JP2008270984A - Signal relay amplifier - Google Patents

Abstract

[PROBLEMS] To perform relay amplification by interposing on a metal line transmission line in which uplink signals and downlink signals of the same frequency are transmitted in a time division manner, from one relay line with an inexpensive configuration and simple control. To obtain a signal relay amplifier having a mechanism for preventing the formation of a loop that goes around both relay paths even if a signal wraps around the other relay path or noise is mixed.
A comparator compares a magnitude relationship between signal levels Vd and Vu detected by a downstream signal level detector 6a and an upstream signal level detector 6b, and the comparison result is In parallel with the down path switch 5a and the up path switch 5b, a signal wraps around from one relay path to the other by simply outputting one as an open circuit and the other as a control signal. However, even if noise is mixed in, it is possible to prevent the formation of a loop that makes a round of both relay paths and to prevent oscillation.
[Selection] Figure 1

Description

  The present invention relates to a signal relay amplifier that performs relay amplification by interposing on a metal line transmission line through which upstream and downstream signals of the same frequency are transmitted in a time division manner.

  In a signal relay amplifier that performs relay amplification by interposing on a metal line transmission line in which upstream and downstream signals of the same frequency are transmitted in a time division manner, an upstream signal relay path and a downstream signal relay path are provided in parallel. When one signal leaks into the other signal relay path when one signal is relayed, a loop that makes a round of both relay paths is formed and oscillation occurs.

  Therefore, measures are taken to prevent the upstream signal from leaking to the downstream signal side when relaying the upstream signal, and to prevent the downstream signal from leaking to the upstream signal side when relaying the downstream signal. (For example, Patent Document 1).

  That is, the relay amplifier for bidirectional communication disclosed in Patent Document 1 is a first cable (one side) that bisects a metal line transmission line through which time-division transmission of upstream signals and downstream signals of the same frequency ( A first distributor / coupler connected to the upstream metal line transmission line) and a second distributor / coupler connected to the second cable (downstream metal line transmission line) on the other side. ing.

  Each of the first and second distribution / combining units distributes and outputs signals input from the corresponding cables to the two internal relay paths, and combines the signals input from the two internal relay paths and sends them to the corresponding cables. Two internal input / output ports.

  A first attenuating unit, a first amplifying unit, and a first attenuating path from one internal input / output port of the first distributing / combining unit to one internal input / output port of the second distributing / combining unit. Are connected in this order, and the second attenuation is applied to the upstream relay path from the other internal input / output port of the second distribution / combining unit to the other internal input / output port of the first distribution / combining unit. , The second amplifying unit, and the second switch unit are arranged in this order.

  A first detection unit that controls the first switch unit and the second attenuation unit, and a second detection unit that controls the second switch unit and the first attenuation unit are provided.

  The first detection unit controls the first switch unit to be in a conductive state when the downstream signal is detected in the output of the first attenuation unit, and the attenuation amount of the second attenuation unit is set to the first attenuation unit. Control is performed so as to increase more than the predetermined attenuation. This prevents the downstream signal from leaking into the upstream relay path when the downstream signal is relayed.

  The second detection unit controls the second switch unit to be in a conductive state when an upstream signal is detected at the output of the second attenuation unit, and the attenuation amount of the first attenuation unit is set to the first attenuation unit. Control is performed so as to increase beyond a predetermined attenuation of 2. This prevents the upstream signal from leaking into the downstream relay path when the upstream signal is relayed.

JP 2005-184272 A

  However, in the above-described conventional technique, it is troublesome to set the attenuation amount in the attenuation unit provided in each of the upstream signal relay path and the downstream signal relay path appropriately at the time of relaying and at the time of non-relaying. When one signal is relayed, the control for preventing the one signal from leaking into the other signal relay path is complicated as a two-stage configuration of switch opening / closing control and attenuation increase / decrease control, As a result, the cost increases.

