JPS62275Y2 - - Google Patents

Description

[Detailed explanation of the idea]

The present invention relates to a surface wave filter using lithium tantalate LiTaO ₃ single crystal as the surface wave propagation medium. A video intermediate frequency (hereinafter abbreviated as VIF) filter of a television receiver is usually composed of 5 to 8 LC tuning circuits. As this VIF filter, a surface wave filter that achieves no adjustment and a solid stay has been developed and is being replaced by the above-mentioned LC tuning circuit. Figure 1 shows the surface wave filter. In the figure, 1 is a LiTaO ₃ single crystal plate that propagates surface waves, 2 is a transmitting electrode that excites surface waves when an electric signal is applied, and 3 is a surface wave filter. The transmitting/receiving electrodes 2 and 3 are interdigital electrodes and are arranged on the LiTaO ₃ single crystal plate 1 so as to face each other. 4 is above
The stem 5 on which the LiTaO ₃ single crystal plate 1 is placed is an adhesive made of epoxy resin that adhesively fixes the stem 4 and the LiTaO ₃ single crystal plate 1. In the surface wave filter having such a configuration, it is not only the surface waves that are excited from the transmitting electrode 2;
A small amount of bulk waves are also generated, which propagate in a direction perpendicular to the propagation direction of the surface waves, that is, in the thickness direction of the LiTaO ₃ single crystal plate 1. This bulk wave constitutes a bulk wave resonator that resonates at a frequency determined by the thickness of the LiTaO ₃ single crystal plate 1. When resonance due to bulk waves occurs within the passband of the surface wave filter, ripples appear in the frequency characteristics as shown by the arrows in FIG. This ripple causes a drop in the resolution of images on a television receiver. Considering that the bulk wave is a resonant mode in the thickness direction, the present inventors measured the fundamental resonant frequency of the bulk wave both before and after the application of the adhesive 5. It was discovered that the above fundamental resonance frequency fluctuates depending on the state of application of the agent. That is, in FIG.
The bulk wave excited from the transmitting electrode 2 propagates and resonates not only through the LiTaO ₃ single crystal plate 1 but also through the adhesive 5, as shown schematically showing the resonance state of the bulk wave with and without the LiTaO 3 single crystal plate 1. It is. In view of this point, the surface wave filter of the present invention prevents bulk wave resonance within the pass band of the filter, and specific embodiments thereof will be described in detail below. Generally, the fundamental resonant frequency of the bulk wave in the thickness direction
₀ is ₀ = Vb/λ... (1) where λ: co-frequency wavelength of bulk wave. The above λ is λ ₀ when the adhesive 5 is not included and the thickness of the LiTaO ₃ single crystal plate 1 is t.
= 2t. That is, equation (1) can be rewritten as ₀₁ =Vb/2t...(2). Similarly, when the adhesive 5 is included, and the thickness of the adhesive 5 is Δt, λ ₂ is given by λ ₂ =2(t+Δt), and formula (1) is ₀₂ =vb/2(t+Δt)...(3 ) becomes. Here, the X-cut LiTaO ₃ with thickness t = 0.4 (mm)
Using the single crystal plate 1, the above substrate resonance frequency ₀₁ ,
When I measured ₀₂ , ₀₁ = 5.350 (MHz), ₀₂
=5.284 (MHz) was obtained. In addition, LiTaO ₃ single crystal plate 1
The propagation velocity Vb of the bulk wave determined by the cut plane is Vb=
4280 (m/sec), and the thickness Δt of the adhesive 5 made of epoxy resin was 0.1 (mm). Comparing this experimental result with the results obtained by substituting numerical values into both equations (2) and (3), we get

