JPH11266136A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-surface acoustic wave filter which has a required band width and out-of-band attenuation amount and is small in spuriousness of time response and short in delay times. SOLUTION: This filter has a first interdigital electrode 1 and a second interdigital electrode 2 for converting an electric signal into a surface accoustic wave and again converting the surface acoustic wave into the electrical signal. A standard-shaped electrode is used for the first interdigital electrode 1, and weighing of an cross width is applied to the second interdigital electrode 2 by a function which multiplies the Hamming function by a cosine function, both ends of which are cut in the middle of it.

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 used for a circuit of a communication device or the like.

[0002]

2. Description of the Related Art FIG. 8 shows, for example, a document "Acoustic Surface".
Waves ”(edited by AAOliner, Springer-Verlag, 197
This is the structure of a conventional surface acoustic wave filter of this type shown in 8 years, p. 68). In FIG. 8, 1
Is a first interdigital electrode, 2 is a second interdigital electrode, 3 is a piezoelectric substrate, 4 is an input terminal, 5 is an output terminal,
The interdigital transducer 1 and the second interdigital transducer 2 are formed on a piezoelectric substrate 3, and the first interdigital transducer 1 has an input terminal 4, and the second interdigital transducer 2 has an output terminal. 5 are respectively connected.

Next, the operation will be described. When an electric signal is input to the input terminal 4, the electric signal is converted into a surface acoustic wave in the first IDT 1, and the surface acoustic wave is excited on the piezoelectric substrate 3. This surface acoustic wave reaches the second interdigital electrode 2, where it is converted into an electric signal again and output from the output terminal 5.

The efficiency of conversion between an electric signal and a surface acoustic wave at the IDTs 1 and 2 has a frequency characteristic determined by the structure of the IDTs 1 and 2. That is, the center frequency, which is the frequency at which the conversion efficiency is maximized, is determined by the arrangement period of the electrode fingers constituting the interdigital electrodes 1 and 2 and the propagation speed of the surface acoustic wave on the piezoelectric substrate 3. In addition, the passband width and the shape of the passband in the frequency characteristics and the out-of-band characteristics are determined by the number of electrode fingers and the shape of the weight given to the intersection width of the electrode fingers.

Accordingly, the electric signal from the input terminal 4 to the output terminal 5 is affected by the frequency characteristics of the first and second interdigital electrodes 1 and 2 and therefore has a specific center frequency. Have specific frequency characteristics. Therefore, the surface acoustic wave filter shown in FIG. 8 operates as a band-pass filter.

In FIG. 8, a normal electrode is used as the first interdigital electrode 1. Further, as the second interdigital transducer 2, an electrode in which the intersection width of the electrode fingers is weighted depending on the location is used. Thus, the interdigital transducers 1, 2
The frequency characteristics of a surface acoustic wave filter using an electrode in which the cross width of the electrode fingers is uniform regardless of the location on one side and the electrode in which the cross width of the electrode fingers is weighted by the location on the other side are as follows: , The frequency characteristics of the first interdigital transducer 1 and the second
It is known that it is represented by the product of the interdigital transducer 2 and the frequency characteristic. Such a configuration is generally widely used because the design is simple and a desired frequency characteristic is easily obtained.

As the interdigital electrode 1 in which the cross width of the electrode fingers is uniform irrespective of the location, a thinned-out electrode, a phase inversion type electrode, and the like are known in addition to the normal type electrode shown in FIG. And these electrodes are sometimes used.

As the second interdigital transducer 2, an electrode in which the intersection width of the electrode fingers is weighted depending on the location is used. Various functions are used for such electrode weighting. In FIG. 8, a function having a shape similar to the Hamming function is used. It is known that the Hamming function has a small side lobe level of the frequency characteristic. Therefore, a surface acoustic wave filter having a large out-of-band attenuation can be obtained by using the second interdigital transducer 2 weighted by the Hamming function. be able to.

As the weighting function to be applied to the second interdigital transducer 2, various functions other than the Hamming function are used according to the required frequency characteristics. Also, instead of an analytical function, a function created by numerical calculation by a computer may be used. Such a weighting function is often designed especially considering only frequency characteristics. Various methods have been devised for designing a weighting function to obtain a required frequency characteristic.

