JPH01123515A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To make the band of a low loss filter wide by specifying a product of a surface acoustic wave reflecting quantity per electrode finger and the logarithm of the electrode finger constituting an interdigital converter. CONSTITUTION:Assuming surface acoustic wave reflecting quantity per one of electrode fingers 303 and 304 constituting an interdigital converter 3 as epsilon, and the logarithm of the interdigital converter 3 as N, then the interdigital converter 3 is constituted so as to satisfy epsilonN>=15. Thus, the reflection of the surface acoustic wave in electrode in the electrode fingers 303 and 304 constituting the interdigital electrode is actively used, two different electrodes of the electrode fingers 303 and 304 constituting the interdigital electrode are made into a pair, the logarithm is enlarged extremely, and thus, the impedance of the interdigital converter 3 can be extremely minimized over a wide frequency band. Thus, a low loss filter is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a surface acoustic wave device, and particularly to the structure of an interdigital transducer thereof.

(Prior Art) Generally, TV-IF filters and the like use surface acoustic wave devices. However, it was configured using this surface acoustic wave device. Filters have a large insertion loss.

Hearing is becoming a problem.

Therefore, conventional methods of configuring filters with low insertion loss have been devised. As a representative example, a method is known in which a surface acoustic wave resonator is constructed and a circuit is constructed using the surface acoustic wave resonator in a ladder type or lattice type, similar to a crystal filter using a crystal resonator.

This method utilizes the fact that the surface acoustic wave device exhibits inductive reactance between the resonant frequency (hereinafter referred to as fr) and the anti-resonance frequency (hereinafter referred to as fa), and in the filter, the Q Acts as a high inductance. However, since the frequency interval between fr and fa (hereinafter referred to as Δf) of a surface acoustic wave resonator generally does not have a large value, it is difficult to construct a wideband filter.

That is, Δf of the surface acoustic wave resonator can be approximated by Δf:=, fr-, where r is the capacitance ratio of the surface acoustic wave strong vibration element. On the other hand, surface acoustic wave 2
r π2 ), and there is naturally a limit to widening the band if r≧8.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned problems, and provides a surface acoustic wave device in which a low-loss filter has a wider band.

(Structure of the Invention) (Means for Solving the Problems) In order to achieve the above-mentioned object, the surface acoustic wave device of the present invention has the following features: The basic gist is that this interdigital converter satisfies εN2, where ε is the logarithm of this interdigital converter and N is the logarithm of this interdigital converter.

(Function) With the above-described configuration, the surface acoustic wave device of the present invention actively utilizes the reflection of surface acoustic waves on the electrode fingers constituting the interdigital electrodes, and the surface acoustic wave device of the present invention actively utilizes the reflection of the surface acoustic waves on the electrode fingers constituting the interdigital electrodes. By forming a pair of poles and making the logarithm extremely large, the impedance of the interdigital converter can be made extremely small over a wide frequency band, and this is used to construct a low-loss filter.

(Example) Hereinafter, an example of the surface acoustic wave device of the present invention will be described with reference to FIG.

In FIG. 1, a one-port surface acoustic wave resonator (meaning one in which one interdigital transducer is formed) has a piezoelectric substrate, for example, a (110) cut L+ 2 B.
A pair of reflectors (A) and (A) are formed on the 4O7 substrate (2) so as to sandwich the interdigital converter (2). This interdigital converter (■) is made of aluminum and has comb-shaped electrodes (301) with different polarities,
(302) intersect with each other at 0.02λ. The comb-shaped electrodes (301) and (302) each have 3
It is composed of 00 electrode fingers (303), (304), which are adjacent to each other and have different polarities (303), (
304), this interdigital converter (2) has 300 pairs of logarithms. The difference width of this interdigital converter (3) is 1λ, and the line width of its electrode fingers (303) and (304) is 4. It is a solid electrode. On the other hand, the structural material of the reflector is aluminum, and its film thickness is 0.02λ. Then, the grid electrode fingers (4
The line width a of 01) is d, and the grid electrode finger (401)
The distance between k and the reflector is formed by photolithography.

It is constructed by electrically connecting a plurality of surface acoustic wave resonators (1) in series.

By the way, when the surface acoustic wave reflection amount of the electrode fingers constituting the interdigital electrode cannot be ignored, as the logarithm of this interdigital converter increases, the stop band (band with high surface acoustic wave reflectance of the electrode fingers) increases. ) A steep resonance occurs near the lower end frequency. (Research and Practical Application Report Vol. 27, No. 8 (1978), pp. 35-4
(See “A single-terminal pair surface acoustic wave resonator using a multi-pair IDT and its application to a narrowband filter” on page 9)
Based on this technique, it has been found that when the logarithm of the interdigital converter is further increased, in addition to the above-mentioned resonance, an extremely low impedance region is generated in the band corresponding to the stop band.

