JPH07212178A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To miniaturize a TV and a VTR, etc., by defining the distance between the power leaders of adjacent normalized type electrodes of plural normalized type electrodes as a specified value and obtaining a surface acoustic wave filter where the suppression degree of a passing band end trap is not deteriorated. CONSTITUTION:Each pair of two weighing electrodes 2 and 3 and normalized type electrodes 4 and 5 opposed to each of the electrodes 2 and 3 are formed on a sheet of piezoelectric substrate 1 by the electrode pitch and the number of electrode according to a desired frequency characteristic. The shortest distance alpha between the shortest between the normalized type electrodes 4 and 5 is set to 1.7lambda<alpha<800mum when the wavelength of a surface acoustic wave is defined as lambda. Between the weighing electrodes 2 and 3 and the normalized type electrodes 4 and 5, a shield electrode 6 is arranged. In the characteristic figure of a BPF for IF of video frequency 38.9 MHz TV, a signal is inputted between the input terminals 20 and 21 of a surface acoustic wave filter and the signal is taken out from output terminals 41 and 51. At this time, an input terminal 31 is grounded. In the example of an NTSC-US channel, the signal is inputted between the terminals 20 and 31, and the signal is outputted from terminals 41 and 51.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a television (TV).
The present invention relates to a surface acoustic wave filter used in a receiver, a video tape recorder (VTR), a CATV and the like.

[0002]

2. Description of the Related Art In a surface acoustic wave filter used in an audio circuit or a video circuit of a TV receiver or the like, a specific filter characteristic is selected from a plurality of filter characteristics in which center frequencies are close to each other and pass bands are partially overlapped with each other. In the surface acoustic wave filter to be selectively taken out, as shown in FIGS. 8A and 8B for audio, and as shown in FIGS. 9A and 9B for video. , A filter characteristic having a pass band peak at _two adjacent frequencies f ₁ and f ₂ corresponding to an audio signal or a video signal and having sufficiently attenuated amplitude outside each pass band is required.

In response to such a demand, conventionally, a plurality of surface acoustic wave filters housed in individual packages have been used. In this case, the individual surface acoustic wave filters have the structure shown in FIG.
As shown in FIG. 3, on the piezoelectric substrate 1, a weighting electrode 2 having a variable crossing length is arranged on the input side, and a normal type electrode 4 having a constant crossing length is arranged on the output side so as to face the weighting electrode.
A shield electrode 6 is arranged between both electrodes, and the substrate is housed in a separate package.

On the other hand, like the video signal filter shown in FIG. 10 (a) and the audio signal filter shown in FIG. 10 (b), although the pass bands partially overlap, the center frequencies f ₁ and f ₂ are In the surface acoustic wave filter which takes out both signals independently at the same time when they are sufficiently separated, as shown in FIG.
On one piezoelectric substrate 1, a plurality of weighting electrodes 2 and 3 are provided on the input side, and regular electrodes 4 and 5 facing each of the weighting electrodes are provided on the output side. The electrodes are connected in parallel, and an input signal is applied between both ends 21, 31 of the electrodes and the intermediate electrode pad 20, and both ends 41, 42 or 51, 52 of each of the plurality of normal type electrodes are applied.
A type in which an audio signal is taken out from one side and a video signal is taken out independently from the other side simultaneously is used.

Further, as shown in FIGS. 11 (a) and 11 (b), in the surface acoustic wave filter for a voice signal in which the center frequencies f ₁ and f ₂ are sufficiently separated and the pass bands are not overlapped, As shown in FIG. 7, a plurality of weighting electrodes 2 and 3 are provided on the input side on one piezoelectric substrate 1, and normal type electrodes 4 and 5 facing each of the weighting electrodes are provided on the output side. , An arbitrary weighting electrode is selected from the plurality of weighting electrodes, an input signal is applied to the weighting electrode, the plurality of normal type electrodes are connected in parallel to each other, and both ends 4 thereof are connected.
There is used a type in which a plurality of types of filter characteristics are selectively obtained for each weighting electrode to which an input signal is applied by extracting an output signal from Nos. 1 and 51.

