JP2522516B2 - Balanced transmission equipment - Google Patents

Description

The present invention relates to a balanced transmission device for transmitting an AC signal such as a video signal by superimposing it on a DC power source via a parallel two-wire cable such as a telephone line or an intercom line. It is about.

[Prior Art] As an example, FIG. 9 shows a signal transmission system of a surveillance camera system. In this signal transmission system, a transmitter 10 to which a television camera is connected and a receiver 20 to which a television monitor is connected are connected via a parallel two-wire cable 1. The transmitter 10 is generally arranged outdoors such as a front door, and the receiver 20 is generally arranged indoors. Therefore, DC power is supplied to the receiver 20 and power is supplied to the transmitter 10 via the parallel two-wire cable 1. That is, the parallel two-wire cable 1 is transmitted in a form in which an AC signal is superimposed on a DC power supply, and the transmitter 10 and the receiver 20 need to separate the DC power supply and the AC signal.

A pair of power supply superimposing circuits 11 'and 21' having high impedance with respect to an AC signal are inserted in the power supply side and the power receiving side of the DC power supply, respectively, and this is realized by a choke coil and a constant current circuit. In addition, the unbalanced signal output from the TV camera is converted into a balanced signal and sent to the parallel two-wire cable 1, and the output terminal of the balanced signal transmission circuit 13 and the balanced signal from the parallel two-wire cable 1 are received to monitor the television. Capacitors C ₀₁ and C ₀₂ for blocking direct current are respectively inserted at the input ends of the balanced signal receiving circuit 23 that outputs an unbalanced signal.
Further, in the transmitter 10, the matching resistor R ₀₁ is connected in series with the capacitor C _01, and between the terminals connected to the parallel two-wire cable 1 in the receiver 20, the matching resistor R ₀₃ and the DC blocking are provided. A series circuit with the capacitor C ₀₃ for is connected.

[Problems to be Solved by the Invention] The reason why signals are balanced-transmitted via the parallel two-wire cable 1 is to prevent the entry of external noise.
This is because when unbalanced transmission is performed, the signal line is not shielded, so that external noise is mixed and the S / N ratio of the signal is reduced. Especially in the case of video signals, AM radio broadcasting becomes the most problematic, and if this extraneous noise is mixed, beat fringes will appear on the TV monitor. The circuit configuration shown in Fig. 9 also realizes balanced transmission to some extent, but in the strong electric field area of AM radio, AM
There is a problem that radio broadcast waves are mixed. This is because the impedances of the power supply superimposing circuits 11 'and 21' of the transmitter 10 and the receiver 20 have variations. For example, as shown in FIG. 10, when a pair of lines are arranged close to each other, one line is grounded through impedance element Z ₁ and the other line is connected through impedance element Z _2. It shall be grounded. If the resistances of both lines are not grounded, the ground impedances of both lines are equal, so this is set to Z. If both lines are placed in an electromagnetic field and the resistor is not grounded, the ground voltage of both lines is equal, so this is V ₀ . When the resistor is grounded, the ground voltage V ₁ , V ₂ of each line is as follows.

V ₁ = V ₀ Z ₁ / (Z + Z ₁ ) V ₂ = V ₀ Z ₂ / (Z + Z ₂ ) Here, if V ₁ = V ₂ , there is no potential difference between both lines of the parallel 2-wire cable. It means that there is no radio wave mixing. To satisfy this condition, Z ₁ = Z ₂ must hold. In other words, if there are variations in the impedance of the power supply superposition circuit, Z ₁ ≠ Z ₂ and radio waves will be mixed.

Thus, despite the use of the parallel two-wire cable, the interference eliminating capability is not so high in the conventional configuration. Further, the power supply superimposing circuit requires a pair of a transmitter and a receiver, so that the circuit configuration becomes complicated.

The present invention is intended to solve the above-mentioned problems, and in a transmission system for transmitting an AC signal superimposed on a DC power source via a signal line formed of a parallel two-wire cable,
It is an object of the present invention to provide a balanced transmission device that has a high capability of eliminating interference against external noise and that can be configured at low cost by providing only one power supply superimposing circuit for each circuit.

