JP6846789B2 - Notch filter - Google Patents

Description

The present invention relates to a notch filter that blocks a signal of a specific frequency from passing through.

In recent years, it has been theoretically proved that a weak electromagnetic state change occurs in the ground space several days to several weeks before the earthquake occurs as a precursor of an earthquake, and by observing this electromagnetic state change, Several studies have been conducted to predict earthquakes.

For example, there is a method of observing an electromagnetic state change in terrestrial space via radio waves in the VHF band (very high frequency band) such as FM radio broadcasting. As shown in FIG. 3A, the FM radio wave transmitted by the broadcasting station H1 is normally not reflected by the ionosphere D, so that it reaches only a radius of about 100 km and is received at the observation base K located at a distance of 100 km or more. Not done. However, as shown in Fig. 3 (b), when pressure is applied to the crust as a precursor to the occurrence of an earthquake and micro-fracture of the fault progresses, a large number of charged microparticles are generated from the destroyed rocks and the like, and the surface E Appears in. Further, due to this effect, the electromagnetic state of the ionosphere D and the like in the sky also changes. When the electromagnetic state of the surface E or the ionosphere changes, the FM radio waves transmitted by the broadcasting station H1 are reflected or scattered by the ionosphere D or the like, and can propagate directly above the electric charge on the surface E, and are separated by 100 km or more. It will also be received at the observation base K. In this way, by continuously observing the FM radio waves of the distant broadcasting station H1, it is possible to capture the precursory phenomenon of an earthquake.

As shown in FIG. 4, the observation devices used at the observation base K are, for example, a receiver 3 that receives FM radio waves through the antenna 1 and a notch filter 2, and an analysis device 4 (computer or the like) that analyzes the received signals. Consists of. The notch filter 2 is provided to remove noise unrelated to the precursory phenomenon of an earthquake, but the particularly loud noise is the FM radio wave transmitted by the broadcasting station H2 near the observation base K.

On the other hand, as a filter for blocking a specific frequency, a band blocking filter using a dielectric resonator and a microstrip line (Patent Document 1), a band blocking filter using an elastic surface wave resonator (Patent Document 2), and a strip line Band-blocking filters using resonators (Patent Document 3) and the like are disclosed in each patent document.

Japanese Unexamined Patent Publication No. 10-23362 Japanese Unexamined Patent Publication No. 2004-129238 JP-A-2002-185208

The FM radio wave of the broadcasting station H1 received as a precursory phenomenon of an earthquake is very weak, and the frequency is close to that of the FM radio wave of the broadcasting station H2. Therefore, the attenuation characteristic of the notch filter 2 is required to have a sufficiently large amount of attenuation at the frequency of the FM radio wave of the broadcasting station H2, and a very high selectivity Q so as not to attenuate the FM radio wave of the broadcasting station H1. However, conventionally, it has not been possible to construct a notch filter 2 having excellent performance.

For example, a winding inductor in which an electric wire is wound around a magnetic core and a general capacitor element are connected in series to form a notch filter 2, and a signal is attenuated by utilizing the series resonance of the winding inductor and the capacitor element. A method is conceivable. However, in the winding inductor, when it becomes a frequency band such as the VHF band, the resistance value of the electric wire increases due to the skin effect or the like, and the hysteresis loss and the eddy current loss of the magnetic core increase. Further, since the stray capacitance between windings cannot be reduced, the self-resonant frequency is lowered, and the equivalent loss is also increased. Then, due to these phenomena, the Q value of the winding inductor alone decreases. Therefore, the notch filter 2 using the winding inductor cannot increase the selectivity Q of the attenuation characteristic so much.

It is considered that the band-stop filter disclosed in Patent Documents 1 to 3 can increase the selectivity Q of the attenuation characteristic to some extent, but it is used in the UHF band (about 850 MHz, 900 MHz, 2 GHz, respectively). Is assumed, so it is difficult to apply it to the notch filter 2 used in the VHF band.

The present invention has been made in view of the above background art, and provides a notch filter which can greatly increase the selectivity of damping characteristics and is particularly suitable for use in earthquake prediction in the VHF band. With the goal.

The present invention is mounted on a receiver for receiving FM radio waves in the VHF band to be used for earthquake prediction, in the FM radio wave reception signal passing through the signal transmission path the receiver has, the FM of the specific frequency A notch filter that attenuates received signals of radio waves, and is a low-loss type coaxial cable with a length of 1 m or less and the ends of the center conductor and the outer conductor are short-circuited, and the center conductor or the open end of the outer conductor. It has a low-loss type capacitor element to which one end is connected, and both ends of a series circuit composed of the coaxial cable and the capacitor element are connected between the signal transmission line and ground, and an inductance component of the coaxial cable is provided. It is a notch filter that attenuates a signal of the specific frequency by a series resonance characteristic between the capacitor element and the capacitance of the capacitor element.

