JP2004222146A - Catv protector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce streamed noise in a protector for a two-way digital communication system using a CATV. <P>SOLUTION: This CATV protector arranged at a leading-in entrance of a CATV subscriber and for protecting the a device of the subscriber from a surge, an input hot side terminal located at an unbalanced side of an unbalanced-balanced transformer B is set to be an input terminal of the protector, an input ground side terminal is grounded, and a pair of output terminals located on the balanced side are respectively connected to the hot side of an output terminal of the protector and the ground side through a capacitor. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a protector used as a service entrance for a subscriber in a cable television system (hereinafter, referred to as CATV), and particularly to a countermeasure against inflow noise when performing bidirectional digital communication for use of the Internet or the like. Related to those subjected to.
[0002]
[Prior art]
As shown in FIG. 13, the CATV protector is provided to absorb a surge voltage superimposed on the cable due to induced lightning or the like, and has a gas tube / arrestor AR, a choke coil CH, and the like, and has a core wire of a coaxial cable. Surge voltage between the ground and the ground is suppressed.
[0003]
Further, in order to prevent the surge voltage from invading the subscriber's equipment along the coaxial cable, high-voltage capacitors C1 and C2 for DC cut are inserted to cut off the DC component.
[0004]
As shown in FIG. 14, the coaxial cable connected to the protector covers the core wire W with the shield S connected to the ground as shown in FIG.
[0005]
When connecting the coaxial cable CC to the protector, as shown in FIG. 15, the coaxial cables CC1 and CC2 are connected via two capacitors C1 and C2 inside the protector.
[0006]
As shown in FIG. 15, the general protector is insulated by a high-voltage capacitor on both the core wire of the coaxial cable on the subscriber side and the ground side, and the shield S1 on the CATV side is grounded. S2 is floating from the ground by the capacitor C2. As a result, under the condition that the high-frequency impedance of the high-withstand-voltage capacitor C2 cannot be ignored, the external noise gets on the core wire W.
[0007]
As a countermeasure, a conventional Internet-capable CATV uses a filter to remove noise of 10 to 50 MHz, which is an upstream frequency for the Internet. That is, a high-pass filter that cuts 70 MHz or less is provided on the TV side of the protector at the subscriber's house and the two-partition type protector to reduce inflow noise from a television receiver or the like.
[0008]
However, the protector connected to the CATV cable modem passes all frequency bands of 10 MHz or more. That is, as conceptually shown in FIG. 16, the TV side noise N1 is blocked by the protector P, but the cable modem side noise N2 goes to the coaxial cable CC. For this reason, external noise in the subscriber's house is mixed into the upstream signal of the cable modem, and transmitted as upstream noise of the CATV system through the cable.
[0009]
Here, the magnitude of the ingress noise is determined by the high-frequency impedance of the ground-side capacitor C2 (FIG. 15). For this reason, as shown in Patent Document 1, a ring-shaped capacitor is mounted on the output side of the earth-side capacitor of the protector so as to eliminate the impedance of the lead wire, thereby improving the characteristics.
[0010]
FIG. 17 shows the relationship between the capacitance and the impedance of 10 MHz, and FIG. 18 shows the relationship between the capacitance of a capacitor having a resistor (75 ohm) connected in series and the noise power generated between both ends of the capacitor. . As can be seen from FIG. 18, even if the capacitance of the capacitor is increased by a factor of 10, the noise level decreases only by 10 dB.
[0011]
A more detailed examination of how noise is generated is as follows. FIG. 19 shows a noise measuring circuit in which the hot side of the input terminal IN and the hot side of the output terminal OUT are connected by a capacitor C1 (1,000 pF), and the ground between the ground side of the output terminal OUT and the ground. It is assumed that the output terminal OUT is connected by a capacitor C2 (10,000 pF) and terminated with a 75 ohm resistor.
[0012]
In order to measure the ingress noise, there is a method of measuring the noise generated at the input terminal IN. However, in a passive circuit, the transmission characteristics do not change even if the direction of input-output is changed. Therefore, here, the noise power is measured by inputting a signal of 0 dB from the input terminal.
[0013]
At this time, as shown in FIG. 20, the frequency characteristic of noise generated between both ends of the capacitor C2 is such that a low frequency is greatly affected and a large noise of -36 dB is generated at 10 MHz.
