JPS5997216A - Acoustic surface wave filter - Google Patents

Abstract

PURPOSE:To prevent the degradation of an out-band attenuation quantity without putting restrictions on the structure of a filter itself, by arranging a reed screen-type electrode for exciting unnecessary waves on a propagation line except a main propagation line of surface waves and connecting this electrode to a reed screen-type electrode for input and output. CONSTITUTION:Two reed screen-type electrodes 34 and 36 for transmission are arranged on both sides of a central reed screen-type electrode 35 for reception formed on a piezoelectric substrate 30. The electrode 35 and a reed screen-type electrode 37 for exciting unnecessary waves are connected in parallel, and they are so arranged that the surface wave propagation path of the electrode 37 exists in a place different from main propagation paths of surfaces waves of electrodes 34, 35, and 36. Electrodes 34 and 36 are connected to a high frequency power source 12 having an internal impedance 11 to constitute one input converter, and the electrode 35 is connected to a load impedance 13 to constitute an output converter, thus constituting the filter. Intervals of electrode fingers of the electrode 37 are made different from those of electrodes 34, 35, and 36.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a filter using surface acoustic waves.

A surface acoustic wave filter consists of a piezoelectric substrate having a small number of electrodes formed on the surface (at least a pair of opposing interdigital electrodes), and the surface waves excited by the transmitting interdigital electrode are received by the receiving interdigital electrode, thereby forming a filter. The characteristics of the surface wave filter are almost determined by various parameters such as the logarithm, period, or length of the electrode fingers that make up the interdigital electrode.For example, the band of the filter is inversely proportional to the number of electrode fingers that make up the interdigital electrode. The center frequency is inversely proportional to the spacing between the electrode fingers.

In addition, in order to obtain desired amplitude frequency characteristics,
Weighting by apodization with different electrode finger lengths is designed.

FIG. 1 is a diagram showing one embodiment of a surface acoustic wave filter obtained by a conventional technique. In the figure, 10
Two interdigital electrodes 14 and 16 are arranged on both sides of a central interdigital transducer 15 formed on a piezoelectric substrate. Here, the central interdigital electrode 15 is the input transducer.
is a high frequency source 12 with internal impedance 11
It is connected to the. Also, the interdigital electrodes 14, 16 on both sides, which are output transducers, are connected to the load impedance 13. Further, the central interdigital electrode 15 is weighted by apodization in order to obtain the required frequency characteristics.

By the way, a drawback of the conventional technique shown in FIG. 1 is that a large amount of attenuation outside the band cannot be achieved because surface acoustic waves having a frequency outside the band propagate between the transmitting and receiving interdigital electrodes.

The main cause is the effect of diffraction. When there is no diffraction, the number of pairs of electrode fingers and the weighting of the interdigital electrode in the out-of-band attenuation region are determined by the method, etc., and there exists a frequency at which the amount of attenuation is very large. When a wave is received, a voltage is generated at each electrode finger, but
At such frequencies, where the amount of attenuation becomes very large,
When the voltages at each electrode finger are added together, they cancel each other out due to the phase shift relationship, and the total output becomes zep. However, when the surface wave beam spreads due to the effect of diffraction, the propagation distance of the surface wave differs at each point of the aperture of the interdigital electrode, and the phase shift relationship of the voltages generated at each electrode finger shifts. An output voltage is generated at the interdigital electrodes.

Thus, due to the effect of diffraction, the actual out-of-band attenuation is worse than the out-of-band attenuation determined by the electrode index and weighting method.

FIG. 2 is a boot 2 diagram illustrating an embodiment of a surface acoustic wave filter obtained by the prior art that has improved the above-mentioned drawbacks. In the figure, two interdigital lightning electrodes are formed on both sides of a central interdigital electrode 25 formed on a piezoelectric substrate 20.
4 and 26 are arranged. Here, the central interdigital electrode 25 is used as an input transducer, and the interdigital electrodes 24 on both sides
．． It has a three-electrode configuration in which 26 wires are connected to form one output converter. Furthermore, the interdigital electrodes 24 and 26 on both sides are weighted by a design that takes into account the effect of diffraction. The weighting method in this case will be described below.