The capacitor element is preferably a low-loss type ceramic capacitor.

Further, the specific frequency is fp, the capacitance of the capacitor element is Cs, the inductance component required for the coaxial cable is Lk, the light speed is Co, the characteristic impedance of the coaxial cable is Zo, and the central conductor and the outer conductor of the coaxial cable. The effective refractive index of the insulator existing between the two is set to neff, and the length A of the coaxial cable is set based on the following equations (1) and (2).

The notch filter of the present invention can have a very high selectivity of damping characteristics and is particularly suitable for use in earthquake prediction in the VHF band. Further, since the coaxial cable and the capacitor element can use general-purpose products that are widely available on the market, they can be easily obtained. Further, the coaxial cable is easy to handle because its characteristics hardly change even if it is rolled up and bundled, and it can be stored compactly even if it becomes a little long because the inductance component is adjusted.

It is a figure (a) and the equivalent circuit (b) which show the structure of one Embodiment of the notch filter of this invention. The figure (a) which shows the structure of the prototype of the notch filter of this embodiment, the figure (b) which shows the structure of a comparative product, and the graph (c) which measured the attenuation characteristic (signal transmission characteristic) of a prototype and a comparative product is there. It is a figure (a), (b) which shows the outline of the earthquake prediction technology using FM radio waves. It is a block diagram which shows the structure of the observation apparatus used in the observation base of FIG.

Hereinafter, an embodiment of the notch filter of the present invention will be described with reference to FIGS. 1 and 2. The notch filter 10 of this embodiment is a filter that attenuates a signal having a specific frequency (hereinafter, referred to as an attenuation target signal) passing through the signal transmission line 12. The attenuation target signal is a signal that has received a radio wave in the VHF band such as FM radio broadcasting.

As shown in FIG. 1A, the notch filter 10 includes a coaxial cable 14 and a capacitor element 16. The coaxial cable 14 is cut out to a predetermined length, and the ends of the central conductor 14a and the outer conductor 14b on the same side are short-circuited by a short-circuit line 18. One end of the capacitor element 16 is connected to the opposite end of the central conductor 14a. As a result, a series circuit of the coaxial cable 14 and the capacitor element 16 is formed, and both ends of the series circuit are connected between the signal transmission line 12 and the ground 20.

As the coaxial cable 14, a general-purpose product that is widely available on the market can be used. For example, low-loss types such as 8D-FB, 10D-FB, and 12D-SFA specified by the Japanese Industrial Standards (JIS standard) are suitable. ing. The capacitor element 16 preferably has a small equivalent series resistance, and for example, a ceramic capacitor that is relatively inexpensive and easily available is suitable.

The equivalent circuit of the notch filter 10 can be represented as shown in FIG. 1 (b). Cs is the capacitance of the capacitor element 16, and Lk is the inductance component of the coaxial cable 14. Since the coaxial cable 14 has a short circuit at the end and the impedance of the line is inductive, it can be regarded as an inductor equivalently.

As can be seen from this equivalent circuit, when the coaxial cable 14 and the capacitor element 16 are connected in series, the capacitor element 16 may be connected to the open end of the central conductor 14a or to the open end of the outer conductor 14b. You may. Further, when connecting the ones connected in series between the signal transmission line 12 and the ground 20, the capacitor 16 may be arranged on the signal transmission line 12 side, or the coaxial cable 14 may be arranged on the signal transmission line 12 side. May be good.

The notch filter 10 attenuates the signal to be attenuated by the series resonance characteristic of the inductance component Lk of the coaxial cable 14 and the capacitance Cs of the capacitor element 16. Therefore, the inductance component Lk and the capacitance Cs are set so that the frequency of the series resonance matches the frequency fp of the signal to be attenuated.

The inductance component Lk is adjusted by changing the length A of the coaxial cable 14. The appropriate length A of the coaxial cable 14 can be easily determined based on, for example, the above equations (1) and (2). The length A of the coaxial cable 14 may be adjusted while actually measuring the attenuation characteristics, instead of being calculated using the equations (1) and (2). Further, the capacitance Cs of the capacitor element 16 may be adjusted by connecting a plurality of capacitor elements in parallel.

Next, with respect to the prototype 10 (1) of the notch filter 10 actually manufactured by the inventor, a detailed configuration and measurement results of damping characteristics will be described with reference to FIG. Here, the frequency fp of the signal to be attenuated is set to 82.7 MHz. When designing the prototype 10 (1), the length A was set to 1 m or less (Lk ≈ several μH or less) so that the coaxial cable 14 could be stored compactly. In order to set the frequency of the series resonance to 82.7MHz, the capacitance Cs of the capacitor element 16 was set to an affordable 3.3pF, and the length A of the coaxial cable 14 was determined to be 0.653m based on the equations (1) and (2). The inductance component Lk is 1.23 μH. As the capacitor element 16, a low-loss type multilayer ceramic chip capacitor having a small equivalent series resistance was used. FIG. 2A shows the configuration of the prototype 10 (1).

