JPS61157112A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To decrease variance in the oscillated frequency by forming an electrode constituted to generate a static capacitance on a substrate and connecting the static capacitance of the electrode in parallel with an interdigital converter electrically so as to suppress the reduction in the capacitance ratio. CONSTITUTION:The interdigital converter 2 where two comb tooth electrodes 2a, 2b are combined together is provided on the piezoelectric substrate 1 made of LiB4O7 and a parallel capacitance correcting electrode 4 providing comb tooth electrodes 4a, 4b as a pair in parallel with both sides of the converter 2 is connected in parallel with the interdigital converter 2 electrically, An electrode line width (d) and a pitch (p) of the parallel capacitance correcting electrode 4 are selected nearly equal to grating electrodes 3a, 3b. Thus, the variance of the oscillating frequency due to the variance in the constant of the oscillating circuit is reduced by using a piezoelectric substrate having a small dielectric factor, especially an LiB4O7 substrate having a large electromechanical coupling coefficient also to some degree, increasing the capacitance ratio (gamma) and decreasing the resonance/antiresonance frequency difference.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a surface acoustic wave resonator in which variations in oscillation frequency due to impedance variations in an oscillation circuit are reduced.

[Technical background of the invention and its problems] Conventionally, as a one-point surface acoustic wave resonator, as shown in FIG.
, 2b are inserted therein, and grating reflectors 3a and 3b are provided on both sides of the interdigital converter 2, which is generally used. L is a lead wire, and king is a terminal.

The interdigital transducer 2 is formed of a conductive film such as Al1, and the grating reflector is similarly formed of a conductive thin film such as Al1 or a group of grooves on the substrate. is Li TaO3, Li
NbO3, crystal, etc. are used.

Among the above piezoelectric substrates, especially Xcut-112°YLiT
The a03 board has a surface wave delay time temperature coefficient of -18p.
The electromechanical coupling coefficient, which is a parameter that determines the electrical impedance of an interdigital converter, is about 0.8%, which is a value between that of a crystal substrate and a LiNb 03 substrate in pm/''C. This is also crystal and LiNbO
3, the chip size can be made smaller than that of crystal, and the frequency temperature drift also meets the system specifications, so it is suitable for carrier oscillators used in RF modulators of VTRs in particular. It was widely used as a material suitable for surface acoustic wave resonators.

Recently, LiB4O7 (lithium tetraborate) has been attracting attention as a new substrate material for surface acoustic wave devices. This material has an electromechanical coupling coefficient of 2 x cut
-112℃Y It has a value similar to that of the LiTa Q3i board, and the delay time temperature coefficient becomes 0 near room temperature, so the frequency temperature drift is small and the chip size is also comparable to that of the LiTa Q3i board.
It can be made to the same level as in the case of an aO3 substrate, and from this point of view, a wide range of applications are being considered. In particular, the RF
When considering application to a resonator for a modulator, since k2 is about the same, a resonator with the same figure of merit M can be created with the same chip size as the 1-iTao3 board. It is possible to realize an element with small frequency temperature drift.

However, regarding the dielectric constant of the material, LiB4O7
For , the value of LiTaO3 is about 176, and if we design an interdigital transducer with the same number of electrode pairs and crossing width, the electrode capacitance of the transducer is 1i3407, for LiT
The value is 176 for aO3, which is extremely small. This electrode capacitance is a value that determines matching of phase oscillation conditions with the circuit side of the oscillator when used as an oscillator, and 3 to 60 F is considered appropriate for IC circuits currently in practical use. The glue actance on the circuit side can be changed to some extent by changing the circuit constants, but 1
When considering oscillation in an IC oscillation circuit equivalent to iTaQ3, it is desirable that the electrode capacitance of the resonator be approximately the same as the value described above.

The number of electrode pairs of the interdigital converter in the case of a 1iTa03 substrate is N1, and if the crossing width is W, then in order to obtain the same electrode capacitance as in the case of a 1iB*o7 substrate, the number of electrode pairs N should be equal and the crossing width W should be multiplied by 6. , or increase the number of electrode pairs N by 6 while keeping the crossing width W constant, or N(j; 1o
1)XW(LikOt)=6×N(LiTaO3)X
%; There is a method of increasing both the number of electrode pairs N and the crossing width W so as to satisfy the TxOB voice. In this case, the intersection width W
Increasing is undesirable because the chip width increases almost proportionally. Therefore, it is necessary to suppress the °C1 intersection width W to be equal to or somewhat larger than that in the case of LiTaO3, and to greatly increase the number of electrode pairs N (to a value close to 6 times).