[0006]

As described above, when a specific filter characteristic is selectively extracted from a plurality of filter characteristics in which the center frequencies are close to each other and a part of the pass band is overlapped, each filter characteristic is selected. The surface acoustic wave filter corresponding to is formed on an individual piezoelectric substrate and used in a plurality of individual packages, but this increases the number of parts, increases the cost of the product, and increases the entire device. It also hinders the miniaturization of, and is not preferable in practice.

On the other hand, when a surface acoustic wave filter for selectively extracting a specific filter characteristic from a plurality of filter characteristics whose center frequencies are close to each other and whose pass bands partially overlap each other is formed on one piezoelectric substrate, Since the center frequencies of the filters are close and the electrodes of adjacent filters are close to each other, normal electrodes other than the normal electrodes facing the weighted electrodes to which the input signal is applied are excited by the surface acoustic wave interference. However, when this interference signal is mixed with the original signal and output, and surface acoustic wave filters corresponding to each filter characteristic are formed on individual piezoelectric substrates and packaged in individual packages, multiple units can be used. In comparison, the degree of suppression of the pass band edge trap becomes worse. When the suppression degree of the pass band edge trap is deteriorated, the suppression of the interfering signal becomes insufficient, which is a serious problem in practical use.

However, regarding the problem of surface acoustic wave interference, in the surface acoustic wave filter of the type whose characteristics are shown in FIGS. 10 and 11, the center frequencies f ₁ and f _{2 of the two} filter characteristics are sufficient. Since they were separated from each other, there was relatively little interference of surface acoustic waves with each other, which was not a problem.

The present invention relates to a surface acoustic wave filter of the type whose characteristics are shown in FIGS. 8 and 9, that is, a specific filter characteristic from a plurality of filter characteristics in which center frequencies are close to each other and a part of pass bands are overlapped. The present invention provides a surface acoustic wave filter that selectively removes the surface acoustic wave filter even if the surface acoustic wave filter is formed on a single piezoelectric substrate without deteriorating the degree of suppression of pass band edge traps.

[0010]

In a surface acoustic wave filter according to the present invention, a plurality of weighting electrodes having varying crossing lengths are provided on a single piezoelectric substrate, and a constant crossing length is provided to face each of the weighting electrodes. A plurality of normal type electrodes are provided, an arbitrary weighting electrode is selected from the plurality of weighting electrodes, an input signal is applied to the weighting electrodes, and the plurality of normal type electrodes are connected in parallel and output from both ends thereof. In a surface acoustic wave filter from which a signal is taken out, the distance d between the electrode finger ends of adjacent normal type electrodes among the plurality of normal type electrodes is 1.7λ <d when the wavelength of the surface acoustic wave is λ. <800 μm, more preferably, the gap between adjacent weighting electrodes among the plurality of weighting electrodes is
It is characterized in that it is expanded with distance from the plurality of normal type electrodes, and a sound absorbing material is arranged in the gap.

[0011]

In the surface acoustic wave filter having the above structure, one weighting electrode corresponding to a desired filter characteristic is selected from the plurality of weighting electrodes, an input signal is applied, and a plurality of normal type electrodes connected in parallel are selected. Output signals are taken out from both ends, and desired filter characteristics are selectively obtained by the selected weighting electrode and the normal type electrode facing the weighting electrode.

At this time, if the distance d between the electrode finger ends of the adjacent normal type electrodes is 1.7 λ or less, the suppression of the pass band edge trap is deteriorated due to the interference effect of the surface acoustic wave.
If 1.7λ <d, the interference can be suppressed to a level at which practically no problem occurs.

On the other hand, as the distance d increases, the influence of interference decreases, but the size of the piezoelectric substrate also increases, so that it cannot be stored in a standard package for TV or VTR IF band (30 to 60 MHz). There is a problem in terms of downsizing and cost reduction. When the distance d is 800 μm or less, it can be stored in a standard package having a width of 6 mm, a length of 16 mm and a height of 4 mm.

[0014]

EXAMPLES Examples of the present invention will be described below.

In the surface acoustic wave filter according to the first embodiment of the present invention, as shown in FIG. 1, each pair of the two weighting electrodes 2 and 3 and the normal type electrodes 4 and 5 facing each other is shown in FIG. 8 (a) and 8 (b) are formed on one piezoelectric substrate 1 with the electrode pitch and the number of electrodes according to the desired frequency characteristics, and the normal type electrodes 4 and 5 are connected in parallel to each other, The shortest distance d between the electrode finger ends of the electrodes 4 and 5 is set to be 1.7λ <d <800 μm, where λ is the wavelength of the surface acoustic wave. Further, the shield electrode 6 is arranged between the weighting electrodes 2 and 3 and the normal type electrodes 4 and 5.