[0010]

However, surface acoustic wave filters used in some communication systems sometimes require characteristics such as time response characteristics and delay time characteristics in addition to frequency characteristics. That is, in addition to the general required characteristics of having a required bandwidth and satisfying a required out-of-band attenuation, it is required that spurious response in time is small and delay time is short.

However, a design method for satisfying such requirements has not been completely established yet. Although a method based on numerical calculation by a computer is conceivable, the design efficiency is poor and a good function cannot always be obtained. As described above, the conventional surface acoustic wave filter has a problem that it is difficult to simultaneously satisfy various requirements such as frequency characteristics and time response characteristics.

The present invention has been made to solve the above problems, and has a required bandwidth and a required out-of-band attenuation, a small spurious response in time, and a small surface acoustic wave. The purpose is to get a filter.

[0013]

SUMMARY OF THE INVENTION A surface acoustic wave filter according to the present invention comprises a first interdigital transducer for converting an electric signal into a surface acoustic wave or the above-mentioned surface acoustic wave into an electric signal again, and a second interdigital transducer. The first interdigital transducer has an interdigital electrode, and the first interdigital electrode uses a uniform interdigital electrode regardless of the location of the electrode fingers, and the second interdigital electrode includes:
In a surface acoustic wave filter using interdigital electrodes in which the cross width of the electrode fingers is weighted according to location, the first interdigital electrode is a normal electrode and the second interdigital electrode is applied to the second interdigital electrode. The weighting performed is a function obtained by truncating both ends of a function obtained by multiplying a cosine function by a Hamming function.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a configuration diagram showing a surface acoustic wave filter according to Embodiment 1 of the present invention. In FIG. 1, reference numeral 1 denotes a first interdigital electrode in which the cross width of the upper and lower electrode fingers is a uniform electrode regardless of the location,
Reference numeral 20 denotes a second weighting method in which the intersection width of the upper and lower electrode fingers is weighted according to the location, and the weighting is performed by a function obtained by truncating both ends of a function obtained by multiplying the cosine function by the Hamming function. The IDTs 3 are piezoelectric substrates, 4 is an input terminal, and 5 is an output terminal. The first IDT 1 and the second IDT 20 are formed on the piezoelectric substrate 3. . An input terminal 4 is connected to the first interdigital electrode 1, and an output terminal 5 is connected to the second interdigital electrode 20.

Next, the operation will be described. When an electric signal is input to the input terminal 4, the electric signal is converted into a surface acoustic wave in the first IDT 1, and the surface acoustic wave is excited on the piezoelectric substrate 3. This surface acoustic wave reaches the second interdigital electrode 20, where it is converted again into an electric signal,
Output from the output terminal 5.

In FIG. 1, the first interdigital transducer 1 uses an electrode whose electrode finger cross width is uniform regardless of the location,
As the interdigital electrode 20, an electrode in which the cross width of the electrode finger is weighted depending on the place is used. Therefore,
As in the conventional case, the frequency characteristics of the surface acoustic wave filter are the same as those of the first IDT 1 and the second IDT 20.
Of the respective frequency characteristics.

Here, as the first interdigital transducer 1, a regular electrode is used as an electrode in which the intersection width of the electrode fingers is uniform regardless of the location. Further, the second interdigital transducer 20 is weighted by a function obtained by truncating both ends of a function obtained by multiplying the cosine function by the cosine function as an electrode in which the intersection width of the electrode finger is weighted depending on the location. The frequency characteristics of the IDTs 1 and 20 are represented by a Fourier transform of a weighting function applied to the electrodes.

Therefore, the frequency characteristics of the first interdigital transducer 1 using the normal-shaped electrodes are represented by the following equation: centered on the center frequency determined by the type of the piezoelectric substrate 3 and the arrangement period of the electrode fingers.
It has n (x) / x-like characteristics. At this time, the side lobe level is about −13 dB with respect to the level at the center frequency, and so good out-of-band attenuation cannot be obtained.
However, the normal type electrode has a feature that the delay time can be reduced because the number of electrode fingers required for the electrode can be reduced.

