JP2000165196A - Tapped delay line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a tapped delay line with a stable temperature characteristic of a center frequency where a propagation loss can be reduced and miniaturization is attained by decreasing a cross width. SOLUTION: An interdigital transducer IDT electrode section 3 and a tap section 4 are formed on a piezoelectric substrate 2, the tap section 4 has taps 5-10 and one end of electrode fingers 5a, 5b-10a, 10b configuring each tap is connected to a bus bar 11 or 12 and a metallic film 13 is formed on the piezoelectric substrate among the taps 5-10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tapped delay line used as a correlator or a matched filter in, for example, a demodulation unit of a spread spectrum communication type receiver, and more particularly to a tapped delay line constituted by using a surface acoustic wave element. Regarding delay lines.

[0002]

2. Description of the Related Art Conventionally, various detection surface wave elements using a tapped delay line have been proposed as detection elements used in a spread spectrum communication system (for example, Japanese Patent Application Laid-Open No. Hei.
77445, JP-A-7-283761, JP-A-8-18483, etc.).

FIG. 11 shows a basic structure of a tapped delay line used as the above-described surface acoustic wave element for differential detection. In the tapped delay line 51 shown in FIG.
2, an interdigital electrode (IDT electrode portion) 53 and a tap portion 54 are formed. IDT electrode section 53
Has a pair of comb electrodes 53a and 53b having electrode fingers interposed therebetween. The tap section 54 has a plurality of taps 55 to 60. Each of the taps 55 to 60 is provided with a sign, and a pair of electrode fingers 55a and 55, respectively.
5b to 60a and 60b.

[0004] One end of each of the electrode fingers 55a, 55b to 60a, 60b constituting the taps 55 to 60 is connected to the bus bar 61 or the bus bar 62. In the tapped delay line 51, a surface wave is excited by a signal input to the IDT electrode unit 52, the surface wave propagates from the IDT electrode unit 52 to the tap unit 53 side, and the delay output encoded in the tap unit 53 is encoded. Is taken out.

[0005]

In the tapped delay line 51, the delay output encoded as described above is taken out, but the propagation loss is small, and the size can be reduced.
In addition, it is strongly required that the temperature coefficient of the center frequency be small.

GW Judd et al .: "An improved tapping
transducer geometry for surfacewave phase coded d
elay lines. "1972 Ultrasonics Symposium Proceeding
s On page 373, in a delay line with a tap using a LiNbO ₃ substrate propagating in the Y-cut Z-direction, the propagation loss is improved by providing a grating electrode with multiple electrode fingers short-circuited at both ends between taps. Is described.

However, this prior art merely shows that the propagation loss is improved by forming the grating electrode in a tapped delay line using a Y-cut Z-direction propagation LiNbO ₃ substrate. No mention is made as to whether the propagation loss can be improved in various piezoelectric substrates. Further, in this prior art, there is no description about a change in frequency temperature characteristic or the like due to the formation of the grating electrode.

[0008] Further, as with other electronic components, there is a strong demand for downsized tapped delay lines as well as other electronic components. If the width of the piezoelectric substrate of the tapped delay line is reduced in order to reduce the size, the electrode finger crossing width in the IDT electrode portion and the tap portion must be reduced. However, when the cross width is reduced, the propagation loss increases. Therefore, it has been very difficult to reduce the size of the tapped delay line while preventing deterioration of the propagation loss.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned disadvantages of the prior art, and to use a piezoelectric substrate having a small propagation loss, a small size, and a first-order negative frequency temperature coefficient. An object of the present invention is to provide a delay line with a tap capable of reducing a value.

[0010]

According to the present invention, there is provided a delay line with a tap provided by the present invention, comprising: a piezoelectric substrate; an interdigital electrode portion formed on the piezoelectric substrate; and an interdigital electrode formed on the piezoelectric substrate. An electrode portion and a tap portion separated in a surface wave propagation direction, wherein the tap portion has a pair of busbars and a plurality of taps each having one end connected to the busbar, and a piezoelectric substrate is provided between the taps of the tap portion. It further comprises a metal film formed thereon.