FIG. 12 is a characteristic diagram of a bandpass filter for a TV IF having a video frequency of 38.9 MHz according to the first embodiment of the present invention. FIG. 12A shows PAL-B / G,
I, D / K channels, (b) of FIG. 12 is NTSC-U
The filter characteristics for each audio signal used in the S channel multi-system are shown. The solid line in the figure is
Adopting the configuration of the first embodiment of the present invention shown in FIG. 1, the shortest distance d of the electrode finger ends of the adjacent normal type electrodes is 400 μm (PA
Center frequency of L-B / G, I, D / K channels 32.
7 is a frequency characteristic of a surface acoustic wave filter in which d = 3.37λ at 7 MHz). The broken line adopts the configuration of the comparative example shown in FIG. 7, and the distance d is 200 μm (32.7 in the above).
(Equivalent to d = 1.68λ in MHz).

Here, as the material of the piezoelectric substrate 1 of these surface acoustic wave filters, L of 128 ° Y cut-X propagation is used.
iNbO ₃ (sound velocity = 3870 m / s) is used, and the crossing length (aperture length) of the normal type electrodes is PAL-B / G, I, D /
The K channel is 500 μm, the NTSC-US channel is 400 μm, and the chip size is 1.8 mm × 1.
The size is 0.4 mm, which is a size that can be stored in a standard TV or VTR package.

PAL-B / G, I, D / of FIG. 12 (a)
In order to obtain the K channel filter characteristic, a signal is input between the input terminals 20 and 21 of the surface acoustic wave filter shown in FIG. 1, and a signal is taken out from the output terminals 41 and 51. At this time, the input terminal 31 is grounded. On the other hand, in order to obtain the filter characteristics of the NTSC-US channel of FIG. 12B, the input terminals 20 and 3 of the surface acoustic wave filter shown in FIG.
A signal is input during 1 and a signal is taken out from the output terminals 41 and 51. At this time, the input terminal 21 is grounded.

As can be seen from FIG. 12A, in the first embodiment of the present invention (solid line), PAL-B / G,
The trap suppression degree of 34.47 MHz, which is a high-frequency side trap in the pass band of the I and D / K channels, is improved by 8 to 12 dB as compared with the comparative example (broken line). this is,
Center frequencies of both channels are 32.7 MHz and 34.4
Since it is close to MHz, a comparative example (d = 1.68
In the configuration of λ), the normal type electrode 5 of the NTSC-US channel, which should not be excited originally, is excited by the interference of the surface acoustic wave, and the frequency characteristic by the normal type electrode 4 of the PAL-B / G, I, D / K channels is obtained. While the characteristics are deteriorated because they are combined and output, the first embodiment of the present invention (d = 3.3).
This is because the 7λ) configuration suppresses the excitation of the NTSC-US channel due to the above interference.

Further, in the first embodiment of the present invention (solid line) and the comparative example (broken line), the waveform in the band is different depending on the presence or absence of the interference. That is, although the filter characteristic of the comparative example (d = 1.68λ) has a larger deviation from the theoretical calculated value,
As described above, if the degree of agreement between the theoretical calculation value and the actual filter characteristic is poor, it becomes difficult to design a filter having a desired characteristic, and in this respect also, the configuration of the first embodiment of the present invention is advantageous.

FIG. 13 is a graph showing the relationship between the trap suppression degree of 34.47 MHz and the shortest distance d between the electrode finger ends of the adjacent normal type electrodes. As can be seen from FIG. 13, the larger the distance d, the better the trap suppression degree.

FIG. 14 is a characteristic diagram of a bandpass filter used in a multi-system for a TV IF having a video frequency of 38.0 MHz according to the second embodiment of the present invention. FIG. 14A shows PAL-B. / G, I, D / K channel audio signal filter characteristics, (b) of FIG. 14 is NTSC-U
The filter characteristic for the audio signal of S channel is shown.