[0014]
Here, it is thought that the magnitude of noise can be improved by increasing the capacitance of the capacitor, but this is due to the increase in the size of the capacitor and the influence of the lead inductance, which increases the impedance in the high frequency range and broadens the circuit bandwidth. Inhibits.
[0015]
On the other hand, as shown in FIG. 21, the input terminal IN and the output terminal OUT may be DC-separated by a secondary coupling transformer T so that the subscriber side coaxial cable does not have a DC potential with respect to the ground. (See Patent Document 2).
[0016]
[Patent Document 1]
Registered Utility Model No. 3073455 [Patent Document 2]
JP 2001-10347 A
[Problems to be solved by the invention]
However, when the transformer T is inserted, the coupling at a low frequency is weak, and the insertion loss is large over the entire frequency band as shown in FIG. There is also a method of increasing the size of the core and increasing the number of turns of the winding in order to increase the degree of coupling, but the high-frequency characteristics are significantly deteriorated.
[0018]
The present invention has been made in consideration of the above points, and has as its object to reduce ingress noise in a protector for a bidirectional digital communication system using CATV.
[0019]
[Means for Solving the Problems]
To achieve the above object, the present invention provides:
In a CATV protector disposed at an entrance of a CATV to a subscriber to protect equipment in the subscriber from surge, an input hot side terminal on an unbalanced side of an unbalanced-balanced transformer is connected to an input of the protector. A CATV protector wherein the input ground side terminal is grounded, and a pair of output terminals on the balanced side are respectively connected to the hot side and the ground side of the output terminal of the protector via capacitors. ,
Is provided.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows an ingress noise elimination circuit for a protector in which noise is reduced using the unbalanced-balanced transformer according to the present invention. This unbalanced-balanced transformer is generally called a balun, and is used to couple a television antenna balanced circuit with an unbalanced circuit formed by a 75Ω coaxial cable. One end of the primary winding is an unbalanced input terminal, one end of the secondary winding is an unbalanced ground terminal, and the primary winding and the secondary winding are connected in parallel with each other. Each other end of the winding is a balanced output terminal. Here, each winding has 2.5 turns and is wound around a core to form two sets of transformers.
[0021]
Then, the input terminal of the balun B is connected to the input terminal IN of the protector, the ground terminal of the balun B is grounded, and the two output terminals of the balun B are connected to the hot side and the ground side of the output terminal OUT of the protector. ing. Since the turn ratio of the balun is 1 to 1, the output terminal of the protector has a characteristic impedance of 75Ω.
[0022]
FIG. 2 shows the insertion loss characteristics of the balun B in the protector shown in FIG. Looking at the frequency range of 10 MHz to 1000 MHz used in CATV, the loss between the input terminal IN and the output terminal OUT tends to increase toward the high frequency side, which is almost 0 dB at 10 MHz and about 1.0 dB at 1,000 MHz. It is.
[0023]
3 (a), 3 (b) and 3 (c) show circuit configurations for comparing and measuring the effect of the transformer mounted on the output side of the protector. FIG. 3A shows a configuration in which the input terminal IN and the output terminal OUT are connected by connecting a capacitor C11 and a 75Ω resistor in series, and the output terminal OUT is grounded by a capacitor C12. However, elements not particularly related to the present invention, such as an arrester, are not shown. FIG. 3B shows a configuration in which a secondary transformer T is inserted between the capacitor C11 in FIG. 3A and a resistor of 75Ω. FIG. 3C shows a balun B inserted in place of the secondary transformer T shown in FIG. 3B.
[0024]
FIG. 4 shows the measurement of the ingress noise level for these three circuit configurations. When viewed at a frequency of 10 MHz, (a) the case without a transformer is -36 dB, but when (b) the secondary transformer T is inserted and (c) the balun B is inserted, both become -70 dB or less. ing.
[0025]
When the insertion loss of the circuit and the increase in the bandwidth are taken into consideration, it can be seen that the configuration (c) in which the balun B is inserted exhibits the best characteristics.
[0026]
FIGS. 5 (a) and 5 (b) are obtained by examining what effect appears on the inflow noise when a DC blocking capacitor is combined with the circuit of FIG. 3 (c).