[Table] becomes. That is, the fundamental resonance frequency ₀₂ including the adhesive 5 is
It decreases compared to ₀₁ , but as shown in equation (3), ₀₂ does not change and decreases only slightly. This is thought to be due to the difference in the materials, mainly hardness, between the LiTaO ₃ single crystal plate 1 and the adhesive 5, and the inventors of the present invention determined the thickness of the adhesive 5.
It is recommended to use a value converted to the thickness of the LiTaO ₃ single crystal plate 1. This converted value and
The effective sum of the thickness t of LiTaO ₃ single crystal plate 1 is
If the value of LiTaO ₃ single crystal plate 1 is Teff, then this Teff
is determined by Teff=t+cΔt. However, c
is a correction coefficient that changes depending on the material of the adhesive 5. Therefore, the fundamental resonance frequency ₀₂ including the adhesive 5 is ₀₂ =Vb/2Teff=Vb/2(t+cΔt
)……(4) is given. Therefore, the correction coefficient c is obtained as c=1/Δt(Vb/2 ₀₂ −t) (5). In order to obtain the above c, the thickness Δt of the adhesive 5 was varied and the fundamental resonance frequency ₀₂ was measured.
The results and the calculated value of c from equation (5) are shown below.

[Table] Therefore, the effective thickness of LiTaO ₃ single crystal plate 1
Teff can be found as Teff=t+0.05Δt. Next, in order to investigate bulk wave resonance in more detail, the present inventors measured the reflection characteristics of surface waves using a network analyzer, and found that strong and weak resonances periodically alternated, as shown in Figure 4. It turned out that this was happening. Considering this resonance, the strong resonance mentioned above is because the odd-order harmonics all have the same phase and overlap each other as they travel back and forth within the propagation medium, and the weak resonance is because the even-order harmonics have opposite phases and cancel each other out. Conceivable. The strong resonance frequency n at which the above strong resonance occurs is determined from equation (4): n=(2n+1) ₀₂ =(2n+1)Vb/2T
eff...(6) However, n: 0, 1, 2,... is expressed. Here, if the lowest frequency of the passband of the filter is l and the highest frequency is h, then in order to prohibit the strong resonance frequency n from entering the passband of l and h, the LiTaO ₃ unit must be set so that the following formula is satisfied. What is necessary is to determine the effective thickness Teff of the crystal plate 1. n=(2n+1)Vb/2Teff<l...
(7) n=(2n+3)Vb/2Teff>h...
(8) Therefore, Teff is (2n+1)Vb/2l<Teff<(2n+3)Vb/
If it is set within the range of 2h...(9), bulk wave resonance within the passband of the filter can be suppressed. To show specific numbers, the filter passband of the VIF filter is l = 52.78 (MHz), h = 60.22
(MHz) and n = 5, equation (9) becomes 446 (μm) < Teff < 462 (μm), and the thickness t of the LiTaO ₃ single crystal plate 1 is the thickness Δt of the adhesive 5, Δt = 100 (μm). ), then 441 (μm)<t<457 (μm). As is clear from the above explanation, the present invention
Conditional expression (2n+1)Vb/2l<Teff<(2n+3) is given as an effective value for the thickness of the LiTaO ₃ single crystal plate considering the thickness of the adhesive.
By satisfying Vb/2h, it is possible to prevent the occurrence of bulk wave resonance within the passband of the filter, suppress ripples in the frequency characteristics, and obtain a surface wave filter with excellent frequency characteristics. .

[Brief explanation of the drawing]

Figure 1 is a perspective view of a basic surface wave filter, Figure 2 is a schematic diagram explaining bulk wave resonance, Figure 3 is a frequency characteristic diagram of the surface wave filter affected by the bulk resonance, and Figure 4. 1 is a reflection characteristic diagram illustrating the influence of harmonics of bulk wave resonance. 1 is a LiTaO ₃ single crystal plate, 2 and 3 are transmitting/receiving electrodes, 4 is a stem, and 5 is an adhesive, respectively.

Claims (1)

[Claim for Utility Model Registration] A surface wave in which a lithium tantalate LiTaO ₃ single-crystal plate with transmitting and receiving electrodes arranged on its surface for transmitting and receiving surface waves is adhesively fixed to a stem using an adhesive made of epoxy resin. In the filter, the above
The thickness of the LiTaO ₃ single crystal plate is t, and the thickness of the adhesive is Δ
The effective thickness Teff of the LiTaO ₃ single crystal plate given by t+0.05Δt is given by the following formula (2n+1)Vb/2l<Teff<(2n+3)Vb
/2h However, n: 0, 1, 2,... l: The lowest frequency of the filter passband h: The highest frequency of the filter passband Vb: Satisfies the propagation speed of the bulk wave propagating in the thickness direction of the LiTaO ₃ single crystal plate A surface wave filter characterized by being set within a range that allows