In this embodiment, as shown in FIG.
Adopting a surface acoustic wave filter configuration like this,
The shortest distance d between the electrode fingers of the standard electrode is 400 μm (PA
Center frequencies of LB / G, I, D / K channels 31.
At 8 MHz (corresponding to d = 3.27λ), the piezoelectric substrate 1
As a material, LiNbO of 128 ° Y cut-X propagation ₃
(Sound velocity = 3870 m / s) is used and the opening of the normal type electrode
Length is 600 for PAL-B / G, I, D / K channels
μm, NTSC-US channel is 400 μm,
Chip size is 1.9 x 10.4 mm, standard T
It becomes the size that can be stored in the package for V and VTR.
There is.

PAL-B / G, I, D / of FIG. 14 (a)
In order to obtain the K channel filter characteristic, a signal is input between the input terminals 20 and 21 of the surface acoustic wave filter shown in FIG. 1, and a signal is taken out from the output terminals 41 and 51. At this time, the input terminal 31 is grounded. On the other hand, in order to obtain the filter characteristics of the NTSC-US channel of FIG. 12B, the input terminals 20 and 3 of the surface acoustic wave filter shown in FIG.
A signal is input during 1 and a signal is taken out from the output terminals 41 and 51. At this time, the input terminal 21 is grounded.

As can be seen from FIG. 14, in FIG. 14 (a), 33.57M, which is a high frequency side trap in the pass band of the PAL-B / G, I, D / K channels.
For the Hz trap, in FIG.
25d for the 34.42 MHz trap, which is the high frequency side trap in the pass band of the TSC-US channel
A good degree of suppression of B or higher is obtained.

The surface acoustic wave filter according to the third embodiment of the present invention is obtained by further improving the structure of the surface acoustic wave filter shown in FIG. 1, and as shown in FIG. The inclination angle of each weighting electrode is set so as to spread away from the normal type electrodes 4 and 5 (toward the left side of the drawing), and the sound absorbing material 7 is applied to the gap.

According to this structure, the degree of suppression of the pass band edge trap is further increased as compared with the structure shown in FIG. 3 in which the gap between the weighting electrodes 2 and 3 is narrow and no sound absorbing material is arranged in the gap. Be improved.

FIG. 15 is a characteristic diagram of a bandpass filter for an IF of a TV having a video frequency of 38.0 MHz according to the fourth embodiment of the present invention. FIG.
25MHz PAL-B / G, I, D / K channels,
FIG. 15B shows NTSC with a center frequency of 35.75 MHz.
-Shows the filter characteristics for each video signal used in the multi-system of the US channel.

The solid line in the figure indicates the surface acoustic wave filter according to the fourth embodiment of the present invention, that is, as shown in FIG. 4, as the gap between the weighting electrodes 2 and 3 moves away from the normal type electrodes 4 and 5 (see the figure). The inclination angle of each weighting electrode is set so as to expand (toward the left side), the sound absorbing material 7 is applied to the gap, and the distance d between the electrode finger ends of the output-side regular electrodes 4 and 5 is set to 500 μm. Is a frequency characteristic of the surface acoustic wave filter, and the broken line indicates the inclination angle of each weighting electrode so that the gap between the weighting electrodes 2 and 3 becomes wider as it goes away from the normal electrodes 4 and 5 (toward the left side of the figure). The distance d between the electrode fingers of the normal electrodes 4 and 5 on the output side is set.
Is also set to 500 μm, but this is the frequency characteristic of the surface acoustic wave filter in which the sound absorbing material 7 is not applied to the gap. Here, the material of the piezoelectric substrate 1 of these surface acoustic wave filters is 128 ° Y. Cut-X propagation Li
NbO ₃ (sound velocity = 3870 m / s) is used, and the aperture length of the normal type electrode is 1340 μm for PAL-B / G, I, D / K channels and 1180 for NTSC-US channels.
μm, the chip size is 3.7 × 10.6 mm,
It is a size that can be stored in a standard TV or VTR package.

PAL-B / G, I, D / of FIG. 15 (a)
In order to obtain the K channel filter characteristic, a signal is input between the input terminals 20 and 21 of FIG.
The signal is taken out from 51. At this time, the input terminal 31 is grounded. On the other hand, in order to obtain the filter characteristic of the NTSC-US channel of FIG. 12B, the input terminal 20 of FIG.
A signal is input between 31 and a signal is taken out from the output terminals 41 and 51. At this time, the input terminal 21 is grounded.

