JPH045291B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a surface acoustic wave filter that realizes novel characteristics as an intermediate frequency filter for a color television receiver.

[Prior art]

As shown in FIG. 1, a surface acoustic wave filter generally has an input electrode 2 for converting an electric signal into a surface acoustic wave on the surface of a piezoelectric substrate 1, a propagation path 4 for the surface acoustic wave, and a surface acoustic wave. Furthermore, an output electrode 3 for converting into an electric signal is constructed. The input and output electrodes 2 and 3 are interdigital transducers (hereinafter referred to as
(abbreviated as IDT) is usually used. The frequency characteristics of this surface acoustic wave filter are determined by the shape of the IDT, but
Normally, either input IDT or output IDT is
In most cases, a so-called normal IDT with constant intersection width Wi and pitch Pi shown in FIG. 2 is used, and desired characteristics are obtained by changing the shape of the other IDT.

By the way, the intersection width Wi and pitch Pi of IDT are
For the direction parallel to the propagation direction of surface acoustic waves,
When changing symmetrically in both directions from the center of the IDT, the frequency specification of the group delay time has a flat characteristic.

[Problem that the invention seeks to solve]

In conventional color television receivers that use surface acoustic wave filters as described above, even if the frequency characteristics of the group delay time of the intermediate frequency filter are flat, when viewed as a whole circuit, it is not necessarily flat. In particular, there were problems with the characteristics near the color signal band. That is, the group delay characteristics change due to the effects of the chromavant pass filter, 4.5MHz trap, etc., resulting in inconveniences such as cross color interference and deterioration of chroma response characteristics.

SUMMARY OF THE INVENTION An object of the present invention is to provide an intermediate frequency filter using surface acoustic waves with a small number of electrodes and having characteristics that compensate for the group delay characteristics of an external circuit in order to obtain good group delay characteristics as a whole circuit. be.

[Means for solving problems]

According to the inventors' research, the color signal is delayed due to the effects of the chroma bandpass filter and 4.5MHz trap, so in order to achieve the above objective, the group delay characteristics of the intermediate frequency filter must be reversed. Just correct it. 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). As a concrete group delay characteristic of an intermediate frequency filter, the color signal carrier frequency is calculated from the video carrier frequency p.
In the frequency band between c + 500 KHz, it has a flat characteristic with a deviation of about ±50 ns or less, and at frequencies below c + 500 KHz, it progresses sequentially, and at c - 500 KHz, it is about several hundred ns ahead compared to the above flat region. is desirable, but such characteristics
It is extremely difficult to achieve this with an LC filter, but it has been found that it is possible to easily achieve such a characteristic by using the surface acoustic wave filter described below. That is, the intersection width Wi
If the changes in pitch and pitch Pi are not symmetrical, the inventors' theoretical study shows that the frequency characteristics of the group delay time will not be flat, and the intersection width Wi and pitch
It was found that it is possible to obtain the desired group delay characteristics by changing the value of Pi.

Based on one of the results, in intermediate frequency filters for color TVs in particular, IDTs with the following characteristics, in which the group delay time around the color signal carrier is advanced compared to around the center frequency, are used.
By creating patterns, products with desired characteristics can be obtained. In other words, (1) The position of the maximum intersection width is the other normal form IDT
However, it is located far from the center.

(2) The intersection width and pitch change asymmetrically in the direction toward and away from the normal form IDT from the position of the maximum intersection width.

Point (2) above is necessary to obtain arbitrary amplitude characteristics and group delay characteristics, and (1) is necessary to obtain characteristics in which the group delay time near the color signal carrier is more advanced than around the center frequency with a small number of electrodes. It is necessary to achieve this goal. The reason for this will be explained in the [Effect] section below.

[Operation] In this surface acoustic wave filter, the group delay time near the color signal carrier is advanced compared to the center frequency, so when viewed on the IDT pattern, the information near the color signal carrier is in the other normal form.
It exists closer to the IDT, and more information near the center frequency is gathered in a farther location.

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

Therefore, the information in the vicinity of the color signal carrier can be sufficiently captured as needed, and the number of electrode pairs can be limited (e.g. 50
In order to obtain the desired characteristics in pair), it is important to shift the position where the intersection width is maximum to a position far from the center with respect to the normal form IDT. To explain this in more detail, by bending the group delay time characteristics as described above, the electrode intersection and pitch become asymmetrical with respect to the maximum electrode intersection position, and in this case, the lobe with less information is Since it was found that the characteristics hardly changed even if the lobe was deleted, the inventors deleted the lobe with less information in order to make the lobe smaller. 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 weighted so that it is located at the center of the converter relative to the normal IDT. and move it to the far side.

