JPS5828770B2 - surface acoustic wave device - Google Patents

Description

[Detailed description of the invention] The present invention relates to a surface acoustic wave device.

It is well known that surface acoustic wave devices are applied to PIF filter circuits of television receivers.

It is well known to apply surface acoustic wave devices to picture intermediate frequency (PIF) filter circuits in television receivers.

In this case, ceramic or LiNb0a plates were generally considered as piezoelectric substrates for surface acoustic wave devices.

However, ceramics have poor manufacturing yields and are unsuitable for industrial use, and LiNbO3 plates have poor temperature characteristics and are unsuitable for use as PIF filters for television receivers.

Therefore, the present inventor developed the use of a LiTaO3 plate.

Conventionally, this L s T a Os board was expensive and no suitable cut had been found, so it was not put into practical use like the L IN b Q B board, but the inventor of the present invention changed the propagation direction of the surface acoustic wave to L It has been found that by setting the temperature in the range of 67.8 degrees to 142 degrees with respect to the Y axis of the i Ta 03 crystal, a surface acoustic wave device with extremely good temperature characteristics and bulk spurious characteristics can be obtained.

However, in this case as well, it has been found that if the substrate shape is not appropriate, the surface acoustic wave device will have poor spurious characteristics, resulting in poor yield and hindering its practical use.

For example, as a piezoelectric substrate, an X-cut LiTaQ
We manufactured a PIF surface wave filter for television receivers in which surface waves were propagated in a direction of 112 degrees from the Y axis using plate B, and measured its frequency (MHz) vs. relative amplitude response output characteristics. If you look at it, it will look like Figure 1.

Curve A is a frequency characteristic curve of the pass band of the PIF filter when the relative response output value (dB) is 10 dB per division.

In this case, it doesn't seem like a very large ripple appears, but if you increase the sensitivity by io times, the l scale is 1d.
When measurements were made using curve B, it became clear that a relatively large ripple occurred within the same pass band from 56 MHz to 57 MHz, as shown in curve B.

When the reflection characteristics of the filter having the frequency characteristics shown in FIG. 1 are measured using a network analyzer, the results are as shown in FIG.

As can be seen from Figure 2, in addition to the fundamental wave of the surface wave and the excitation of the second harmonic, 6.108 MHz%18.823 MHz,
31.376MHz, 43.959MHz, 56
．． It can be seen that strong resonance occurs at multiple frequency positions such as 489 MHz.

When this resonance phenomenon occurs within the passband of the PIF filter, the surface wave excitation energy is absorbed by this resonance vibration, and as a result it appears as a rhinople within the passband of the PIF filter.

Furthermore, if this resonant frequency matches a trap frequency that requires a large amount of attenuation, the required amount of attenuation cannot be obtained.

The present invention was made in view of the above points, and based on the consideration of FIGS. 1 and 2, by preventing absorption of surface wave excitation energy due to the resonance phenomenon, ripples
That is, the object is to provide a surface acoustic wave device with reduced spurious components.

According to this invention, as a piezoelectric body, an X-cut L iTa
The present invention provides a surface acoustic wave device with improved spurious characteristics by using a Q3 substrate and configuring the substrate so that the thickness d satisfies the following equation.

Before entering into the description of the embodiments, the principle of this invention will first be analyzed and explained.

In order to find out the spurious vibration mode shown in FIG. 2, the interdigital transducer was excited using the following three methods.

(4) As shown in FIG. 3A, one electrode 32 of the interdigital transducer 31 is connected to LiTaO3.
The chip 30 is connected to the metal base 33 of the old piano cage and grounded, and an excitation voltage is applied between this and the other electrode 34.

As shown in FIG. 3, one electrode 34 of the interdigital transducer 31 is made a floating electrode without being connected to anything, and an excitation voltage is applied between the other electrode 32 and the metal base 33 of the pinocage. Apply.

0 As shown in FIG. 3C, both electrodes 32 and 34 of the interdigital transducer 31 are connected in parallel, and an excitation voltage is applied between one of the parallel connection ends and the metal base 33.

(4), (B), (O) The electric field distribution in each case is also shown wedge-shaped in FIG. 3 (4), (B), and (C).

Arrows indicate the direction of the electric field lines.

FIG. 4 shows examples of actual measurements of reflection characteristics using each of the excitation methods A, B, and C.

The following can be said from this measurement result. 1. Surface wave excitation is performed only in excitation method A.

2. The resonant frequency of spurious is the same for all excitation methods.

3. Strong excitation of odd harmonics.

4. Spurious excitation intensity has the relationship A<B<C.

From the above, it is considered that spurious excitation is caused by the electric field in the X-axis direction of the substrate.

In order to further confirm this, when the piezoelectric chip was separated from the metal base 33, the spurious response was extremely reduced as expected.

Next, Tables 1 to 5 and FIG. 5 show the results of an experiment in which the relationship between the thickness d of the piezoelectric chip and the shape of the electrodes of the interdigital transducer and the resonance frequency was examined.

Here, the shape of the electrode is shown as the name of the mask used when manufacturing the electrode.

Table 1 shows the measurement results for sample I with a thickness of 0.34011 m using IMT-25B-3288 as a mask, where frn (MHz) is the n-th resonance frequency, n
indicates the order.

Table 2 also shows I MT-25B-3288 as a mask.
The measurement results are shown for sample H with thickness d=0.3401.