[Means for Solving the Problem] In order to achieve the above object, the present invention connects impedance elements of equal value between a pair of terminals to which a parallel two-wire cable is connected and a ground point, and connects them in the same direction. The same extreme of each winding of a transformer having a pair of windings wound with the same number of turns is connected to the above terminals respectively, the other end of one winding of the transformer is connected to the power supply superposition circuit, and the other winding of the transformer is connected. The other end of the line is connected to the ground point.

[Operation] According to the above configuration, the transformer can be inserted so as not to affect the transmission system, and the transformer acts as a high impedance element with respect to the in-phase external noise, and the external noise To eliminate. Further, since only one power supply superimposing circuit is required, the circuit configuration is simplified as compared with the conventional configuration.

[First Embodiment] As shown in FIG. 1, a transmitter 10 having a television camera is provided.
Comprises a transformer T ₁ having a pair of windings n _1, n ₂ of the same number of turns, winding n _1, n ₂ are respectively inserted into both lines of two parallel wire cable 1. Here, the terminals t ₁ and t ₂ connected to the parallel two-wire cable 1 are formed at the same extreme of both windings n ₁ and n ₂ . A series circuit of DC blocking capacitors C ₁ and C ₂ and resistors R ₁ and R ₂ is inserted between terminals t ₁ and t ₂ , and the series circuit that is the connection point of both resistors R ₁ and R ₂ is connected. The middle point of the circuit is connected to the other end of one winding n ₂ and serves as the ground point of the transmitter 10. here,
The resistors R ₁ and R _{2 have} the same resistance value, and the resistance value obtained by adding both the resistors R ₁ and R ₂ is set equal to the characteristic impedance of the parallel two-wire cable 1. The other end of the other winding n ₁ is connected to the power supply superimposing circuit 11
Of the unbalanced transmission circuit 12 is connected via the capacitor C ₃ for blocking direct current. Unbalanced transmission circuit 12 is set the output impedance is sufficiently large, for example, as shown in FIG. 2 (a), constructed of a transistor Q _1. However, in the transmitter 10, the output terminal of the power superimposing circuit 11, the ground point, and the unbalanced transmitting circuit 12
3 terminals t ₃ , t ₄ , t ₅ at the input end of are formed.
DC power is obtained from t ₃ and t ₄ , power is supplied to the transmitter 10, and video signals are input to terminals t ₄ and t ₅ .

On the other hand, the receiver 20 is provided with a transformer T ₂ having a pair of windings n ₃ and n ₄ of the same number of turns, which are respectively inserted into the respective lines of the parallel two-wire cable 1 like the transmitter 10. Terminals where the same extremes of n ₃ and n ₄ are connected to each line of parallel 2-wire cable 1
t ₆ and t ₇ . In addition, a series circuit of DC blocking capacitors C ₄ and C ₅ and resistors R ₃ and R ₄ is inserted between both terminals t ₆ and t ₇ ,
The middle point of this series circuit, which is the connection point of R ₃ and R ₄ , becomes the ground point. The other end of one winding n ₄ is connected to the ground point. Here, the resistance values of the resistors R ₃ and R ₄ are set equal, and the added value of the resistors R ₃ and R ₄ is set equal to the characteristic impedance of the parallel two-wire cable 1. The other winding n ₃
The output terminal of the power supply superimposing circuit 21 is connected to the other end of the unbalanced receiving circuit via the DC blocking capacitor C _6.
22 inputs are connected. As shown in FIG. 2B, the unbalanced receiving circuit 22 is an emitter follower using a transistor Q ₂ and has a large input impedance. Thus, the receiver 20, the terminal t ₈ the DC power source is powered, it's are provided with terminal t ₉ the television monitor is connected. Since the DC potentials across the resistors R ₂ and R ₄ are equal, the capacitors C ₂ and C ₅ can be omitted.

According to the above configuration, the video signal has the terminal t ₅ -the unbalanced transmission circuit 12 -the winding n ₁ -the winding n ₃ -the unbalanced reception circuit 22 -the winding n ₄
-Transmitted through the path of the winding n _{2, and} the magnetic flux due to the video signal in each transformer T ₁ , T ₂ will cancel each other,
The transformers T ₁ and T ₂ do not affect the transmission. Further, DC power supply, the power superposition circuit 21 winding n ₃ - winding n ₁ - power line circuit 11-winding n ₂ - from passing through the path from the winding n _4, again the transformers T _1, T ₂ The magnetic fluxes cancel each other out, preventing magnetic saturation. In this way, the transformers T ₁ and T ₂ do not affect normal transmission.