Regarding this, the impedance characteristics of the surface acoustic wave device in the embodiment shown in FIG. 1 will be explained with reference to FIG. In FIG. 2, there is a steep resonance at the lower end of the stop band 0 of the impedance characteristic curve QO, but independently of this there is a region with extremely low impedance over a wide range near the stop band (support). According to the inventor's experiments (hereinafter referred to as this phenomenon), this phenomenon did not exist when the number of interdigital converters was 250 pairs or less.

The above phenomenon appears in interdigital converters.
It depends on the product of the surface acoustic wave reflection amount of the electrode fingers (303) and (304) constituting the electrode fingers and the logarithm of the interdigital converter. For example, in the embodiment shown in FIG. 1, the electrode fingers (303) and (30
4) The reflectance per one is 0.05, and the logarithm is 300 pairs, so the product is 15. This phenomenon occurs when the product is approximately 15 or more. That is, if the amount of surface acoustic wave reflection per electrode finger constituting an interdigital converter is ε, and the logarithm of this interdigital converter is N, this phenomenon occurs when εN≧15. For example, the reflectance of one electrode finger of an interdigital converter is 0.01.
This phenomenon also occurs in the case of an interdigital converter with 1500 logarithmic pairs. The inventor believes that this phenomenon occurs because a plurality of resonant circuits are closely coupled to each other and connected in parallel.

Conventionally, surface acoustic wave resonators having multi-pair interdigital transducers have been reported, but there are no examples in which the number of logarithms is extremely increased compared to the amount of reflection per electrode finger of the interdigital transducer as in the present invention. This phenomenon was not known to occur.

However, in the surface acoustic wave resonator shown in Fig. 1, there is a frequency range with extremely low impedance apart from the resonance point, so it is not possible to construct a filter by inserting these surface acoustic wave resonators in series. can. The passband width at this time is determined by the amount of reflection per electrode finger of the interdigital converter (that is, the frequency width of the stop band), so the electrode film thickness should be adjusted so that the amount of reflection becomes thicker with a wideband filter. It is sufficient to design the substrate to be thicker or to have deeper grooves (groups) in the substrate between the electrode fingers of the interdigital transducer. On the other hand, in the case of a narrowband filter, the amount of reflection may be small and the logarithm of the interdigital converter may be designed to be sufficiently large. Further, in order to increase the amount of attenuation sufficiently, it can be realized by connecting a plurality of these surface acoustic wave resonators in series in multiple stages.

In addition, in the above-mentioned example, the method of forming an aluminum electrode on the L+ 2 B4O7 substrate was described, but the L+
The aluminum strip (reflector) on the 28407 substrate has a high reflection efficiency and is therefore extremely effective in implementing the present invention. However, even on a quartz substrate with relatively excellent temperature characteristics, it is possible to construct an interdigital converter with aluminum thin film electrodes of sufficient thickness, or to remove the quartz substrate between the electrode fingers of the aluminum electrode using etching technology. It can be used sufficiently if grooves (groups) or the like are formed to increase the amount of reflection. Further, the amount of reflection can be increased by forming a reflective strip made of dielectric or the like on an interdigital transducer made of aluminum electrodes or between two or more interdigital transducers.

Furthermore, although the above example uses Rayleigh mode surface acoustic waves, the present invention can also be applied to surface acoustic wave devices using pseudo surface waves, 5SBW, SH waves, and the like. Roughly speaking, in the above-mentioned embodiments, a plurality of surface acoustic wave resonators were described, but the present invention is not limited to these, and may be a single surface acoustic wave resonator.

〔Effect of the invention〕

The surface acoustic wave device of the present invention has the effect of being able to function as an interdigital loss filter.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway schematic plan view for explaining the present invention in detail, and FIG. 2 is a characteristic curve diagram for explaining the present invention in detail. ■...Surface acoustic wave resonator■...L+ 2 B4O7 substrate (piezoelectric base)■...
Interdigital converters (301), (302)...comb-like electrodes (303)
), (304)...Electrode agent patent attorney rules
Ken Chika Yudo Kikuo Takehana

Claims (1)

[Claims] (1) In a surface acoustic wave device comprising a piezoelectric substrate and an interdigital transducer formed by intersecting interdigitated electrodes made up of a plurality of electrode fingers arranged on the main surface of the piezoelectric substrate, the electrode fingers 1 The true amount of surface acoustic wave reflection is ε, and the logarithm of the electrode fingers constituting the interdigital converter is N.
A surface acoustic wave device, wherein: the interdigital converter satisfies εN≧15.