JP2005136643A - Communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a communication system for stably reducing the signal level of a loopback signal. <P>SOLUTION: A magnitude relation between the impedance of the total of resistances R1 and R2 for impedance matching between a transmitting part 10 and a transmission line C20 and the characteristics impedance of a transmission line C20 and a magnitude relation between the impedance of the total of resistances R3 and R4 for impedance matching, between the transmitting part 10 and a terminal resistance RT and the impedance of the terminal resistance RT are matched in the frequency range of the signal to be transmitted/received by a hybrid circuit 1. Thus, since phase change at reflection between the transmission line C20 and phase change at reflection between the terminal resistance RT can be made the same, and differential components are taken by a receiving part 11 so that components due to reflection can be offset at the two parts the voltage level of a loopback signal can be stably suppressed low. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a communication system that transmits and receives signals between communication terminals via a transmission line.

  Conventionally, there is a CSMA / CD (Carrier Sense Multiple Access with Collision Detect) method as a method used in a communication system in which a plurality of communication terminals perform multiple access to a common transmission line. This method is a communication method in which any communication terminal can transmit if a no-signal state continues on the transmission line for a certain time (gap between frames), and the communication terminal is a carrier for detecting no signal. It has a signal detection function and a collision detection function for detecting signal collision. Specifically, the collision detection function loops back the transmission signal transmitted by the communication terminal when the communication terminal transmits the signal, monitors the transmission signal transmitted by itself and the transmission signal on the transmission line, If the transmission signal and the transmission signal match, it is determined that no signal collision has occurred, and if the transmission signal and the transmission signal do not match, it is determined that a signal collision has occurred. When a signal collision is detected, the communication terminal temporarily stops transmission and tries to transmit the signal again after a predetermined time (see, for example, Patent Document 1).

  Here, when a transmission signal is directly looped back, for example, in a communication system with a large attenuation employing a multi-drop method in which a large number of communication terminals are branched and connected to a common transmission line, the voltage level of a received signal from another communication terminal On the other hand, the voltage level of the loopback signal becomes too high, and the received signal is buried in the loopback signal, making it difficult to separate. For this reason, the transmission signal can be directly looped back only when the attenuation of the signal from the other communication terminal is relatively small (for example, about 20-40 dB).

  In order to solve this problem, a communication system using a communication terminal X having the circuit configuration shown in FIG. 4 is provided. The communication terminal X includes a transmission / reception circuit 2 that transmits and receives signals and a hybrid circuit 1 that performs 2-wire to 4-wire conversion. The hybrid circuit 1 includes a transmission unit 10 and a reception unit 11.

  The transmission unit 10 is a balanced output, and is connected to the transmission transformer T1 via the impedance matching resistors R1 and R2. The transmission signal output from the transmission / reception circuit 2 is transmitted between the transmission transformer T1 and the input / output terminal 3. To the transmission line C20. Further, a termination resistor RT having an impedance matched to the characteristic impedance of the transmission line C20 is connected between the transmission unit 10 and the impedance matching resistors R1 and R2 via the impedance matching resistors R3 and R4.

  The receiving unit 11 is composed of a differential amplifier. The other end of the impedance matching resistor R6, one end of which is connected between the impedance matching resistor R1 and the transmission transformer T1, and the impedance matching resistor R3 are connected to the inverting input terminal. Is connected in parallel with the other end of the impedance matching resistor R5 having one end connected between the impedance matching resistor R2 and the transmission transformer T1. Are connected in parallel with the other end of the impedance matching resistor R7 connected to the other end of the impedance matching resistor R8, one end of which is connected between the impedance matching resistor R4 and the termination resistor RT.

