JPS5857926B2 - Dansei Hiyoumenhafilta - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to surface acoustic wave filters, and particularly to improving the frequency characteristics of an asymmetric bandpass filter using surface acoustic waves.

FIG. 1 is a simplified configuration diagram of a conventional asymmetric bandpass filter.

In the same figure, an electric signal applied by a signal source 1 is converted into a surface acoustic wave by a comb-shaped output transducer 3 formed on a piezoelectric substrate 2, and propagates on the piezoelectric substrate 2 to form a comb-shaped The output converter 4 converts the signal back into an electrical signal to drive the load 5 .

Furthermore, the input transducer 3 has a frequency that is antisymmetric to that of the symmetrical transducer 6 which has symmetrical frequency characteristics.

It is composed of an antisymmetric converter 7 having numerical characteristics.

The symmetrical transducer 6 is a comb-shaped interdigital electrode in which the interdigital interval is constant and the envelope 8 of the interdigital width changes symmetrically with respect to the electrode center 9 (hereinafter referred to as weighting), The antisymmetric transducer 7 has a constant interdigital spacing, and the envelope 10 of the interdigital width is weighted antisymmetrically with respect to the electrode center 11.
Moreover, it is a comb-shaped interdigital electrode in which the first thickest value 12 gives the maximum intersection with respect to the center of the electrode.

On the other hand, the output converter 4 is a comb-shaped converter (hereinafter referred to as a regular converter) 13° 14 whose interdigital spacing and intersecting width are constant.
It consists of

The crossing widths of the normal converters 13 and 14 are set to be equal to or greater than the maximum crossing of the opposing symmetric converters 6 and antisymmetric converters 7, respectively, and the logarithm has a sufficiently wide band. Therefore, one pair or a small number of pairs are selected.

Also, the surface acoustic wave 1 excited by the symmetrical transducer 6
5 and the surface acoustic wave 16 excited by the antisymmetric transducer 7, the propagation distance 17 of the antisymmetric transducer 7 is equal to the propagation distance 1 of the symmetric transducer 6.
8, the interdigital spacing (i.e., the center frequency f of the filter).

The wavelength λ of surface acoustic waves at .

It is shortened by 1/2 of 1/2).

In order to clarify the meanings of symmetry and antisymmetry in the comb-shaped interdigital electrode, further explanation will be given using the characteristic diagrams of FIGS. 2 and 3.

In the case of the comb-shaped transducer 6 of the symmetrical type, FIG. The tenth interdigital finger 21 has electrodes weighted so that both the polarity and the intersecting width are always equal, and this is called a symmetric transducer.

On the other hand, Figure 3 shows the case where the anti-symmetrical transducer 7 has a comb-shaped shape, and similarly to Figure 2, the cross width in the vertical direction and the distance from the electrode center 22 in the horizontal direction are shown at equal distances from the electrode center 22. The electrodes weighted so that a certain n-th interdigital finger 23 and a -n-th interdigital finger 24 have opposite polarities and equal crossing widths are called an antisymmetric converter.

FIG. 4 is a frequency characteristic diagram of the filter shown in FIG. 1, with relative loss (dB) plotted on the vertical axis and frequency plotted on the horizontal axis.
．． Reference numerals 26 indicate the characteristics of the symmetrical type 6 and antisymmetric type converter 7, respectively, and the overall characteristic 27 is asymmetrical with respect to the center frequency due to the effect of the antisymmetric type converter 7.

However, when such a conventional asymmetrical bandpass surface acoustic wave filter is used as a video intermediate frequency filter of a television receiver, the frequency characteristics of the filter are monotonically asymmetrical, resulting in the following drawbacks.

As is well known, the frequency characteristics of a video intermediate frequency circuit are mostly determined by the video intermediate frequency filter used in the circuit.

In the frequency characteristics of the filter, if the attenuation in the audio carrier fs is insufficient, a 920 KHz beat signal between the audio carrier and the color subcarrier will be generated in the video detection circuit, causing significant disturbance to the image.

In a video intermediate frequency circuit that uses a synchronous detection method, there are usually 26
Attenuation of approximately dB or more is required.

On the other hand, the ink carrier reception method is generally used for audio signals, and if the audio carrier wave is attenuated too much, the 4 to 5 MHz beat signal, which is the audio intermediate frequency signal, becomes smaller, making it difficult to operate the audio circuit. do.

Therefore, it is necessary to suppress the attenuation of the audio carrier wave to about 30 dB.

