JP3297215B2 - Electromagnetic interference prevention device for balanced transmission line - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for preventing electromagnetic interference of a balanced transmission line, which suppresses electromagnetic waves generated by a balanced transmission line bridged between stands in a telephone station or the like.

[0002] For example, if a cable connected to a device at a telephone station emits electromagnetic waves, it may cause a reception failure of a television receiver in a peripheral area. Therefore, it is necessary to suppress the generation of electromagnetic waves as much as possible. ing.

[0003]

2. Description of the Related Art In a telephone station, transmission and reception of signals between intra-office devices are generally performed using a balanced transmission line. In the balanced transmission method, signals are transmitted by placing signals having opposite polarities from a balanced transmission driver on two balanced transmission lines. If the shield jacket of the balanced transmission line is grounded to the signal, it is theoretically possible. The signal transmitted through the balanced transmission line does not leak to the outside as an electromagnetic wave.

[0004]

In a typical example, one balanced transmission line used in a central office is several tens to ten.
About 3000 meters per station are used per station. When a signal having a clock frequency of about several tens of MHz is transmitted to this balanced transmission line, the balanced transmission line itself may become an antenna having a short wavelength and emit electromagnetic waves. Theoretically, balanced transmission lines are designed to prevent leakage of electromagnetic waves by grounding the shield jacket to the signal.However, in actual operation, leakage of electromagnetic waves is often not completely suppressed. In this case, since the number of balanced transmission lines used is very large, there is a possibility that VHF television reception failure may occur in the surrounding area.

The present invention has been made in view of such circumstances, and has as its object to suppress electromagnetic waves radiated from a balanced transmission line as much as possible.

[0006]

FIG. 1 is an explanatory view of the principle according to the present invention. In order to solve the above-mentioned problems, in one aspect of the present invention, there is provided a device for preventing electromagnetic interference of a balanced transmission line, which suppresses an electromagnetic wave generated by a balanced transmission line 40 bridged between mounts. Impedance element 41 which exhibits a low impedance for low frequencies and a high impedance for high frequencies in series in an electric path connecting the signal ground SG of FIG.
And a device for preventing electromagnetic interference of a balanced transmission line.

In another embodiment of the present invention, the signal lines 44, 4 and 4 of the balanced transmission line are provided at the transmitting end of the balanced transmission line.
5 is further connected in series with another impedance element 43 that exhibits low impedance at low frequencies and high impedance at high frequencies.

In another embodiment of the present invention, the signal line 4 of the balanced transmission line is provided at the transmitting end of the balanced transmission line.
A configuration in which a capacitor is connected between 4, 45 and the signal ground SG is further added.

In still another embodiment of the present invention, the signal line 4 of the balanced transmission line is provided at the transmitting end of the balanced transmission line.
The configuration in which the common mode choke coil 42 is connected in series with the components 4 and 45 is further added.

[0010]

The present inventor has conducted research on the cause of leakage and emission of electromagnetic waves from a balanced transmission line. As a result, the cause of the high-frequency current flowing through the shield jacket of the balanced transmission line and the cause of the high-frequency current flowing further Is caused by an AC potential difference between the frames, high frequency current flows through the shield jacket of the balanced transmission line bridged between the frames due to this AC potential difference, and crosstalk and reflection due to impedance mismatch of the connector of the balanced transmission line A high-frequency current flows through the shield jacket of the balanced transmission line due to the common-mode potential, and a high-frequency current flows through the shield jacket of the balanced transmission line based on the poor balance of the balanced transmission signal. It was determined that it was.

Based on such knowledge, the electromagnetic interference prevention device of the present invention has a low impedance with respect to low frequency and a high frequency with high frequency in series in an electric path connecting the signal ground of each stand and the shield jacket 46 of the balanced transmission line. By connecting the impedance element 41 having a high impedance to the high frequency current, the high frequency current corresponding to the cause flowing to the shield jacket 46 is suppressed, thereby preventing the leakage of the electromagnetic wave.