Table 3 shows the measurement results for sample (2) with a thickness of d=0.323 using IMT-25B-3288 as a mask.

Table 4 shows the measurement results for sample (1) with thickness d=0.4587'm using IMT-301A-3291 as a mask.

Table 5 shows the measurement results for sample V of d2 0.290M using I MT-301A-3291 as a mask.

As can be seen from Tables 1 to 5, the value of frnXd/n is about 2.13, which is a substantially constant value.

This means that the relationship between frnXd and n is approximately a straight line as shown in Figure 5, and the slope frnXd/n is 2°13
This can be confirmed from the fact that

The fact that the value of frnXd/n is approximately constant in this way suggests that these spurious resonance vibrations are so-called thickness vibrations (, n-1, n=3, n=5,
The displacement distribution of thickness vibration in the case of n=7 is shown in FIGS. 6A, B, C, and D, respectively.

Regarding the thickness vibration of the X-cut LITa03 board, the coupling coefficient of thickness shear vibration is as large as 44.

And its resonance frequency constant is 1.906 Ml-(z-
111.

When the above-mentioned reflection minimum point is regarded as the resonant frequency, the frequencies in this invention are as shown in Tables 1 to 5 or 2.2 from FIG.
It became 13 MHz-71m.

In this way, the commonly used frequency constant is 1.906
MHz -7Nk and the frequency constant of the present invention is 2.13 M
Although the cause of the difference from Hz-V is not clear, it is reasonable to consider that the spurious resonance mode targeted by the present invention is thickness shear vibration.

The frequency constant obtained in this way (2,13MHz
-Wh ), the thickness d of the piezoelectric substrate and the resonant frequency fr
The results of determining the relationship with n are shown in FIG.

As explained above, it was found that the spurious vibration of the LiTaQ3 piezoelectric substrate of the X-cut is thickness shear vibration in the X-axis direction.

Therefore, it is advisable to select the thickness d of the substrate so that this vibration does not affect the characteristics of the surface wave filter and the resonance does not fall within the passband of the filter.

That is, Figure 8 shows the P for Japanese color television receivers.
The relationship between the passband and thickness d of a PIF filter when the present invention is applied to an IF filter is shown.

The pass band of the PIF filter is, for example, from 54 MHz (lowest pass frequency ft) to 60 MHz (highest pass frequency fu).
), the two straight lines indicated by fz and fu in Fig. 8 and the resonant frequency characteristic curve (n = 1, 3
, 5,...) Dotted line side line area P excluding the intersection range W
By selecting the thickness d within the range, spurious noise can be eliminated.

In other words, the thickness d of the X-cutLiTa03 piezoelectric substrate is an odd number of n-1, 3, 5, 7, . . . .

It is sufficient to choose within the range of .

A PIF surface wave filter piezoelectric substrate for a color television receiver made based on the principle of the present invention described above is an X-cut L r TaQ3 with a thickness d=0.330.
An input transducer configured to form a PIF filter characteristic and a filter frequency band on one surface using a piezoelectric plate, and an output transducer configured to have a flat frequency characteristic wider than the filter frequency band. A juicer is provided at a predetermined distance.

Here, the surface wave propagation direction propagating on the surface of the piezoelectric plate connecting the input and output transducers is, for example, 11
It is set in the direction of 2°.

With this configuration, frequency characteristics as shown in FIG. 9 can be obtained.

Figure 9 shows the attenuation-frequency characteristics of the PIF filter,
It can be seen that spurious is well suppressed within the 6-MHz passband from 54 MH2 to 60 MH2.

In addition, the surface wave propagation direction of X-cut T, 1Tao3 plate is Y
For example, it is already stated in Japanese Patent Application No. 51-29889 that good temperature characteristics can be obtained if the temperature is selected within the range of 67 degrees to 142 degrees from the axis.

[Brief explanation of drawings]

Figure 1 is a frequency characteristic curve diagram of a PIF filter using a conventional LiT a03 substrate, Figure 2 is a reverse characteristic curve diagram of the network analyzer shown in Figure 1, and Figure 3 is for explaining the principle of the device of the present invention. 4 is a reflection characteristic curve diagram of Figure 3, Figure 5 is a characteristic diagram showing the relationship between the order n and the product of the resonance frequency and thickness of Figure 3, and Figure 6 is a diagram of the reflection characteristic curve of Figure 3. Figure 1 shows the displacement distribution diagram of the thickness vibration of the device of the present invention.
i A characteristic diagram showing the relationship between the resonance frequency and the thickness of the TaO2 plate, FIG. 8 is a characteristic diagram of an embodiment applied to the PIF filter of FIG. 7, and FIG. 9 is a frequency characteristic curve diagram of FIG. 8. P: Spurious generation area, W: Spurious non-occurrence area.

Claims (1)

[Claims] 1. An X-cut LiTaO3 plate whose thickness d satisfies the following formula, and the LiTa0a
A surface acoustic wave device comprising input and output electrodes provided on a plate. n: 1, 3.5...Odd number fL: Lowest frequency of filter passband (MHz) fu: Highest frequency of filter passband (■h) d: LiTaO3
The surface acoustic wave device according to claim 1, wherein the surface acoustic wave propagation direction of the LiTa0a plate is in the range of 67.8 degrees to 142 degrees from the Y axis.