From the above, the level of the audio carrier wave is approximately 26 to 30 dB.
must be controlled within the range of

However, in the case of a conventional asymmetric bandpass filter, the audio carrier wave has a slope part 2 with an asymmetric band characteristic as shown in FIG.
It will be placed at 8.

Therefore, there is a margin of tolerance for parallel shifts in the frequency characteristics of the filter due to variations in material constants and temperature characteristics of the piezoelectric substrate 2, and variations in interdigital spacing caused by the process of forming the comb-shaped interdigital electrodes. This was a small problem and was a big problem in practice.

In particular, when a single crystal such as LiNbO3 is used as the substrate material, the effect of variations in temperature characteristics is significant, and in the case of ceramics such as PZT, the effects of variations in material constants are significant.

Furthermore, surface wave filters with a Barker cord type electrode arrangement have been used in the past, but this is a type of phased code in which a pair of interdigitated electrodes are arranged at a fixed pitch, and the intersecting width and pitch are constant. Since the sign, that is, the phase of the electrode follows an arrangement called a Barker code, the linearity of the phase is not guaranteed.

Therefore, in order to guarantee phase linearity, the pitch of the weighting electrodes is constant, and the cross width is symmetrical (antisymmetric) with respect to the electrode center.
is required.

Taking these points into consideration, the present invention provides at least one input/output transducer consisting of comb-shaped interdigital electrodes formed on a piezoelectric substrate, where the interdigital spacing is constant and the envelope of the intersecting width is the electrode. It consists of two types: a so-called symmetrical transducer with weights that change symmetrically with respect to the center, and an antisymmetric transducer with a constant interdigital spacing and an envelope of the interdigital width that is weighted antisymmetrically with respect to the center of the electrode. The bandpass characteristic of the filter is locally stepped, and at least one extremely thick value is formed between the maximum value of the envelope of the cross width in the antisymmetric converter and the center of the electrode. It is designed to have
It is also designed to have a portion where the crossing width is zero at a location other than the center of the electrode, and also to have at least one or more points of inflection.

In particular, when this is used in a video intermediate frequency filter of a television receiver, the above-mentioned defects can be eliminated by adding a step of a required level near the audio carrier wave.

Note that the surface acoustic wave filter according to the present invention can also be applied to cases where a plurality of signals having different levels are selectively extracted from a composite signal.

An embodiment of the present invention shown in FIG. 5 will be described below.

In the figure, the same or corresponding parts as in FIG. 1 are indicated by the same reference numerals.

Also, for convenience, it is assumed that the case is the same as that shown in FIG. 1 except that the envelope characteristic of the cross width of the antisymmetric converter is different.

That is, 2 is a piezoelectric substrate, 6 is a symmetrical transducer formed from 61 pairs of comb-shaped interdigital electrodes that constitute the main half of the input transducer 3, and 13 and 14 are respectively the output transducer 4. 1
This is a regular transducer consisting of four pairs of comb-shaped interdigital electrodes.

Others 5 are loads, 15.16 are surface acoustic waves, 17
18 is the propagation distance as shown in FIG.

However, in the present invention, as described above, the antisymmetric converter 7'
consists of 51 pairs of comb-shaped interdigital electrodes, which together with the symmetrical transducer 6 constitute the input transducer 3 as in the case of FIG. The line 29 indicates the point 30 of the maximum value outside the electrode center 11.
is set so that it is maximum.

6 and 7 are logarithmic (dB) representations of the frequency characteristics of the embodiment shown in FIG. 5 according to the present invention, with the frequency MHz on the horizontal axis and the relative loss (dB) on the vertical axis. There is.

In FIG. 6, 31 is a frequency characteristic line of the symmetrical converter 6, and 32 is a frequency characteristic line of the antisymmetric converter 7' according to the present invention.

The main band 32a and each sidelobe band of the frequency characteristic of the antisymmetric converter 7' are sufficiently narrower than the band characteristics of the symmetric converter 6, and the main band 32a and the first sidelobe band are is sufficiently attenuated compared to the band characteristics of the symmetrical converter 6, and does not give significant ripples to the in-band characteristics.

On the other hand, since the second side lobe 32b is at almost the same level as the band characteristic, the center frequency f.

In contrast, the addition effect on the low frequency side and the subtraction effect on the high frequency side appear prominently.

Therefore, the overall characteristic becomes an asymmetrical frequency characteristic with a step only near the audio carrier on the low frequency side, and a narrow band on the high frequency side.