Further, in the electromagnetic interference prevention device of the present invention, the signal line 4 of the balanced transmission line is provided at the transmitting end of the balanced transmission line.
By connecting a common mode choke coil in series with 4, 45, the degree of balance of the balanced transmission signal is corrected, high-frequency current corresponding to the cause flowing to the shield jacket 46 is suppressed, thereby preventing leakage of electromagnetic waves. ing.

Further, in the electromagnetic interference prevention device of the present invention, the signal line 4 of the balanced transmission line is provided at the transmitting end of the balanced transmission line.
By connecting an impedance element 43 that exhibits low impedance at low frequency and high impedance at high frequency in series with 4, 45, or by connecting a capacitor between the balanced transmission line and signal ground, The high-frequency current corresponding to the cause of the current flowing to 46 is suppressed, thereby preventing leakage of electromagnetic waves.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a diagram showing an apparatus for preventing electromagnetic interference of a balanced transmission line as one embodiment of the present invention. In this embodiment, the electromagnetic interference prevention device of the present invention is applied to a configuration in which a balanced transmission line 2 is bridged between stands 10 and 11 installed in a telephone station to transmit and receive signals. In the figure, balanced transmission line 2
Is composed of two twisted pair wires 20 and 21 and a shield jacket 22 that covers them, and is connected to the internal circuits of the gantry 10 and 11 via the connector 3 respectively. In the embodiment, a case where a transmission circuit is installed on the gantry 10 side and a reception circuit is installed on the gantry 11 side is shown.

The transmission side is provided with a balanced transmission driver 1 for outputting a balanced transmission signal. The balanced transmission driver 1 is configured to transmit output signals having opposite polarities to the two twisted pair lines 20 and 21 in response to the input pulse signal. The output signal of the balanced transmission driver 1 is sent to the twisted pair wires 20 and 21 of the balanced transmission line 2 via the ferrite bead inductor 7, the common mode choke coil 8, and the connector 3. On the transmission side, the shield jacket 22 of the balanced transmission line 2 is connected to the signal ground SG (signal ground) of the transmission circuit printed circuit board via the connector 3, the drain line 31, and the ferrite bead inductor 9. . The signal ground SG is not connected to the frame ground of the gantry. Reference numeral 5 denotes a load circuit.

The above-described ferrite bead inductors 7 and 9
Is an impedance element that exhibits low impedance at low frequency and high impedance at high frequency. For example, the ferrite bead inductor 7 has a very small DC resistance of 100 MHz.
It shows an impedance value of about 15Ω for a high frequency of about 1, and the DC resistance of the ferrite beads 9 is a very small value of 1Ω.
For a high frequency of about 00 MHz, one having an impedance value of about 75 to 300Ω is used.

The above-mentioned common mode choke coil 8
Is an existing choke coil for removing common mode noise, which is of a two-line type and compensates for a mismatch in the balance of a balanced transmission signal.

On the receiving side, a balanced transmission receiver 4 for receiving a balanced transmission signal transmitted through the balanced transmission line 2 is provided. Also on this receiving side, the balanced transmission line 2
Is connected to the signal ground SG of the printed circuit board for the receiving circuit via the contactor 3, the drain line 31, and the ferrite bead inductor 9. Note that, also on the receiving side, this signal ground is not connected to the frame ground of the gantry. Reference numeral 6 denotes a termination circuit.

The operation of this embodiment will be described below.
First, the cause of leakage and emission of electromagnetic waves from the balanced transmission line 2 in the conventional circuit can be explained by the following three with reference to FIG. Note that among these causes, the magnitude of the cause of the generation of the electromagnetic wave is in the following order.

