JPH025328B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a surface acoustic wave device, and in particular to an improvement for obtaining a broadband unidirectional transducer for surface acoustic waves that has good characteristics and is easy to manufacture. Conventional surface acoustic wave unidirectional transducers include (a) those that use a reflector, (b) those that use a 120° phase shifter, and (c) those that use a 90° phase shifter to achieve unidirectionality. There are some that use However, in a unidirectional transducer using a reflector (b), the characteristics of the reflector greatly contribute to the characteristics of the transducer, so it becomes unidirectional only at a predetermined frequency and has a narrow band. Becomes the property of
Furthermore, since the reflector portion requires a fairly large area, the piezoelectric substrate becomes large, resulting in high cost and complicated manufacturing. Next, (b) a unidirectional transducer using a 120° phase shifter has a phase shifter of 0°, 120°, and
In order to intersect the electrodes P ₁ , P ₂ and P ₃ having a phase of 240°, a gap S must be provided at the intersection. There are also devices in which the void portion is replaced with an insulating film, but in either case, the manufacturing is very complicated, and the yield is low and the cost is high. Furthermore, the unidirectional transducer (c) using a 90° phase shifter has a narrow band because it becomes unidirectional only at a predetermined frequency. Moreover, it has the disadvantage that it is extremely difficult to realize a broadband 90° phase shifter. The present invention has been made in order to improve the above-mentioned drawbacks of the prior art, and provides a surface acoustic wave device having at least two sets of electrode sections each consisting of one or more pairs of electrode pieces that intersect with each other. is shifted in the surface acoustic wave propagation direction by 1/4 of the wavelength of the surface acoustic wave at the center frequency, and one electrode piece of each of the electrode parts contacts along the electrode cross width and is commonly grounded, and the other electrode It is an object of the present invention to provide a unidirectional and broadband surface acoustic wave transducer by connecting one piece to a phase shifter that provides a phase difference having predetermined frequency characteristics. The present invention will be described below with reference to embodiments shown in the drawings. In FIG. 2, 1 is a piezoelectric substrate, 2 and 3 are electrode parts disposed on the piezoelectric substrate, 4 is a phase shifter, 5 is a signal source. The electrode parts 2 and 3 are composed of a plurality of pairs of electrode pieces 6 to 9 that intersect with each other and are made by vapor-depositing metal such as aluminum on a piezoelectric substrate, and one electrode piece 6 and 7 of each electrode part has the electrode crossing width. b and are commonly grounded, and both electrodes are shifted in the surface acoustic wave propagation direction by 1/4 (λ _p /4) of the wavelength of the surface acoustic wave at the center frequency. A phase shifter 4 is connected between the other electrode pieces 8 and 9 to provide a phase difference having predetermined frequency characteristics, for example, a phase difference of φ( ) below. φ()=−2π(N+1/4)/ _p +2nπ where N is the number of pairs of electrode pieces of the electrode portions 2 and 3, and in the above embodiment, N=2. _p is the center frequency of the signal frequency, and n is an integer. Surface acoustic waves generated by applying frequency input signals to the electrode parts 2 and 3 propagate in the left and right directions on the piezoelectric substrate 1. For example, surface acoustic waves propagating to the right are It is a combination of waves from parts 2 and 3. Therefore, in this case, the electrode part 2
Whether the combined waves strengthen each other or cancel each other is determined depending on whether the phase difference between the waves from the electrode section 3 and the waves from the electrode section 3 is 0° or 180°. As mentioned above, the electrode parts 2 and 3 are arranged with a shift _of λ _p /4 (corresponding to a phase difference of 90°).
is the wavelength at the center frequency, so if the frequency of the input signal deviates from the center frequency _p ,
As it is, the deviation between electrode parts 2 and 3 is λ/4, so
This does not correspond to a phase difference of 90°. That is, in the transducer shown in FIG. 2, the distance l between the electrode parts 2 and 3 is: l=(Nλ ₀ +λ ₀ /4) The phase angle (transition angle) θ corresponding to this distance l is: θ=kl= 2π/λl (k is the wave number of the surface acoustic wave, λ is its wavelength) From the above equations for θ and l, θ(f) = 2πλ ₀ /λ (N + 1/4) = 2π (N + 1/4)
f/f ₀ It is clear from the above equation of θ(f) that the phase angle between the electrode parts 2 and 3 shows frequency characteristics. However, if the electrical phase condition is changed by the phase shifter so as to follow the above φ() according to the frequency of the input signal, the above θ(f) becomes the phase difference φ(f) of the phase shifter 4. The first term in the equation -2π(N+1/4)f/
Since it is canceled out by f ₀ , the phase difference of the husband's surface acoustic waves traveling in one direction over a wide band is set to 0°.
Furthermore, the phase difference between each surface acoustic wave traveling in the other direction can be set to 180°. Therefore, a broadband unidirectional transducer can be realized. Figure 3 shows N=2 and n=2 in the equation of φ().
This is a graph of the phase difference φ versus / _p when: A phase shifter with such frequency characteristics is generally easy to design. FIG. 13 shows an example of the configuration of the phase shifter 4. In the same figure, L, C ₁ and C ₂ are inductance coils and capacitors that constitute the phase shifter, and C ₁ , L,
Assuming that the susceptances corresponding to C ₂ are g ₃ (=ω ₀ C ₁ ), g ₂ (=1/ω ₀ L), and g ₁ (=ω ₀ C ₂ ), these g ₁ , g ₂ , and g ₃ are As a condition for determination, if the impedance Zin seen from the signal source 5 to the phase shifter 4 is matched at the center frequency f ₀ , then Zin=g ₁ +g ₂ +B _T −jG _T /(g ₂ +g ₃ )G _T +j{(
g ₂ +g ₃ )B _T +g ₁ g ₂ +g ₂ g ₃ +g ₃ g ₁ }=Rg(1) From equation (1), the following condition is obtained. g ₁ +g ₂ +B _T =0 (2) (g ₂ +g ₃ )G _T =0 (3) G _T =−Rg [(g ₂ +g ₃ )B _T +g ₁ g ₂ +g ₂ g ₃ +g ₃ g ₁ ] (4) However, Y _T = G _T + jB _T (5) From (1) to (5), g ₁ = ±√ _T −B _T (6) g ₂ = ±√ _T (7) g ₃ = ±√ _T (8) From (8)