In a specific aspect of the present invention, a piezoelectric substrate having a first-order negative frequency temperature coefficient is used as the piezoelectric substrate. As such a piezoelectric substrate, X-cut 112
An example is a LiTaO ₃ substrate propagating in the Y direction.

In another specific aspect of the present invention, the piezoelectric substrate is constituted by a quartz substrate whose frequency temperature coefficient is essentially close to zero, and the peak temperature of the frequency temperature characteristic is 10 to 5 times.
The cut angle and the metal film thickness / wavelength are selected so as to be in the range of 0 ° C.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail by describing a specific embodiment of a tapped delay line according to the present invention with reference to the drawings.

FIG. 1 is a schematic plan view for explaining a tapped delay line according to first and second embodiments of the present invention. The tapped delay line 1 is configured using a rectangular piezoelectric substrate 2. The piezoelectric material forming the piezoelectric substrate 2 is not particularly limited, and a material made of a piezoelectric single crystal such as LiTaO ₃ , LiNbO _3, or quartz, or a piezoelectric ceramic can be suitably used.

An IDT electrode portion 3 is provided on the upper surface of the piezoelectric substrate 1.
And a tap section 4. IDT electrode part 3
Has a pair of comb electrodes 3a and 3b having one or more electrode fingers interposed therebetween. The tap unit 4 has a plurality of taps 5 to 10 that are provided with encoding. Each of the taps 5 to 10 is a pair of electrode fingers 5a, 5b to 10 respectively.
a and 10b. Each of the electrode fingers 5a, 5b to 10a, 10b of the taps 5 to 10 extends in a direction orthogonal to the surface wave propagation direction.

One end of each of the electrode fingers 5a, 5b to 10a, 10b constituting the taps 5 to 10 is
Alternatively, it is connected to the bus bar 12. Bus bar 11,
Numeral 12 extends in the surface wave propagation direction.

Further, in the delay line with tap 1 of this embodiment,
Between the taps 5 and 10, the metal film 13 is formed on the piezoelectric substrate 2.
Are formed. In the present embodiment, the metal film 13 is formed in all the regions between adjacent taps. However, the metal film 13 may be formed in at least one of the regions between adjacent taps.

The metal film 13 is formed between the taps, that is, formed so as not to contact the electrode fingers 5a to 10b constituting the taps 5 to 10. The dimension of the metal film 13 in the direction perpendicular to the surface wave propagation direction is set to a length that does not contact the bus bars 11 and 12. That is, the metal film 13
Are the electrode fingers 5a unless they contact the bus bars 11 and 12.
The dimension may be greater than or equal to the cross width of 10 to 10b, or may be smaller than the cross width.

The material constituting the metal film 13 is not particularly limited, and an appropriate metal such as aluminum or copper can be used. Preferably, each material constituting the IDT electrode portion 3 and the tap portion 4 is used. It is made of the same material as the electrode material, so that the IDT electrode part 3 and the tap part 4 can be manufactured in the same process.

In the delay line 1 with taps according to the present embodiment, since the metal film 13 is disposed between the adjacent taps, the propagation loss can be reduced. This will be described based on specific experimental examples.

(First Embodiment) As a piezoelectric substrate 2, an X-cut 112 ° Y-propagating LiTaO ₃ substrate (2.0 × 5.5 × thickness 0.3) having a first-order negative frequency temperature coefficient is used.
5 mm). On this piezoelectric substrate, a thickness of 2000
After forming an Al film on the entire surface, patterning is performed to form the IDT electrode portion 3, the tap portion 4, and the metal film 13, and 11 chips having a center frequency f ₀ = 200 MHz and a chip rate = 12.5 Mcps. A delay line 1 with a tap was obtained. In this case, the width of the electrode fingers in the IDT electrode portion 3 (split electrode) was 2.04 μm, the pitch between the electrode fingers was 4.09 μm, the number of pairs of electrode fingers was 16, and the cross width was 700. Further, the width of the electrode fingers 5a to 10b constituting each tap in the tap portion 4 is 2.04 μm, the pitch between the electrode fingers at the tap is 4.09 μm, the cross width is 700 μm, and the same split electrode as the IDT electrode portion 3. And The dimension of the metal film 13 along the surface wave propagation direction was 200 mm, and the dimension in the direction perpendicular to the surface wave propagation direction was 700 mm.