In each mount, the ground potential is not completely zero potential, but a potential that vibrates minutely at a high frequency, so that an AC ground potential difference V1 occurs between the mounts. Therefore, when the balanced transmission line 2 is bridged between the mounts and the shield jacket 22 is connected to the respective signal grounds SG, the high-frequency current I1 flows through the shield jacket 22 due to the AC ground potential V1, and this high-frequency current is Electromagnetic waves are emitted using the shield jacket 22 as an antenna.

If there is an impedance mismatch between the connector 3 and the line, crosstalk occurs at the position of the connector 3,
Reflection occurs, thereby generating a common mode potential V2. Based on the common mode potential V2, the balanced transmission line 2
A high-frequency current flows in a closed loop including the twisted pair wire and the shield jacket 22, and this high-frequency current flows through the shield jacket 2.
Electromagnetic waves are radiated using 2 as an antenna.

Due to the poor balance of the balanced transmission signal, a high-frequency current flows through the shield jacket, and the high-frequency current radiates electromagnetic waves using the shield jacket 22 as an antenna. That is, FIG. 4A is a diagram showing this state, in which the output waveform X,
X ′. As shown in (A-1), a balanced transmission signal transmitted through two twisted pair wires should have a vertically symmetrical balanced waveform in principle, but actually has a vertically symmetrical balance as shown in the figure. Not. As a result of the incomplete balance transmission, a difference (common-mode voltage) X + X ′ occurs in the transmission voltage waveform as shown in (A-2). As a result, as shown in FIG. 3, during the closed loop in which the output of the balanced transmission driver 1 flows from the twisted pair line through the power supply line of the receiver 4, from the signal ground SG to the shield jacket 22 through the drain line 31, and The high-frequency current shown in (A-3) caused by the voltage flows. When a high-frequency current flows through the shield jacket 22 as described above, a back electromotive force is generated due to the inductance of the drain wire, and the shield jacket is electrically vibrated and electromagnetic waves are radiated.

In the embodiment, the high-frequency current flowing through the shield jacket 22 due to the above-described causes is suppressed by inserting the ferrite bead inductor 9 between the shield jacket 22 and the signal ground SG to prevent the emission of electromagnetic waves. I have.

The high-frequency current caused by the operation of the balanced transmission circuit is caused by the fact that the ferrite bead inductor 7 is inserted between the output terminal of the balanced transmission driver 1 and the connector 3 and the common mode is connected in series to the signal line on the transmission side. This can be suppressed by the combination of the insertion of the choke coil 8. That is, the high frequency current flowing through the twisted pair wire can be suppressed by the ferrite bead inductor 7 due to the difference voltage of the above-described cause, and accordingly, the high frequency current flowing through the shield jacket 22 is correspondingly suppressed. In particular, by inserting the ferrite bead inductor 7, as shown in FIG. 5, the ringing component in the output signal of the balanced transmission driver 1 can be suppressed, so that the high-frequency current component that causes electromagnetic wave radiation is further suppressed. be able to. Further, as shown in FIG. 4 (B-1), the degree of balance of the balanced transmission signal is increased, so that the differential voltage is reduced, and the high-frequency current flowing through the shield jacket 22 via the drain line is reduced. Generation of electromagnetic waves can be suppressed.

The high-frequency current caused by the high-frequency current can be suppressed by inserting a ferrite bead inductor 7 having a high impedance with respect to the high frequency in series with the twisted pair line through which the high-frequency current flows.

FIG. 6 is a measurement result diagram for explaining an improved state of electromagnetic interference by applying the present invention. The measurement results are obtained by measuring the spectrum of the electromagnetic wave radiated from the 20 m balanced transmission line using the measurement system shown in FIG. 7 before and after the measures for preventing electromagnetic interference according to the present invention. Here, a balanced transmission signal having a clock frequency of 32 Mbps is used. In FIG. 6, a simple bar indicates the level of electromagnetic wave radiation before the countermeasure, the position of the circle indicates the level of electromagnetic wave radiation after the countermeasure, and the cross indicates the reception level of the television reception radio wave. As is apparent from this figure, after the countermeasure according to the present invention, the level of the radio wave radiated by the electromagnetic wave is remarkably reduced, and it is understood that the electromagnetic interference is prevented.