The frequency response is calculated by including the diffraction effect due to the initial electrode finger length and the position of each electrode finger, and based on this, the initial electrode finger length and the position of each electrode finger are corrected so that the diffraction effect is canceled out. do. The above procedure is repeated until the desired frequency response is obtained to determine the optimum electrode finger length and position of each electrode finger. However, the disadvantage of this conventional technology is that it requires a huge amount of calculation for design, and in order to obtain the effect of diffraction correction, it is necessary to use a three-electrode configuration and weight the electrodes on both sides. In this way, the electrode configuration is limited.

The purpose of the present invention is to eliminate the deterioration of the out-of-band attenuation due to causes such as the aforementioned σ with a simple structure, and to achieve this effect without imposing any restrictions on the structure of the filter itself. The present invention relates to a surface acoustic wave filter consisting of input/output interdigital electrodes arranged on a piezoelectric substrate, in which a main surface wave propagation path in which surface waves excited by the input/output interdigital electrodes propagate is excluded (passing along the propagation path). An interdigital interdigital electrode for exciting unnecessary waves for exciting an out-of-band surface wave signal is arranged on the same piezoelectric substrate together with an input/output interdigital electrode, and the interdigital interdigital electrode for input or output is arranged on the same piezoelectric substrate.

It is characterized by having the interdigital electrodes for excitation of unnecessary waves connected in series or in parallel to the poles, and the amount of attenuation outside the passband is increased in a specific frequency range.

Next, the present invention will be explained with reference to the drawings. Third
The figure is a schematic diagram showing an embodiment of a surface acoustic wave filter according to the present invention, in which two transmitting interdigital electrodes 34 are placed on both sides of a central receiving interdigital electrode 35 formed on a piezoelectric substrate 30. .36, and a receiving interdigital electrode 35
and an interdigital transducer 37 for excitation of unnecessary waves are connected in parallel, and the interdigital transducer 37 has a surface wave propagation path different from the main propagation path of the surface waves of the transmitter/receive interdigital electrodes 34, 35, 36. Arrange it as it is. Here, the transmitting interdigital electrodes 34 and 36 are connected to the high frequency power source 12 having internal impedance 11 to form one input converter, and the receiving interdigital electrode 35 is connected to the load impedance 13 to form an output converter. The above input/output converters constitute a filter. Then, the spacing between the electrode fingers of the interdigital electrode 37 for unnecessary wave excitation connected to the interdigital interdigital electrode 35 for reception is adjusted.
The spacing between the electrode fingers of the reception interdigital electrodes 34, 35, and 36 is different, and the center frequency of the interdigital electrode 37 is placed in the attenuation range of the filter. The surface wave excited by .36 is detected by the interdigital electrode 35, which is an output converter. If the selection is made such that matching is achieved with the interdigital interdigital electrode 35, the electric signal near the center frequency of the interdigital interdigital electrode 37 that is detected by the interdigital interdigital electrode 35 is transmitted to the interdigital interdigital electrode 37, from which it is excited again as a surface wave.On the other hand, the filter Since the impedance of the interdigital electrode 37 is infinite at this frequency, the signal within the band is not transmitted from the interdigital electrode 35 to the interdigital electrode 37, but is outputted from the interdigital electrode 35 as an electrical signal.

For the above reasons, the center frequency determined by the interdigital spacing of the interdigital electrodes 37 is within the attenuation range of the filter, and this specific frequency component is not output as an electrical signal. In other words, the amount of out-of-band attenuation is significantly improved in this specific frequency range.

FIG. 4 is a block diagram showing another embodiment of the surface acoustic wave according to the present invention, in which two interdigital electrodes 44 and 46 are arranged on both sides of a central interdigital electrode 45 formed on a piezoelectric substrate 40. Then, the interdigital electrodes 44 on both sides and the interdigital electrode 47 for excitation of unnecessary waves are connected in parallel, and in the same way, the interdigital electrode 46 and another interdigital electrode 48 for excitation of unnecessary waves are connected in parallel. The wave excitation interdigital electrodes 47, 48 are arranged so that their surface wave propagation paths are different from each other, and are also different from the surface wave main propagation paths of the transmitting/receiving interdigital electrodes 44, 45, 46. Here, the transmitting interdigital electrode 4
4.46 are connected to a high frequency power source 12 having an internal impedance 11 together with interdigitated electrodes 47.48 for unnecessary wave excitation, and the interdigitated electrodes 44.46 are used as one input converter. In addition, by connecting the reception blind *4@45 to the load impedance 13, it is used as an output converter, and the above input and
The output converter constitutes a filter.