That is, the inverting input terminal of the receiving unit 11 has an output from one output terminal of the transmitting unit 10 via the impedance matching resistor R1 and the transmission transformer T1, and an output from the other output terminal of the transmitting unit. The output via the impedance matching resistor R3 and the termination resistor RT is combined and input, and the reception unit 11 transmits the transmission signal transmitted to the transmission line C20 and the termination resistor RT. The differential component with the transmitted signal is input as a loopback signal. The receiving unit 11 amplifies the input loopback signal and the received signal from another communication terminal and outputs the amplified signal to the transmission / reception circuit 2. According to this configuration, the transmission signal transmitted to the transmission line C20 and the transmission signal transmitted to the termination resistor RT cancel each other, so that the voltage level of the loopback signal is lowered, while from other communication terminals Since the received signals are received without canceling each other, the voltage level difference between the loopback signal and the received signal is small, so that the attenuation of signals from other communication terminals is relatively large (for example, about 40-60 dB). Can also be used.
Japanese Examined Patent Publication No. 7-32399 (page 4-12, Fig. 3)

  Here, fixed resistors are used as the impedance matching resistors R1 and R2, and the frequency characteristics are different between the fixed resistor and the transmission line C20. Therefore, the impedance matching resistors are used in the entire frequency range of signals transmitted and received by the hybrid circuit 1. It is impossible to match the impedances of R1 and R2 to the characteristic impedance of the transmission line C20. In many cases, the loopback signal is reflected at the connection with the transmission line C20, the terminating resistor RT, or the pseudo line C21. Due to the presence of components. Usually, the impedances of the impedance matching resistors R1 and R2 are set so as to minimize the reflection as described above, but when the frequency characteristic of the characteristic impedance of the transmission line C20 is high, or for each transmission line C20 If there is a large variation in the characteristic impedance, the difference between the impedances of the impedance matching resistors R1 and R2 and the characteristic impedance of the transmission line C20 becomes large, and it is considered that the influence of the reflection as described above cannot be ignored. .

  The magnitude relationship of the impedances of the impedance matching resistors R1 and R2 with respect to the characteristic impedance of the transmission line C20 is the magnitude relationship of the impedances of the impedance matching resistors R3 and R4 with respect to the impedance of the termination resistor RT or the characteristic impedance of the pseudo line C21. The phase of the component due to reflection from the transmission line C20 and the phase of the component due to reflection from the termination resistor RT or the pseudo line C21 are opposite to each other. For example, when the impedance of the impedance matching resistors R1 and R2 is larger than the characteristic impedance of the transmission line C20 and the impedance of the impedance matching resistors R3 and R4 is smaller than the impedance of the termination resistor RT or the characteristic impedance of the pseudo line C21. The reflection with the transmission line C20 is fixed-end reflection and the phase is shifted by π, while the reflection with the termination resistor RT or the pseudo line C21 is free-end reflection and the phase does not change. In such a case, the voltage level of the differential component between the component caused by reflection between the transmission line C20 and the component caused by reflection between the termination resistor RT or the pseudo line C21 is twice the original voltage level. As a result, the voltage level of the loopback signal becomes relatively high.

  The present invention has been made in view of the above reasons, and an object of the present invention is to provide a communication system capable of stably keeping the voltage level of a loopback signal low.

  The invention of claim 1 is a communication system that transmits and receives signals between a plurality of communication terminals connected to a transmission line, and the communication terminals have been transmitted from a transmission unit that transmits signals and other communication terminals. A hybrid circuit consisting of a receiver that receives a signal and receives a loopback signal from the transmitter, and a termination circuit that matches the impedance to the characteristic impedance of the transmission line, and the receiver outputs from the transmitter to the transmission line A differential component between the transmitted signal and the signal output from the transmission unit to the termination circuit is received as a loopback signal of the transmission signal, and the transmission unit transmits the transmission line via the first impedance matching circuit for impedance matching. To the termination circuit via the second impedance matching circuit for impedance matching, and to the first impedance matching circuit. The magnitude relation of the impedance of the transmission line to the characteristic impedance of the transmission line and the magnitude relation of the impedance of the second impedance matching circuit to the impedance of the termination circuit are matched in the frequency range of the signal transmitted and received by the hybrid circuit. And

  According to this configuration, in the frequency range of the signal transmitted and received by the hybrid circuit, the phase change at the time of reflection between the first impedance matching circuit and the transmission line, and between the second impedance matching circuit and the termination circuit. The phase change during reflection is always the same. That is, in the frequency range of the signal transmitted and received by the hybrid circuit, the phase of the component due to reflection between the first impedance matching circuit and the transmission line, and the reflection between the second impedance matching circuit and the termination circuit Since the phase of the component does not become opposite to each other, the differential component at the receiving unit always cancels out the components due to reflection at the two locations. The voltage level of the loopback signal can be stably kept low compared with the case where the phase changes may be different from each other.

