generally used. In the multi-electrode surface acoustic wave filter using the multi-electrode configuration method, it is possible to reduce the loss by increasing the number of electrodes. Further, as a substrate, a 36 ° Y-cut X-direction propagation LiTaO ₃ substrate having a relatively good temperature characteristic is generally used because of the requirement of high selectivity.

The electrode fingers of the surface acoustic wave filter are shown in FIG.
As shown in, the ratio (Lm between the line width Lm of the electrode fingers and the gap width Lg between the electrode fingers in each of the input and output electrodes (Lm
/ Lg) is normally set to 1. The frequency characteristics of the surface acoustic wave filter thus set are as shown in FIG. Here, FIG. 6 shows an example of a surface acoustic wave filter having seven input electrodes with 18 electrode finger pairs and six output electrodes with 26 electrode finger pairs.

[0004]

However, in the case of a multi-electrode type surface acoustic wave filter using a 36 ° Y-cut X-direction propagating LiTaO ₃ substrate, as shown in FIG. There is a problem that the loss in the high region is large.

The wave propagating in the 36 ° Y-cut X-direction propagation LiTaO ₃ substrate largely changes depending on the electrical state of the substrate surface, so-called SSBW (Surface Skimming Bulk Wafer).
ve) and LSAW (Leaky Surface Acoustic Wave) propagate as complexly coupled grating modes (“Highly coupled rotating Y-cut LiNbO ₃ and LiTaO ₃ propagate SSBW and Leaky SAW” IEICE Transactions.
Vol. J67-C No. 1P. 158-165 (1984)).

However, this grating mode, when coupled with a fast transverse wave, radiates energy into the substrate as a bulk wave. This energy radiation has frequency characteristics,
It is considered that the pass band characteristic is affected when the filter characteristic is used.

The present invention is a 36 ° Y-cut X-direction propagating LiT
An object is to provide a surface acoustic wave filter using an aO ₃ substrate and having improved pass band characteristics.

[0008]

The surface acoustic wave filter of the present invention comprises a substrate made of 36 ° Y-cut X-direction propagating LiTaO ₃ and has a total of three or more input electrodes and output electrodes on the substrate. In the multi-electrode surface acoustic wave filter,
The ratio (Lm / Lg) of the line width Lm of the electrode fingers and the gap width Lg between the electrode fingers in each of the input electrode and the output electrode is set in the range of 1.2 <(Lm / Lg) <3. .

[0009]

According to the surface acoustic wave filter of the present invention, the ratio (Lm / Lg) between the line width Lm of the electrode fingers and the gap width Lg between the electrode fingers at the input electrode and the output electrode is 1.2 <.
Since the range of (Lm / Lg) <3 is set, the emission of the bulk wave is suppressed, the pass band characteristic is improved, and the change of the input impedance is not large, which facilitates the manufacturing.

[0010]

EXAMPLES The present invention will be described below with reference to examples.

FIG. 1 is an enlarged view of an electrode finger portion showing the structure of an embodiment of the present invention.

The surface acoustic wave filter of this embodiment is shown in FIG.
As shown in FIG.
Yes, 36 ° Y-cut X-direction propagation LiTaO₃Board 1
And 36 ° Y cut X direction propagation LiTaO₃On board 1
The formed input electrodes 11a, 11b, 11c, 11d, 1
1e and output electrodes 12a, 12b, 12c, 12d
And its electrode finger E₁, E₂, E₃,…When
Gap between electrode fingers G₁, G ₂, G₃, ... is enlarged in Figure 1.
, The line width of the electrode finger Ei (i = 1, 2, 3, ...)
Lm and the gap width Lg between the electrode finger Ei + 1 and the electrode finger Ei
It is formed so that the ratio (Lm / Lg) is 2.3.
It Although the input electrode side is shown in FIG.
The pole side is also the same.

In the multi-electrode surface acoustic wave filter 10 of the present embodiment constructed as described above, the emission of bulk waves is suppressed, and the number of electrode finger pairs is 18 and the number of electrode finger pairs is 7 and 26 are output electrodes. When the frequency characteristics of the multi-electrode type surface acoustic wave filter 10 composed of 6 pieces were measured, it was as shown in FIG. As is clear from FIG. 2, in the frequency characteristic, the increase in loss disappears in the high frequency region. This is also clear from the comparison with FIG.

Here, the maximum insertion loss in the band of the multi-electrode surface acoustic wave filter 10 when the ratio (Lm / Lg) between the line width Lm of the electrode fingers and the gap width Lg between the electrode fingers is changed is shown in FIG. As shown in FIG. 3, the concave characteristics were exhibited, and it was the smallest when the ratio (Lm / Lg) of the line width Lm of the electrode fingers and the gap width Lg between the electrode fingers was 2.3.

On the other hand, the larger the ratio (Lm / Lg) of the line width Lm of the electrode fingers to the gap width Lg between the electrode fingers is, the more the bulk wave radiation is suppressed, and the attenuation due to the bulk wave radiation is reduced, and the bulk wave radiation is reduced. The flatness of the band characteristic is maintained. However, the ratio of the line width Lm of the electrode fingers to the gap width Lg between the electrode fingers (Lm / L
The larger g), the larger the rate of change of the electrostatic capacity of the electrodes, and the larger the input impedance of the multi-electrode surface acoustic wave filter 10, and the lower the manufacturing yield.

Therefore, the ratio (Lm / Lg) of the line width Lm of the electrode fingers to the gap width Lg between the electrode fingers is 1.2 <(Lm / L
g) When the range is set to <3, the frequency characteristic is almost the same as that of FIG. 2, the flatness of the pass band characteristic is maintained, the maximum insertion loss is suppressed to about 3 dB, and the manufacturing yield is good. It was

The same applies to the case where the electrode fingers are weighted by the apodization method or the tilt type.

[0018]

As described above, according to the present invention, 36
In a multi-electrode type surface acoustic wave filter including a Y-cut X-direction propagation LiTaO ₃ as a substrate and a total of three or more input electrodes and output electrodes formed on the substrate, electrode finger of each of the input electrode and the output electrode is The ratio (Lm / Lg) of the line width Lm to the gap width Lg between the electrode fingers is 1.2 <(Lm
/ Lg) <3, so that the emission of bulk waves is suppressed and the pass band characteristic is improved. Furthermore, the change in the input impedance is small, and the yield is improved.

[Brief description of drawings]

FIG. 1 is an enlarged view of an electrode finger portion showing a configuration of an embodiment of the present invention.

FIG. 2 is a frequency characteristic diagram according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the ratio (Lm / Lg) of the line width Lm of the electrode finger to the gap width Lg between the electrode fingers and the maximum insertion loss in the band in one embodiment of the present invention.

FIG. 4 is a schematic diagram showing a configuration of a multi-electrode surface acoustic wave filter.

FIG. 5 is an enlarged view of an electrode finger portion in a conventional example.

FIG. 6 is a frequency characteristic diagram in a conventional example.

[Explanation of symbols]

1 ... 36 ° Y-cut X-direction propagation LiTaO ₃ substrate 10 ... Multi-electrode type surface acoustic wave filter 11a-11e ... Input electrode 12a-12d ... Output electrode

Claims (1)

[Claims] 1. A multi-electrode type surface acoustic wave filter comprising a 36 ° Y-cut X-direction propagation LiTaO ₃ as a substrate, and a total of three or more input electrodes and output electrodes on the substrate. Ratio (Lm / Lg) of the line width Lm of the electrode fingers and the gap width Lg between the electrode fingers in each electrode
In the range of 1.2 <(Lm / Lg) <3.