JPH10271048A - Balanced transmission line - Google Patents

Abstract

(57) [PROBLEMS] To prevent an unbalanced current from flowing out of a balanced transmission line other than a balanced transmission line in a balanced transmission line that transmits a differential signal having a small amplitude through a pair of signal transmission lines used in a display device. In FIG. 1, a common mode filter is arranged on a transmission line wired on an FPC connected between an LCD panel and a substrate on which an LCD drive circuit is mounted. As a result, the stray capacitance 9 between one line between the two lines and the grounded metal 7 is determined based on the impedance between the two lines of the transmission line 2.
Even if the impedance is lowered by the filter 3, the impedance is inserted between the one wire between the two wires and the grounded metal 7 by the filter 3, so that even if the metal 7 is brought close, the flow through the stray capacitance 9 to the metal 7 may not occur. The balanced current 8 can be reduced, and radiation noise can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a balanced transmission line for transmitting a small-amplitude differential signal used in a display device.

[0002]

2. Description of the Related Art A liquid crystal display (hereinafter abbreviated as LCD) is mainly used as a display device in a portable information terminal such as a notebook personal computer (hereinafter abbreviated as PC). L
In a CD, dots of about 100 μm square are arranged on the display surface, and the dots are turned on / off at high speed to obtain an image. As the shape of the LCD increases and the dot density increases, the total number of dots per LCD increases, and the frequency of the signal for turning on / off the dots increases. In recent years, this signal frequency has been increased to 40 MHz, and a differential signal transmission using a twisted pair line, which is a combination of two signal lines, which has a high speed transmission performance, has been adopted from a conventional method of transmitting with a single signal line. became.

In this method, the impedance is almost constant because the distance between the two wires is constant because the distance between the wires is more closely matched. The impedance of the conductor other than the two wires is higher than the impedance between the two wires. Since the signal current due to the stray capacitance between the conductor and other conductors does not flow through the ground, there is an advantage that the distance of the current path is shortened and radiation noise due to the flow of the high-frequency current is small.

However, in reality, when the twisted pair wire 6 shown in FIG. 3 approaches the grounded metal 7 used as a shield for noise suppression, one of the two wires becomes a two wire due to the stray capacitance 9 for the metal. There is a portion that becomes lower than the impedance between them, and the unbalanced current 8 flows toward the metal through the stray capacitance 9 and causes radiation noise. Particularly in a notebook PC, it is difficult to keep the distance between the twisted pair wire 6 and the metal 7, and the unbalanced current 8 toward the grounded metal through the stray capacitance 9 easily flows. Further, in the case of a notebook type PC, as shown in FIG.
A pair of parallel transmission lines 2 formed above (hereinafter abbreviated as FPC) must be used in place of the twisted pair wires 6.

In the parallel transmission line 2, the stray capacitance 9 between one line and the grounded metal 7 becomes larger than that of the twisted pair, and the impedance between the one line of the transmission line 2 and the grounded metal 7 becomes larger than the impedance between the two lines. The performance as a twisted pair wire is poor due to the lowering of the length of the twisted pair wire.
In some cases, the distance between the wire and the shielded metal 7 is shorter than that of the other wire, and an unbalanced current flows more easily than the twisted pair wire.

[0006] Even if the twisted pair wire 6 is employed as described above, the noise is not sufficiently reduced.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide an FPC.
Another object of the present invention is to not increase the unbalanced current toward the metal even when using a twisted pair wire and approaching a grounded metal.

[0008]

In order to achieve the above object, a common mode filter is connected to at least one end of a balanced transmission line.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram in which a common mode filter is arranged on an FPC.

In FIG. 1, an LCD panel 4 used as a display device of a portable information terminal such as a notebook PC is shown.
A common mode filter 3 is arranged on a transmission line 2 wired on an FPC 1 connected between a substrate 5 on which a driving circuit for turning on / off dots of the LCD is turned on / off. As a result, the impedance between the two lines of the transmission line 2 becomes 1
Even if the stray capacitance 9 between the wire and the grounded metal 7 lowers the impedance, the common mode filter 3 inserts an impedance between one wire between the two wires and the grounded metal 7, so that the grounded metal 7 Even when the distance 7 is reduced, the unbalanced current 8 flowing to the grounded metal 7 through the stray capacitance 9 can be reduced. Chip size 321
When six types of common mode filters are used, the impedance is about 1 compared to when there is no common mode filter.
It becomes 0 times. Thus, radiation noise can be reduced.

The same effect can be obtained by disposing the common mode filter 3 at one end of the twisted pair wire 6 as shown in FIG.

[0013]

According to the present invention, even when the FPC or the twisted pair wire is brought close to the grounded metal by using the common mode filter, the impedance lowered by the stray capacitance between one line of the transmission line and the grounded metal is reduced. Since the current is raised by the mode filter, the unbalanced current toward the metal can be reduced, and the radiation noise can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of the present invention in which a common mode filter is arranged on an FPC.

FIG. 2 is a diagram showing one embodiment of the present invention in which a common mode filter is arranged at one end of a twisted pair wire.

FIG. 3 is a diagram showing a conventional example using a twisted pair wire.

FIG. 4 is a diagram showing a conventional example using an FPC.

[Description of Signs] 1 ... FPC, 2 ... Transmission line, 3 ... Common mode filter, 4 ... LCD panel, 5 ... LCD dot on /
The substrate on which the drive circuit to be turned off is mounted, 6 ... twisted pair wires, 7 ... grounded metal plate, 8 ... unbalanced current, 9 ... stray capacitance.

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Atsushi Hara 1099 Ozenji Temple, Aso-ku, Kawasaki City, Kanagawa Prefecture

Claims (2)

[Claims] 1. A balanced transmission line used for a display device for transmitting a differential signal having a small amplitude, wherein a common mode filter is connected to at least one end of the balanced transmission line. 2. The balanced transmission line according to claim 1, wherein said pair of balanced transmission lines are formed of a flexible substrate.