JPS5824970B2 - Parallel electric field excitation piezoelectric vibrator for overtone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a parallel field-excited piezoelectric vibrator for overtones, which has a low equivalent inductance for high-frequency vibrations of a desired order, and has a high equivalent inductance for vibrations of orders other than the desired order. .

Parallel electric field excitation vibration of a piezoelectric plate has conventionally been used only for measuring material constants because the vibrator impedance is high.

However, recent advances in materials have revealed that LiTa 03 has a cutting direction in which the piezoelectricity is large and the frequency temperature coefficient is small, making it possible to apply it to crystal filters in high frequency bands of several tens of MHz or more.

Conventionally, parallel electric field excitation vibrators have been constructed by providing electrodes on the side surfaces of a piezoelectric plate as shown in Figure 1, or by arranging two electrodes in parallel on the main surface of a piezoelectric plate at appropriate intervals as shown in Figure 2. An energy confinement type is used in which a high frequency voltage is applied between electrodes.

In the figure, 1 is a piezoelectric plate, 2 is an electrode, and due to an externally applied voltage, an electric field E is applied to the piezoelectric plate as shown by the arrowed line in the figure, causing vibrations due to electric field components parallel to the plate surface. is excited.

The current in the vibrator flows along the lines of electric force, and the area through which the current flows is limited by the thickness of the piezoelectric plate, but since the plate thickness is determined by the resonant frequency, the equivalent inductance of the vibrator is limited by the frequency. receive.

Vibrators in the high frequency range of several tens of MHz or higher must use overtones, but since the equivalent inductance is proportional to the square of the overtone order n,
In addition to the restrictions imposed by plate thickness, the resonator impedance becomes high.

In a filter using a piezoelectric vibrator, its terminal impedance is proportional to Ll/Δf, where Ll is the equivalent inductance of the vibrator and Δf is the bandwidth of the filter.

If Δf is large, the terminal impedance becomes high, and conversely, when realizing a filter with low terminal impedance, there is a limit to Δf.

In order to lower the equivalent inductance, the depth dimension W of the electrodes in FIGS. 1 and 2 must be adjusted.

A method is used to increase the

But W. Increasing is not preferable because spurious waves occur due to standing waves in the depth direction.

In addition, it is desirable for a piezoelectric vibrator to excite only overtone vibrations of the order used, and vibrations of other orders to be weakened or not generated, but conventionally, responses of unnecessary orders have been suppressed by an external circuit.

Therefore, we investigated the causes of the increase in equivalent inductance as described above.

Figure 3 is a cross-sectional view showing the displacement distribution in the thickness direction of overtone vibration in a conventional parallel electric field excitation vibrator and the charge distribution generated on the side surface of the vibrator. This is the case with overtones.

As shown in the figure, in overtone vibration, positive and negative charges are simultaneously placed on the same side surface, and some of the charges cancel each other out, so the overall current becomes smaller and the equivalent inductance increases.

FIG. 4 shows the charge distribution of energy-confined parallel field excitation vibration with electrodes on only one side, with 42 ports corresponding to the 3rd and 5th overtones, respectively.

Furthermore, FIG. 5 shows the charge distribution of the energy-confined type parallel electric field excitation vibration with electrodes provided on both sides, and similarly to FIG. 4, the 42 holes are for 3rd and 5th overtones, respectively.

In the case of double-sided electrodes, if we assume that the thickness in the Z-axis direction is 0 at the bottom of the H1 oscillator, then the upper electrode has a stress distribution in the M part, which is the upper half of the piezoelectric plate, that is, H/, ~H part. A charge corresponding to the current is generated, and a charge corresponding to the N portion, which is the portion from O to S, is generated on the lower surface electrode.

The charge distribution on the electrodes in the energy confinement type parallel electric field excitation vibrator as shown in Figs. 4 and 5 is such that the charges that would normally be on the side surfaces are expanded onto the plate surface. Although the positions where the positive and negative values are reversed are not equally spaced as shown in FIG. 3, the fact remains that the charges cancel each other out in overtone vibration.

Therefore, it has been found that if the effects of canceling charges can be reduced or eliminated, the equivalent inductance in overtone vibration can be reduced.

In order to eliminate the conventional drawbacks, the present invention provides a parallel electric field excitation vibrator that applies a high frequency voltage between a pair of rectangular electrodes provided parallel to each other on the main surface of a piezoelectric plate. A slit is provided in the electrode portion where a charge of a different sign from that induced in the electrode portion as a whole is induced, and overtone vibration of a desired order can be strongly excited without increasing the depth of the electrode. The present invention, which aims to lower the equivalent impedance and conversely weaken the electrical response to vibrations of other orders, will be described with reference to the drawings.

FIG. 6 shows an embodiment of the parallel electric field excitation vibrator according to the present invention.

42 mouth is 3rd overtone, Ha. The second type is a vibrator having an electrode structure in which a slit is provided in a portion where canceled charges are generated by fifth-order overtone vibration.

A and C show cross-sectional views, and 2 shows a plan view. Since there is no canceling charge, only charges of the same sign are induced on the electrode in each overtone, so the equivalent inductance is ideally 1/2.5th overtone in the 3rd overtone compared to the case without slits. Then, it can be reduced to 1/3.