  In addition, in the metal line transmission line, it is assumed that as the line length becomes longer, noise is increased. However, in the above-described conventional technology, the distribution / coupling unit arranged on both metal line transmission lines side is the metal line transmission line. Since, for example, when there is noise from the downstream metal line transmission line when relaying an upstream signal, it is erroneously detected as a signal input and relayed downstream. The path may become conductive. In this case, since it corresponds to the upstream signal leaking into the downstream relay path, oscillation may not be prevented.

  This invention has been made in view of the above, and in the case of performing relay amplification by interposing on a metal line transmission line where uplink signals and downlink signals of the same frequency are transmitted in a time division manner, with an inexpensive configuration, In addition, a signal relay amplifier having a mechanism capable of preventing the formation of a loop that goes around both relay paths even if a signal wraps around from one relay path to the other relay path or noise is mixed with simple control. The purpose is to obtain.

In order to achieve the above-described object, the present invention provides a signal relay amplifier that performs relay amplification by interposing on a metal line transmission line in which an upstream signal and a downstream signal of the same frequency are transmitted in a time-sharing manner. An upstream metal line transmission line, which is one side of the line transmission line divided into two, is connected to a downstream output port that outputs a downstream signal from the upstream metal line transmission line to the upstream metal line transmission line An upstream input / output circuit having an upstream input port through which an upstream signal to be transmitted is input is connected to a downstream metal line transmission line which is the other side of the metal line transmission line, and the downstream metal A downlink input / output circuit having an uplink output port for outputting an uplink signal from a line transmission line to the inside, and a downlink input port for inputting a downlink signal to be transmitted to the downlink metal line transmission line from the inside; Downstream signal amplifier for amplifying a signal transmitted from the downstream output port of the upstream side input / output circuit, disposed in the downstream relay path from the downstream output port of the input / output circuit to the downstream input port of the downstream side input / output circuit A downstream signal switch interposed between an output terminal of the downstream signal amplifier and a downstream input port of the downstream input / output circuit; and an upstream input / output from the upstream output port of the downstream input / output circuit An upstream signal amplifier that amplifies a signal transmitted from the upstream output port of the downstream input / output circuit, disposed on an upstream relay path to the upstream input port of the circuit, and an output terminal of the upstream signal amplifier, An upstream signal switch interposed between the upstream input port of the upstream input / output circuit, a downstream signal level detector for detecting the output level of the downstream signal amplifier, and The signal level detector for the upstream signal that detects the output level of the upstream signal amplifier is compared with the magnitude relationship between the signal levels detected by the signal level detector for the downstream signal and the signal level detector for the upstream signal. And a comparator for outputting the comparison result as a control signal for setting one side in an open state and the other in a closed state in parallel with the downstream signal switch and the upstream signal switch. And

  According to the present invention, each of the upstream and downstream relay paths is constituted by a series circuit of an amplifier and a switch, and one of the upstream signal switch and the downstream signal switch is opened, and the other is closed and opened. Since it is performed according to the magnitude relationship between the output levels of the amplifiers in those input stages, it can be configured cheaper than the prior art, and one relay path can be achieved by a simple control method. Even if a signal wraps around the other relay path or noise is mixed, it is possible to prevent the formation of a loop that goes around both relay paths and to prevent oscillation.

  Exemplary embodiments of a signal relay amplifier according to the present invention will be explained below in detail with reference to the drawings.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of a signal relay amplifier according to Embodiment 1 of the present invention. The signal relay amplifier 1 shown in FIG. 1 is disposed on a metal line transmission line through which uplink signals and downlink signals having the same frequency are transmitted in a time division manner. In FIG. One side divided into two is expressed as an upstream metal line transmission line 2a, and the other side is expressed as a downward metal line transmission line 2b.

  As shown in FIG. 1, the signal relay amplifier 1 includes a hybrid circuit 3a that is an upstream input / output circuit to which an upstream metal line transmission line 2a is connected, and a downstream input that is connected to a downstream metal line transmission line 2b. And a hybrid circuit 3b which is an output circuit. The hybrid circuits 3a and 3b are so-called two-wire / four-wire conversion circuits, respectively.