Next, the characteristics of the second IDT 20 will be described with reference to FIG. FIG. 2A shows the amplitude characteristics of the Hamming function. This function is represented by the following equation. 0.54 + 0.46 cos (πx), | x | ≦ 1

FIG. 2B shows a Fourier transform of the Hamming function shown in FIG. 2A. That is, it shows the frequency characteristics of the interdigital transducer when the width of the intersection is weighted by the Hamming function. The side lobe level in this frequency characteristic is a small value of −40 dB or less, and a very good out-of-band attenuation can be obtained. However, the in-band characteristics have an upwardly convex shape, and flat characteristics cannot be obtained.

FIG. 3A shows a cosine function. This function is represented by the following equation. cos (cπx) In the above equation, c is a constant larger than 0.

FIG. 3 (b) shows a Fourier transform of the cosine function of FIG. 3 (a). This frequency characteristic is represented by a so-called delta function.

FIG. 4A is a graph obtained by multiplying the Hamming function of FIG. 2A by the cosine function of FIG. 3A. FIG. 4B is a result of Fourier transform of the function of FIG. 4A. This frequency characteristic is shown in FIG.
FIG. 3B shows a frequency characteristic based only on the Hamming function and FIG.
This is a function obtained by convolving the frequency characteristic with only the cosine function of (b). As shown in the figure, due to the influence of the cosine function,
It can be seen that the in-band characteristics are flattened and the bandwidth is widened.
Moreover, the side lobe level is suppressed to a small value according to the frequency characteristic of only the Hamming function.

FIG. 5A is a function obtained by truncating both ends of the function shown in FIG. 4A, and is a function used for weighting the second IDT 20 in FIG. It is. That is, in the second interdigital transducer 20 of FIG. 1, the cross width at which the upper and lower electrode fingers cross each other is changed depending on the position, and the change due to this position is shown in FIG.
Where the intersection width of the electrode fingers at the center is the largest, and gradually decreases toward both sides,
It becomes 0 on the way, and thereafter the phase is reversed and ends at the end. In fact, the shape of the second interdigital electrode 20 of FIG. 5
It has a shape in which the function shown in (a) and a function obtained by inverting the function on the horizontal axis are superimposed.

This function is represented by the following equation. {0.54 + 0.46cos (πx)} · cos (cπ
x), where │x│ ≦ X _t, is X _t is 1 smaller than a positive constant. By truncating both ends, the overall length is reduced as compared to FIG.

FIG. 5B is a result of Fourier transform of the function of FIG. 5A, and shows the frequency characteristics of the weighting function of FIG. 5A. Due to the effect of the truncation, the in-band characteristics are slightly rounded and the side lobe level is slightly increased as compared with FIG.

Next, consider the time response. An impulse response which is an output waveform when an impulse waveform is input to the interdigital transducer has a waveform similar to a weighting function applied to the interdigital transducer. Therefore, the impulse response of the second interdigital transducer 20 has a shape similar to the function shown in FIG.

At both ends of FIG. 5A, there are portions where the value of the function is negative. This portion indicates that the phase is inverted from that of the central portion, and in some communication devices, spurious that does not require time response may be generated. Therefore, in such a case, the level of the part where the phase is inverted is required to be smaller than the required level.

The impulse response of the surface acoustic wave filter shown in FIG. 1 is the same as the impulse response of the second interdigital transducer 20 shown in FIG. Is represented by the convolution of

The impulse response of the first interdigital transducer 1 has a uniform rectangular waveform as shown in FIG. Therefore, the impulse response of the surface acoustic wave filter is shown in FIG.
The waveform is as shown in FIG. That is, the phase inversion region is much smaller than the waveform shown in FIG. That is, the spurious response of the time response can be reduced.

Next, the delay time characteristic will be considered. Since the first interdigital transducer 1 has a regular shape, the number of electrode fingers can be reduced. In addition, since the second interdigital transducer 20 is also weighted by the above-described function, the number of electrode fingers can be reduced due to the effect of truncation.

When the piezoelectric substrate 3 is determined, the delay time of the surface acoustic wave filter is proportional to the center-to-center distance between the position of the first IDT 1 and the position of the second IDT 20. Therefore, if the center-to-center distance between the first interdigital transducer 1 and the second interdigital transducer 20 is reduced, a surface acoustic wave filter with a small delay time can be obtained.

At this time, in order to suppress the influence of the so-called direct wave, a certain interval is required between the electrodes. But,
In the surface acoustic wave filter shown in FIG.
However, since the number of electrode fingers of both the second interdigital electrodes 20 can be reduced, the distance between the centers of the two electrodes can be sufficiently short. Therefore, the delay time can be sufficiently reduced.