〔Example〕

Hereinafter, specific embodiments of the present invention will be described according to the drawings.

As an example of the present invention, specifically created input
When IDTs are illustrated with their intersecting widths and pitches corresponding to IDT numbers, they are as shown in FIGS. 3 and 4. 3 and 4 contain the two main points of the invention described above.

In other words, the pitch of the side lobe formed by the 10th to 30th IDTs is the same as that of the 40th to 80th IDTs.
The pitch of the main lobe formed by the IDT is slightly larger than that of the main lobe, and as a result, 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 low frequency components of the video signal. It responds to frequency components, 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 form IDT of the output opposite to this direction is arranged. Further, 7a and 7b are terminals for inputting high-frequency electric signals, and 8 and 9 are ground electrodes for preventing electromagnetic coupling between the sound absorbing material for preventing reflected waves at the end face of the piezoelectric substrate and the input/output IDT, respectively.

According to the inventors' research, as mentioned above, due to the effects of the chroma bandpass filter and 4.5MHz trap placed after this filter, the color signal is reduced by several hundred nanoseconds.
It turned out that I would be late. Said Japanese Unexamined Patent Application Publication No. 1973-
Publication No. 3038 indicates 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.

Figure 6 shows the amplitude delay and group delay time characteristics with respect to frequency when 49 pairs of input IDTs having the IDT shapes shown in Figures 3 and 4 and 15 pairs of normal IDTs are used as output IDTs. It shows. As is clear from Figure 6, the frequency characteristics of the group delay time vary from the video carrier frequency p to the color signal carrier frequency.
In the frequency band between c + 500KHz, approximately 750±50ns
It has good flat characteristics, and at frequencies below c + 500KHz, c is about 650ns, and c - 500KHz is about 650ns.
300ns, progressing sequentially, and the flat region and c-500K
The difference in values in Hz is about 450ns.

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.

〔Effect of the invention〕

As is clear from the above explanation, by employing the surface acoustic wave filter of the present invention in the intermediate frequency filter of a color television receiver, characteristics that compensate for the group delay characteristics of the external circuit can be easily obtained with a small number of electrodes. Furthermore, compared to conventional LC filters, it is smaller and requires no adjustment.

[Brief explanation of drawings]

Figure 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 is a diagram showing the relationship between the intersecting width of the IDT and the IDT number, which is an embodiment of the present invention, and FIG. 4 is also an embodiment of the present invention. Figure 5 is a pattern diagram of the input IDT, and Figure 6 is a diagram showing the relationship between the pitch of the IDT and the IDT number as shown in Figures 3 and 4. FIG. 7 is a characteristic diagram showing amplitude characteristics and group delay time characteristics with respect to frequency in another embodiment. 1...Piezoelectric substrate, 2...Input IDT, 3...Output IDT, 4...Surface acoustic wave propagation path.

Claims (1)

[Claims] 1. A piezoelectric substrate, an input interdigital transducer provided on the piezoelectric substrate and converting an electrical signal into a surface acoustic wave, and an input interdigital transducer formed in the piezoelectric substrate and converting the input interdigital transducer into a surface acoustic wave.・Equipped with a propagation path for propagating the surface acoustic waves generated by the transducer and an output interdigital transducer for converting the surface acoustic waves propagated in the propagation path back into an electrical signal, In a surface acoustic wave filter used as an intermediate frequency filter in a color television receiver, either the input interdigital transducer or the output interdigital transducer is connected to the main lobe section. It is composed of weighted electrodes including a side lobe portion, and the maximum position of the interdigital transducer is located farther from the center of the electrode than the other interdigital transducer, and The width and pitch of the electrodes change asymmetrically in the direction toward and away from the other interdigital transducer from the position of the maximum crossing width, and the pitch of the electrode is closer to the other interdigital transducer than the center. The group delay characteristic of the intermediate frequency filter of the television receiver is approximately ±50 nsec or less in the frequency band between the video carrier frequency _p and the color signal carrier frequency fc + 500 kHz. It is a flat characteristic of the deviation of fc + 500 kHz, and it progresses sequentially at frequencies below fc + 500 kHz, and at fc - 500 kHz, it is several hundred
1. A surface acoustic wave filter characterized in that the surface acoustic wave filter is formed so as to have a lead time of about nanoseconds, and is configured to compensate for group delay characteristics caused by other circuits inside a color television receiver.