JPS6077509A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To decrease voltage loss by taking number of electrode finger pairs of one electrode as <20 and taking the percentage of the width of electrode finger to 1/2 of the repetitive period to the electrode finger as >=76% or <=24% so as to make the admittance close to that of a device to be connected. CONSTITUTION:A comb line electrode 2 as an input electrode, a comb line electrode 3 being an output electrode and a shield electrode 4 are formed on a piezoelectric substrate 1 forming a surface acoustic wave device. The number of pairs of the electrode finger in the comb line electrodes 2, 3 is taken as <20 and also the percentage of the width of electrode finger to 1/2 of the repetitive period of the electrode finger is taken as >=76% or <=24%. Moreover, the percentage of the width of the electrode finger of the comb line electrodes 2, 3 arranged oppositely onto the substrate 1 is taken as <=24% for the electrode 2 or 3 and >=76% for the other electrode 3 or 2. Then the admittance of the surface acoustic wave device is approached to the device to be connected so as to decrease the voltage loss.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of Weight] The present invention relates to a surface acoustic wave device that approaches the admittance of a coupling circuit, improves voltage loss, and achieves miniaturization.

[Background of the invention]

FIG. 1 shows a row of conventional surface acoustic wave devices.

1 is a piezoelectric substrate, 2.3 is a comb-shaped electrode, and 4 is a shield electrode. Comb-shaped electrodes generally have a double-electrode structure (split type) in which electrode fingers of the same polarity are divided into two adjacent ones and arranged with alternating polarities, in order to suppress the reflection of surface acoustic waves. electrodes) are often used. Also, usually
The metallization ratio, which is the percentage ratio of 1/2 of the repetition period of the electrode fingers to the width of the electrode fingers, is 50%.

The surface acoustic wave device is one comb type! When a signal is applied to a pole, for example, the comb-shaped electrode 2 in FIG. 1, the electrical signal is converted into a surface acoustic wave. This surface acoustic wave propagates on the substrate and is again converted into an electric signal by the other comb-shaped electrode, for example, the comb-shaped electrode 3 in FIG. At this time, desired frequency characteristics are obtained by changing the crossing width or repetition period of at least one of the comb-shaped electrodes and applying weighting. The shield electrode directly suppresses leakage waves.A surface acoustic wave device for the purpose of compensating the characteristics of a video head used for V'I'R has a center frequency of 15.times.2.degree. and a relative bandwidth of 0.57. The signal source side has regular electrodes with 35 pairs of logarithms, and the load side has weighted electrodes with 7.5 pairs of logarithms. In addition, both electrode fingers have a double electrode structure with 1150% metallization lacy, the aperture length is 1890 μn1, and the chip size is 9 wi.
n×2.5B. At this time, the radiation conductance Gi % radiation susceptance Bt of the regular electrode is 0.07 m 9 and 0.45 m U at the center frequency, respectively.
, and the radiation conductance of the electrode and the radiation susceptance Bo are respectively 0.011 m&' at the center frequency.
, 0.75m17.

When this surface acoustic wave device is used in a VTR, the admittance of the coupling circuit seen from the surface acoustic wave device is as follows: signal source side Gs is 50 m, load side voltage is Oゝ0,2 +n U〇Next, this elastic surface Consider voltage loss in wave devices. The power loss LbB of the surface acoustic wave device is expressed as follows. Here, K is the bidirectional loss of the electrode, which is 1/4 when the opening lengths of the input and output electrode fingers are equal. Also. Other variables are as described above.

The voltage loss can be expressed as follows using the power loss determined earlier.

According to this equation, the voltage loss of the surface acoustic wave device described above is calculated to be 25.4 dB.

When this surface acoustic wave device was used in a VTR, the reproduced images had a very poor S/N ratio, and the device could not fully demonstrate its performance. This is because the surface acoustic wave device has a large voltage loss.

Voltage loss can be reduced by bringing the admittance of the coupling circuit and surface acoustic wave device closer to the optimum conditions according to the equation expressing voltage loss above, but the admittance of the coupling circuit can be reduced by approaching the conditions that minimize voltage loss. Is pleased. However, as can be seen from the above formula for voltage loss, the admittance of a surface acoustic wave device has a large %Gi and a G
It is desirable to make □ smaller.

The signal source side radiation conductance GL of this surface acoustic wave device
The reason why is small is that the frequency is low and the number of electrode finger pairs is small. However, the number of electrode finger pairs is determined by the fractional bandwidth and cannot be changed. Furthermore, Gt can be increased by increasing the aperture length of the electrode fingers;
This method results in an increase in device size.

In this way, in conventional surface acoustic wave devices, the admittance freedom 1 (of the electrode fingers is small, the voltage loss is large,
Otherwise, there was a problem that the size of the device increased.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface acoustic wave device that improves the above-mentioned conventional problems, has an admittance close to connected equipment, and reduces voltage loss while remaining small.

[Summary of the invention]

In order to achieve the above object, in the present invention, the number of pairs of electrode fingers of at least one electrode is less than 20, and 1/2 of the repetition period of the electrode fingers (dimension 5
) to the electrode finger width (dimension 6 shown in Figure 2), that is, the metallization ratio is set to 76% or more or 24% or less to bring the admittance of the surface acoustic wave device close to that of the coupling circuit. .

