Low-EMI circuit board and low-EMI cable connector

Details Low-EMI circuit board and low-EMI cable connector Low-EMI circuit board and low-EMI cable connector

1998-12-23

2001-02-13

Lee, Benny (Department: 2817)

Wave transmission lines and networks

Transmission line inductive or radiation interference...

C333S02200F, C333S260000

Reexamination Certificate

active

06188297

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a low-EMI circuit board on which circuit elements such as an IC and a LSI are mounted, or more in particular to a low-EMI cable connector for suppressing the unrequired radiation in the low-EMI circuit board and the transmission cable in such a manner as to suppress the radiation mainly of differential mode from the mounted parts.
BACKGROUND ART
Conventionally, in a multilayer circuit board having arranged therein signal lines, a power line and a ground line and having arranged the surface thereof an IC elements, a LSI element and circuits, a serious problem is posed by the fact that with the increase in speed and density, the unrequired radiation is liable to occur due to high harmonics which have an effect on other devices.
The unrequired radiation is roughly divided into two types, the common-mode radiation caused by the resonance due to the potential fluctuations of the power layer and the ground layer and the radiation of differential mode caused by the signal line layers and the component parts mounted. In the prior art, various methods have been proposed to reduce these unrequired radiation.
A method generally employed for reducing the radiation of differential mode is by shielding, and a method specifically employed is by coating a conductive paste containing a resistance material on the surface of the board.
In order to send a signal to the circuit board from an external source, a transmission line such as a coaxial cable is connected by a connector with an external signal source. Such a connection is schematically shown in FIG.
13
.
In this diagram, the signal source is designated as a transmitting terminal unit
100
and the circuit board receiving signals from the transmitting terminal unit
100
is designated as a receiving terminal unit
101
, with a coaxial cable
102
connected between them. The circuit board constituting the receiving terminal unit
101
is connected to the coaxial cable
102
by a connector not shown. The transmitting terminal unit
100
is also connected with the coaxial cable
102
by a connector not shown.
In the transmitting terminal unit
100
, an outward line
100
a
connected to a signal source
100
c
for generating a pulse-like signal of frequency &ohgr;j and voltage V
0
is connected to an internal conductor
102
a
of the coaxial cable
102
, and an inward line
100
b
is connected to an external conductor
102
b
of the coaxial cable
102
, each by a connector not shown. Also, the receiving terminal unit
101
is equivalently expressed by a receiving line
110
a
, a return line
101
b
and a load impedance ZL connected between them. This receiving line
100
a
is connected to the internal conductor
102
a
of the coaxial cable
102
, and the inward line
101
b
is connected to the external conductor
102
b
of the coaxial cable
102
, respectively, by a connector not shown. The inward line
100
b
of the transmitting terminal unit
100
and the return line
101
b
in the receiving terminal unit
101
are grounded, and the external conductor
102
b
of the coaxial cable
102
is used as a grounding line.
In this configuration, a signal line is formed of the outward line
100
a
from the signal source
100
c
of the transmitting terminal unit
100
, the internal conductor
102
a
of the coaxial cable
102
, the receiving line
101
a,
the load resistor R, the return line
101
b
of the receiving terminal unit
101
, the external conductor
102
b
of the coaxial cable
102
and the inward line
100
b
of the transmitting terminal unit
100
.
In this signal line, the signal output from the signal source
100
c
in the transmitting terminal unit
100
is sent to the internal conductor
102
a
of the coaxial cable
102
as a voltage V
1
a
and a current i
1
a,
and received at the receiving terminal unit
101
as a voltage V
1
b
and a current il
b,
respectively. Also, in the return path of this signal line, a signal of a voltage V
2
b
and a current i
2
b
flows from the receiving terminal unit
101
along the inner surface of the external conductor
102
b
of the coaxial cable
102
. Not only that, the current is reflected by an equivalent impedance at a junction point B between the coaxial cable
102
and the receiving terminal unit
101
, so that the current leaks out to the outer surface of the external conductor
102
b
of the coaxial cable
102
. This flows as a leakage current i
3
b
along the outer surface of the external conductor
102
b
of the coaxial cable
102
. The signal flowing along the inner surface of the external conductor
102
b
is input to the transmitting terminal unit
100
as a voltage V
2
a
and a current i
2
a
. The current is also reflected by an equivalent impedance at a junction point A between the coaxial cable
102
and the transmitting terminal unit
100
. As a result, part of the current i
2
a
leaks out to the outer surface of the external conductor
102
b
of the coaxial cable
102
and flows along the outer surface of the external conductor
102
b
of the coaxial cable
102
as a leakage current i
3
a.
The coaxial cable
102
forming this signal line has a resonance point of a wavelength &lgr; satisfying the relation L=(2n−1)·&lgr;/4 (n: a positive integer) where L is the length of the coaxial cable
102
. Therefore, as long as the wavelength of the currents i
3
a
, i
3
b
flowing along the outer surface of the external conductor
102
b
of the coaxial cable
102
is sufficiently away from the wavelength &lgr;, the currents i
3
a
, i
3
b
which are originally very small pose no problem. In the case where the wavelength of the currents i
3
a
, i
3
b
is proximate to the resonance point of the coaxial cable
102
, however, the coaxial cable
102
develops a resonance, operates as a mono-pole antenna, and thus generates an unrequired electromagnetic radiation. Let the length L of the coaxial cable
102
be 1 m, for example. A resonance point occurs at a resonance point of frequency equivalent to odd multiples of f=3×108/4×1 =75 MHz.
The leakage current described above could be eliminated, if the case of the interior of the transmitting terminal unit
100
, the interior of the coaxial cable
102
and the interior of the case of the receiving terminal unit
101
could be completely hermetically closed by integrating the case of the transmitting terminal unit
100
completely with the outer surface of the external conductor
102
b
of the coaxial cable
102
and also by integrating the case of the receiving terminal unit
101
completely with the outer surface of the external conductor
102
b
of the coaxial cable
102
. Actually, however, such a configuration is substantially impossible to realize. Therefore, the occurrence of the unrequired radiation described above is unavoidable.
In view of this, according to the prior art, in order to suppress the unrequired radiation, a ferrite core
103
a
called a common mode core or a common mode choke is arranged on the side end of the transmitting terminal unit
100
of the coaxial cable
102
, and in similar manner, a ferrite core
103
b
is arranged on the side end of the receiving terminal unit
101
.
The provision of the ferrite cores
103
a
,
103
b
is equivalent to the insertion of a series circuit including an inductance and a resistor in the signal line along the outer surface of the external conductor
102
b
of the coaxial cable
102
due to the inductance and the polarization derived from the ferrite cores
103
a
,
103
b
. It follows, therefore, that the leakage currents i
3
a
, i
3
b
flowing along the same outer surface are suppressed. The absolute value of the impedance of the ferrite cores
103
a
,
103
b
is conventionally set to about 100&OHgr; from the viewpoint of the material and structure.
According to the above-mentioned conventional method, however, a conductive paste is coated on a comparatively flat portion of the surface of the board but cannot be coated on the component parts mounted or the portion where they are mounted. Even in the case wh

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