JPS5947494B2 - surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter that has improved group delay characteristics and is miniaturized.

Various types of filters, which are elements of electronic circuit components, have been developed and put into practical use by utilizing various physical and chemical phenomena, but two main types of characteristics are generally considered important.

One of these is the frequency characteristic of amplitude, and the second is the frequency characteristic of group delay time, and each characteristic is determined to meet the required specifications and put into practical use.

Even with conventionally used filters, such as solid filters, it is difficult to control both of the above characteristics completely and obtain the most preferable filter characteristics due to the nature of the physical and chemical phenomena used. Often accompanied by sex,
In many cases, it is not possible to be free.

On the other hand, so-called surface acoustic wave filters that utilize surface acoustic waves propagating on the surface of piezoelectric materials have the advantage that both of the above characteristics can be controlled relatively independently. For example, in principle, it is easy to obtain a filter in which the frequency characteristic of amplitude changes in a very complex manner, and the frequency characteristic of group delay time is very flat.

However, further investigation reveals that if a filter is designed to keep the frequency characteristics of the group delay time flat over the entire frequency band in a filter whose amplitude frequency characteristics change in a complex manner, the surface acoustic wave filter's input end or output end Configure,
The so-called interdigital transducer c is abbreviated as IDT.

) requires a very large number of electrode pairs, which not only increases various unnecessary secondary effects, but also has the disadvantage that it is not possible to reduce the size of the filter.

Taking an intermediate frequency filter for a television receiver as a specific example, and explaining it according to the drawings, Fig. 1 shows the amplitude frequency characteristic 1 (hereinafter abbreviated as amplitude characteristic) of a conventional filter.
and the frequency characteristic 2 of the group delay time (hereinafter abbreviated as delay time characteristic).As can be seen from the figure, the frequency characteristic 2 of the group delay time is flat.

The amplitude characteristic 1 of the intermediate frequency filter of the television receiver has a center frequency f as shown by curve A in FIG. 1 near the audio carrier frequency f8.

It is desirable to have an asymmetric amplitude characteristic with respect to the input voltage, but for this purpose, the number of electrode pairs of the input side IDT to be weighted (equal to half the maximum electrode number) is required to be about 70 pairs.

In this case, the electrodes of the output IDT were normal and 15 logarithmic.

Furthermore, when the number of electrode pairs of the input side IDT is 50, the amplitude characteristic 1 shown in FIG. 1 becomes as shown by curve B,
Center frequency f.

It is difficult to obtain desired amplitude characteristics by simply reducing the number of electrode pairs.

That is, in order to realize a complex amplitude characteristic as shown by curve A in FIG. 1, it is necessary to provide about 70 pairs of electrodes as the input side IDT, which makes it impossible to downsize the surface acoustic wave filter.

An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a surface acoustic wave filter suitable for miniaturization.

The present invention utilizes the fact that the frequency characteristic of the group delay time of a surface acoustic wave filter is determined independently of the frequency characteristic of the amplitude. The group delay time in at least one of the upper and lower frequency regions where the amplitude is reduced by 20 dB is advanced by 100+1 seconds or more as a group delay time difference compared to the value in the flat region, or In order to delay, in the distribution of the intersection widths of the comb-shaped electrodes inserted into each other, the maximum intersection width position of the intersection width distribution component corresponding to the main frequency wave and the maximum intersection of the intersection width distribution component corresponding to the main sub-frequency wave. By creating a cross-width distribution that is separated and synthesized by an interval at least equal to the product of the group delay time difference of 100 ns and the surface acoustic wave velocity, the group within the remaining frequency band is This reduces the frequency characteristics of the delay time and realizes the burden by reducing the number of electrode pairs of the IDT.

The principle will be briefly explained below using the drawings.

FIG. 2 is an explanatory diagram showing model characteristics of the frequency characteristics of amplitude and the frequency characteristics of group delay time, which are assumed for convenience of explanation, on the same coordinate axis.

In the figure, 1 shows the amplitude characteristics, and the nose shows the group delay time characteristics, and Fig. 3 a, b, and c show the model characteristics shown in Fig. 2 for each frequency band with amplitudes of 31.32, 33 and group delay times of 41.42. This possibility is easily shown from the integral theorem inherent in the mathematical expression described below.

Now, generally speaking, to know the frequency characteristics of a certain circuit, it is sufficient to apply a signal containing all frequency components, that is, an impulse, to the circuit, and then look at the frequency components of the response, that is, the Fourier transform.

Conversely, in order to obtain a circuit with amplitude characteristics and two-phase characteristics (if the frequency characteristics of the group delay time are given, it can be expressed as the integral of the group delay time over the frequency), find the inverse transform of the Fourier transform, It is clear that the circuit can be designed to have that impulse response.

Therefore, when an inverse Fourier transform is applied to obtain an impulse response function for each of the above component characteristics, the resulting impulse response function has a group delay time t1 with respect to the model characteristics shown in FIG. and the amplitude is a
and two of the first and second waves corresponding to 1 (respectively Fig. 3a).

(shown on the right side of FIG. 3C) and a third wave (shown on the right side of FIG. 3C) having group delay time t2 and having an amplitude of 1.

Further listing the wave characteristics of each term, the first wave is t
= wave packet with maximum amplitude at t1, and the second wave is at t = t
1 is the wave packet with the maximum amplitude.

The third wave is a wave packet having a maximum amplitude at approximately t=t2.

Therefore, when the second wave is a main frequency wave and the first and third waves are sub-frequency waves, it is the third wave that has a large influence on the group delay time t1, as t2 has a large effect, As described above, it can be seen that the group delay time difference path t2-t□ has a practical influence.