FIG. 8 shows another embodiment of the present invention. This embodiment uses a three-terminal capacitor between the output of the balanced transmission driver 1 and the signal ground SG instead of the ferrite bead inductor 7 inserted in series between the balanced transmission driver 1 and the common mode choke coil 8 in the above-described embodiment. The ground lead GND of the three-terminal capacitor is connected to the signal ground SG of the printed circuit board. With such a configuration, the high-frequency current flowing through the twisted pair wire and thus the shield jacket can be suppressed.

In addition to the above, various modifications are possible in implementing the present invention. For example, the mounting location of the ferrite bead inductor 9 corresponding to the above-described cause is not limited to being inserted in the path between the drain line 31 and the signal ground SG as described in the embodiment. The position may be used to be built in the connector 3. Also, as high frequency resistance,
Instead of the ferrite bead inductor, another impedance element exhibiting low impedance at low frequency and high impedance at high frequency can be used.

[0029]

As described above, according to the present invention, it is possible to suppress the high-frequency current flowing through the shield jacket of the balanced transmission line, and thus suppress the leakage radiation of electromagnetic waves from the balanced transmission line. Can be.

[Brief description of the drawings]

FIG. 1 is an explanatory view of the principle according to the present invention.

FIG. 2 is a diagram illustrating a device for preventing electromagnetic interference of a balanced transmission line as one embodiment of the present invention.

FIG. 3 is a diagram illustrating a cause of occurrence of electromagnetic interference from a balanced transmission line.

FIG. 4 is a diagram showing output waveforms of a balanced transmission driver before and after application of the present invention.

FIG. 5 is a diagram showing output waveforms of a balanced transmission driver before and after mounting a ferrite bead inductor.

FIG. 6 is a measurement result diagram illustrating an improved state of electromagnetic interference by applying the present invention.

FIG. 7 is a diagram showing a measurement system.

FIG. 8 is a diagram showing another embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 balanced transmission driver 2 balanced transmission line 3 connector 4 balanced transmission receiver 5 load circuit 6 termination circuit 7, 9 ferrite bead inductor 8 common mode choke coil 10, 11 mount 20, 21 twisted pair wire 22 shield jacket 31 drain wire 12 3 terminal Capacitor

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-6-13191 (JP, A) JP-A-5-90779 (JP, A) JP-A-2-288429 (JP, A) JP-A-49-49 39309 (JP, A) JP-A-4-218214 (JP, A) JP-A 63-185330 (JP, U) JP-A 56-102130 (JP, U) JP-A 62-26959 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 15/00

Claims (4)

(57) [Claims] An apparatus for preventing electromagnetic interference of a balanced transmission line, which suppresses an electromagnetic wave generated by a balanced transmission line bridged between mounts, wherein a signal ground of each mount is connected to a shield jacket of the balanced transmission line. A device for preventing electromagnetic interference of a balanced transmission line, comprising an impedance element having a low impedance for low frequencies and a high impedance for high frequencies connected in series in an electric path. Series to 2. A signal line of the <br/> balanced transmission line at the transmitting end of the balanced transmission line, the other showing a high impedance to the low frequency low impedance for high frequencies
The electromagnetic interference prevention apparatus for a balanced transmission line according to claim 1 , wherein the impedance element is connected to the impedance element. Wherein electromagnetic interference preventing apparatus of the balanced transmission line wherein <br/> formed by connecting a capacitor between the signal line and the signal ground of the balanced transmission line according to claim 1 balanced transmission line described in the transmitting end of the. 4. The transmission line of the balanced transmission line further comprises:
4. The apparatus according to claim 2 , wherein a common mode choke coil is connected in series with the signal line of the balanced transmission line.