On the other hand, external noise coming from the outside to the parallel two-wire cable 1 is input to both lines as an in-phase signal,
In each of the transformers T ₁ and T ₂ , current flows in the same direction with respect to the windings n _{1 to} n ₄ , resulting in high impedance, and the current due to external noise flows to the ground point through the resistors R _{1 to} R _4. It will be. Since R _{1 to} R ₄ are equal to each other, there is no potential difference between both lines of the parallel two-wire cable 1, and as a result, the influence of external noise on the transmission system is prevented. When the external noise has a relatively low frequency, the impedances of the transformers T ₁ and T ₂ are not so large, but there is no problem because the voltage induced in the parallel two-wire cable 1 is not large.

Second Embodiment In the first embodiment, if there is a leakage magnetic flux in the transformers T ₁ and T ₂ ,
Although the impedances of the transformers T ₁ and T ₂ also affect the transmission signal, in the present embodiment, as shown in FIG. 3, the terminal t _{1 is} passed through the balanced transmission circuit 13 that performs unbalanced-balanced conversion. , t ₂ is applied with a video signal, and terminals t ₆ and t ₇ are connected to the input terminals of a balanced receiving circuit 23 for performing balanced-unbalanced conversion through capacitors C ₇ and C ₈ . Is. In this configuration, since the transformers T ₁ and T ₂ are not present in the video signal transmission system, even if there is a leakage magnetic flux in the transformers T ₁ and T _2, it can be used without any problem.

In the configurations of the first and second embodiments, when viewed from the parallel two-wire cable 1, the impedances and transmission characteristics of the transmitter 10 and the receiver 20 change depending on the frequency, so that they may not be usable depending on the usage conditions. . For example, the inductance of each winding n _{1 to} n ₄ of transformers T ₁ and T ₂ is
Assuming that the resistance value of the resistors R _{1 to} R ₄ is 100 Ω and 0.5 mH, the impedance characteristic is as shown in FIG. 4 (a), and the transmission characteristic is as shown in FIG. 4 (b). Here, the fourth
The solid line in the figure (a) shows resistance value, and the broken line shows reactance.
That is, the impedance increases in the high frequency region and the transmission characteristics are not flat. Therefore, it cannot be used under the use condition in which the characteristic impedance of the parallel two-wire cable 1 and the impedance between the terminals t ₁ , t ₂ , t ₆ and t ₇ must be matched. However, it is not necessary to match the characteristic impedance of the parallel two-wire cable 1 with the impedance between the terminals t ₁ , t ₂ , t ₆ and t ₇ , for example, the center frequency is 1 MHz and the frequency deviation is 20 kH.
It can be used in the case of transmitting an FSK-modulated signal such as z, and the purpose of eliminating external noise can be achieved.

[Embodiment 3] In this embodiment, as shown in FIG. 5, compensation circuits 14 and 24 are added to a receiver 10 and a transmitter 20 in addition to the circuit configuration shown in FIG. Compensation circuits 14 and 24 are obtained by connecting resistors R ₂₁ and R ₂₂ and capacitors C ₂₁ and C ₂₂ in series at both ends of a parallel circuit of capacitors C ₁₁ and C ₁₂ and resistors R ₁₁ and R ₁₂ , respectively, They are inserted between the grounding point and the connection point between the windings n ₁ and n ₃ and the power supply superimposing circuits 11 and 21, respectively. Here, the resistance values of the resistors R _{1 to} R ₄ are
And 200 [Omega, the resistance value of the resistor R _21, R ₂₂ and 133Omu, resistance
Set the resistance of R ₁₁ and R ₁₂ to 67Ω and the capacitance of capacitors C ₁₁ and C ₁₂ to 10
Assuming 0nF, as shown in FIG.
The transmission characteristics could be made almost flat. This compensation circuit
Since 14, 24 are inserted on the opposite side of the transformers T ₁ and T ₂ with respect to the parallel two-wire cable 1, the ability to eliminate external noise is equivalent to that of the first embodiment.