On the other hand, the impulse response (distribution) is proportional to the intersection width (distribution) of the comb-shaped electrodes of the surface acoustic wave filter, so once the frequency characteristics of the amplitude and group delay time are determined as above, the surface acoustic wave filter The scale (size) of the electrode is required.

This means that the frequency band of the necessary group delay time remains the same.
The other frequency bands are changed in order to reduce the electrode size using the group delay time difference.

In other words, it is shown that it is possible to appropriately determine the group delay time difference to suit the purpose and eliminate the tail of the cross width distribution in order to downsize the electrode scale.

What has been described above is related to the model characteristics shown in Figure 2, but the basic technical ideas described here are also essential for the required characteristics of surface acoustic wave filters used in practical applications. It will be easily understood that this applies to

Embodiments according to the present invention will be described below with reference to the drawings.

In Fig. 4, a and b are input side IDs which are embodiments according to the present invention.
FIG. 4A shows the relationship between the IDT crossover width (arbitrary scale) and the electrode number, and FIG. 4A shows the relationship between the electrode pitch and the electrode number.

The number of electrode pairs is 50, which is 1/2 of the maximum IDT number.

In this case, the output side IDT is a normal type, and the number of electrode pairs is 15.

FIG. 5 shows the amplitude characteristic 5 and group delay time characteristic 6 of a device having the shape and dimensions shown in FIG. , the group delay time in the low frequency side band and the high frequency side band of the 20 dB lower frequency is a surface acoustic wave filter that is delayed by about 100 ns compared to the above-mentioned flat region, and at the same time, the center wave number f.

It has an asymmetric amplitude characteristic 5 with respect to

The number of electrode pairs has been reduced by a little less than 30 inches, and the size has actually been reduced by 30 inches.

FIG. 6 shows another embodiment of the invention, in which the input side I
FIG. 7 is a characteristic diagram showing amplitude characteristics 7 and group delay time characteristics 8 when the DT has 50 pairs of electrodes and the output side IDT has a configuration of 15 normal pairs.

In other words, the group delay time in the frequency bandwidth where the amplitude characteristic is reduced by 3 dB is flat, and the group delay time in the low frequency band where the frequency is reduced by 20 dB is approximately 750n compared to the flat region.
It is a surface acoustic wave filter that advances by s, and at the same time has a center frequency f
.

It can be seen that it has an asymmetric amplitude characteristic 7 with respect to FIG.

Further, as in the previous embodiment, the size is reduced by about 30%.

Therefore, we conducted a fundamental study on the conventional technical ideas regarding group delay time characteristics in filter design, and used the results to address the strong requirements for surface acoustic wave filters, such as asymmetric amplitude characteristics and a reduction in the number of IDT electrode pairs, i.e., miniaturization. This was achieved at the same time.

It goes without saying that even if the input side IDT and the output side IDT are exchanged, the same result can be obtained due to their bidirectional nature.

As is clear from the above, we have made a fundamental study of the technical ideas in conventional filter design, and based on that knowledge, we have created a filter that suppresses the influence of secondary effects and has asymmetric amplitude characteristics. This is aimed at miniaturization, and more specifically, the group delay time is made flat only in the necessary frequency band, and in unnecessary frequency bands, the group delay time value is made larger or smaller as appropriate than the flat group delay time value. The number of electrode pairs is reduced compared to a filter with a constant group delay time over the entire frequency band, that is, a flat filter, which reduces the number of electrode pairs and achieves the desired complex amplitude characteristics. There are significant contributions to be made to the advancement of surface acoustic wave filter design technology and, ultimately, to the development of usage technology.

[Brief explanation of the drawing]

FIG. 1 is a frequency characteristic diagram of the amplitude and group delay time of a conventional intermediate frequency filter, and FIG. 2 is a model frequency characteristic diagram of the amplitude and group delay time used to explain the principle of the present invention. Figures 3 ab and 3 c are explanatory diagrams of the model characteristics shown in Figure 2 divided into frequency band components with respect to amplitude and group delay time, and surface acoustic wave wave model diagrams predicted for each frequency band component. Figures a and b are design dimension diagrams of electrode numbers, their intersection widths, and pitches, respectively, of surface acoustic wave filters according to embodiments of the present invention, and Figure 5 is amplitude and group diagrams of embodiments having the design dimensions shown in Figure 4. A frequency characteristic diagram of delay time, FIG. 6 is a similar frequency characteristic diagram of another embodiment. 1.3, 5.7... Frequency characteristics of amplitude, 2. soil. 6.8... Frequency characteristics of group delay time, 31,3
2゜33...Amplitude characteristic for each frequency band component of frequency characteristic l of amplitude, 41.42.43......Group delay time for each frequency band component of frequency characteristic diagram of group delay time Characteristics, A... Features of asymmetric amplitude characteristics, B...
・・・・＼i Features of symmetrical amplitude characteristics.

Claims (1)

[Claims] 1 Consists of a piezoelectric substrate, an input-side interdigital transducer, and an output-side interdigital transducer provided on the piezoelectric substrate, and sandwiching the first frequency region and the first frequency region in which the amplitude attenuation is within 3D. , the second one with an amplitude attenuation of 20 dB or more. A surface acoustic wave filter exhibiting an amplitude wavenumber characteristic consisting of a group delay time in the first frequency domain and a second frequency domain. The difference between the group delay time of at least one of the third frequency regions is 100
A surface acoustic wave filter characterized in that electrodes of an interdigital transducer are configured to have a time difference of ±1 second or more.