Regarding the relationship between the metallization ratio of electrode fingers and surface acoustic wave devices, various analyzes and experiments have been conducted in the past.
ns, MTT-25, No. 11.1975) et al., the relationship between metallization ratio, capacitance, and radiation conductance is that by increasing the metallization ratio, the capacitance of the electrode finger increases, but the radiation conductance increases as the metallization ratio increases. It has been reported that the rate is maximum when 50% of the order is 50%, and decreases at other times. However, according to the inventor's experiments, the above report concerns the case where the number of electrode finger pairs is large (20 or more), and the VT
It is not suitable for cases where the number of pairs of electrode fingers is small, such as surface acoustic wave devices for R.

In the case of a WL pole pair number of 35 electrode finger opening length of 1890 μm and a solid electrode, the relationship between metallization radiation and radiation conductance obtained by the inventor as a result of experiments is expressed in the third example.
In addition, the relationship between the metallization ratio and the capacitance is shown in FIG.

Here, the capacitance and radiation conductance are values per pair of electrode fingers and 1 μm of aperture length.

In this way, the capacitance and radiation conductance increase with the metallization Raysilo, and the metallization Raysilo increases the admittance of the device by about 0.1 to 2 times compared to the case of 50% metallization Raysilo. I found out that it can be changed. At this time, the voltage loss can be improved or the admittance can be kept unchanged and the aperture length can be reduced to achieve miniaturization.

Furthermore, it is clear that a double electrode structure has similar effects.

[Embodiments of the invention]

As an example, a surface acoustic wave device used in an FM reproduction phase equalizer of a VTR will be described.

This device is used to compensate for the characteristics of a video head, and when creating the device, the video signal is up-converted and frequency-converted.

In the multi-array, the upconversion frequency was 1 iMHz, and the center frequency of the device was 15 MITz. The first embodiment of the present invention is described in the fifth [i! ! This will be explained using I.

As the piezoelectric substrate 1, a 128-degree rotated Y-axis cut, X-propagating lithium niobate single crystal with a plate thickness of 0.5 m was used.

The input electrode 2 is a regular type electrode with 3.5 pairs of solid electrodes, and the metallization ratio is 90%, and the output electrode 5 is 8.5%.
The weighting of the 5-pair dual electrode structure l (the metallization reciprocity at the poles is 50%. Also, these electrodes have a
ooo An aluminum vapor deposited film of A was formed by photolithography technology. The opening length of the electrode finger is 1890 μm. At this time, the admittance of the device is Qj=0.12m0, Bi=0.76mty at the center frequency,
Go=0.041m 5r, B□=0.74m c'
Therefore, the admittance on the input side has increased approximately 1.7 times compared to the conventional one, and when the admittance on the signal source side is 50 m-r and the admittance on the load side is 0.2 mQ-, the voltage loss is 20
JdB, which is 5 dB lower than before.

Next, a second embodiment of the present invention will be described.

The second example differs from the first example in that the electrode finger aperture length is 1000 μm.
The only difference is that it is set to m, and the rest are the same.

At this time, the admittance of the device is the center frequency te Gt=
=0.104m', B L=0.66m ;,,
Q = = 0.035 m2: , BO = 0.64 m;
Voltage loss is 21.1dB, which is 2.3d lower than before.
B is reduced, and the piezoelectric substrate dimensions of the device are 9 HHX
This was a 12% reduction to 2.2 mm.

〔Effect of the invention〕

As explained above, according to the present invention, the radiation conductance can be changed from about 0.1 times to 2 times that of the conventional method, thereby improving voltage loss and downsizing the device.

[Brief explanation of the drawing]

Figure 1 is an electrode pattern diagram of a conventional surface acoustic wave device;
The figure is an explanatory diagram of metallization Reishiro, Fig. 3 is a diagram showing the relationship between metallization Reishiro and radiation conductance, Figure 4 is a diagram showing the relationship between metallization Reishiro and capacitance, and Figure 5 is a diagram showing the relationship between metallization Reishiro and capacitance. FIG. 3 is a diagram of the ¥IL pole pattern of one embodiment. 1... Piezoelectric substrate, 2.3... Comb-shaped electrode, 7...
・Characteristic curve showing the relationship between metallization Reishiro and radiation conductance, 8...Characteristic curve showing the relationship between metallization Reishiro and capacitance. Condolences 1 Diagram 2 Da
Method) Director General of the Patent OfficeJljIndication 1982 Patent Application No. 185002 Title of Invention Person who corrects surface acoustic wave device IIfl! :σ匍fii Patent applicant r, title 151su) 11th ceremony! Hitachi Manufacturing Agent

Claims (1)

[Scope of Claims] 1) In a surface acoustic wave device in which at least one set of manual and output comb-shaped electrodes are disposed facing each other on a piezoelectric substrate, at least one of the comb-shaped electrodes has a number of pairs of electrode fingers. 20
and a percentage ratio of the electrode finger width to 1/2 of the repetition period of the electrode fingers is 76% or more or 24% or less. 2) The electrode finger width percentage ratio of the electrode fingers of the comb-shaped electrodes arranged facing each other is 24% or less for one electrode and 76% or more for the other electrode. Surface acoustic wave device.