Title: Method and apparatus for reducing radiant noise energy by radiating noise energy from a quasi-ground into a signal wire
Abstract: An EMI suppression apparatus allowing a common mode radiant noise energy to be reduced without attenuating a normal mode signal level is disclosed. A communication LSI has an earth terminal connected to the housing via a common mode impedance. A common mode choke composed of two coils having a mutual inductance is provided in the housing. One coil connects a signal terminal of the communication LSI to a signal transmission wire. The other coil connects the earth terminal of the communication LSI to the quasi-ground surface of the housing. The common mode chokes causes a common mode radiant noise generated by the common mode impedance to be removed from the signal transmission wire without affecting a normal mode signal flowing through the signal transmission wire.
Patent Number: 6,885,258 Issued on 04/26/2005 to Suzuki
| Inventors:
|
Suzuki; Mitsuo (Tokyo, JP)
|
| Assignee:
|
NEC Corporation (Tokyo, JP)
|
| Appl. No.:
|
858604 |
| Filed:
|
May 17, 2001 |
Foreign Application Priority Data
| May 17, 2000[JP] | 2000-144988 |
| Current U.S. Class: |
333/12; 333/181 |
| Intern'l Class: |
H04B 003//26 |
| Field of Search: |
333/12,181,177,182,185
361/818
|
References Cited [Referenced By]
U.S. Patent Documents
| 3697896 | Oct., 1972 | Sarkozi et al.
| |
| 3832646 | Aug., 1974 | Szabo et al.
| |
| 5220298 | Jun., 1993 | Nagase.
| |
| 5321373 | Jun., 1994 | Shusterman et al.
| |
| 5969583 | Oct., 1999 | Hutchison.
| |
| Foreign Patent Documents |
| 11-307986 | Nov., 1999 | JP.
| |
Primary Examiner: Lee; Benny
Attorney, Agent or Firm: Young & Thompson
Claims
1. A method for suppressing a radiant noise of an electronic circuit having at
least one signal transmission wire connected thereto, wherein the electronic circuit
includes an electronic device having an earth terminal and at least one signal
terminal, the earth terminal being connected to a housing of the electronic circuit
via a common mode impedance, comprising the steps of:
forming a plurality of current paths corresponding to one of said at least one
signal transmission wire;
transmitting an output signal of an electronic device to a corresponding signal
transmission wire through a corresponding one of the current paths;
flowing a current between an earth terminal of the electronic device and a quasi-ground
through said one path; and
radiating noise energy from the quasi-ground into said at least one signal transmission
wire through the plurality of current paths to increase a noise level of said at
least one signal transmission wire so that a noise level of said at least one signal
transmission wire is equal to a noise level of the quasi-ground.
2. The method according to claim 1, wherein said plurality of current paths are
formed by a common mode choke having a plurality of coils corresponding to respective
ones of the current paths.
3. The method according to claim 2, wherein the current flows in a first coil
of the plurality of coils of the common mode choke and the output signal flows
in a second coil of the plurality of coils of the common mode choke, so that an
energy of the at least one signal transmission wire is equal to that of the quasi-ground
so that the at least one signal transmission wire is equivalently fixed to the
quasi-ground by a single screw.
4. The method according to claim 2, wherein the common mode choke is provided
at a position near a plate of the quasi-ground from which the signal transmission
wire extends outside of the housing.
5. The method according to claim 1, wherein said plurality of current paths are
formed by a respective resistor and at least one operational amplifier, which correspond
to respective ones of the current paths, wherein
the resistor is inserted one of the plurality of current paths to generate a
reference voltage from the current flowing in said one of said plurality of current
path, and
said at least one operational amplifier inputs the reference voltage at a non-inverting
input terminal thereof.
6. The method according to claim 1, wherein there are a plurality of said at
least one transmission wire, said plurality of signal transmission wires are accommodated
in a single cable, wherein the plurality of signal transmission wires are equivalently
fixed to the quasi-ground at a single point to suppress a radiant noise caused
by a current loop antenna equivalently formed between the plurality of signal transmission wires.
7. The method according to claim 1, wherein the noise energy radiates from the
quasi-ground to said at least one signal transmission wire, so that a radiant noise
energy is not increased independently of a number of signal transmission wires.
8. The method according to claim 1, wherein the electronic device is an integrated
circuit, wherein the common mode impedance includes impedances of connecting members
connected between the earth terminal of the integrated circuit and the housing.