  The invention of claim 2 is a communication system that transmits and receives signals between a plurality of communication terminals connected to the transmission line, and includes a pseudo line that matches the characteristic impedance to the characteristic impedance of the transmission line. A hybrid circuit including a transmission unit that transmits a signal and a reception unit that receives a signal transmitted from another communication terminal and receives a loopback signal from the transmission unit; A differential component between the signal output to the transmission line and the signal output from the transmission unit to the pseudo line is received as a loopback signal of the transmission signal, and the transmission unit includes a first impedance matching circuit for impedance matching. Connected to the transmission line via the second impedance matching circuit for impedance matching, and connected to the pseudo line via the second impedance matching circuit. The magnitude relationship of the impedance of the matching circuit to the characteristic impedance of the transmission line and the magnitude relationship of the impedance of the second impedance matching circuit to the impedance of the pseudo line are matched in the frequency range of the signal transmitted and received by the hybrid circuit. It is characterized by.

  According to this configuration, in the frequency range of the signal transmitted and received by the hybrid circuit, the phase change at the time of reflection between the first impedance matching circuit and the transmission line, and between the second impedance matching circuit and the pseudo line. The phase change during reflection is always the same. That is, in the frequency range of the signal transmitted / received by the hybrid circuit, the phase of the component due to reflection between the first impedance matching circuit and the transmission line and the reflection between the second impedance matching circuit and the pseudo line Since the phase of the component does not become opposite to each other, the differential component at the receiving unit always cancels out the components due to reflection at the two locations. The voltage level of the loopback signal can be stably kept low compared with the case where the phase changes may be different from each other. In addition, the characteristic impedance of the pseudo line is easy to match the frequency characteristic to the characteristic impedance of the transmission line compared to the impedance of the fixed resistance, so the voltage level of the loopback signal can be set in a wider frequency range than when using the fixed resistance. It can be kept low.

  The invention according to claim 3 is the invention according to claim 1 or 2, wherein the impedance of the first impedance matching circuit is the maximum value and the minimum value of the characteristic impedance of the transmission line in the frequency range of the signal transmitted and received by the hybrid circuit. And the impedance of the second impedance matching circuit between the maximum value and the minimum value of the impedance of the termination circuit or the characteristic impedance of the pseudo line in the frequency range of the signal transmitted and received by the hybrid circuit. It is characterized by being set equal to either one.

  According to the above configuration, while suppressing the voltage level of the loopback signal to be low, the reflection between the first impedance matching circuit and the transmission line, and between the second impedance matching circuit and the termination circuit or the pseudo line. And the S / N ratio can be improved.

  The present invention relates the magnitude relationship of the impedance of the first impedance matching circuit to the characteristic impedance of the transmission line and the magnitude relationship of the impedance of the second impedance matching circuit to the impedance of the termination circuit or the characteristic impedance of the pseudo line. Because the frequency range of the signal transmitted and received by the hybrid circuit is matched, the phase of the component due to reflection between the transmission line and the phase of the component due to reflection between the termination circuit or the pseudo line matches. By taking the differential component, the component due to reflection can be removed, and therefore the voltage level of the loopback signal can be stably kept low.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  In the following description, a balanced line is assumed as the transmission line C20. However, the transmission line C20 is not necessarily a balanced line, and the transmission line C20 may be an unbalanced line such as a coaxial cable.

(Embodiment 1)
Hereinafter, the communication terminal X of the present embodiment will be described with reference to FIG. Since the basic configuration of this embodiment is the same as that of the conventional example, common portions are denoted by the same reference numerals, description thereof is omitted, and only different portions are described.

  The communication terminal X of this embodiment includes a termination circuit including a termination resistor RT connected to the impedance matching resistors R3 and R4 via the pseudo load terminal 4. The impedance Z1 of the termination resistor RT is matched to the characteristic impedance Z0 of the transmission line C20. Actually, since the characteristic impedance Z0 of the transmission line 20 has variation and frequency dependence for each transmission line C20, the impedance Z1 of the termination resistor RT is, for example, the center of the range of values that the characteristic impedance Z0 of the transmission line C20 can take. Set to value. For example, when the characteristic impedance of the transmission line C20 is 60-90Ω, the impedance Z1 of the termination resistor RT is set to about 75Ω. The impedance matching resistors R1 and R2 form a first impedance matching circuit for impedance matching with the transmission line C20, and the impedance matching resistors R3 and R4 are second impedance matching resistors for the termination resistor RT. An impedance matching circuit is formed. The impedances of the impedance matching resistors R1 to R4 are all set to the same value Z / 2, and the impedance of the first impedance matching circuit and the impedance of the second impedance matching circuit are the same value Z. Yes. The impedances of the impedance matching resistors R5 to R8 whose one end is connected to the receiving unit 11 are set higher than the impedances of the impedance matching resistors R1 to R4 whose one end is connected to the transmitting unit 10.