As can be seen from FIG. 15, FIG.
(A) PAL-B / G, I, D / K channels, FIG.
In any of the NTSC-US channels of FIG. 5 (b), when the sound absorbing material is not applied to the gap between the weighting electrodes, the amplitude characteristic and the group delay characteristic in the pass band are distorted and a ripple appears. When the material is applied, the ripple is sufficiently suppressed. This is because the center frequencies of both channels are 35.25MHz and 35.75.
If the sound absorbing material is not applied because it is close to MHz, the normal type electrode, which should not be excited originally, does not face the weighting electrode to which the input signal is applied and is excited by the surface acoustic wave interference. This is because the characteristics are deteriorated because they are combined with the frequency characteristics of the normal type electrodes facing the applied weighting electrodes and output, whereas in the configuration where the sound absorbing material is applied, the excitation due to the interference action is suppressed. is there.

The case where the surface acoustic wave filter from which two kinds of filter characteristics are selectively extracted is formed on one piezoelectric substrate has been described above. However, the present invention selectively selects three or more kinds of filter characteristics. It can also be applied to the case where the surface acoustic wave filter to be taken out is formed on one piezoelectric substrate.

[0033]

According to the present invention, in a surface acoustic wave filter for selectively extracting a specific filter characteristic from a plurality of filter characteristics whose center frequencies are close to each other and whose pass bands partially overlap, one piezoelectric substrate is used. Provided is a surface acoustic wave filter which does not deteriorate the suppression of pass band edge traps even when formed above, and a TV using the surface acoustic wave filter,
The miniaturization and cost reduction of VTRs are promoted.

[Brief description of drawings]

FIG. 1 is a plan view showing the structure of a surface acoustic wave filter according to an embodiment of the present invention.

FIG. 2 is a plan view showing the structure of a surface acoustic wave filter according to an embodiment of the present invention.

FIG. 3 is a plan view showing a configuration of a surface acoustic wave filter according to an embodiment of the present invention.

FIG. 4 is a plan view showing a configuration of a surface acoustic wave filter according to an embodiment of the present invention.

FIG. 5 is a plan view showing a configuration of a surface acoustic wave filter according to a conventional example.

FIG. 6 is a plan view showing a configuration of a surface acoustic wave filter according to a conventional example.

FIG. 7 is a plan view showing a configuration of a surface acoustic wave filter according to a conventional example.

FIG. 8 is a filter characteristic diagram to be realized by the present invention.

FIG. 9 is a filter characteristic diagram to be realized by the present invention.

FIG. 10 is a filter characteristic diagram to be realized by the conventional example.

FIG. 11 is a filter characteristic diagram to be realized by the conventional example.

FIG. 12 is a filter characteristic diagram according to an embodiment of the present invention.

FIG. 13 is an experimental result diagram according to an example of the present invention.

FIG. 14 is a filter characteristic diagram according to an embodiment of the present invention.

FIG. 15 is a filter characteristic diagram according to an embodiment of the present invention.

[Explanation of symbols]

 1 Piezoelectric substrate 2, 3 Weighting electrode 4, 5 Normal type electrode 6 Shield electrode

Claims (3)

[Claims] 1. A plurality of weighting electrodes having varying crossing lengths are provided on one piezoelectric substrate, and a plurality of normal type electrodes having a constant crossing length are provided to face each of the weighting electrodes. Any weighting electrode is selected from the weighting electrodes, an input signal is applied to the weighting electrode, the normal electrodes are connected in parallel, and an output signal is extracted from both ends of the surface acoustic wave filter, When the distance d between the electrode finger ends of the adjacent normal type electrodes among the plurality of normal type electrodes is the wavelength of the surface acoustic wave,
A surface acoustic wave filter having a setting of 1.7λ <d <800 μm. 2. A gap between adjacent weighting electrodes among the plurality of weighting electrodes is widened with increasing distance from the plurality of normal type electrodes, and a sound absorbing material is arranged in the gap portion. The surface acoustic wave filter according to claim 1. 3. A plurality of types of filter characteristics in which the center frequencies are close to each other within 2.5 MHz and a part of the pass band is overlapped are selectively obtained for each weighting electrode to which the input signal is applied. The surface acoustic wave filter according to claim 1 or 2.