For comparison, a tapped delay line of Conventional Example 1 was manufactured in the same manner as above, except that the metal film 13 was not formed. In the delay line with tap of the embodiment and the conventional example 1, an impulse is input and the tap unit 4
The extension waveform taken out from was measured. FIG. 2 shows an expansion waveform due to the tapped delay line of Conventional Example 1 having no metal film 13, and FIG. 3 shows an expansion waveform due to the tapped delay line of the embodiment.

The time scale on the horizontal axis in FIGS. 2 and 3 is 100 ns, and one scale on the vertical axis is 5 ns.
mV. As is clear from a comparison of FIGS. 2 and 3, the tapping delay line of the embodiment has a constant amplitude with respect to the time axis as compared with the tapping delay line of the conventional example 1, so that the propagation loss is reduced. You can see that it is.

The center frequencies of the tapped delay lines of the above embodiment and Conventional Example 1 were measured at various temperatures, and the temperature change of the center frequency was measured. FIG. 4 shows the results. In FIG. 4, the broken line indicates the result of the tapped delay line of the embodiment,
The solid line shows the result of the tapped delay line of Conventional Example 1.

As can be seen from FIG. 4, the center frequency decreases as the temperature increases in both the tapped delay lines of the embodiment and the conventional example 1. This is because an X-cut 112 ° Y-direction propagating LiTaO ₃ substrate having a negative first-order temperature coefficient was used as the piezoelectric substrate.

However, as is apparent from FIG.
In the example (broken line), it can be seen that the decrease in the center frequency due to the temperature rise is smaller than in the conventional example (solid line). That is, it can be seen that the temperature change of the center frequency is suppressed by providing the metal film 13. When the temperature coefficient of the center frequency in the conventional example and the embodiment was calculated based on the measurement results shown in FIG.
pm / ° C., whereas in the example, -13.6 pp
m / ° C.

Thus, when a piezoelectric substrate having a negative first-order temperature coefficient is used, the temperature coefficient of the center frequency can be reduced by forming the metal film 13 as compared with the conventional example. Second Embodiment A quartz substrate is used as the piezoelectric substrate 2,
A tapped delay line according to the second embodiment was manufactured. That is, an ST cut quartz substrate of 1.5 × 15.5 × 0.4 mm thick is formed on the entire surface of the piezoelectric substrate 2 so that Al is formed to a thickness of 2000 °, and the aluminum film is patterned. The IDT electrode part 3, the tap part 4, and the metal film 13 were formed. Here, center frequency 200
A delay line with a 15-chip tap having a MHz and a chip rate of 4 Mcps was configured.

In this case, the width of the electrode fingers in the IDT electrode section 3 (split electrode) was 2.0 μm, and the pitch between the electrode fingers was 3.94 μm. The width of the electrode fingers constituting each tap of the tap portion 4 is 2.0 μm, the pitch between the electrode fingers in the tap is 3.94 μm, and the IDT electrode portion 3
The same split electrode was used. The electrode finger crossing width in the IDT electrode portion and the tap portion was 470 μm.
And The size of the metal film 13 was 400 μm along the surface wave propagation direction, and 470 μm in the direction perpendicular to the surface wave propagation direction.

For comparison, the metal film 13 was not formed, and the electrode finger cross width was 119.3λ (λ:
A surface acoustic wave device of Conventional Example 2 was obtained in the same manner as in the second embodiment except that the wavelength was 15.76 μm. A tapped delay line of Conventional Example 3 was obtained in the same manner as in the second embodiment, except that the metal film was not formed.