  The hybrid circuit 3a includes a downlink output port D1 that outputs a downlink signal from the uplink metal line transmission path 2a to the downlink relay path, and an uplink signal to which an uplink signal to be transmitted to the uplink metal line transmission path 2a is input from the uplink relay path. And an input port U2. Further, the hybrid circuit 3b receives an upstream output port U1 that outputs an upstream signal from the downstream metal line transmission path 2b to the upstream relay path and a downstream signal that is transmitted to the downstream metal line transmission path 2b from the downstream relay path. And a downstream input port D2.

  The downlink relay path is a path from the downlink output port D1 of the hybrid circuit 3a to the downlink input port D2 of the hybrid circuit 3b. From the downlink output port D1 side, an amplifier (Amp) 4a and a switch 5a are arranged in this order. Yes. The upstream relay path is a path from the upstream output port U1 of the hybrid circuit 3b to the downstream input port D2 of the hybrid circuit 3a. From the upstream output port U1 side of the hybrid circuit 3b, the amplifier (Amp) 4b and the switch 5b Are arranged in this order.

  As a control system for the switches 5a and 5b, signal level detectors 6a and 6b and a comparator 7 are provided. The signal level detector 6 a detects the output level of Amp 4 a and supplies it to the comparator 7. The signal level detector 6 b detects the output level of Amp 4 b and supplies it to the comparator 7. The comparator 7 compares the magnitude relationship between the signal level Vd detected by the signal level detector 6a and the signal level Vu detected by the signal level detector 6b, and the comparison result is parallel to the switches 5a and 5b. , One is opened, and the other is closed as a control signal.

  Next, the operation will be described with reference to FIGS. FIG. 2 is a diagram showing the relationship between the two signal levels input to the comparator shown in FIG. 1 when the downlink signal is relayed. FIG. 3 is a diagram for explaining an operation in which the comparator shown in FIG. 1 controls opening and closing of two switches when relaying a downlink signal. FIG. 4 is a diagram showing a relationship between two signal levels input to the comparator shown in FIG. 1 when relaying an uplink signal. FIG. 5 is a diagram for explaining an operation in which the comparator shown in FIG.

  Since the uplink signal and the downlink signal are transmitted in a time division manner on the metal line transmission path, the hybrid from the downlink metal line transmission path 2b during the period when the downlink signal is input from the uplink metal transmission path 2a to the hybrid circuit 3a. No upstream signal is input to the circuit 3b. Similarly, in a period in which an upstream signal is input from the downstream metal transmission line 2b to the hybrid circuit 3b, no downstream signal is input from the upstream metal line transmission line 2a to the hybrid circuit 3a. However, noise can be mixed. Note that the initial state of the switches 5a and 5b is an open / closed state depending on the comparison result of the comparator 7 as described above.

  The downlink signal relay operation is performed as follows. The downstream signal from the upstream metal transmission line 2a is input from the downstream output port D1 of the hybrid circuit 3a to the switch 5a and the signal level detection unit 6a via the Amp 4a. At this timing, a noise component is input to the signal level detector 6b. Therefore, as shown in FIG. 2, since the signal level Vd input from the signal level detector 6a is larger than the signal level Vu input from the signal level detector 6b, the comparator 7 immediately closes the switch 5a, A control signal for opening the switch 5b is output to both the switches 5a and 5b in parallel (FIG. 3). As a result, the downlink signal amplified by Amp 4a is transmitted to the downlink input port D2 of the hybrid circuit 3b via the switch 5a, and is transmitted from the hybrid circuit 3b onto the downlink metal line transmission line 2b.

  In this case, when the impedance of the downstream metal line transmission line 2b viewed from the hybrid circuit 3b is a constant impedance that is matched, the hybrid circuit 3b has a sufficient gap between the downstream input port D2 and the upstream output port U1. Therefore, the amount of the downstream signal leaking from the upstream output port U1 to the Amp 4b side is small.

