JP2539549B2 - Surface acoustic wave filter - Google Patents

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 device filter.

[0002]

2. Description of the Related Art As shown in FIG. 1, a surface acoustic wave device filter generally includes an input electrode 2 for converting an electric signal into a surface acoustic wave, a surface acoustic wave propagation path 4 and a surface acoustic wave on a surface of a piezoelectric substrate 1. It is composed of an output electrode 3 for further converting the surface acoustic wave into an electric signal. As the input and output electrodes 2 and 3, an interdigital transducer (hereinafter abbreviated as IDT) having a structure in which the toothless electrodes 5 and 6 intersect each other as shown in FIG. 2 is usually used. The frequency characteristic of this surface acoustic wave filter is determined by the shape of the IDT,
Usually, one of the input IDT and the output IDT is a so-called normal type IDT in which the cross width Wi and the pitch Pi shown in FIG. 2 are constant, and in most cases, a desired characteristic is obtained by the shape of the other IDT.

By the way, the cross width Wi and the pitch P of the IDT
i is ID with respect to the direction parallel to the propagation direction of the surface acoustic wave.
When it changes symmetrically in both directions from the center of T, the frequency characteristic of the group delay time becomes a flat characteristic.

[0004]

For example, in the conventional color television receiver using the 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 as a whole is not always flat, and there is a problem particularly in the characteristics near the color signal band. That is, the group delay characteristic changes due to the influence of the chroma band pass filter, the 4.5 MHz trap, etc., causing inconveniences such as cross color interference and deterioration of the chroma response characteristic, and the group delay time characteristic of the surface acoustic wave filter becomes non-flat. It was necessary to do so (in the above case, the low frequency side of the intermediate frequency filter is advanced).

An object of the present invention is to provide a surface acoustic wave filter having a non-flat group delay characteristic with a small number of electrodes.

[0006]

The case of the above television receiver will be described below as an example. According to the research conducted by the present inventors, the aforementioned chroma band pass filter and 4.5 MH
Since the color signal is delayed due to the influence of the z-trap, it has been found that the group delay characteristic of the intermediate frequency filter may be corrected in reverse to achieve the above object. As mentioned above,
Generally, the color signal is delayed in the television receiver, but this is also clear, for example, in Japanese Patent Application Laid-Open No. 48-3038 (see page 2, upper left column, lines 8 to 15 of the same). As a concrete group delay characteristic of the intermediate frequency filter, from the video carrier frequency fp to the color signal carrier frequency fc
± 50 ns in the frequency band up to +500 KHz
Shows a flat characteristic with the following deviation, fc + 500KH
The frequency is progressively lower than z, and fc-500KH
It is desirable that z is several hundreds of nanoseconds ahead of the flat region, but it is extremely difficult to realize such a characteristic with a conventional LC filter, and if a surface acoustic wave filter described below is used, It has been found that such characteristics can be easily realized. That is, when the changes in the cross width Wi and the pitch Pi are not symmetrical, the frequency characteristics of the group delay time are not flat from the theoretical examination by the present inventors, and the cross width Wi and the pitch Pi are not flat.
It was found that the desired group delay characteristic can be obtained by the value of.

Based on one of the results, by producing an IDT pattern having the following characteristics, one having desired characteristics can be obtained with a small number of electrodes.
That is, 1) The position where the crossing width of the electrode fingers is maximized is made different from the center of the electrode.

2) The crossing width of the electrode fingers is asymmetrical in the direction approaching the normal form IDT and in the direction moving away from the position where the crossing width of the electrode fingers is the maximum , and the main lobe.
The electrode finger pitches of the
The electrode finger pitch of the main lobe is the same as the electrode finger pitch of the main lobe.
Composed of different pitches.

The above point 2) is necessary to obtain an arbitrary amplitude characteristic and group delay characteristic, and the point 1) is a characteristic that the group delay time near the chrominance signal carrier is advanced from near the center frequency with a small number of electrodes. It is necessary to realize it. The reason
It will be described in detail in the "Operation" section below .

[0010]

Now, the intermediate frequency filter of the television receiver will be described as an example. Based on point 2)
The width of the electrode fingers that consists of
The main lobe and the size are
Setting the electrode finger pitch of the drobe, as well as the main lobe and
Group delay of the signal to be processed depending on the position of the idrobe
It becomes possible to correct the characteristics to desired characteristics. vice versa,
If the electrode finger cross width is symmetrical, set the electrode finger pitch.
Fixed, and changing the position of the main lobe and side lobe
The group delay characteristics of the signal to be processed to the desired characteristics.
Cannot be corrected.