  The impedance Z of the first impedance matching circuit and the second impedance matching circuit is such that Z ≦ Z0 and Z ≦ Z1 in the frequency range (for example, several hundred k to several MHz) of the signal transmitted and received by the hybrid circuit 1 or It is set so as to satisfy one of the conditions Z ≧ Z0 and Z ≧ Z1. Here, when the difference between the impedance Z of the first impedance matching circuit and the second impedance matching circuit and the characteristic impedance Z0 of the transmission line C20 and the impedance Z1 of the termination resistor RT increases, reflection increases. Signal attenuation increases. When Zmin ≦ Z0 ≦ Zmax and Zmin ≦ Z1 ≦ Zmax in the frequency range of a signal transmitted / received by the transmission / reception circuit 2, setting Z = Zmin or Z = Zmax makes it possible to reduce the voltage level of the loopback signal while keeping the voltage level low. S / N ratio can be improved by minimizing.

  According to the above configuration, when Z ≦ Z0 and Z ≦ Z1, the phase change at the time of reflection between the first impedance matching circuit and the transmission line C20, the second impedance matching circuit, and the termination resistance Both the phase change during reflection with RT is 0, and when Z ≧ Z0 and Z ≧ Z1, the phase change during reflection between the first impedance matching circuit and the transmission line C20. The phase change during reflection between the second impedance matching circuit and the termination resistor RT is both π. That is, in the frequency range of the signal transmitted / received by the hybrid circuit 1, the phase change at the time of reflection between the first impedance matching circuit and the transmission line C20, and between the second impedance matching circuit and the termination resistor RT. The phase change during reflection is always the same. That is, in the frequency range of the signal transmitted / received by the hybrid circuit 1, between the phase of the component due to reflection between the first impedance matching circuit and the transmission line C20, and between the second impedance matching circuit and the termination resistor RT. Since the phase of the component due to reflection is not opposite to each other, by taking the differential component at the receiving unit 11, the component due to reflection at the two locations always cancels each other. The voltage level of the loopback signal can be stably kept low compared with the case where the phase changes at the time of reflection are different from each other. Therefore, this embodiment can also be used when the attenuation of a signal from another communication terminal is, for example, 60-80 dB.

(Embodiment 2)
Since the basic configuration of the present embodiment is the same as that of the first embodiment, common portions are denoted by the same reference numerals, description thereof is omitted, and only different portions are described.

  In this embodiment, instead of the termination resistor RT of the first embodiment, as shown in FIG. 2, the other end of the pseudo line C21 whose one end is matched with the characteristic impedance of the transmission line C20 is used as the transmission transformer. It is characterized in that it is connected to the pseudo load terminal 4 via T2. It is desirable to use the same line type as the transmission line C20 for the pseudo line C21. Furthermore, when an air pair in the same sheath as the transmission line C20 is used as the pseudo line C21, the characteristic impedance can be easily matched to the characteristic impedance of the transmission line C20.

  According to the above configuration, the characteristic impedance of the transmission line C20 and the characteristic impedance of the pseudo line C21 can be matched not only to a value at a specific frequency but also to a frequency characteristic. As a result, the voltage level of the loopback signal can be stably kept low.

  Note that the first impedance matching circuit and the second impedance matching circuit may be circuits that automatically match the impedance by a known technique using a training signal, for example, instead of being configured by a fixed resistor. Further, a phase compensation circuit may be incorporated in the first impedance matching circuit and the second impedance matching circuit.