With respect to the tapped delay lines of the second embodiment and the tapped delay lines of Conventional Examples 2 and 3, an impulse was input, and the expanded waveform taken out from the tap portion was measured. The results are shown in FIGS.

In FIG. 5 to FIG. 7, one division of time on the horizontal axis is 900 ns, and one division of voltage on the vertical axis is 20 mV. 5 shows the result of Conventional Example 2 and FIG. 6 shows the result of Conventional Example 3.
FIG. 7 shows the result of the second embodiment.

From the comparison between FIG. 5 and FIG. 6, when the electrode finger cross width is changed from 119.3λ to 29.8λ, that is, when the electrode finger cross width is reduced to １ ／, the amplitude is reduced and the propagation loss is reduced. It turns out that it becomes large.

On the other hand, the second
In the delay line with taps of the embodiment of FIG.
7, the electrode finger cross width is 119.3 λ in both the amplitude and the propagation loss, as is apparent from FIG.
It can be seen that this is equivalent to Conventional Example 2 which is

Therefore, by forming the metal film 13, it is possible to prevent the propagation loss and the deterioration of the amplitude, so that the electrode finger crossing width can be reduced, whereby the size of the tapped delay line can be reduced. It turns out that it gets.

The center frequency of the tapped delay line of the second embodiment and the center frequency of the conventional example 3 were measured at various temperatures. FIG. 8 shows the results. In FIG. 8, the broken line indicates the second
And the solid line shows the result of the tapped delay line of Conventional Example 3.

In a delay line with a tap using a quartz substrate, the center frequency is lowered when the temperature rises or falls after a certain temperature (peak temperature). In FIG. 8, in the delay line with tap of the conventional example 3,
While the apex temperature is around 30 ° C., the tapped delay line according to the second embodiment has (calculated −43.5 ° C.)
There is a peak temperature near. That is, the formation of the metal film 13 lowers the peak temperature in the center frequency temperature characteristic by about −73.5 ° C.

However, the peak temperature in the temperature characteristics of the center frequency can be increased by reducing the cut angle of the crystal. This will be described with reference to FIGS. In the tapped delay line according to the second embodiment, using quartz substrates having various cut angles,
In addition, the thickness of the metal film 13 was variously changed, and the peak temperature of the center frequency temperature characteristic was measured. The results are shown in FIGS. FIG. 10 shows the same data as FIG. 9, but the horizontal axis shows film thickness / wavelength. As is clear from FIG. 9, the thickness of the metal film is set to 2000 ° and the cut angle of the quartz crystal is 33.4 ° to 37.4 °. It can be seen that the peak temperature can be increased from -43.5C to 10-50C.

Accordingly, since a quartz substrate having a small cut angle can be used, the unit cost of a wafer can be reduced.
Cost can be reduced. In addition, by using a quartz substrate having a small cut angle, the Al film becomes an epitaxial film, thereby improving power durability and extending operating life.

Also, in FIG. 9, it can be seen that the apex temperature of the center frequency can be changed by changing the thickness of the metal film 13. For example, when trying to obtain a tapped delay line having a vertex temperature in the range of about 10 to 50 ° C. with a 37.4 ° Y-cut crystal, in the second embodiment,
It can be seen that the thickness of the metal film 13 should be in the range of 740 to 2000 °.

Therefore, the cut angle and the thickness of the metal film 13, that is, the thickness / wavelength of the metal film are determined so that the peak temperature on the center frequency temperature characteristic is in the range of 10 to 50 ° C. It can be seen that a tapped delay line with a small change in center frequency with respect to temperature can be provided.

The electrode structure of the tapped delay line according to the present invention is not limited to that shown in FIG. 1 and can be variously modified. That is, JP-A-7-28376
As described in Japanese Unexamined Patent Application Publication No. Hei. 11-18483 and Japanese Unexamined Patent Publication No. H8-18483, a plurality of tap portions may be formed for one IDT electrode portion, or a plurality of ID portions may be formed for one tap portion.
A structure in which a T electrode portion is formed may be used.

