JPH0133969B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave resonator, and in particular to a surface acoustic wave resonator that can significantly improve resonance resistance by defining the widths of excitation electrodes and reflective electrodes. be.

For example, in VTR, it depends on the viewer's preference.
A method is used that receives television radio waves sent in the UHF and VHF ranges, records them, and plays the recorded signals on the empty channel of the television equipment, for example, channel 2 in the Kanto region, when necessary. ing. For this reason, it is necessary to oscillate the same carrier wave as the carrier wave of the empty channel, and an RF converter is inevitably installed inside.

Conventionally, this RF converter required a complex circuit consisting of a crystal resonator, a multiplier circuit, an amplifier, a harmonic removal circuit, and a VSB circuit, but with the development of surface acoustic wave resonators, direct oscillation is now possible. It has become possible to obtain an extremely simple RF converter that connects a surface acoustic wave resonator directly to a VSB circuit.

Next, an example of a surface acoustic wave resonator will be explained with reference to FIG.

That is, a pair of common electrodes 2a and 2b are formed on one main surface of a piezoelectric substrate 1 made of LiTaO ₃ , LiNbO ₃ , crystal, etc.
An excitation electrode 2 in which comb-like electrodes 2a ₁ and 2b ₁ are formed so as to mesh with each other, and a pair of common electrodes 3a, 3b and 4 on both sides of this excitation electrode 2, respectively.
It consists of reflective electrodes 3 and 4 arranged in parallel with each other at _a predetermined distance between a _and 4b. The surface waves are reflected by the reflective electrodes 3 and 4 on both sides and fed back to the excitation electrode 2 again to directly oscillate at a predetermined frequency.

However, in a surface acoustic wave resonator having such a structure, the pitch P between the comb-shaped electrodes 2a ₁ and 2b ₁ and between the reflective electrodes 3 and 4 is determined by the frequency ( ) of the resonator and the propagation of the surface wave. It is 1/2 of the wavelength (λ) determined from the velocity (v), but the comb-shaped electrode 2a ₁ ,
The widths of the 2b ₁ and the reflective electrodes 3 and 4 are not particularly defined, and this has been a cause of deteriorating the resonance characteristics of the surface acoustic wave resonator.

The present invention has been made in view of the above-mentioned problems, and provides a surface acoustic wave resonator with extremely good resonance characteristics by establishing a certain relationship between the width of the comb-shaped electrode and the width of each reflective electrode. It is intended to.

First, the equivalent circuit of a surface acoustic wave element will be explained with reference to Fig. 2. It consists of one arm in which an equivalent series reactance (L), an equivalent series capacitance (C), and a resonant resistance (R) are connected in series, and an electrode capacitance (C _T ) is connected to the other arm at terminals 12 ₁ and 12 _2. In a surface acoustic wave resonator, it is necessary to have a large figure of merit (M). , this (M) and each dimension of the equivalent circuit have the following relationship.

M=Q/γ=1/ωC _T R ...(1) γ=C _T /C ...(2) Q=1/wCR=wL/R ...(3) That is, the above 3 equations (1)( The performance of the resonator is evaluated by 2) and (3). Therefore, to make (M) large, (1)
From the formula, it is necessary to make (C _T ) small and (R) small.

Based on this idea, the inventors conducted various experiments and obtained the following results.

First, the surface acoustic wave resonator of the present invention will be explained with reference to FIGS. 3 and 4.