When the number of electrode pairs N is increased, the resonant resistance R1 becomes smaller and the figure of merit M becomes larger, making it possible to obtain a resonator that oscillates easily, but the capacitance ratio γ becomes smaller accordingly. The capacitance ratio γ is the frequency range in which the resonator impedance becomes inductive, that is, the resonance frequency f
It is a value that determines the interval between r and anti-resonant frequency fa, and < fa
-fr )//r=1/2γ, and as γ becomes smaller,
As shown in FIG. 7, the interval fa-fr widens. R
A simple and general oscillator such as an excavator in an F modulator oscillates at a frequency between fa and fr,
When γ becomes smaller and fa-fr becomes wider, the frequency range in which oscillation can be performed becomes wider, and along with this, the variation in excavated frequency due to the variation in constants on the circuit side also becomes larger.

This shows that even if the resonator is manufactured with high precision to obtain a predetermined resonant frequency, the oscillation frequency will vary widely, which is particularly undesirable when used as an oscillator.

Next, what has been explained so far will be quantitatively explained using an equivalent circuit of a resonator. FIG. 8 is an equivalent circuit of a one-bottom surface acoustic wave resonator. Each constant of this equivalent circuit is
It is expressed as follows.

Parallel capacity ffi: Co = C5WN
...---------(4) k2 electromechanical coupling coefficient C3: one pair of interdigital electrodes, capacitance per unit length W: intersection width of interdigital electrodes N: logarithm of interdigital electrodes lfl Reflectance of total rating reflector Jleff: Equivalent capacity length of SAW co-conception.

Below, the number of interdigital electrode pairs of an oscillator for an RF modulator on a LiTaO3 substrate is N I N and the intersection width is Wl.
With the resonant frequency fo equal to this, the parallel capacitance CO is also equal L! An example of designing a B<07 substrate SAW resonator will be described.

The electromechanical coupling coefficient of 2 is the same for both boards, and C8 is L! B*
C) In the case of y, it is 1/6 of LiTaO5, so the intersection width W1 is made equal and the number of interdigital electrode pairs is designed to be six times as large. Regarding IT'1 and J2eH, the grating reflectors are designed to be equal on both substrates. In addition, when a grating reflector is formed with a β film, the reflectance of one reflector is L! Although B<07 is larger than in the case of 1iTaO3, by reducing the thickness of the AA film, the reflectance per reflector on both substrates can be made equal.

Organizing the above conditions, fO(-kov)=/o(UTh(h)k”(L;
140y) = ' ``L city 01) C5 (Lis4oT
) = (1/6) X Cs<t+r*os>W
(L;840)) -W(LiTaO2)N(LiB
4O7) -6xN(LrrsQ*>+7' +(
68401do111(mu7aO@>f16+t(mukh
) - β e 4 (Lilies).

Here, the only difference between both boards is Cs and N, and since the others are equal, rewriting equations (1) to (7) focusing only on C8 and N, and comparing the equivalent circuit constants of both boards, we get .

As is clear from the above table, in order to equalize the parallel capacitance ffi Co on both boards, if N is increased by 6, R1 becomes 1/6 and M becomes 6 times, resulting in a resonator that oscillates more easily. However, since γ becomes 1/6, the resonance/anti-resonance frequency interval becomes six times as large, and the problem arises that the oscillation frequency variation due to the constant variation of the oscillation circuit also becomes about six times as large.

In this way, piezoelectric substrates with relatively small dielectric constants, especially L!
In a surface acoustic wave resonator using a B4O7JS board, when the number of interdigital electrode pairs is increased to increase the electrode capacitance, the capacitance ratio of the resonator decreases, and as a result, the resonance/anti-resonance frequency interval widens. When incorporated into an oscillation circuit and used as an oscillator, there is a drawback that the oscillation frequency varies greatly due to the influence of constant variations on the circuit side.

[Object of the Invention] Therefore, the present invention provides a surface acoustic wave resonator in which the number of interdigital electrodes is not increased significantly (the electrode capacitance is set to a desired value, the capacitance ratio is suppressed, and the variation in oscillation frequency is reduced). The purpose is to provide.

[Summary of the Invention] That is, the surface acoustic wave resonator of the present invention is a surface acoustic wave resonator equipped with an interdigital transducer and a grating reflector, in which electrodes are configured to generate capacitance on the same substrate. By connecting the capacitance of this electrode in parallel to an interdigital converter, the capacitance can be set to the desired value without significantly increasing the number of logarithms of the interdigital electrode, suppressing the decrease in capacitance and increasing the oscillation frequency. This reduces the variation in .

[Embodiments of the invention] FIG. 1 is a plan view showing the configuration of an embodiment of the present invention.

In the following figures, parts common to those in FIG. 6 are denoted by the same reference numerals and redundant explanations will be omitted.

The surface acoustic wave resonator in this example is 18407
Two comb-like electrodes m2a are mounted on a piezoelectric substrate 1 consisting of
A lυ-shaped interdigital converter 2 is provided, and two comb teeth 4a and 4b are installed on both sides of the interdigital converter 2.
As a set, parallel capacitance correction electrodes 4 are provided which are alternately connected to a pair of parallel busbars, and the parallel capacitance correction electrodes 4 are electrically connected in parallel to the interdigital converter 2. has been done. The electrode line width d and pitch p of these parallel capacitance correction electrodes 4 are set to values equivalent to those of the grating reflectors 3a and 3b.

