JPH04211507A - Acoustic surface wave filter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter that achieves novel characteristics as an intermediate frequency filter for color television receivers.

0002

[Prior Art] Generally, surface acoustic wave device filters are shown in FIG.
As shown in FIG. 1, an input electrode 2 for converting an electric signal into a surface acoustic wave, a propagation path 4 for the surface acoustic wave, and an output electrode 3 for further converting the surface acoustic wave into an electric signal are provided on the surface of the piezoelectric substrate 1. Consists of. Input and output electrodes 2, 3
For example, comb-like electrodes 5 and 6 as shown in FIG.
An interdigital transducer (hereinafter abbreviated as IDT) having a structure in which the transducers cross each other is usually used. The frequency characteristics of this surface acoustic wave filter are determined by the shape of the IDT, but usually, either the input IDT or the output IDT is a so-called normal type IDT with a constant intersection width Wi and pitch Pi shown in FIG. In most cases, desired characteristics are obtained by changing the shape of the other IDT.

By the way, when the intersection width Wi and pitch Pi of the IDT change symmetrically in both directions from the center of the IDT with respect to the direction parallel to the propagation direction of the surface acoustic wave,
The frequency characteristic of the group delay time becomes a flat characteristic.

0004

[Problems to be Solved by the Invention] In a conventional color television receiver using a surface acoustic wave filter as described above, even if the frequency characteristic of the group delay time of the intermediate frequency filter is flat, the circuit When viewed as a whole, it was not necessarily flat, and there were problems particularly with the characteristics near the color signal band. That is, the group delay characteristics change due to the influence of the chroma bandpass filter, the 4.5 MHz trap, etc., resulting in problems such as cross color interference and deterioration of the chroma response characteristics.

An object of the present invention is to provide an intermediate frequency filter using a surface acoustic wave, which has characteristics that compensate for the group delay characteristics of an external circuit, with a small number of electrodes, in order to obtain good group delay characteristics for the entire circuit. , to provide.

[0006]

[Means for Solving the Problem] According to the research conducted by the present inventors, since the color signal is delayed due to the effects of the aforementioned chroma band pass filter and 4.5MHz trap, in order to achieve the above object, What is necessary is to reversely correct the group delay characteristic of the intermediate frequency filter. As mentioned above, color signals are generally delayed in television receivers, and this is also clear, for example, in Japanese Patent Application Laid-Open No. 48-3038 (page 2, upper left column, lines 8 to 15). (see line). Specifically, the group delay characteristic of an intermediate frequency filter is a flat characteristic with a deviation of about ±50 ns or less in the frequency band from the video carrier frequency fp to the color signal carrier frequency fc + 500 KHz, and progressively progresses at frequencies below fc + 500 KHz. , at fc-500KHz, it is desirable to have a characteristic that is several hundred ns ahead of the above-mentioned flat region, but it is extremely difficult to achieve such characteristics with conventional LC filters, so the surface acoustic wave filter described below It has been found that such characteristics can be easily achieved by using . In other words, if the changes in the crossover width Wi and pitch Pi are not symmetrical, the frequency characteristics of the group delay time will not be flat according to the theoretical study of the present inventors, and the desired value will be determined by the values of the crossover width Wi and pitch Pi. It turns out that it is possible to obtain group delay characteristics.

[0007] Based on one of the results, an intermediate frequency filter for a special color television has the following characteristics in which the group delay time near the color signal carrier is advanced compared to the vicinity of the center frequency. By creating an IDT pattern, one having desired characteristics can be obtained. That is, 1) The position of the maximum crossing width is far from the center of the other normal IDT. 2) The crossing width and pitch change asymmetrically in the direction toward and away from the normal IDT from the position of the maximum crossing width.

Point 2) above is necessary to obtain arbitrary amplitude characteristics and group delay characteristics, and 1) is necessary to obtain a characteristic in which the group delay time near the color signal carrier is advanced from the vicinity of the center frequency by using a small number of electrodes. It is necessary to realize this. The reason for this will be explained in the [Effect] section below.

[0009]

[Operation] In this surface acoustic wave filter, the group delay characteristics near the color signal carrier are advanced compared to those near the center frequency, so when viewed on the IDT pattern, the information near the color signal carrier is compared to the other normal form IDT. information near the center frequency is concentrated in locations farther away.

Furthermore, since the amplitude level near the center frequency is higher than that near the color signal carrier, the width of the intersection is large on the IDT pattern. Therefore, this satisfies the above-mentioned condition 1), that is, "the position of the maximum crossing width is
It is located far from the center with respect to the other normal form IDT. ” can be derived.