[0042]

In the delay line with tap according to the present invention, since the metal film is formed on the piezoelectric substrate between the taps in the tap portion, the propagation loss is improved by forming the metal film. Therefore, not only can the propagation loss of the tapped delay line be improved, but also if the propagation loss is equalized, the electrode finger cross width can be reduced, thereby reducing the size of the tapped delay line. Can be.

When a piezoelectric substrate having a first-order negative frequency temperature coefficient is used as the piezoelectric substrate, the temperature change of the center frequency is suppressed by forming the metal film. Accordingly, a tapped delay line having stable temperature characteristics can be provided.

In particular, an X cut 112 ° as a piezoelectric substrate
When a Y-direction propagating LiTaO ₃ substrate is used, since the substrate has a first-order negative frequency temperature coefficient, by forming a metal film according to the present invention, the propagation loss is improved and the temperature characteristics of the center frequency are improved. A stable tapped delay line can be provided.

In the present invention, when a quartz substrate whose center frequency tends to decrease in both directions of increasing and decreasing the temperature at the apex temperature is used as the piezoelectric substrate, the cut angle and the metal film thickness are reduced. Since the peak temperature changes depending on the thickness, the peak temperature of the frequency temperature characteristic is 10 to 50.
If the cut angle and the metal film thickness / wavelength are selected so as to fall within the range of ° C., a tapped delay line with a small change in center frequency with respect to temperature can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view showing a tapped delay line according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an extension waveform of a tapped delay line of Conventional Example 1.

FIG. 3 is a diagram illustrating an expanded waveform of a tapped delay line according to the first embodiment.

FIG. 4 is a diagram showing the relationship between the amount of deviation from the center frequency of the delay line with tap and the temperature in the first embodiment and the conventional example 1;

FIG. 5 is a diagram showing an extension waveform of a tapped delay line of Conventional Example 2.

FIG. 6 is a diagram showing an extension waveform of a tapped delay line of Conventional Example 3.

FIG. 7 is a diagram showing an expanded waveform of a tapped delay line according to a second embodiment.

FIG. 8 is a diagram showing the relationship between the temperature and the amount of deviation from the center frequency in the tapped delay lines of Conventional Example 3 and the second embodiment.

FIG. 9 is a diagram showing the relationship between the thickness of the metal film and the apex temperature in the tapped delay line of the second embodiment.

FIG. 10 is a diagram showing the relationship between the metal film thickness / wavelength and the peak temperature in the delay line with taps according to the second embodiment.

FIG. 11 is a plan view showing a conventional tapped delay line.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Delay line with a tap 2 ... Piezoelectric substrate 3 ... IDT electrode part 4 ... Tap part 5-10 ... Tap 5a, 5b-10a, 10b ... Electrode finger 11, 12 ... Bus bar 13 ... Metal film

Claims (4)

[Claims] A piezoelectric substrate; an interdigital electrode portion formed on the piezoelectric substrate; and a tap portion formed on the piezoelectric substrate and separated from the interdigital electrode portion in a surface wave propagation direction. Wherein the tap portion has a pair of bus bars, and a tap formed of a plurality of electrode fingers having one end connected to any one of the bus bars, and a metal formed on the piezoelectric substrate between the taps of the tap portion. A tapped delay line, further comprising a film. 2. The tapped delay line according to claim 1, wherein the piezoelectric substrate has a first-order negative frequency temperature coefficient. 3. The delay line with tap according to claim 2, wherein the piezoelectric substrate is an X-cut 112 ° Y-direction propagating LiTaO ₃ substrate. 4. The method according to claim 1, wherein the piezoelectric substrate is a quartz substrate, and the cut angle and the metal film thickness / wavelength are selected so that the peak temperature of the frequency temperature characteristic is in the range of 10 to 50 ° C. Delay line with tap.