Based on the above, the electrode finger cross width is made asymmetrical.
The electrode finger pitch of the main lobe to the center of the intermediate filter.
The pitch is responsive to signals near the frequency, and evenly spaced.
The electrode finger pitch of the side lobe near the color signal carrier.
Of the main lobe.
Place the probe close to the other normal form IDT.
With, group delay characteristics near the color signal carrier with center frequency
It is possible to proceed compared to the near.

Luminance signals, such as color television signals
And processes signals that are composed by frequency-multiplexing color signals, etc.
In the central frequency band to be processed (corresponding to the luminance signal)
Shows that the group delay characteristics are relatively free of distortion,
The group delay characteristic of the signal (corresponding to the color signal) is not flat.
Often, there is no group delay characteristic in the entire signal band.
There are cases where the desired frequency characteristic does not become the desired characteristic.

In this case, the frequency characteristic in the entire band of the signal is
The desired characteristic is the frequency characteristic of the filter.
Can be corrected. However, frequencies in the center frequency band
If the number characteristic is not distorted, it is not necessary to correct it.

On the other hand, the frequency characteristics in the peripheral band are as described above.
Since it often distorts like this, the frequency characteristics of this band are corrected.
do it. Due to the correction, the filter output is the signal
The frequency characteristic becomes a desired characteristic without distortion in all bands.

In the surface acoustic wave filter, the total delay
If the extension characteristic is non-flat, the main lobe is
The signal is processed and the side lobes are in the peripheral frequency band.
Can be configured to process signals. From this,
Signal to obtain the desired frequency characteristics over the entire band of the signal .
To make it a characteristic, set each lobe as follows.
Yes.

In the present invention, the frequency characteristics of the center frequency band are
Since the property is flat, there is no need to correct it.
The electrode finger pitch of is set to the pitch corresponding to the signal of the center frequency.
Make it evenly spaced. Also, the frequency characteristics in the peripheral band are distorted.
Therefore, the electrode finger pitch of the side lobe is set as the main lobe.
Are electrode fingers corresponding to the above-mentioned peripheral band at different pitches.
Arrange as a pitch. This allows for this peripheral band
It is possible to correct the frequency characteristic.

The electrode finger pitch of the main lobe should be evenly spaced.
The frequency characteristics of the group delay time near the center frequency
It has flat characteristics. The luminance signal located near this center frequency
Since the frequency characteristic of the group delay time of signal is flat,
The image quality on the surface does not deteriorate.

Next, based on the above point 1), downsizing is planned.
In order to obtain the desired characteristics with a limited number of electrode pairs (eg 50 pairs), all of the electrode finger patterns are provided in order to ensure that the information near the color signal carrier is sufficient.
Rather than the other normal form IDT
Electrode finger crossing width position side, since it has an average small-Pas <br/> turn from the opposite side, small Ri yo electrodes intersecting side
Remove the causes the crossing width of the electrode fingers shifted <br/> from around the position of the maximum.

Conventionally, the main lobe portion sandwiched between the side lobe portions is usually weighted so as to be located at the center of the transducer, whereas in the present invention, the electrode crossing is smaller.
The main lobe portions from deleting the old side is shifted from the transducer center can be realized miniaturization of the surface acoustic wave filter.
An embodiment will be described below by taking an intermediate filter of a television receiver as an example.

[0020]

DETAILED 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, the cross widths and pitches of the input IDTs specifically created are shown in FIGS. 3 and 4 in correspondence with the IDT numbers. FIG.
And FIG. 4 has the two main points of the invention described above.

That is, as is clear from FIG.
~ The pitch of the main lobe part formed by the 80th IDT
Since the pitch is a constant value, the pitch of the main lobe is
Become. Further, the pitch of the side lobe portion formed by the 10th to 30th IDTs is made slightly larger than the pitch of the main lobe portion formed by the 40th to 80th IDTs. The main lobe portion mainly responds to the high frequency component of the video signal and the signal near the carrier of the color signal, and the main lobe portion mainly responds to the low frequency component of the video signal, that is, the signal near the center frequency of the filter. Furthermore, since the positional relationship between the input and output IDTs is determined so that the side lobe portion is closer to the output IDT than the main lobe portion, the signal near the color signal carrier is advanced in time with respect to the signal near the center frequency of the filter. The above intermediate filter
Intermediate frequency stage provided with (frequency characteristics illustrated in FIG. 6)
Is the spectrum of the baseband signal
On the other hand, the high and low frequencies are reversed.