The interdigital interdigital electrodes 47.4B for unnecessary wave excitation connected to the interdigital interdigital electrodes 44.46 for transmission are made different from the electrode finger spacing of the interdigital interdigital electrodes 44.45.46 for transmission/reception. The center frequency of 47.48 is placed in the attenuation range of the filter (.According to the above configuration, the electrical signal input to the interdigital electrode 44.46, which is the input transducer, is converted into an acoustic signal and excited there. Surface waves are detected by the interdigital electrode 45, which is an output converter. If the impedance is selected to match the signal and the external load, the input electrical signal will be interdigitated, pole 47
．． The electrical signal near the center frequency of 48 is transmitted to the interdigital electrode 4
7.48, it is converted into a surface wave as an unnecessary wave. Therefore,
The surface waves excited from the transmission interdigital electrodes 44, 46 do not include a specific electrical signal near the center frequency of the unnecessary wave excitation interdigital N5 poles 47, 48. For the above reasons, the center frequency of the interdigital interdigital electrodes 47 and 48 is within the attenuation range of the filter, and this particular frequency component is not excited as a surface wave from the transmitting interdigital electrode 44 and 46, and this frequency component is electrically It is not output as Shingetsu. In other words, even with such a configuration, the amount of out-of-band attenuation can be significantly improved in a specific frequency range.

FIG. 5 is a block diagram showing another embodiment of the surface acoustic wave according to the present invention, in which two transmitting interdigital electrodes are placed on both sides of a central receiving interdigital electrode 55 formed on a piezoelectric substrate 50.
Pole 54 and 56 are arranged, and further a receiving type, pole 5 is placed.
5 and an interdigital transducer 57 for excitation of unnecessary waves are connected in series, and the interdigital transducer 57 has a surface wave propagation path different from the main propagation path of the surface waves of the transmitter/receive interdigital electrodes 54, 55, 56. Place it as shown. Here is the transmission wire $i! 54 and 56 are connected to a high frequency power source 12 having an internal impedance 11 to form one input converter, and the receiving interdigital electrode 55 is connected to a load impedance 13 to form an output converter, and the above input/output converter The filter is configured by: This is the embodiment shown in Figure 3. 55 (3s) and the unnecessary wave excitation interdigital electrode 57 (37) are connected in series with the external load. Even with such a configuration, unnecessary waves outside the band of the filter are excited from the interdigital electrode 57 and have the effect of increasing the amount of attenuation outside the band. In this way, even when the interdigital electrode for unnecessary wave excitation and the interdigital electrode for transmission/reception are connected in series with the signal source and the external load, the amount of out-of-band attenuation increases.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of a surface acoustic wave filter according to the prior art, FIG. 2 is a block diagram showing an embodiment of a surface acoustic wave filter according to another prior art, and FIG. 3 is a block diagram showing an embodiment of a surface acoustic wave filter according to the present invention. FIG. 4 is a block diagram showing a first embodiment of a surface acoustic wave filter, FIG. 4 is a block diagram showing an embodiment of a surface acoustic wave whistle 2 according to the present invention, and FIG. 5 is a block diagram showing a third embodiment of a surface acoustic wave filter according to the present invention. It is a block diagram showing an example of. In the figure, 10.20.30.40.50 is piezoelectric substrate 1
1 is the internal impedance of the high frequency power supply, 12 is the high frequency power supply, 13 is the load impedance, 14, 1.5.16
．． 24.25.26.34.35.36.44.45.
46.54.55.56 are interdigital electrodes for input/output converters. Further, 37.47.4B and 57 are one pole of a blind current collection pole for excitation of unnecessary waves. 51 Box Year 2 Kasumi No. b Record No. 4/2'

Claims (1)

[Claims] In a low-loss filter that uses surface acoustic waves, the main propagation path of the surface waves, where the surface waves excited by the input/output interdigital electrodes propagate, is removed (the interdigital interdigital electrodes for input and output remove the main propagation path of the surface waves, in which the surface waves are excited outside the passband). The input/output interdigital electrodes are arranged on the same piezoelectric substrate together with the input/output interdigital electrodes, and the unnecessary wave excitation interdigital electrodes are connected in series or parallel to the input or output interdigital electrodes. A surface acoustic wave filter that increases the amount of attenuation outside the band in a specific frequency range.