  However, depending on the transmission path characteristics of the downstream metal line transmission path 2b, the hybrid circuit 3b may not see the impedance of the downstream metal line transmission path 2b as a constant impedance. In this case, since the hybrid circuit 3b cannot secure sufficient isolation between the downstream input port D2 and the upstream output port U1, the downstream signal is sent from the upstream output port U1 to the Amp 4b according to the lack of isolation. The amount that leaks to the side increases.

  That is, although there is an excessive amount, a part of the downlink signal transmitted from the hybrid circuit 3b onto the downlink metal line transmission line 2b leaks from the uplink output port U1 to the Amp 4b side. When the line length of the metal line transmission line is long, the degree of noise mixing increases. The leaked down signal on which noise is superimposed is amplified by Amp 4b and input to switch 5b and signal level detector 6b.

  The signal level detector 6b detects the level of the leaked downstream signal amplified by Amp 4b and supplies it to the comparator 7. Even if noise is superimposed on the downstream signal leaking from the upstream output port U1 to Amp4b, the level Vd of the downstream signal amplified by Amp4a and the downstream leakage signal amplified by Amp4b are input to the comparator 7. The level relationship with the level Vu is the same as Vd> Vu shown in FIG. Therefore, the comparator 7 outputs a control signal (comparison result) for maintaining the state shown in FIG. 3 in parallel to both the switches 5a and 5b. As a result, the switch 5a on the downstream relay path is controlled so as to maintain the closed state, and the switch 5b on the upstream relay path is controlled to maintain the open state.

  Since the switch 5b is in the open circuit state, the leaked downstream signal amplified by the Amp 4b is prevented from being input to the upstream input port U2 of the hybrid circuit 3a. In the hybrid circuit 3a, even if the isolation between the upstream input port U2 and the downstream output port D1 is not sufficient, there is no leaked downstream signal input to the upstream input port U2. Is not formed and does not oscillate.

  As described above, the relay amplification operation of the downlink signal is stably executed without oscillation within the transmission period of the downlink signal.

  Next, when the signal to be transmitted is switched from the downstream signal to the upstream signal, the following operation is performed. The upstream signal from the downstream metal transmission line 2b is input from the upstream output port U1 of the hybrid circuit 3b to the open switch 5b and the signal level detection unit 6b via the Amp 4b.

  In this case, in the situation where the isolation between the uplink output port U1 and the downlink input port D2 of the hybrid circuit 3b is not sufficient as described above, the amount of the uplink signal leaking from the downlink input port D2 to the switch 5a side increases. . Since the switch 5a is in a closed state, the leaked up signal on which noise is superimposed is input to the signal level detector 6a via the switch 5a.

  However, even if noise is superimposed on the upstream signal leaking from the downstream input port D2 of the hybrid circuit 3b to the signal level detector 6a side, as shown in FIG. 4, the signal level Vd detected by the signal level detector 6a. Is lower than the signal level Vu detected by the signal level detector 6b, the comparator 7 immediately opens a control signal (comparison result) for opening the switch 5a and closing the switch 5b to both the switches 5a and 5b. Output in parallel (FIG. 5). As a result, the uplink signal amplified by Amp 4b is transmitted to the uplink input port U2 of the hybrid circuit 3a via the switch 5b, and is transmitted from the hybrid circuit 3a onto the uplink metal line transmission line 2a.

  In this case, as in the case of relaying the downlink signal described above, an amount of downlink signal corresponding to the degree of isolation between the uplink input port U2 and the downlink output port D1 of the hybrid circuit 3a is transferred from the downlink output port D1 to the Amp 4a side. It leaks and is amplified and input to the switch 5a and the level detector 6a.

  However, since the switch 5a is already in the open circuit state, the leaked upstream signal amplified by the Amp 4a is not transmitted to the downstream input port D2 of the hybrid circuit 3b. Therefore, in the hybrid circuit 3b, even if the isolation between the downlink input port D2 and the uplink output port U1 is not sufficient, the leaky uplink signal is not input to the downlink input port D2, so that both relay paths are made a round. The loop is not formed and does not oscillate.