[Formula] Therefore

【formula】 From (7)

[Formula] Therefore

【formula】 From (6)

[Formula] Therefore Thus, from (9) to (11), the values of the capacitor and inductance may be determined as follows. However, _p is the center frequency, Y _T is the equivalent admittance of the transducers 2 and 3, G _T and B _T are the equivalent conductance and equivalent susceptance at the center frequency _p , respectively, and R _g is the internal resistance of the signal source 5. Note that the two electrode parts constituting the interdigital transducer do not have to be of the same shape, or they can be apotized with the cross width b of the electrode pieces changed according to the position x, as shown in FIG. The electrode structure may be a chirp electrode structure in which the electrode spacing a is varied depending on the position x as shown in FIG. 12. Furthermore, FIGS. 4 to 10 show other embodiments of the present invention. The embodiment shown in FIG. 4 has a piezoelectric film 1 on a piezoelectric substrate 10.
1 is deposited, and the electrode portions 12 and 13 described above are provided. In the embodiment shown in FIG. 5, the above-mentioned electrode parts 12 and 13 are arranged on a piezoelectric substrate 10, and a piezoelectric film or a non-piezoelectric film 14 is deposited thereon. The embodiment shown in FIG. 6 includes a semiconductor substrate 14, an insulating film 15,
The above-mentioned electrode parts 12 and 13 are arranged on a laminate consisting of a piezoelectric film 16 and a piezoelectric film 16. In the embodiment shown in FIG. 7, the electrode portion 1 is
2 and 13 are disposed on an insulating film 15, and a piezoelectric film 16 is deposited thereon. The embodiment shown in FIG. 8 includes a semiconductor substrate 14, an insulating film 1
8. The above-mentioned electrode parts 12 and 13 are arranged on a laminate consisting of a metal film 19 and a piezoelectric film 20. In the embodiment shown in FIG. 9, the electrode parts 12 and 13 are arranged on the insulating film 18, and the piezoelectric film 20 and the metal film 19 are deposited thereon. In the embodiment shown in FIG. 10, the electrode parts 12 and 13 described above are arranged on a piezoelectric substrate 21, and a piezoelectric film or a non-piezoelectric film 22 and a metal film 23 are deposited thereon. As is clear from the above description, according to the present invention, a unidirectional and broadband transducer can be provided, and the practical effects are great.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing a conventional surface acoustic wave unidirectional transducer, Fig. 2 is a schematic diagram showing an embodiment of the present invention, and Fig. 3 shows the frequency characteristics of a phase shifter in the embodiment. 4 to 10 are schematic sectional views showing other embodiments of the present invention, FIG. 11 is a schematic diagram showing an apotizing electrode, and FIG. 12 is a schematic diagram showing a chirp electrode. FIG. 13 is a circuit diagram showing an example of the configuration of a phase shifter. 1... Piezoelectric substrate, 2, 3... Electrode part, 4...
Phase shifter, 5...signal source, 6-9...electrode pieces.