As described above, the surface acoustic wave filter shown in FIG. 1 has the required bandwidth and out-of-band attenuation only by using a simple analytical function, and has a spurious time response. Small and delay time is small. Further, since only a simple analytical function is used as the weighting function, the design is easy, and the time and cost required for the design can be reduced. In addition, since the delay time is small, the size of the entire filter can be reduced as a result. Therefore, the weight of the filter can be reduced, and the cost of the material can be reduced.

In the present embodiment, a so-called double electrode is used for the first IDT 1 and the second IDT 20. However, the present invention is not limited to this. A single electrode or another configuration may be used.

Further, a shield electrode for reducing the influence of a direct wave may be arranged between the first interdigital electrode 1 and the second interdigital electrode 20.

Further, in FIG. 1, the input terminal 4 is connected to the first interdigital electrode 1 and the output terminal 5 is connected to the second interdigital electrode 20, but in this type of surface acoustic wave filter, Generally, even if the input and the output are reversed, exactly the same frequency characteristics and time response characteristics can be obtained. Therefore, the effect of the present invention can be obtained even if the output terminal 5 is connected to the first IDT 1 and the input terminal 4 is connected to the second IDT 20.

[0038]

As described above, according to the present invention, the first interdigital transducer and the second interdigital transducer for converting an electric signal into a surface acoustic wave or converting the above surface acoustic wave into an electric signal again. The first interdigital electrode has a uniform interdigital width regardless of the location, and the second interdigital electrode has an interdigital width depending on the location. In the surface acoustic wave filter using the weighted interdigital transducer, the first interdigital transducer is used as a normal electrode, and the weight applied to the second interdigital transducer is defined as a Hamming function. Having a function obtained by truncating both ends of the function obtained by multiplying the cosine function by halfway, has a required bandwidth and a required out-of-band attenuation, and
There is an effect that a surface acoustic wave filter having a small spurious response in time and a small delay time can be easily realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a surface acoustic wave filter according to Embodiment 1 of the present invention.

FIG. 2 is an explanatory diagram for explaining the operation of the surface acoustic wave filter according to Embodiment 1 of the present invention, showing an amplitude characteristic and a frequency characteristic of a Hamming function.

FIG. 3 is an explanatory diagram for explaining an operation of the surface acoustic wave filter according to Embodiment 1 of the present invention, showing an amplitude characteristic and a frequency characteristic of a cosine function;

FIG. 4 is a diagram for explaining the operation of the surface acoustic wave filter according to Embodiment 1 of the present invention, and is an explanatory diagram showing amplitude characteristics and frequency characteristics of a function obtained by multiplying a Hamming function and a cosine function.

FIG. 5 is a diagram for explaining the operation of the surface acoustic wave filter according to the first embodiment of the present invention, which is an amplitude characteristic of a function obtained by truncating both ends of the function shown in FIG. FIG. 4 is an explanatory diagram showing an impulse response and frequency characteristics of a second interdigital transducer 20.

FIG. 6 is an explanatory diagram for explaining an operation of the surface acoustic wave filter according to Embodiment 1 of the present invention, showing an impulse response of the first interdigital transducer 1;

FIG. 7 is an explanatory diagram showing an impulse response of the surface acoustic wave filter according to Embodiment 1 of the present invention.

FIG. 8 is a configuration diagram showing a conventional surface acoustic wave filter.

[Explanation of symbols]

1 first interdigital electrode, 20 second interdigital electrode,
3 Piezoelectric substrate, 4 input terminals, 5 output terminals.

Claims (1)

[Claims] A first interdigital electrode for converting an electric signal into a surface acoustic wave or a second interdigital electrode for converting the surface acoustic wave into an electric signal again; In addition to the use of interdigital electrodes in which the cross width of the electrode fingers is uniform regardless of the location, the second interdigital electrode is
In a surface acoustic wave filter using interdigital electrodes in which the cross width of the electrode fingers is weighted according to location, the first interdigital electrode is a normal electrode and the second interdigital electrode is applied to the second interdigital electrode. A surface acoustic wave filter characterized in that the weighting is a function obtained by truncating both ends of a function obtained by multiplying a Hamming function by a cosine function.