FIG. 2 shows an electrical equivalent circuit of the surface acoustic wave resonator constructed in this manner. In the figure, C indicates the capacitance of the interdigital converter 2, and CR^ indicates the capacitance of the parallel capacitance correction electrodes 4 provided on both sides thereof.

The surface acoustic wave resonator of this embodiment has a parallel capacitance and capacitance γ equivalent to the ψ column capacity ffi Co of the surface acoustic wave resonator using a LiTaC)3 substrate, which was explained in the technical background section of the invention. The number of electrode pairs of the interdigital converter 2 is F, and the capacitance of the parallel capacitance correction electrodes on both sides is C3 (
L;340v) xWx N<t;Thoz)X (
6-F D) = C8 (Lil'10B) XWx N
<t=no*)X has a value of (1-1/('V)>.

A comparison of the Lie407J3 board surface acoustic wave resonator of the present invention with a LiTa03M board resonator is as follows.

As shown in the table above, the logarithm of the interdigital converter is F1 times that of the LiTaO3 resonator, and the parallel capacitance is corrected using the parallel capacitance correction electrode to be equal to that of the LiTaO5 resonator. It is possible to obtain a LiB+07 resonator with an equivalent circuit constant of .

Since the electrodes for parallel capacitance correction in this example have the same electrode line width and pitch as the grating reflector,
It can be considered as a part of a reflector, and even if the number of grating reflectors is reduced by the number of comb teeth of this parallel capacitance correction electrode, the reflectance will not be affected. Furthermore, these parallel capacitance correction electrodes are connected alternately to the upper and lower busbars in pairs, and these are n/8 width split electrodes that excite surface waves at a frequency that is 1/2 of the resonant frequency. However, since almost no surface waves are excited at the resonant frequency, the electrical equivalent circuit at this frequency can be considered to be simply the electrode capacitance.

Regarding the reflection characteristics, as shown in Fig. 3, when the standing wave W is generated, the same displacement occurs in the comb teeth 4a and 4b connected to the upper and lower busbars, so the distance between the busbars is The potentials become equal. Therefore, this is equivalent to the case where all the comb teeth 4a and 4b are electrically short-circuited, and there is almost no effect on the reflection characteristics.

FIG. 4 is a plan view showing the configuration of another embodiment, in which capacitance correction electrodes 4 are provided on both sides of grating reflectors 3a and 3b.

FIG. 5 is a plan view showing the configuration of still another embodiment, in which a capacitance correction electrode 4 is formed on the side of one grating reflector 3a that is remote from the interdigital converter 3. The number of comb teeth of the capacitance correction electrode 4 is the same as the total number of the two capacitance correction electrodes 4 in the embodiments shown in FIGS. 1 and 4.

In the embodiments shown in FIGS. 4 and 5, the same excellent effects as in the embodiment shown in FIG. 1 can be obtained.

[Effects of the Invention] As explained above, the present invention provides a piezoelectric substrate with a small dielectric constant, especially 1i3407 with a relatively large electromechanical coupling coefficient.
Electrodes that generate capacitance are provided on the same substrate of the surface acoustic wave resonator using a substrate, and this is connected in parallel with the interdigital converter to increase the equivalent parallel capacitance of the resonator. By increasing the ratio γ and reducing the resonance/anti-resonance frequency difference, it is possible to reduce variations in the oscillation frequency due to variations in the constants of the oscillation circuit. In particular, when the electrostatic capacitance ω is generated and the electrode also serves as a grating reflector as shown in the embodiment, the chip area does not become large.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a plan view showing the configuration of an embodiment of the present invention, FIG. 2 is an equivalent circuit diagram thereof, FIG. 3 is a diagram showing standing waves generated in the piezoelectric substrate, and FIG. Figures 4 and 5 are plan views showing the configuration of other embodiments, Figure 6 is a plan view showing the configuration of a conventional surface acoustic wave resonator, and Figure 7 is the relationship between the resonant frequency and the non-resonant frequency. FIG. 8 is an equivalent circuit diagram of the surface acoustic wave resonator shown in FIG. 6.

Claims (4)

[Claims] (1) A surface acoustic wave resonator including at least one set of interdigital transducers and at least one set of grating reflectors on a piezoelectric substrate, configured so that capacitance is generated on the same substrate. A surface acoustic wave resonator characterized in that an electrode is formed, and the capacitance of the electrode is electrically connected in parallel with the interdigital converter. (2) The surface acoustic wave resonator according to claim 1, wherein the electrode provided to generate the capacitance constitutes at least a part of the grating reflector. . (3) The electrodes provided to generate the capacitance have a comb-teeth shape in which a set of two comb teeth are connected alternately to a pair of parallel busbars. A surface acoustic wave resonator according to claim 2 characterized by: (4) The surface acoustic wave resonator according to any one of claims 1 to 3, wherein the piezoelectric substrate is made of lithium tetraborate.