JP3175830B2 - One-way SAW filter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a SAW filter, and more particularly to a low-loss SAW filter using an internal reflection type unidirectional IDT having a floating electrode.

(Prior Art) Conventionally, a transversal SAW filter has been widely used as a wideband low-loss filter in a high frequency range of a VHF to UHF band.

However, transversal SAW filters have the disadvantage that insertion loss increases due to bidirectional loss.
In order to reduce the loss, it is necessary to use a unidirectional IDT.

There are three widely used methods for this one-way IDT.
There is a one-way IDT and a group-type one-way IDT.

However, the former requires a complicated phase shifter to create the three phases, and the pattern formed on the piezoelectric substrate has a bridge-over part, making it difficult to manufacture. The latter also requires a 90 ° phase shifter. In addition, there is a defect that it is virtually impossible to cope with an increase in the IDT logarithm when a narrow band filter is required due to an ohmic resistance based on a long meander line grounded.

On the other hand, the internal reflection type one-way ID that allows the wave to travel in one direction using the reflection inside the electrode of the excited SAW
T has the advantage of not requiring an external phase shifter, but one side of the IDT made of double electrodes is made of aluminum (Al) and the other is made of gold (Au) to make it heavier. The center position of the reflection is shifted so that the reflected wave has the same phase in the forward direction of the SAW and has the opposite phase in the backward direction to obtain unidirectionality. Therefore, the number of manufacturing processes increases and accurate electrode alignment is required. There was a disadvantage that the production yield was poor.

On the other hand, Yamanouchi et al. Can obtain unidirectionality by inserting an open or short-type floating electrode between the positive and negative electrode fingers of the IDT and shifting the position of the floating electrode from the center of the positive and negative electrode fingers. I found something. (Literature-Technical Report of the Institute of Ultrasonics, IEICE, US84-18, p.95 (1984).) Yamanouchi et al.
It has been shown that the simultaneous insertion of the open floating electrode 3 and the short-circuit floating electrode 4 between the positive and negative electrode fingers 2, 2 'of the IDT enhances the unidirectionality and is effective in reducing the loss.

The internal reflection type unidirectional IDT with this floating electrode is
Like the AW filter, there is an advantage that the electrode can be formed by only one evaporation and photo-etching process and the manufacturing is easy.However, when the IDT logarithm is reduced, the reflection effect is weakened and the unidirectionality is deteriorated, and the insertion loss is increased Has the disadvantage of becoming
It was still insufficient for use in broadband low-loss filters.

(Object of the Invention) The present invention has been made to solve the above-mentioned drawback that when the logarithm of the internal reflection type unidirectional IDT having a floating electrode is reduced, the unidirectionality is weakened and the insertion loss is increased. It is another object of the present invention to provide a low-loss SAW filter having a wide band in a high frequency region by further strengthening the unidirectionality.

(Summary of the Invention) In order to achieve the above object, in the present invention, the line width h of the floating electrode of the internal reflection type unidirectional IDT having the floating electrode is described.
The ratio h / p of the electrode pitch p to the open floating electrode
The configuration is 0.6 to 0.8, and the short-circuit type floating electrode is 0.1 to 0.3.

Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings.

Prior to the description of the embodiment, the configuration and characteristics of a conventional internal reflection type unidirectional IDT by Yamanouchi will be described a little in order to facilitate understanding of the present invention.

FIG. 2 is a view showing the structure of the above-mentioned conventional internal reflection type unidirectional IDT, in which an open floating electrode 3 and a short type floating electrode are interposed between positive and negative IDT electrode fingers 2, 2 'formed on the surface of a piezoelectric substrate 1. The electrodes 4 are arranged, and the line width of each electrode and the width of the space between the electrodes are all the same, that is, both the line width and the space width are λ / 1.
2 (λ is the wavelength of the excited SAW).

The internal reflection type one-way ID configured as above is 128 ゜ Y
Cut was excited to form a LiNbO ₃ substrate of X-propagating
Most of the SAW propagates in the direction indicated by 5 in the figure. Generally, the direction indicated by 5 is referred to as “forward direction”, and the direction indicated by 6 on the opposite figure is referred to as “reverse direction”.

By the way, as a result of experimentally producing a filter using the internal reflection type one-way IDT having such a configuration as an input / output IDT and examining its characteristics, as shown in FIG. 3, a ripple of about 0.4 dB was generated in a pass band.

This is considered to be due to the triple transit eco (TTE) of the internal reflection type unidirectional IDT used in this experiment, and as described above, the unidirectionality of SAW propagation was still insufficient. .

In order to solve this problem, first, when the ratio h / p of the line width h of the IDT electrode finger or the reflector to the electrode pitch (inter-electrode center distance between adjacent electrodes) p is changed, the reflection of the propagating wave is reduced. The literature on the behavior of the so-called mode coupling coefficient K ₁₂ P indicating the degree is shown in the literature (Takeuchi et al., Proceedings of the Acoustical Society of Japan, 2-2-12, p.645, March 1985). Reference is made to the figure (FIG. 4 herein).

As is clear from this figure, for an open electrode, h / p, in other words, when the electrode line width h is larger than the interelectrode space, | K ₁₂ P | becomes larger, and the wave reflection is increased accordingly. It has been shown.