Claims (1)

[Scope of Claims] 1. In a surface acoustic wave device in which at least two sets of electrode sections each consisting of one or more pairs of intersecting electrode pieces are disposed, the electrode sections are set at 1/1/2 of the wavelength of the surface acoustic wave at a center frequency. 4 in the surface acoustic wave propagation direction, one electrode piece of each of the electrode parts contacts along the electrode crossing width, is commonly grounded, and the other electrode piece is connected to a phase difference having a predetermined frequency characteristic. φ(f)=-2π(N+1/4)f/ _f0 +2nπ(N: number of electrode pieces, f: input signal frequency, _f0 : center frequency,
A surface acoustic wave device characterized in that it provides a phase difference (n: an integer). 2. The surface acoustic wave device according to claim 1, wherein a piezoelectric film is deposited on a piezoelectric substrate, and the electrode portion is disposed on the piezoelectric film. 3. The surface acoustic wave device according to claim 1, wherein the electrode portion is disposed on a piezoelectric substrate, and a piezoelectric film or a non-piezoelectric film is deposited thereon. 4. The surface acoustic wave device according to claim 1, wherein a semiconductor substrate, an insulating film, and a piezoelectric film are sequentially laminated, and the electrode portion is disposed on the piezoelectric film. 5. The surface acoustic wave according to claim 1, characterized in that an insulating film is laminated on a semiconductor substrate, the electrode portion is disposed on the insulating film, and a piezoelectric film is deposited thereon. Device. 6. The surface acoustic wave device according to claim 1, wherein an insulating film, a metal film, and a piezoelectric film are sequentially laminated on a semiconductor substrate, and the electrode portion is disposed on the piezoelectric film. 7. Claim 1, characterized in that an insulating film is laminated on a semiconductor substrate, the electrode portion is disposed on the insulating film, and a piezoelectric film and a metal film are sequentially deposited thereon. surface acoustic wave device. 8. The surface acoustic wave device according to claim 1, wherein the electrode portion is disposed on a piezoelectric substrate, and a piezoelectric film, a non-piezoelectric film, and a metal film are sequentially deposited thereon. 9. The surface acoustic wave device according to any one of claims 1 to 8, wherein both electrode portions have the same shape. 10. The surface acoustic wave device according to any one of claims 1 to 8, wherein both the electrode portions have different shapes. 11. The surface acoustic wave device according to any one of claims 1 to 8, characterized in that the crossing width of the electrode pieces is changed. 12. The surface acoustic wave device according to any one of claims 1 to 8, characterized in that the spacing between the electrode pieces is changed.