[0027]
FIG. 5A shows a circuit configuration in which a DC blocking capacitor C13 is combined with the circuit shown in FIG. 3C. That is, a circuit configuration is shown in which a DC cutoff capacitor C13 is inserted between the output terminal on the ground side of the balun B in the circuit of FIG. 3C and a point where a 75Ω resistor is connected to the capacitor C12. . However, two types of capacitors C13 of 1,000 pF and 10,000 pF were separately connected.
[0028]
When the level of the ingress noise was measured for this circuit configuration, as shown in FIG. 5B, 1,000 pF was still insufficient, and at least 10 pF was required to approach the case without a capacitor (direct connection). 2,000 pF was found to be necessary.
[0029]
FIG. 6 shows a two-distribution type protector in which a protector is combined with a distributor for distributing signals into two in order to investigate the noise reduction effect of the noise reduction circuit shown in FIG. FIG. 6 is a connection diagram when the ingress noise is measured by connecting the noise reduction circuit illustrated in FIG. 5A to the protector.
[0030]
This two-partition type protector has an input terminal IN and two output terminals DATA and TV, supplies a television signal from the input terminal IN to a television output terminal TV, and also includes an input terminal IN and a data terminal DATA. It transmits Internet signals to and from the Internet.
[0031]
The television output terminal TV incorporates a high-pass filter HPF that blocks noise components of 70 MHz or less generated on the television receiver side.
[0032]
FIG. 7 shows the result of measuring the level of the ingress noise between the input terminal IN, the television output terminal TV, and the data terminal DATA by the method shown in FIG.
[0033]
The noise level between the input terminal IN and the television output terminal TV is represented by an IN-TV curve indicated by a solid line, and the noise level between the input terminal IN and the data terminal DATA is indicated by an alternate long and short dash line IN-DATA. It is represented by a curve.
[0034]
As can be seen from these curves, when the frequency is 10 MHz, the inflow noise between the IN and the TV is -80 to -90 dB, while the inflow noise between the IN and DATA passing the entire frequency band is -40 dB. is there.
[0035]
FIGS. 8A and 8B show a noise level measuring circuit for determining the capacity of a DC cutoff capacitor to be combined with a noise reducing circuit, and a measurement result thereof. This measurement was performed in the frequency range (1 to 100 MHz) including the lowest frequency range in the measurement range shown in FIG.
[0036]
Here, the circuit shown in FIG. 8A has basically the same configuration as the circuit shown in FIG. 5A, and is inserted between the balun B and the input terminal IN in FIG. A capacitor C11 'was inserted between the balun B and the data terminal DATA in place of the capacitor C11.
[0037]
Then, as shown in FIG. 8 (b), as shown in FIG. 8 (b), when the frequency is 10 MHz, the noise level is insufficient at −13 dB at C13 = 1,000 pF, and a satisfactory value of −77 dB at 10,000 pF. became.
[0038]
FIG. 9 shows the result of measuring the level of the ingress noise in the case where 10,000 pF is used as the DC cutoff capacitor C13 in the circuit configuration shown in FIG. 8A. As is clear from the measurement results, in the frequency range of 10-50 MHz used on the Internet, the level of the ingress noise is -80 dB or less, which indicates that the noise reduction effect is sufficient for practical use. I understand.
[0039]
FIG. 10 shows the configuration of the noise reduction circuit according to the present invention obtained through the above-described process. In this circuit, between an input terminal IN and an output terminal OUT, a balun B as an unbalanced-balanced transformer, and DC blocking capacitors C11 'and C12 are connected in series with two balanced-side output terminals of the balun B. It has an inserted configuration.
[0040]
FIG. 11 shows a first embodiment according to the present invention, and is an example of a CATV protector configured by using an ingress noise reduction circuit based on the noise reduction circuit shown in FIG. .
[0041]
In the first embodiment, the input terminal IN is grounded by the arrester AR and the choke coil CH, and is connected to the input terminal of the balun B via the high-voltage capacitor C1. The hot side of the output terminal OUT is connected to one of the output terminals of the balun B via a high voltage capacitor C2, and the ground side is connected to the other of the output terminals of the balun B via a high voltage capacitor C3.
[0042]
FIG. 12 shows a second embodiment of the present invention, and is an example of a CATV protector using an ingress noise reduction circuit as in the first embodiment.
[0043]
In the second embodiment, a two-divider DIV is interposed between the high withstand voltage capacitor C1 and the non-ground side winding of the balun B in the embodiment of FIG. 11, and one of the divided outputs is connected to a data terminal DATA. The output is supplied to the balun B, and the other output is supplied to the television output terminal TV grounded by the high-voltage capacitor C7 via the high-pass filter HPF and the high-voltage capacitor C6.