  Then, even if noise is superimposed on the downstream signal leaked from the downstream output port D1 of the hybrid circuit 3a and amplified by the amplifier 4a, the signal detected by the signal level detection unit 6a input to the comparator 7 The magnitude relationship between the level Vd and the signal level Vu detected by the signal level detection unit 6b is the same as Vd <Vu shown in FIG. Therefore, the comparator 7 outputs a control signal (comparison result) for maintaining the state shown in FIG. 5 in parallel to both the switches 5a and 5b. Accordingly, the switch 5a on the downstream relay path is controlled so as to maintain the open circuit state, and the switch 5b on the upstream relay path is controlled to maintain the closed circuit state.

  As described above, the relay amplification operation of the uplink signal is stably performed without oscillation during the transmission period of the uplink signal.

  At the time of designing to realize the above relay operation, the isolation determined by the relationship with the impedance of the metal line transmission line in the hybrid circuits 3a and 3b is appropriately adjusted, and the amplification gains of the Amps 4a and 4b are In consideration of the magnitude of noise mixed in the metal line transmission path, it is adjusted so that Vd> Vu is reliably established during downlink relay and Vd <Vu is reliably established during uplink relay.

  As described above, according to the first embodiment, each upstream and downstream relay path is configured by a series circuit of an amplifier and a switch, so that it can be configured at a lower cost than the conventional technology. Then, the open / close control for switching one of the down relay switch and the up relay switch to the open state and the other to the closed state is performed according to the magnitude relationship of the output levels of the amplifiers in the input stages. Therefore, even if a signal wraps around from one relay path to the other relay path or noise is mixed by a simpler control method than the prior art, it is possible to prevent the formation of a loop that goes around both relay paths. Therefore, it is possible to prevent oscillation and realize a stable relay amplification operation.

Embodiment 2. FIG.
FIG. 6 is a block diagram showing a configuration of a signal relay amplifier according to Embodiment 2 of the present invention. In FIG. 6, components that are the same as or equivalent to the components shown in FIG. 1 (Embodiment 1) are assigned the same reference numerals. Here, the description will be focused on the portion related to the second embodiment.

  As described in the first embodiment, the initial state of the downstream signal switch 5a and the upstream signal switch 5b is an open / close state depending on the comparison result in the comparator 7. In principle, this may be sufficient, but depending on the level of noise mixed in the metal line transmission line, an erroneous opening / closing operation may be performed on the downstream signal switch 5a and the upstream signal switch 5b during a period when the upstream / downstream signal is not transmitted. There is a possibility of doing it.

  Therefore, in the signal relay amplifier 10 according to the second embodiment shown in FIG. 6, in the configuration shown in FIG. 1 (the first embodiment), the signal level and the noise level are given to the comparator 11 in place of the comparator 7. When a threshold level that can be identified is set, and the switching operation for the downstream signal switch 5a and the upstream signal switch 5b is performed by the method described in the first embodiment, it is not affected by noise. .

  FIG. 7 is a diagram for explaining the relationship between the threshold level set in the comparator shown in FIG. 6 and the noise level detected by the two signal level detectors shown in FIG. 6 during a period when there is no uplink / downlink relay signal. FIG. 8 is a diagram for explaining an operation in which the comparator shown in FIG. 6 controls opening / closing of two switches in a period when there is no uplink / downlink relay signal.

  During the period when the upstream signal and the downstream signal are not transmitted on the metal line transmission path, the hybrid circuit 3a sees noise existing on the upstream side metal line transmission path 2a, and the hybrid circuit 3b Noise present on the downstream metal line transmission line 2b is visible.