That is, a pair of common electrodes 12 are formed on one main surface of a piezoelectric substrate 11 made of LiTaO ₃ , LiNbO ₃ , crystal, etc.
an excitation electrode 12 formed such that comb-shaped electrodes 12a ₁ and 12b ₁ from a and 12b mesh with each other;
1 provided on both sides of this excitation electrode 12, respectively.
Pair of common electrodes 13a, 13b and 14a, 14b
Reflective electrodes 1 arranged in parallel with a predetermined interval therebetween
3 and 14, the terminals 12 ₁ of the excitation electrode 12,
Only the signal of a predetermined frequency applied between 12 _{and 2} is reflected by the reflective electrodes 13 and 14 on both sides using the surface waves of the piezoelectric substrate 11, and fed back to the excitation electrode 12 again, and directly oscillates at a predetermined frequency. This is almost the same as a conventional surface acoustic wave resonator, but in this embodiment, between the comb-shaped electrodes 12a ₁ and 12b ₁ and between the reflective electrodes 13 and 14.
The pitch P between each electrode is set to 1/2 of the wavelength (λ) of the surface acoustic wave resonator, and the comb-shaped electrode 12
The width of each electrode of a ₁ and 12b ₁ (W _T ), and the width of each electrode of reflective electrodes 13 and 14 (W _G )
It is characterized in that it satisfies 0.54<W _T /P≒W _G /P<0.62.

The width of this electrode was determined based on various experiments conducted by the inventors and the manufacturing method normally used when forming excitation electrodes and reflective electrodes on a piezoelectric substrate. This includes the error range when forming a predetermined electrode using photolithography after depositing it by vapor deposition or the like.

Next, an example of the inventor's experimental results will be explained with reference to FIG. In addition, in the figure, the rate of change is shown based on the case of W _T (W _G )/P=0.5.

That is, assuming that W _T /P and W _G /P are almost the same, 0.38
If the figure changes from 0.66 to 0.66, the figure of
The rate of change in merit (M) peaks at approximately 0.55,
The curve becomes a downward slope on both sides, and the rate of change in the electrode capacitance (C _T ) is W _T /P. As W _G /P increases, the curve rises gently, and the rate of change in the resonant resistance (R) is W _T /P, W _G When /P becomes large, a curve gradually descends. Accordingly
By setting W _T /P and W _G /P to approximately 0.55, (M) can be maximized and the resonance resistance (R) can be lowered, so that a surface acoustic wave element with good characteristics can be obtained.

That is, as shown in Figure 5, the figure of
The merit (M) reaches its maximum when W _T /P≒W _G /P=0.55, and at 0.5, M becomes a monotonically increasing curve. Therefore, among those where M starts to become larger than 100% shown in Fig. 5, those that are sufficiently large (W _T /P≒W _G /P = 0.54) are larger, and the upper limit is when M becomes 100%. % (W _T /P≒
W _G /P=0.62), i.e. 0.54<
By making the width of the excitation electrode and the width of the reflective electrode within a range that satisfies W _T /P≒W _G /P<0.62,
The figure of merit (M) increases, making it easier for the surface acoustic wave resonator to oscillate, and since the resonance resistance (R) can be lowered, it is possible to obtain a surface acoustic wave resonator with extremely good characteristics. It became.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an example of a conventional surface acoustic wave resonator, FIG. 2 is an equivalent circuit diagram of the surface acoustic wave resonator, and FIGS. 3 to 5 are one example of the surface acoustic wave resonator of the present invention. FIG. 3 is a diagram showing an example, and FIG. 3 is a plan view;
Figure 4 is an enlarged view of the main part of Figure 3, and Figure 5 is W _T /
FIG. 3 is a curve diagram showing changes in figure of merit, electrode capacitance, and resonance resistance when the horizontal axis is the value of P·W _G /P and the vertical axis is the rate of change. 1, 11... Piezoelectric substrate, 2, 12... Excitation electrode, 3, 4, 13, 14... Reflection electrode.

Claims (1)

[Claims] 1. An excitation electrode formed such that comb-like electrodes from a pair of common electrodes on one main surface of the piezoelectric substrate mesh with each other, a pair of common electrodes provided on both sides of this excitation electrode, and 1. In a surface acoustic wave resonator comprising at least a pair of common electrodes and reflective electrodes arranged in parallel with each other at a predetermined distance,
Let P be the pitch between the comb-shaped electrodes formed to mesh with each other and between the reflective electrodes arranged in parallel, and let W _T be the width of the comb-shaped electrodes formed to mesh with each other.
When the width of the reflective electrode is W _G , 0.54＜W _T /P≒
A surface acoustic wave resonator characterized by having an electrode width that satisfies W _G /P<0.62.