Reference numeral 33 in FIG. 7 is a comprehensive characteristic based on the frequency characteristics of the symmetric converter and the frequency characteristics of the antisymmetric converter.

Thus, according to the present invention, a stage 33a of about 30 dB is provided near the audio carrier due to the band characteristics of the surface acoustic wave filter.

Compared to the conventional case where the point of the audio carrier wave is placed on the sloped portion, by placing the point on the stepped portion of the band characteristic, a remarkable effect can be obtained in terms of the tolerance to parallel shift of the frequency characteristic.

Furthermore, the operation according to an embodiment of the present invention is described in the eighth and ninth degrees io,
This will be explained according to Figure i1.

As is well known, there is a similar relationship between the shape of the envelope of the cross width of the comb-teeth progressive transducer and the shape of the impulse response of the transducer.

On the other hand, the frequency characteristics of just any network can be expressed as the Fourier transform of the impulse response of said network.

Therefore, the frequency characteristics of the transducer can be obtained by Fourier transforming the shape of the envelope of the transverse width of the transducer, which is a function of position, expressed as a time function, that is, the impulse response, via the sound velocity of the surface acoustic wave. I can do it.

FIGS. 8 and 9 illustrate this relationship.

First, in Figure 8, λ. is equal to twice the intersecting spacing of the comb-shaped interdigital electrodes, and the center frequency f of the bandpass characteristic of the transducer.

This is the wavelength of surface acoustic waves corresponding to .

is an integer representing the logarithm from the center of the electrode of the transducer, and the number of the transducers is 2N+1 pairs.

The curve 34 is the envelope h/(λ0+
), normalized within the maximum intersection.

Now, if the speed of sound of a surface acoustic wave is ■, then the horizontal axis λ represents the position in FIG.

i can be converted to a time axis according to the relationship , and FIG. 9 is obtained.

In FIG. 9, curve 35 is the impulse response h(t) of the transducer, and the envelope h'(λ) of the cross-width of the transducer of FIG.

1) It is connected to 34 by equation (1).

Also, T is equal to 2λ6N/V (=2N/fo).

In this way, the transducer cross-width envelope can be expressed as the impulse response of said transducer.

In order to quantitatively demonstrate the effects of the present invention, Figure 10 shows the impulse response, that is, the envelope of the cross width, of the transducer determined in (A)-(B) of Table 1 below, in the right half from the center of the electrode. is shown in absolute value.

Since the impulse response h(t) shown in Table 1 is an odd function, the frequency characteristic 1 jH(f) l can be obtained by Fourier sine transformation, which is shown in Table 2.

(A')-(B') in Table 2 correspond to (A) and (B) in Table 1, respectively.

FIG. 11 illustrates Table 2.

The conventional type shown in FIG. 1 corresponds to type B, which is close to A in FIG. 10, and the embodiment of the present invention shown in FIG. 5 corresponds to type D, which is close to E in FIG.

With the conventional characteristics AI and B/ shown in Figure 11, it is difficult to locally step the frequency characteristics because the main band is too wide and the attenuation of the side lobe is too large compared to the main band. However, according to the characteristics c/, D/, and E/ of FIG. 11 according to the present invention, the main band and each sidelobe band are narrow, and the sidelobe level is not significantly attenuated.

These features make it possible to locally step the in-band characteristics as shown in one embodiment of the present invention in FIGS. 6 and 7.

From the above, by using an antisymmetric converter having an envelope with a cross width characterized by C-E in Fig. 10,
A filter having locally stepped frequency characteristics suitable for a video intermediate frequency filter of a television receiver can be obtained.

Note that the shape of the envelope shown in FIG. 10 used in the above description does not limit the present invention, and any shape having the same tendency may have the same effect as the embodiment described above. Needless to say.

That is, (1) there is at least one local maximum value between the maximum value of the envelope of the cross width and the center of the electrode; (n) the maximum value of the envelope of the cross width and the center of the electrode; If there is a part in between where the crossing width is zero (1), there is at least one inflection point, if necessary, between the thickest value giving the maximum value of the crossing width envelope and the center of the electrode. All cases are included in the present invention.

Here, the three conditions mentioned above are shown diagrammatically in Figure 12, where Figures A and B are for (1), and Figures C and D are for (II).
The conditions of , Figures E and F indicate the conditions of
Figure G shows the conventional antisymmetric zero pole case.

The above description has been about a filter for selecting an audio carrier signal from a video intermediate frequency signal at a special level compared to other signals in a television receiver. The present invention can also be applied to a filter that selects one or more of the plurality of signals at a specific level, ie, a stepped film other than for television.