In addition to the prototype 10 (1), a comparative product 22 which is a conventional type notch filter was also manufactured. FIG. 2B shows the configuration of the comparative product 22. The comparative product 22 replaces the coaxial cable 14 with a winding inductor 24. The winding inductor 24 is an RF choke coil in which an electric wire is spirally wound around the side peripheral surface of a cylindrical ferrite core. An inductance of 1 μH is selected, and a capacitor is selected according to the frequency fp of the signal to be attenuated at 82.7 MHz. The capacitance Cs of the element 16 was set to 3.0 pF. The winding resistance (DC resistance) of the winding inductor 24 is 0.08Ω, and the self-resonant frequency is 200MHz.

When the damping characteristics of the comparative product 22 and the prototype 10 (1) were measured, the results shown in FIG. 2 (c) were obtained. The comparative product 22 had a peak attenuation of about 2.2 dB, a selectivity Q of about 20, and a very wide frequency band in which the signal was attenuated (about 15 to 20 MHz). On the other hand, in the prototype 10 (1), the peak attenuation was about 6.3 dB, the selectivity Q was very high, about 150, and the width of the frequency band in which the signal was attenuated could be made very narrow (). About 2-3MHz).

In the case of the comparative product 22, as described above, since the Q value of the winding inductor 24 alone decreases in the VHF band, it is considered that the selectivity Q of the attenuation characteristic becomes a low value. On the other hand, in the prototype 10 (1), since the characteristics of the coaxial cable 14 hardly change even in the VHF band, the selectivity Q of the attenuation characteristics is very high.

As described above, the notch filter 10 can make the selectivity Q of the attenuation characteristic very high, and is particularly suitable for use in the VHF band. Further, since the coaxial cable 14 and the capacitor element 16 can use general-purpose products that are widely available on the market, they can be easily obtained. Further, the coaxial cable 14 is easy to handle because its characteristics hardly change even if it is rolled up and bundled, and it can be stored compactly even if it becomes a little long because the inductance component Lk is adjusted.

Further, if a plurality of notch filters 10 having the same characteristics are prepared and connected in parallel between the signal transmission line 12 and the ground 20, a notch filter unit having substantially the same attenuation characteristic selectivity Q and a large attenuation peak can be configured. can do. Further, if a plurality of notch filters 10 having different frequencies of series resonance are prepared and connected in parallel between the signal transmission line 12 and the ground 20, a notch filter unit corresponding to a plurality of attenuation target signals having different frequencies can be configured. Can be done. By combining the plurality of notch filters 10 in this way, it is possible to easily obtain a notch filter unit suitable for various uses such as an earthquake prediction system using FM radio waves.

The notch filter of the present invention is not limited to the above embodiment. If the coaxial cable can obtain the desired damping characteristics (selectivity Q), the thickness of the center conductor, the inner diameter of the outer conductor, the characteristic impedance (50Ω / 75Ω, etc.), and the type of insulator (polyethylene / foamed polyethylene). / Highly foamed polyethylene, etc.), the shape of the outer conductor (single conductor braid, double conductor braid, conductor braid with aluminum foil tape, etc.) are not particularly limited.

As the capacitor element, a ceramic capacitor having a small equivalent series resistance is preferable, but a film capacitor or another capacitor may be used as long as a desired attenuation characteristic (selectivity Q) can be obtained.

10, 10 (1) Notch filter 12 Signal transmission line 14 Coaxial cable 14a Center conductor 14b External conductor 16 Capacitor element 18 Short-circuit line 20 Ground

Claims (3)

A reception signal of the FM radio wave having a specific frequency among the reception signals of the FM radio wave that is mounted on a receiver that receives FM radio waves in the VHF band used for earthquake prediction and passes through a signal transmission path of the receiver. a notch filter to attenuate,
A low-loss type coaxial cable having a length of 1 m or less and short-circuiting the ends of the central conductor and the outer conductor, and a low-loss type capacitor element having one end connected to the open end of the central conductor or the outer conductor. Have and
Both ends of the series circuit including the coaxial cable and the capacitor element are connected between the signal transmission line and the ground, and the specific frequency is determined by the series resonance characteristic of the inductance component of the coaxial cable and the capacitance of the capacitor element. A notch filter characterized by attenuating a signal. The notch filter according to claim 1, wherein the capacitor element is a low-loss type ceramic capacitor. The specific frequency is fp, the capacitance of the capacitor element is Cs, the inductance component required for the coaxial cable is Lk, the light speed is Co, the characteristic impedance of the coaxial cable is Zo, and between the central conductor and the outer conductor of the coaxial cable. The notch filter according to claim 1 or 2, wherein the effective refractive index of the insulator existing in the above is neff, and the length A of the coaxial cable is set based on the following equations (1) and (2).

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