FIG. 5 shows the input IDT used at this time. The arrow A indicates the propagation direction of the surface acoustic wave, and the output normal form IDTs are arranged opposite to this direction. Figure
The same parts as those in FIG. 1 are designated by the same reference numerals. Comb-shaped
The electrodes 5'and 6'correspond to 5 and 6 in FIG. Again 7
Reference numerals a and 7b denote terminals for inputting a high-frequency electric signal, and reference numerals 8 and 9 denote a sound absorbing material for preventing reflected waves at the end face of the piezoelectric substrate and a ground electrode for preventing electromagnetic coupling between the input and output IDTs. This input IDT satisfies the above points 1) and 2)
I have. That is, "the position where the crossing width of the electrode fingers is maximized,
"Different from the center of the electrode", "The cross width of the electrode fingers
Approaching the normal form IDT from the position where the finger cross width is maximum
Is asymmetrical in both direction and away, and
The electrode finger pitch of the main lobe is arranged at equal intervals,
The electrode finger pitch of the side lobe is the electrode of the main lobe.
It is composed of a pitch different from the finger pitch "
It

According to the research conducted by the present inventors, as described above, due to the influence of the chroma band pass filter and the 4.5 MHz trap arranged in the subsequent stage of the present filter, the color signal is several hundreds.
It turned out that it would be delayed by ns. JP-A-48-3
Japanese Patent Laid-Open No. 038 discloses that the color signal is delayed by 600 ns due to the above-mentioned influence. Therefore, in order to correct the delay of the color signal, the surface acoustic wave filter described above was created. FIG. 6 has a shape of the IDT of FIGS. 3 and 4.
15 pairs of normal form IDs as a pair of input IDT and output IDT
FIG. 6 shows amplitude characteristics and group delay time characteristics with respect to frequency when T and T are used. As is apparent from FIG. 6, the frequency characteristic of the group delay time is a good flat characteristic of about 750 ± 50 ns in the frequency band from the video carrier frequency fp to the color signal carrier frequency fc + 500 KHz, and the frequency characteristic of fc + 500 KHz or less. Then, fc is about 650 ns , fc-500 KHz is about 300 ns, and the difference between the values in the flat region and fc-500 KHz is about 450 ns.

The color signal carrier frequency and the center frequency
If the difference in group delay time between
Intersection composed of electrode pitch corresponding to carrier frequency
Place the part closer to the other IDT side and set the maximum intersection position.
The IDT patterns so that they are farther from the center.
By changing the width and pitch of the group delay time
The degree of progress can be increased. Also, one of the I
If the characteristics cannot be borne only by the DT, both IDTs
Needless to say, it is also the purpose of the present invention to apply a fixed ratio to the above.

[0026]

As is apparent from the above description, according to the present invention, a surface acoustic wave filter having a non-flat group delay time characteristic can be easily obtained with a small number of electrodes and can be miniaturized.

[Brief description of drawings]

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

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

FIG. 3 is an IDT crossing width and ID according to an embodiment of the present invention.
It is a figure which shows the relationship with T number.

FIG. 4 is a pitch and ID of an IDT according to an embodiment of the present invention.
It is a figure which shows the relationship with T number.

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

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

[Explanation of symbols]

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshihiko Noro, 292 Yoshida-cho, Totsuka-ku, Yokohama, Kanagawa, Ltd. Home Appliances Research Laboratory, Hitachi, Ltd. (56) Reference JP-A-51-76946 (JP, A) JP 48-3038 (JP, A) JP-A-52-135646 (JP, A) Japanese Patent Publication No. 4-5291 (JP, B2) R.K. L. Miller & A. J. Devices "A SIMPLE (BUILDING BLOCK) METH OD FOR THE DESIGN OF SAW FILTERS HAVING NON-LINEAR PHA SE RESPNSE. 1976, UITrathonics Symposium. IEEE Cat, # 76CH 1120-5SU, P. 533-557 IEEE TRANSACTIONS, ON MICRWAVE THEO RY AND TECHNIQUES, Vol. MTT-24, No. 1, JA N, 1976. P. 5

Claims (1)

(57) [Claims] 1. A surface acoustic wave filter comprising a piezoelectric substrate and an input interdigital transducer and an output interdigital transducer provided on the piezoelectric substrate, wherein the input interdigital transducer and the output interdigital transducer are provided. One of the interdigital transducers described above is such that the crossing width of the electrode fingers varies depending on the crossing position of the electrode fingers.
Is formed by an electrode composed of a main lobe and a side lobe, and the position where the crossing width of the electrode fingers is maximum is different from the center of the electrode, and the size of the crossing width of the electrode fingers is The envelope function obtained from is asymmetric with respect to the center of the electrode, and the electrode of the main lobe
The finger pitch is evenly spaced and the side lobe voltage is
The pole finger pitch is different from the electrode finger pitch of the main lobe
A surface acoustic wave filter characterized by being composed of a pitch .