Further, in the configuration diagram of an embodiment of the present invention as shown in FIG. 5, the signal source and the load are connected in parallel, but the connection can be made either in parallel or in series.

Also, one side of the antisymmetric converter is used as the input converter,
Of course, it can also be used as an output converter.

Furthermore, the surface wave 36°37 radiated in the opposite direction to the output transducer
In order to utilize It is also possible to improve the loss.

Further, it is not necessary to divide the output normal type converter into two for a symmetric type converter and an antisymmetric type converter, and it is also possible to use a single converter having a cross width that covers both types.

Furthermore, the same applies when both input and output converters are weighted respectively.

As described above, according to the present invention, when used as an intermediate frequency filter for a color television receiver, an asymmetrical bandpass characteristic with a step of a desired level near the audio carrier wave can be obtained, and piezoelectric It is possible to significantly expand the tolerance for parallel shifts in frequency characteristics due to variations in temperature characteristics and material constants of a substrate having a .

Furthermore, the electrode according to the present invention is not designed using coded electrodes such as Parker code or Golay code, but is designed using the least squares method.
It can be designed using approximate methods such as the minimax method, linear programming, or Fourier series expansion method.

[Brief explanation of drawings]

Figure 1 is a simplified configuration diagram of a conventional asymmetric bandpass filter.
2 and 3 are characteristic diagrams of the comb-shaped interdigital electrodes of the symmetrical transducer and the antisymmetrical transducer, respectively; FIG. 4 is the frequency characteristic diagram of the filter of FIG. 1; and FIG. 5 is the characteristic diagram of the filter according to the present invention. A simplified configuration diagram of an embodiment of a stepped asymmetric band-pass filter, FIG. 6 is a frequency characteristic diagram of the symmetrical converter and anti-symmetrical converter of the filter in FIG. 5, and FIG. 7 is a diagram of the frequency characteristics of the filter in FIG. FIG. 8 is a cross-width envelope diagram of the antisymmetric converter according to the present invention, FIG. 9 is an impulse response diagram of the converter, and FIG. 10 is a detailed diagram of the present invention. Various impulse response diagrams for explanation; Figure 11 is a frequency characteristic diagram corresponding to the various impulse responses in Figure 10; Figures 12 A-F are 3
Figure G is a conventional tube line diagram diagrammatically showing the conditions. In the figure, 1 is a signal source, 2 is a piezoelectric substrate, 3 is an input transducer, 4 is an output transducer, 6 is a symmetrical transducer, 7' is an antisymmetric transducer, and 13 and 14 are normal transducers. vessel.

Claims (1)

[Claims] 1. At least one of the input/output transducers consisting of comb-shaped interdigital electrodes formed on a piezoelectric substrate has a constant interdigital interval and an envelope of the interdigital width relative to the center of the electrode. The filter is composed of two types: a symmetrical transducer in which the interdigital spacing is constant and the envelope of the intersecting width is weighted in an antisymmetrical manner with respect to the center of the electrode. The bandpass characteristic is locally stepped, and has at least one extremely thick value between the maximum value of the envelope of the cross width in the antisymmetric converter and the center of the electrode. A surface acoustic wave filter characterized by: 2 At least one of the input/output transducers formed from a comb-shaped interdigital electrode formed on a piezoelectric substrate has a constant interdigital interval and an envelope of the interdigital width that changes symmetrically with respect to the center of the electrode. It is composed of a weighted symmetrical transducer and an antisymmetric transducer in which the interdigital spacing is constant and the envelope of the intersecting width is weighted antisymmetrically with respect to the center of the electrode, and the bandpass characteristic of the filter is localized. The cross width of the antisymmetric transducer is stepped and has a portion where the cross width becomes zero between the maximum value of the envelope of the cross width and the center of the electrode other than the center of the electrode. A surface acoustic wave filter featuring: 3 At least one of the input/output transducers operated from comb-shaped interdigital electrodes formed on a piezoelectric substrate has a constant interdigital interval and an envelope of the interdigital width that changes symmetrically with respect to the center of the electrode. It is composed of two weighted symmetrical transducers and an antisymmetric transducer in which the interdigital spacing is constant and the envelope of the intersecting width is weighted antisymmetrically with respect to the electrode center, and the bandpass characteristic of the filter is localized. It is characterized by having at least one inflection point between the maximum value of the envelope of the cross width in the antisymmetric transducer and the center of the electrode. surface acoustic wave filter.