Furthermore, for overtones of orders other than the desired order, the presence of the slit reduces the total amount of charges induced, so the equivalent inductance increases, and the electrical response becomes weaker than in the case without the slit.

Although FIG. 6-2 shows an example in which each electrode is provided with slits at two locations, it is also effective to provide only one slit.

In the case of an overtone vibrator of seventh order or higher, the equivalent inductance of overtone vibration of a desired order can be lowered by similarly using an electrode with one or more slits.

Next, an example of a parallel electric field excitation vibrator with double-sided electrodes will be described.

In the case of double-sided electrodes, as shown in Figure 5A, in the case of third-order overtone, the upper half of the piezoelectric plate, that is, H/2<
The region where Z<H corresponds to the portions of electrodes a to C, and canceling charges are induced in the portion corresponding to the outer portion of the electrode b = c.

If we consider the case where we remove the part b=c, we will get 1 new piece a.
Since the =b part corresponds to the area H/2 to H of the piezoelectric plate, it is meaningless to remove the electrode over the entire area of the b=c part, leaving the part near the outer edge of the electrode. If this is necessary, the electrode between the hatched portion b and d in FIG. 5A can be removed.

However, in this case, a sufficient effect cannot be expected because a charge having a different sign from that of the portions a to b is induced in the electrode at the portion d=c and acts as a canceling charge.

In the case of a fifth-order overtone, the induced charge distribution is as shown in Figure 5, so by providing a slit in the portion where canceling charges are induced, the resonance characteristics can be greatly improved.

FIG. 7 shows the electrode structure of a parallel electric field excitation vibrator aimed at improving the characteristics of fifth-order overtone vibration, and Table 1 shows the electrode dimensions, the piezoelectric substrate used, etc.

Table 2 shows the measurement results for the fifth-order and seventh-order overtone vibrations of this vibrator in comparison with the case where no slit is provided in the electrode.

Regarding the 5th overtone, clearly the resonance characteristics did not improve significantly when a slit was provided, whereas the 7th overtone did not significantly improve the resonance characteristics.
For the next overtone, the slit position is not appropriate, resulting in an increase in resonance resistance and equivalent inductance, and a decrease in resonance characteristics.

The position and width of the slit can be determined by conformal mapping as shown in FIG.

Namely. For the vibrator shown in FIG. 8A, the electric lines of force connecting the left and right electrodes are mapped to obtain an electrode shape that produces linear lines of electric force as shown in the opening of FIG.

This has the same shape as shown in FIG.

On the map shown in the opening of Figure 8, the positions t'~d, v/~w' where canceling charges are induced (t' is (to, tl), U' is (u□, ul), l is (v□, Vl)
W is (wO.

Wl ) is displayed.

) can be easily found, so by performing inverse mapping, it is generally possible to find the actual slit position (to, tl) on the electrode, (
u□, ul), (vo.

vl) and (w□, wl) can be determined.

In general, the width of the slit becomes finer as the frequency is higher or the overtone order is higher, but according to calculations, the slit width when obtaining a 100200 MHz resonator with a fifth overtone is 5 to 10 μm.

Slits with a width of about 10 μm or more can be formed by laser cutting after electrode attachment, and even finer patterns can be formed by photoetching, and slits with a width of 200 to 300 MH using high-order overtones can be formed.
It is possible to realize frequencies up to about z.

With the above configuration, the piezoelectric vibrator of the present invention can have a low equivalent inductance for overtone vibration of a desired order, and a high equivalent inductance for vibrations of other orders.

Further, due to the above characteristics, the electrical response in unnecessary orders is small in a frequency band of several tens of MHz or more, and a voltage filter with low terminal impedance can be realized.

[Brief explanation of drawings]

Figures 1 and 2 are perspective views of a conventional parallel field-excited piezoelectric vibrator, Figure 3 is a diagram showing the displacement distribution and charge distribution on the side surface of the parallel field-excited vibrator during overtone vibration, and Figure 4 The figure is a diagram showing the charge distribution on the electrode during overtone vibration of a parallel field excitation resonator with a single-sided electrode.
The figure shows the 3rd and 5th overtones, Figure 5 shows the charge distribution in the case of double-sided electrodes, Figures A and 0 show the 3rd and 5th overtones, and Figure 6 shows the overtone parallel electric field according to the present invention. An example of an excitation piezoelectric vibrator: Figure A, Figure 0 is cubic, Figure C,
The varnish is a 5th order overtone, Figures A and C are cross-sectional views, 0
FIG. 7 shows an example of a fifth-order overtone of double-sided electrodes, and FIG. 8 shows a method for determining the position where a slit is to be provided. 1: piezoelectric plate, 2: electrode, H: plate thickness, Sl, 81'. S2. S! ! , S3,83' Nislit, d: electrode spacing, g Nislit width.

Claims (1)

[Claims] 1. In a parallel electric field excited vibrator that applies a high frequency voltage between a pair of rectangular electrodes arranged in parallel and spaced apart on the main surface of a piezoelectric plate, the electric charge induced in the entire electrode part due to the high-order thickness vibration and A parallel electric field excitation piezoelectric vibrator for overtones, characterized in that a slit is provided in an electrode portion where charges of different signs are induced.