  Noise existing on the upstream metal line transmission line 2 a enters the amplifier 5 a from the downstream output port D 1 of the hybrid circuit 3 a, is amplified, detected as the signal level Vd by the signal level detector 6 a, and input to the comparator 11. . Further, noise existing on the downstream side metal line transmission line 2b enters the amplifier 5b from the upstream output port U1 of the hybrid circuit 3b, is amplified, detected as the signal level Vu by the signal level detector 6b, and input to the comparator 11 Is done.

  As shown in FIG. 7, the signal level Vd detected by the signal level detector 6a and the signal level detector 6b are detected in a period in which the upstream signal and the downstream signal are not transmitted on the metal line transmission path. The signal level Vu is almost the same size, but the magnitude relationship between them is constantly changing. In this case, since the comparator 7 shown in FIG. 1 cannot output a stable comparison result, the switches 5a and 5b may repeatedly open and close repeatedly.

  Therefore, the comparator 11 has a threshold level Vth on the metal line transmission line that can discriminate between the levels of the uplink signal and the downlink signal transmitted on the metal line transmission line and the noise level superimposed on the uplink and downlink signals. In a period in which the upstream signal and downstream signal are not transmitted, the signal level Vd detected by the signal level detector 6a and the signal level Vu detected by the signal level detector 6b are both set to be larger. Has been.

  The comparator 11 first compares the signal levels Vd and Vu detected by the signal level detectors 6a and 6b with the threshold level Vth, and the signal levels Vd and Vu are both as shown in FIG. When it is smaller than the threshold level Vth, as shown in FIG. 8, a control signal for opening both the switches 5a and 5b is output in parallel to the switches 5a and 5b. That is, the switches 5a and 5b are prevented from performing an unnecessary opening / closing operation in a period in which the upstream signal and the downstream signal are not transmitted on the metal line transmission path.

  FIG. 9 is a diagram showing the relationship between the two signal levels input to the comparator shown in FIG. 6 and the threshold level when relaying the downlink signal. FIG. 10 is a diagram showing the relationship between the two signal levels input to the comparator shown in FIG. 6 and the threshold level when the uplink signal is relayed.

  When an up signal or a down signal is transmitted on the metal line transmission line, as shown in FIGS. 9 and 10, the two signal levels Vd and Vu input to the comparator 11 are both higher than the threshold level Vth. Can be understood from the description in the first embodiment.

  Therefore, as described above, the comparator 11 first compares the signal levels Vd and Vu detected by the signal level detectors 6a and 6b with the threshold level Vth. The signal levels Vd and Vu are both threshold values. Since it is higher than the level Vth, the magnitude relationship between the signal levels Vd and Vu is compared next time as described in the first embodiment.

  As a result, as shown in FIG. 9, when Vd> Vu, a control signal for setting the switches 5a and 5b to the state shown in FIG. 3 is output in parallel to the switches 5a and 5b. As shown in FIG. 10, when Vd <Vu, a control signal for setting the switches 5a and 5b to the state shown in FIG. 5 is output to the switches 5a and 5b in parallel.

  As described above, according to the embodiment, since the threshold level for distinguishing between the noise level and the signal level is provided, even in an installation environment in which the line length is long and noise is mixed, the uplink and downlink relay signals are It is possible to avoid an erroneous switch open / close control during a period of only a noise level, and to perform an appropriate relay amplification operation on the uplink / downlink relay signal.

  As described above, the signal relay amplifier according to the present invention has a low-cost configuration when relay amplification is performed on a metal line transmission line in which uplink signals and downlink signals of the same frequency are transmitted in a time division manner. In addition, it is useful for preventing oscillation even when a signal wraps around from one relay path to the other relay path or noise is mixed in with simple control. It is suitable for a signal relay amplifier in a metal line transmission line that is wired in an environment in which such a mixture is expected, for example, in a building or a train.