[0044]
The two divider DIV includes an impedance adjustment transformer T1, a distribution transformer T2, an impedance compensation capacitor C5 for grounding a connection point between the transformers T1 and T2, and a resistance R between output terminals of the distribution transformer T2. Signal transmission / reception with the data terminal DATA and transmission of a television signal from the input terminal IN to the terminal TV are performed.
[0045]
【The invention's effect】
As described above, according to the present invention, the input hot side terminal on the unbalanced side of the unbalanced-balanced transformer is used as the input terminal of the protector, the input ground side terminal is grounded, and a pair of output terminals on the balanced side are provided. Are connected to the hot side and the ground side of the output terminal of the protector via capacitors, respectively, so that the CATV protector can be sufficiently suppressed, so that the noise flowing from the Internet can be sufficiently suppressed, and the CATV system is adapted to the Internet. It can be done.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a configuration of an unbalanced-balanced conversion circuit using a balun as a high-frequency transformer.
FIG. 2 is a measured characteristic diagram showing insertion loss of a balun B in the circuit of FIG. 1;
3 (a) to 3 (c) show a configuration example when a transformer is added to the output side of the protector. FIG. 3 (a) shows a circuit composed only of a resistor and a capacitor. FIG. 3B is a circuit diagram showing a configuration using a secondary transformer, and FIG. 3C is a circuit diagram showing a configuration using an unbalanced-balanced transformer.
FIG. 4 is a graph showing actual measurement characteristics of inflow noise in the circuits shown in FIGS.
5A is a diagram showing a configuration of a circuit in which a DC blocking capacitor is added to the circuit of FIG. 3C, and FIG. 5B is a diagram showing a combination of the circuit of FIG. 5A; FIG. 9 is a graph showing actually measured characteristics of noise.
FIG. 6 is a connection diagram for measuring ingress noise of a two-partition type protector.
7 is an actual measurement characteristic diagram of an ingress noise level measured by the connection in FIG. 6;
8 (a) is a connection diagram for noise measurement when a noise reduction circuit similar to that shown in FIG. 5 (a) is attached to a protector, and FIG. FIG. 4 is a graph showing measured inflow noise characteristics for a frequency portion.
FIG. 9 is a graph showing measured ingress noise characteristics over the entire frequency range of the two-distribution noise reduction protector including the part shown in FIG. 8 (b).
FIG. 10 is a diagram showing an ingress noise reduction circuit according to the present invention.
FIG. 11 is a circuit diagram showing a first embodiment of the present invention.
FIG. 12 is a circuit diagram showing a second embodiment of the present invention.
FIG. 13 is a circuit diagram showing a circuit configuration of a general protector.
FIG. 14 is a diagram showing how noise affects a coaxial cable.
FIG. 15 is a diagram illustrating how noise is applied to a coaxial cable when a protector is provided.
FIG. 16 is an explanatory diagram showing the flow of ingress noise in CATV.
FIG. 17 is a characteristic diagram showing the relationship between the capacitance of a capacitor used in a protector as a DC cutoff element and its 10 MHz impedance.
FIG. 18 is a characteristic diagram showing a relationship between the capacitance of the capacitor and noise power generated between both ends.
FIG. 19 is a circuit diagram showing a circuit for measuring noise power generated in a capacitor as a DC blocking element.
FIG. 20 is a diagram illustrating frequency versus noise characteristics measured by the circuit of FIG. 19;
FIG. 21 is an explanatory diagram of a secondary transformer used to disconnect an input terminal and an output terminal of the protector in a DC manner.
FIG. 22 is a characteristic diagram showing insertion loss of the secondary transformer shown in FIG. 21;
[Explanation of symbols]
IN Input terminal OUT Output terminal B Balun DATA Data terminal TV Television terminal

Claims (1)

A CATV protector arranged at an entrance to a CATV subscriber to protect equipment in said subscriber from surges,
The input hot side terminal on the unbalanced side of the unbalanced-balanced transformer is used as the input terminal of the protector, the input ground side terminal is grounded, and the pair of output terminals on the balanced side is connected to the security terminal via a capacitor. A CATV protector which is connected to a hot side and an earth side of an output terminal of the device.

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