[0011] Therefore, information in the vicinity of the color signal carrier can be sufficiently contained as necessary, and with a limited number of electrode pairs (for example, 5
In order to obtain the desired characteristics with 0 pairs), it is important to shift the position where the crossing width is maximum to a position far from the center with respect to the normal IDT. In other words, normally the main lobe section sandwiched between the side lobe sections is weighted so that it is located at the center of the converter, but in the present invention, the main lobe section is positioned from the center of the converter to the normal IDT. As a result, the group delay time near the center frequency to which the main lobe portion mainly responds is delayed from the group delay time near the color signal frequency to which the side lobe portion mainly responds.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below with reference to the drawings.

As an embodiment of the present invention, specifically created input IDTs are shown in FIGS. 3 and 4, with their crossing widths and pitches corresponding to IDT numbers. 3 and 4 have the two main points of the invention mentioned above.

That is, the pitch of the side lobe portions formed by the 10th to 30th IDTs is 40th to 8th IDTs.
The pitch is slightly larger than the pitch of the main lobe formed by the 0th IDT, so that the side lobe mainly responds to the high frequency components of the video signal and signals near the carrier of the color signal, and the main lobe mainly responds to the signal near the carrier of the video signal. It responds to low-frequency components of the signal, that is, signals near the center frequency of the filter. Furthermore, since the positional relationship of the input and output IDTs is determined so that the side lobe section is closer to the output IDT than the main lobe section, the signal near the color signal carrier is temporally advanced from the signal near the center frequency of the filter.

FIG. 5 shows the input IDT used at this time. Arrow A represents the propagation direction of the surface acoustic wave, and the normal IDTs with outputs opposing this direction are arranged. Also,
7a and 7b are terminals for inputting high frequency electric signals;
Each represents a ground electrode that prevents electromagnetic coupling between the sound absorbing material for preventing reflected waves at the end surface of the piezoelectric substrate and the input/output IDT. According to the research of the present inventors, as mentioned above, the chroma bandpass filter and the chroma bandpass filter placed after this filter,
．． It was found that the color signal was delayed by several hundred ns due to the influence of the 5MHz trap. Said Japanese Unexamined Patent Publication No. 48-3038
The publication discloses that the color signal is delayed by 600 ns due to the above effect. Therefore, in order to correct the delay of the color signal mentioned above, the above surface acoustic wave filter was created. FIG. 6 shows 49 pairs of input IDTs having the shapes of the IDTs in FIGS. 3 and 4, and 15 pairs of normal-form IDTs as output IDTs.
This shows the amplitude characteristics and group delay time characteristics with respect to frequency when using As is clear from FIG. 6, the frequency characteristic of the group delay time is a good flat characteristic of approximately 750±50 ns in the frequency band between the video carrier frequency fp and the color signal carrier frequency fc+500 KHz, and is flat at frequencies below fc+500 KHz. About 65 at fc
0 ns and about 300 ns at fc-500 KHz, and the difference between the values in the flat region and fc-500 KHz is about 450 ns. It goes without saying that the degree of advance of the group delay time can be changed as necessary by changing the width and pitch of the IDT patterns. Furthermore, it goes without saying that the gist of the present invention is to have both IDTs bear the burden in a fixed proportion if it is not possible to bear the burden on only one IDT.

[0016]

[Effects of the Invention] As is clear from the above explanation, by adopting the surface acoustic wave filter of the present invention as an intermediate frequency filter of a color television receiver, characteristics that compensate for the group delay characteristics of an external circuit can be achieved with a reduced number of electrodes. Furthermore, compared to conventional LC filters, it is smaller and requires no adjustment.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the principle of a surface acoustic wave filter.

FIG. 2 is a model diagram showing the structure of an IDT.

FIG. 3: Intersection width and ID of an IDT that is an embodiment of the present invention
It is a figure showing the relationship with T number.

FIG. 4 is a diagram showing the relationship between IDT pitch and IDT number, which is also an embodiment of the present invention.

FIG. 5 is a pattern diagram of an input IDT.

6 is a characteristic diagram showing amplitude characteristics and group delay time characteristics with respect to frequency of the Taichi embodiment in which an input IDT having the IDT crossing width and pitch shown in FIGS. 3 and 4 is provided; FIG.

[Explanation of symbols]

1... Piezoelectric substrate, 2... Input IDT, 3... Output IDT, 4... Surface acoustic wave propagation path

Claims (1)

[Claims] a piezoelectric substrate; an input interdigital transducer provided on the piezoelectric substrate to convert an electrical signal into a surface acoustic wave; and an input interdigital transducer formed in the piezoelectric substrate to propagate the surface acoustic wave generated by the input interdigital transducer. In the surface acoustic wave filter, the input interdigital transducer and the output interdigital transducer are provided. One of the interdigital transducers is composed of weighted electrodes including a main lobe part and a side lobe part, and the position of the maximum crossing width of the electrodes is
The interdigital transducer is located at a position different from the center position of the electrode, and the crossing width and pitch of the electrode are configured to change asymmetrically in the direction toward and away from the other interdigital transducer from the position of the maximum crossing width. A surface acoustic wave filter characterized by: