Noise elimination method and transmission circuit

Pulse or digital communications – Cable systems and components

Reexamination Certificate

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Details

C375S220000, C375S285000, C370S201000

Reexamination Certificate

active

06507620

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to noise elimination methods and transmission circuits for eliminating a crosstalk noise which is generated among a plurality of signal lines, and more particularly to a noise elimination method and a transmission circuit which eliminate a far-end crosstalk noise of a bus transmission by inserting a terminating resistor which has a specific value at a far-end.
2. Description of the Related Art
In electronic equipments such as personal computers, signal transmissions among LSI circuits in most cases are made in units of 32 bits or 64 bits. In such signals, a plurality of bits make transitions at the same timing, thereby causing signal interference among the bits and in many cases generating the crosstalk noise. The value of this crosstalk noise becomes larger as the number of signals which make the transitions simultaneously becomes larger. In addition, the crosstalk noise becomes a large value even in the case of a short line as the signal rise/fall time becomes shorter.
FIGS. 1A and 1B
are diagrams for explaining a backward near-end crosstalk for explaining a background of the present invention.
FIG. 1A
shows a driving line
80
, a driver
81
, a receiver
82
, a passive line
90
, a driver
91
, and a
35
receiver
92
.
FIG. 1B
additionally shows an internal resistance
83
of the driver
81
, a terminating resistor
84
, an internal resistance
93
of the driver
91
, and a terminating resistor
94
for the case shown in FIG.
1
A.
In a case where two lines on which the signal transmitting directions are opposite to each other as shown in
FIG. 1A
, the backward near-end crosstalk refers to the noise which is introduced on the passive line
90
near the driver
81
due to the signal on the driving line
80
.
FIG. 2
is a diagram showing the magnitude of the backward near-end crosstalk which is introduced in the transmission circuit shown in
FIGS. 1A and 1B
. In
FIG. 2
, it is assumed that an internal resistance
83
of the driver
81
has a value r=10&OHgr;, and a resistance R of the terminating resistor
84
is infinitely large. In
FIG. 2
, the ordinate indicates the magnitude of the voltage, and the abscissa indicates the time. In
FIG. 2. a
thin solid line v
1
(near) indicates a voltage change on the driving line
80
on the side of the driver
81
(near-end), a thin dotted line v
1
(far) indicates a voltage change on the driving line
80
on the side of the receiver
82
(far-end), a bold solid line v
2
(near) indicates a voltage change on the passive line
90
on the side of the driver
91
(near-end), and a bold dotted line v
2
(far) indicates a voltage change on the passive line
90
on the side of the receiver
92
(far-end).
The backward near-end crosstalk becomes a considerably large value when the value r of the internal resistance
83
of the driver
81
is sufficiently small compared to the characteristic impedance of the passive line
90
. For this reason, the value r of the internal resistance
83
is conventionally set large so as to eliminate the backward near-end crosstalk noise.
The terminating resistor
94
is provided to make a waveform matching with respect to the output signal, and the resistance of this terminating resistor
94
is set to a value approximately equal to the characteristic impedance of the passive line
90
.
In other words, in a case where the characteristic impedance of the line is 50&OHgr;, the terminating resistor
94
is set to approximately 50&OHgr;.
Conventionally, when signals are transmitted on a plurality of lines in the same direction, no measures were taken with respect to the noise generated at the far-end on the opposite end from the driving side (hereinafter referred to as a forward far-end crosstalk noise) because the amplitude (voltage) of the forward far-end crosstalk noise is small compared to the backward near-end crosstalk noise and the effects of the forward far-end crosstalk noise with respect to the transmission line are small.
Although no measures are conventionally take with respect to the forward far-end crosstalk noise, there is a tendency for the physical distance among the signals to become smaller, due to the increased operation speed of the circuits and the reduced size and weight of the equipments. As a result, there is a tendency for the crosstalk noise to be generated more easily. More particularly, when making a parallel signal transmission of multiple bits such as 32 bits or 64 bits, there exists a case where the logic amplitude changes from a “0” state to a “1” state in all of the bits with the exception of one bit, and in such a case, the effects of the lines on which the logic amplitude of the bits which changed to the “1” state appear at the far-end of the signal line on which the logic amplitude of the bit remained at the “0” state. In some cases, such effects appearing at the far-end become large and no longer negligible. In order to simultaneously achieve the increased operation speed and reduced size and weight of the equipment, it is an object to overcome this crosstalk noise from the point of view of electronic packaging.
But conventionally, in order to reduce the crosstalk noise described above, it was either necessary to increase the physical distance among the signals or to reduce the number of signals which make the transition simultaneously. For this reason, it was either necessary to sacrifice the packaging or mounting density or to sacrifice the performance by relaxing the signal timings.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful noise elimination method and transmission circuit, in which the problems described above are eliminated.
Another and more specific object of the present invention is to provide a noise elimination method and a transmission circuit which can eliminate a far-end crosstalk of a bus transmission when transmitting signals in the same direction, by a simple means.
Still another object of the present invention is to provide a noise elimination method characterized in that when transmitting signals in the same direction on at least two distributed constant lines, a resistance of a terminating resistor at a far-end is set so that voltages propagated to the far-end become equal between two kinds of propagation modes on coupled distributed constant lines, where the two kinds of propagation modes are a common mode which propagates with respect to a ground plane and a differential mode which propagates between the coupled lines. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
A further object of the present invention is to provide a noise elimination method characterized in that when first and second driving sources are coupled to respective ends of at least two distributed constant lines on which signals can be transmitted two ways, and a signal is to be transmitted from the first driving source to the other end or from the second driving source to the other end, a resistance of a terminating resistor is set so that an approximately reciprocal relationship exists between an internal resistance of the first or second driving source normalized by a characteristic impedance of the line, and a terminating resistance at a far-end with respect to the first or second driving source normalized by the characteristic impedance of the line. According to the present invention, it is possible to effectively eliminate the forward far-end crosstalk noise by use of a simple construction.
Another object of the present invention is to provide a transmission circuit having at least two distributed constant lines for transmitting signals in the same direction, characterized in that a terminating resistor is coupled at a far-end of the distributed constant lines, and the terminating resistor has a terminating resistance which is set so that an approximately reciprocal relationship exists between the terminating r

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