[Embodiment 4] This embodiment is a further simplification of the compensation circuits 14 and 24. As shown in FIG. 7, a series circuit of resistors R ₃₁ and R ₃₂ and capacitors C ₃₁ and C ₃₂ is used. It is a constituent. With this configuration, the resistance value of the resistors R _{1 to} R ₄ is 1 kΩ, the resistance R ₃₁ ,
FIG. 8 shows the results when the resistance value of R ₃₂ is 110Ω and the inductances of the windings n _{1 to} n ₄ of the transformers T ₁ and T ₂ are 0.5 mH. In this configuration, the resistance values of the resistors R _{1 to} R ₄ need to be set higher than in the case of the third embodiment, and the ability to remove external noise is slightly inferior to that of the third embodiment, but the circuit configuration is simple. The circuit is practical and has a flat frequency characteristic, which is a practical circuit.

EFFECTS OF THE INVENTION As described above, the present invention connects impedance elements of equal value between a pair of terminals to which a parallel two-wire cable is connected and a grounding point, and winds the same number of turns in the same number of turns. Connect the same poles of each winding of a transformer having a pair of windings to the above terminals respectively, connect the other end of one winding of the transformer to the power supply superposition circuit, and connect the other end of the other winding of the transformer to the ground point. Since it is connected to, a transformer can be inserted so as not to affect the transmission system, and yet
With respect to in-phase external noise, the transformer acts as a high impedance element and has the advantage of eliminating external noise.

Also, when a compensating circuit that flattens the frequency characteristics is connected between both ends of the transformer winding, it is possible to prevent impedance characteristics and transmission characteristics from changing with frequency, and perform more desirable signal transmission. You can

[Brief description of drawings]

FIG. 1 is a circuit diagram showing Embodiment 1 of the present invention, and FIG. 2 (a).
(B) is a circuit diagram showing a configuration example of an unbalanced transmission circuit and an unbalanced reception circuit used in the same as above, FIG. 3 is a circuit diagram showing a second embodiment of the present invention, and FIGS. 4 (a) and (b) are FIG. 5 is a circuit diagram showing Embodiment 3 of the present invention, and FIGS. 6 (a) and 6 (b) are operation explanatory diagrams of the same, respectively.
FIG. 7 is a circuit diagram showing Embodiment 4 of the present invention, and FIG. 8 (a).
(B) is an operation explanatory diagram of the same as above, FIG. 9 is a circuit diagram showing a conventional example, and FIG. 10 is an operation explanatory diagram showing problems of the same. 1 ... Parallel 2-wire cable, 11,21 ... Power supply superimposing circuit, 12
…… Unbalanced transmission circuit, 13 …… Balanced transmission circuit, 14,24 ……
Compensation circuit, 22 ...... unbalanced receiver circuit, 23 ...... balanced receiver circuit, n ₁ ~n ₄ ...... windings, R ₁ to R ₄ ...... resistance, T _1, T ₂ ...... transformer.

Claims (4)

(57) [Claims] 1. A balanced transmission device for superimposing an AC signal on a DC power source for transmission / reception via a parallel two-wire cable, wherein an equal value is provided between a pair of terminals to which the parallel two-wire cable is connected and a grounding point. Connect a pair of impedance elements,
The same extreme of each winding of a transformer having a pair of windings wound in the same direction with the same number of turns is connected to the above terminals respectively, the other end of one winding of the transformer is connected to the power supply superimposing circuit, and the other end of the transformer is connected. A balanced transmission device characterized in that the other end of the winding is connected to a ground point. 2. The balanced transmission device according to claim 1, wherein at least one of the balanced signal transmission circuit and the balanced signal reception circuit is connected to a terminal to which a parallel two-wire cable is connected. 3. An unbalanced signal transmission circuit and an unbalanced signal reception circuit are connected to a connection point between one end of one winding of a transformer and a power supply superposition circuit. Item 2. The balanced transmission device according to item 1. 4. A balanced transmission device for superimposing an AC signal on a DC power source for transmission and reception via a parallel two-wire cable, wherein an equal value is provided between a pair of terminals to which the parallel two-wire cable is connected and a grounding point. Connect a pair of impedance elements,
The same extreme of each winding of a transformer having a pair of windings wound in the same direction with the same number of turns is connected to the above terminals respectively, the other end of one winding of the transformer is connected to the power supply superimposing circuit, and the other end of the transformer is connected. The balanced transmission device is characterized in that the other end of the winding is connected to a ground point, and a compensating circuit for flattening the frequency characteristic is connected between the other ends of both windings of the transformer.