JP2008160471A - Low pass filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low pass filter for a DSL (Digital Subscriber Line) capable of reducing high frequency noise. <P>SOLUTION: Between a public line network and a POTS (Plain Old Telephone Service), the low pass filer for making a sound signal pass comprises inductors L11, L12 and L13, capacitors C11, C12 and C13, and capacitors C14a and C14b, wherein the inductor L12 includes a shield coil L12c, wherein one end is connected to ground E and the other end is free, and inductors L12a and L12b on respective lines of the inductor L12 and the shield coil L12c make high frequency noise pass to the ground E to thereby reduce high frequency noise and improving the communication quality of the low pass filter. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a low-pass filter that passes an audio signal in a DSL (Digital Subscriber Line) system for high-speed data communication.

  The DSL system for high-speed data communication using the existing public line network for voice calls includes an ADSL (Asymmetric Digital Subscriber Line) system, a VDSL (Vary high-speed Digital Subscriber Line) system, etc., and VDSL is 100 Mbps up and down. This technology realizes ultra-high-speed data communication.

  In the DSL high-speed data communication, as shown in FIG. 5, a public line network, POTS (Plain Old Telephone Service), and data communication equipment are connected via a DSL splitter. A low-pass filter (hereinafter abbreviated as LPF) passes POTS audio signals and blocks high-frequency data signals. The high-pass filter (hereinafter abbreviated as HPF) has a function of passing a data signal and blocking a low-frequency audio signal.

  The LPF of the DSL splitter has the circuit configuration shown in FIG. 6 and includes inductors L61, L62, and L63 having a pair of inductors on both the lines A and B, and capacitors C61, C62, C63 and capacitors C64a and C64b. The inductor L61 and the capacitor C61 form one LC filter, and FIG. 6 shows a low-pass filter composed of three stages of LC filters.

  In the low-pass filter circuit shown in FIG. 6, the inductor L63 located on the POTS side has a common mode choke structure, and the inductor L63 having the common mode choke structure suppresses circuit imbalance between the line A and the line B and performs communication. The quality of the was kept.

JP2003-152894A

  The inductor L63 having the common mode choke structure in FIG. 6 has an effect of reducing noise in addition to suppressing circuit imbalance, but because of the winding structure, noise reduction can only be taken up to several MHz. Further, it was difficult to reduce noise in the high frequency band.

  Generally, capacitors are used to reduce noise in the high frequency band. However, adding a capacitor for noise reduction to the low-pass filter shown in FIG. 6 increases the number of components and increases the volume of the low-pass filter. In addition, it is necessary to review the setting constants of all the inductors and capacitors constituting the low-pass filter.

  In view of the above problems, an object of the present invention is to provide a low-pass filter for DSL that can reduce noise of several MHz or more without increasing the number of components.

  The present invention provides a low-pass filter having an inductor and a capacitor as main components, and at least a pair of inductors at symmetrical positions of a line in a filter circuit configured with n stages of inductors and capacitors are provided in a common magnetic core in a transformer structure. A low-pass filter including a shield coil having one end connected to ground and the other end open.

  Further, according to the present invention, the inductor at the symmetrical position of the line located at the second stage from the public network side is composed of a coil provided in a common magnetic core in a transformer structure, and the magnetic core has a shield coil connected to the ground. The shield coil is a low-pass filter that is in an open state in which coil terminals other than the ground connection portion are not connected.

  Further, in the present invention, the shield coil connects the center tap to the ground, and the low pass filter according to claim 1 or 2.

  By reducing noise of several MHz or more without increasing the number of parts constituting the low-pass filter, it is possible to improve the communication quality and reliability of DSL, which is high-speed communication.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic circuit diagram of a low-pass filter according to the present invention. FIG. 2 is a schematic circuit diagram of an inductor having a shield coil according to the low-pass filter of the present invention.

  The low-pass filter of the present invention has a three-stage LC filter configuration including inductors L11, L12, and L13 and capacitors C11, C12, and C13. Among the inductors, the inductor L13 that is closest to the POTS side has a common mode choke structure. did.