It is a block diagram which shows the structure of the signal relay amplifier by Embodiment 1 of this invention. It is a figure which shows the relationship between the two signal levels input into the comparator shown in FIG. 1 at the time of downstream signal relay. It is a figure explaining the operation | movement which the comparator shown in FIG. 1 controls opening / closing of two switches at the time of a downstream signal relay. It is a figure which shows the relationship of two signal levels input into the comparator shown in FIG. 1 at the time of an upstream signal relay. It is a figure explaining the operation | movement which the comparator shown in FIG. 1 controls opening / closing of two switches at the time of an upstream signal relay. It is a block diagram which shows the structure of the signal relay amplifier by Embodiment 2 of this invention. FIG. 7 is a diagram for explaining a relationship between a threshold level set in the comparator shown in FIG. 6 and a noise level detected by two signal level detectors shown in FIG. 6 in a period when there is no uplink / downlink relay signal. It is a figure explaining the operation | movement which the comparator shown in FIG. 6 controls opening / closing of two switches in the period when there is no uplink / downlink relay signal. It is a figure which shows the relationship between the two signal levels input into the comparator shown in FIG. 6 at the time of a downstream signal relay, and a threshold level. It is a figure which shows the relationship between the two signal levels input into the comparator shown in FIG. 6 at the time of an upstream signal relay, and a threshold level.

Explanation of symbols

1 Signal Relay Amplifier 2a Upstream Metal Line Transmission Line 2b Downstream Metal Line Transmission Line 3a, 3b Hybrid Circuit 4a, 4b Amplifier (Amp)
5a, 5b Switch 6a, 6b Signal level detector 7 Comparator 10 Signal relay amplifier 11 Comparator

Claims (3)

A signal relay amplifier that performs relay amplification by interposing on a metal line transmission line in which upstream and downstream signals of the same frequency are transmitted in a time-sharing manner,
An upstream metal line transmission line, which is one side of the metal line transmission line divided into two, is connected, a downstream output port that outputs a downstream signal from the upstream metal line transmission line, and the upstream metal line transmission An upstream input / output circuit having an upstream input port through which an upstream signal to be sent to the road is input;
A downstream metal line transmission line that is the other side of the metal line transmission line divided into two is connected, an upstream output port that outputs an upstream signal from the downstream metal line transmission line, and the downstream metal line transmission A downstream input / output circuit having a downstream input port through which a downstream signal to be transmitted to the road is input;
Downlink that amplifies a signal transmitted from the downstream output port of the upstream input / output circuit, which is disposed on the downstream relay path from the downstream output port of the upstream input / output circuit to the downstream input port of the downstream input / output circuit A signal amplifier, and a downstream signal switch interposed between an output terminal of the downstream signal amplifier and a downstream input port of the downstream input / output circuit;
An upstream that amplifies a signal transmitted from the upstream output port of the downstream input / output circuit, disposed in the upstream relay path from the upstream output port of the downstream input / output circuit to the upstream input port of the upstream input / output circuit A signal amplifier, and an upstream signal switch interposed between the output terminal of the upstream signal amplifier and the upstream input port of the upstream side input / output circuit;
A downstream signal level detector for detecting an output level of the downstream signal amplifier, and an upstream signal level detector for detecting an output level of the upstream signal amplifier;
Compares the magnitude relationship between the signal levels detected by the downstream signal level detector and the upstream signal level detector, and compares the comparison results with the downstream signal switch and upstream signal switch. In parallel, a comparator that outputs as a control signal that opens one circuit and closes the other,
A signal relay amplifier comprising: The comparator is
A threshold level is set that can distinguish between the level of the upstream signal and downstream signal transmitted on the metal line transmission line and the level of noise superimposed on the upstream and downstream signals,
First, a comparison is made between the signal level detected by the downstream signal level detector and the upstream signal level detector and the threshold level, and both detection levels are both lower than the threshold level. Outputs a control signal for opening both in parallel to the downstream signal switch and the upstream signal switch, and when both detection levels are both higher than the threshold level, both detection levels are output. Make a size comparison between
The signal relay amplifier according to claim 1.   3. The signal relay amplifier according to claim 1, wherein each of the upstream side input / output circuit and the downstream side input / output circuit is a hybrid circuit that performs 2-wire / 4-wire conversion.

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