On the other hand, for the short-circuit type electrode, it can be seen that the smaller the electrode line width h, the stronger the wave reflection. If the above-mentioned facts are applied to the above-mentioned conventional internal reflection type unidirectional IDT, if the line width of the open floating electrode is large and that of the short floating electrode is small, the unidirectionality of wave propagation will be improved. It may be possible to make it even bigger.

Therefore, as shown in FIG. 1, the 128 ° Y-cut LiNbO ₃ substrate 1
IDT electrode fingers 2, 2 'are formed of Al on the surface, and the line width of the positive and negative electrode fingers is λ / 12, and the line width R is 3 μm (≒ 0.109λ) at the same pitch (λ / 6) between them. A filter having an IDT as an input / output IDT in which an open floating electrode 7 and a short-circuit floating electrode having a line width S = 1 μm (≒ 0.036λ) are arranged was manufactured, and its frequency transmission characteristics were measured.

The IDT electrode fingers 2, 2 'were not weighted, the cross width was 50λ, and the Al electrodes were all 2% λ.

FIG. 5 is a diagram showing the frequency transmission characteristics of the filter having the input / output IDT of the internal reflection type one-way IDT according to the present invention manufactured as described above.
In comparison with FIG. 3 showing the characteristics of the filter using the IDT, first, the passband is flat and about 0.2 V, which is recognized in FIG.
It can be seen that the ripple of 4 dB has disappeared and the unidirectionality of the wave propagation has been improved.

Also, comparing the insertion loss of the filter with that of the conventional filter was 3.2 dB, while that of the present invention was improved to 2.6 dB, which also supports the improvement of the unidirectionality of the wave propagation.

In order to clearly determine the degree of unidirectionality of wave propagation,
The dashed line shown in FIG. 5 (similarly in FIG. 3) was obtained by manufacturing the conventional filter described above and the filter according to the present invention in which the direction of the unidirectional IDT was reversed and their frequency transmission characteristics were examined. It is.

From this result, the difference in insertion loss between the case where the unidirectional IDT faces in the correct direction and the case where the unidirectional IDT faces opposite is about 13 dB in the conventional filter, whereas in the filter according to the present invention. Showed about 17 dB.

This clearly indicates that the unidirectionality of the SAW propagation excited by the internal reflection type unidirectional IDT according to the present invention is greatly improved as compared with the conventional one. The effectiveness of one-way IDT is supported.

The required specification (spec) is to determine the optimal value of the line width of the open / short floating electrode.
In consideration of FIG. 4, the electrode thickness H / λ = 2%, and for the open type, h / p = 0.6 to
0.8, h / p = 0.1-0.3 for short-circuit type.

The input and output IDTs have been described above with reference to a normal type SAW filter. However, the present invention can be applied to a weighted IDT.
In a transducer type filter, a bidirectional weighted IDT may be used at the center, and a unidirectional IDT according to the present invention may be used at both sides.

Further, the present invention is not limited to a 128 ° Y-cut LiNbO ₃ substrate but also to other substrates having a similar relationship, for example, an X-cut LiNbO ₃ substrate.
゜ It is also applicable to a LiTaO ₃ substrate or the like propagating in the Y direction.

(Effect of the Invention) Since the present invention is configured as described above, there is a remarkable effect in obtaining a low-loss SAW filter in a wide band in a high frequency region.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electrode configuration of an internal reflection type unidirectional IDT having a floating electrode according to the present invention, FIG. 2 is a diagram showing an electrode configuration of a conventional internal reflection type unidirectional IDT having a floating electrode, Third
The figure shows the frequency transmission characteristics of the SAW filter using the IDT shown in FIG. 2, FIG. 4 shows the relationship between the line width and the inter-mode coupling coefficient used for consideration of the present invention, and FIG. FIG. 5 is a diagram illustrating frequency transmission characteristics according to an embodiment of the SAW filter according to the present invention. 1 ... Piezoelectric substrate, 2,2 '... Positive and negative IDT electrode fingers, 3,7 ... Open floating electrode, 4,8 ... Short floating electrode.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References Kazuhiko Yamanouchi, Hiromi Furuyashiki "Surface acoustic wave filter using internal reflection interdigital transducer-directional transducer" IEICE Technical Report, US84-18, PP. 95-100 Masao Takeuchi, Kazuhiko Yamanouchi "Construction of Surface Acoustic Wave Elements with Positive and Negative Reflection Coefficients" Proceedings of the Acoustical Society of Japan, PP645-646.

Claims (1)

(57) [Claims] An input / output digital transducer (IDT) electrode for exciting and receiving a surface acoustic wave (SAW) is disposed on a 128 ° Y-cut X-propagating LiNbO ₃ substrate, and at least one of the IDT electrodes has a positive / negative polarity. Two floating electrodes are provided at the same electrode pitch between the two electrode fingers, one of which is an open floating electrode and the other is a short-circuit floating electrode in which adjacent electrodes are short-circuited. In a SAW filter using a reflective unidirectional IDT, the IDT electrode and the floating electrode are made of aluminum,
The ratio h / p between the line width h of the floating electrode and the electrode pitch p
A SAW filter using an internal reflection type unidirectional IDT having a floating electrode, wherein the floating electrode is set to 0.6 to 0.8 and the short-circuit type floating electrode is set to 0.1 to 0.3.