  In addition, the inductor L12 located at the center of the low-pass filter circuit shown in FIG. 1 includes a shield coil L12c. Further, the shield coil L12c has one end connected to the ground E and the other end unconnected. It was.

  The shield coil L12c has an inter-coil capacitance Cb between the inductor L12a on the line A and the inductor L12b on the line B of the inductor L12 as shown in FIG. Similar to the capacitor, the capacitor Cb has a function of blocking low frequencies and allowing high frequencies to pass therethrough, and allows noise in a high frequency band to pass.

  That is, by providing the inductor L12 constituting the low-pass filter with the shield coil L12c, the shield coil is not connected to the low-pass filter without increasing the number of parts, and the inductors L11, L12, L13 and capacitor C11. High frequency noise components can be reduced without changing the set constants of C12, C13 and C14a, C14b.

  Furthermore, by combining with L13 which is the common mode choke structure shown in FIG. 1, a low-pass filter capable of reducing noise in the entire frequency band excluding the audio signal band can be obtained.

FIG. 3 shows another shield coil configuration of the inductor shown in FIG.
FIG. 3 is a schematic circuit diagram of an inductor having a shield coil as in FIG. 2. The shield coil L22c has a center tap connected to the ground E and both ends of the terminal open. Since the shield coil L22c connects the center tap portion to the ground, unbalance of the inductors L22a and L22b located on the line caused by the shield coil L22c is prevented.

  As an example, the signal attenuation characteristic of an inductor having a shield coil constituting the low-pass filter of the present invention was confirmed. The evaluation of the signal attenuation characteristic is performed by using the inductor L22 having the shield coil L22c that connects the center tap shown in FIG. 3 to the ground, and connecting the left terminal and the right terminal of the inductor L22, respectively. It was measured. As a comparison, data for an inductor having a shield coil that is not connected to ground was also measured. The measurement results are shown in FIG. The solid line is an example of the present invention, and the wavy line is a comparative example in which the shield coil is not connected to the ground.

  From the signal attenuation characteristics shown in FIG. 4, the inductor having the shield coil connected to the ground of the example of the present invention exhibits the signal attenuation characteristics from around 10 MHz, and exhibits characteristics that enable reduction of high-frequency noise, which has been a problem in the past. It was.

  The present invention is a highly effective countermeasure against high-frequency noise caused by coil parts, which has been difficult to take in the past, and enables noise countermeasures by means of coil parts, and can improve the performance and functionality of the low-pass filter. It is.

It is a schematic circuit diagram of the low-pass filter of the present invention. It is a circuit diagram of the inductor which comprises the shield coil which concerns on the low-pass filter of this invention. It is a circuit diagram of the inductor which comprises another shield coil concerning the low-pass filter of the present invention. It is a signal attenuation characteristic figure of an inductor which comprises another shield coil concerning the low-pass filter of the present invention. It is a schematic circuit block diagram of the splitter for DSL. It is a schematic circuit diagram of the low-pass filter of a prior art.

Explanation of symbols

L11, L12, L13: Inductors C11, C12, C13, C14a, C14b: Capacitor L12a: Inductor L12b located on line A L12b: Inductor L12c located on line B L12c: Shield coil E: Ground

Claims (3)

In a low-pass filter including an inductor and a capacitor as main components, at least a pair of inductors at symmetrical positions of a line in a filter circuit configured with n stages of an inductor and a capacitor is composed of a coil provided on a common magnetic core in a transformer structure. The magnetic core includes a shield coil having one end connected to ground and the other end open. The inductor at the symmetrical position of the line located at the second stage from the public line network side is composed of a coil provided in a common magnetic core in a transformer structure, and the magnetic core includes a shield coil connected to the ground, and The low-pass filter according to claim 1, wherein the shield coil is in an open state in which coil terminals other than the ground connection portion are not connected. 3. The low pass filter according to claim 1, wherein the shield coil connects a center tap to the ground.

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