  Said Embodiment 1 or 2 is used for the monitoring call system for apartment houses shown, for example in FIG. This collective housing monitoring call system includes, as communication terminals X, an intercom X1 provided in each dwelling unit and a monitoring camera X2 installed in an elevator or the like. The intercom X1 and the monitoring camera X2 are connected to the main line C1 by a so-called multi-drop method via the branch line C2 and the branch Y1. A plurality of trunk lines C1 are provided, and one end of each trunk line C1 is connected to each other via a distribution device Y2, and the other end is terminated by a terminator T. The distribution device Y2 includes a lobby intercom X3 installed at a common entrance or the like and capable of communicating with each intercom X1, a video monitoring device X4 installed in a management room and displaying a video of the monitoring camera X2, and a lobby intercom installed in the management room. The alarm monitoring device X5 is connected to connect the X3 and the intercom X1, and to the lobby intercom X3 and the intercom X1. The branch line C1 is, for example, a category 5 cable for LAN or a CPEV line. The characteristic impedance of the category 5 cable is, for example, 100Ω, and the characteristic impedance of the CPEV line is, for example, 60-90Ω. In addition to the transmission loss in the branch line C2 and the trunk line C1, the branch loss in the branching unit Y1 is, for example, about 10-30 dB, and a large number of branching units Y1 may be connected to one trunk line C1. As the attenuation increases. In order to use the CSMA / CD method in such a residential call supervisory call system, it is necessary to keep the voltage level of the loopback signal low in accordance with the voltage level of the signal. Therefore, the communication system of Embodiment 1 or 2 is preferable. It is.

It is a circuit diagram which shows Embodiment 1 of this invention. It is a circuit diagram which shows Embodiment 2 of this invention. It is explanatory drawing which shows the apartment house video management system in which this invention is used. It is a circuit diagram which shows a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hybrid circuit 2 Transmission / reception circuit 10 Transmission part 11 Reception part C20 Transmission line C21 Pseudo line R1-R4 Impedance matching resistance RT Termination resistance X Communication terminal

Claims (3)

  A communication system for transmitting and receiving signals between a plurality of communication terminals connected to a transmission line, wherein the communication terminal receives a signal transmitted from a transmission unit that transmits a signal and another communication terminal, and a transmission unit A hybrid circuit composed of a receiver that receives a loopback signal from the terminal, and a termination circuit that matches the impedance of the transmission line to the characteristic impedance of the transmission line, and the receiver includes a signal output from the transmitter to the transmission line, and a transmitter The differential component of the signal output from the signal to the termination circuit is received as a loopback signal of the transmission signal, and the transmission unit is connected to the transmission line via the first impedance matching circuit for impedance matching, and the impedance is The impedance of the first impedance matching circuit is connected to the termination circuit via the second impedance matching circuit for matching. The communication is characterized in that the magnitude relation to the characteristic impedance of the transmission line and the magnitude relation of the impedance of the second impedance matching circuit to the impedance of the termination circuit are matched in the frequency range of the signal transmitted and received by the hybrid circuit. system.   A communication system for transmitting and receiving signals between a plurality of communication terminals connected to a transmission line, comprising a pseudo line whose characteristic impedance is matched to the characteristic impedance of the transmission line, and the communication terminal includes a transmission unit for transmitting signals And a hybrid circuit including a receiving unit that receives a signal transmitted from another communication terminal and receives a loopback signal from the transmitting unit. The receiving unit outputs a signal output from the transmitting unit to the transmission line. And a differential component of the signal output from the transmission unit to the pseudo line as a loopback signal of the transmission signal, and the transmission unit is connected to the transmission line via the first impedance matching circuit for impedance matching. And connected to the pseudo line via the second impedance matching circuit for impedance matching, and the impedance of the first impedance matching circuit. The magnitude relationship of the dance with respect to the characteristic impedance of the transmission line and the magnitude relationship of the impedance of the second impedance matching circuit with respect to the impedance of the pseudo line are matched in the frequency range of the signal transmitted and received by the hybrid circuit. Communication system.   The impedance of the first impedance matching circuit is set equal to either the maximum value or the minimum value of the characteristic impedance of the transmission line in the frequency range of the signal transmitted and received by the hybrid circuit, and the second impedance matching circuit The impedance is set equal to one of the maximum value and the minimum value of the impedance of the termination circuit or the characteristic impedance of the pseudo line in the frequency range of the signal transmitted and received by the hybrid circuit. 2. The communication system according to 2.

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