9. The method according to claim 8, wherein the integrated circuit is a communication
LSI, wherein one of the at least one signal transmission wire extends from the
housing to outside of the housing and is connected to a communication signal output
terminal of the communication LSI.
10. The method according to claim 1, wherein said quasi-ground is set to a radiant
noise energy level equal to or smaller than an EMI regulation value.
11. An electronic apparatus comprising:
a housing having at least one signal transmission wire extending outside the
housing;
an electronic device having an earth terminal and at least one signal terminal
corresponding to the at least one signal transmission wire, wherein the earth terminal
is connected to the housing via a common mode impedance;
at least one common mode choke having a plurality of coils having a mutual inductance,
wherein
one of the plurality of coils connects a signal terminal of the electronic device
to a corresponding signal transmission wire; and
another one of the plurality of coils connects the earth terminal of the electronic
device to a quasi-ground surface of the housing,
wherein noise energy radiates from said quasi-ground into said at least one signal
transmission wire through said at least one common mode choke to increase a noise
level of said at least one signal transmission wire so that a noise level of said
at least one signal transmission wire is equal to a noise level of said quasi-ground.
12. The electronic apparatus according to claim 11, wherein a front plate from
which the at least one signal transmission wire extends is the quasi-ground.
13. The electronic apparatus according to claim 11, wherein the common mode choke
is provided at a position near a plate of the quasi-ground from which the at least
one signal transmission wire extends to outside.
14. The electronic apparatus according to claim 11, wherein the electronic device
has the earth terminal and said at least one terminal includes first and second
signal terminals corresponding respectively to first and second signal transmission
wires that comprise said at least one signal transmission wire, wherein
said at least one common mode choke comprises a first common mode choke having
first and second coils having a mutual inductance, wherein
the first coil connects the first signal terminal of the electronic device to
the first signal transmission wire; and
the second coil connects the earth terminal of the electronic device to a quasi-ground
surface of the housing, and
wherein said at least one common mode choke further comprises a second common
mode choke having third and fourth coils having a mutual inductance, wherein
the third coil connects the second signal terminal of the electronic device to
the second signal transmission wire; and
the fourth coil connects the earth terminal of the electronic device to the quasi-ground
surface of the housing,
wherein the first and second common mode chokes reduce a common mode radiant
noise generated by the common mode impedance without affecting a normal mode signal
flowing through the first and second signal transmission wires in opposite directions
to each other.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to countermeasures against EMI (electromagnetic
interference) in electronic devices, and in particular to a technique of using
a quasi-grounding method to reduce radiant noise energy in an electronic device
connected to a plurality of cables such as communication wires, power source lines,
and interface lines.
2. Description of the Related Art
First, a quasi-grounding method will be described as a background of the invention.
Quasi-Grounding
As shown in FIG. 1, assuming that a hole is dug in the ground
11 and a
wire
12 is buried therein, the wire
12 does not radiate any electromagnetic.
As shown in FIG. 2, a housing
10 enabling a radiant noise energy level
to be reduced below the regulation value level of EMI, can be regarded as "quasi-ground",
and there is no radiated electromagnetic wave from the wire
12 fixed to
the housing
10 by means of screw.
Referring to FIG. 3, a housing
10 accommodates a group of communication
system packages
21 and a group of control system packages
22. Each
of the communication system packages
21 is connected to a communication
wire such as a twisted cable extending from a front plate (quasi-ground surface)
20 of the housing
10. The control system packages
22 each
have no communication wires and include high-speed central processing units (CPU),
memory packages, clock packages, (CLK), and interface packages (INT).
In FIG. 3, a quasi-ground surface
20 is a surface that reduces a radiant
noise energy level below the regulation value level of EMI, and does not radiate
any electromagnetic wave from the wire fixed thereto.
FIG. 4A is a sectional side view of a communication system package having a
communication wire. In FIG. 4A, an earth pin
3E (also referred to as an
earth terminal or "E pin") of a communication LSI
3F, a plurality of LSIs
(Large Scale Integration) implemented in the package, and an E (earth) pin of an
IC component are connected to the housing
10 through an E (earth)
3D
of a printed board (PWB), a connector
3C of a back board (BWB), an E (earth)
layer
3B of the back board, and bosses
3A.
On the other hand, a power pin
3Q of the communication LSI
3F and
a power pin of LSIs and IC components implemented in the package are connected
to the contact point of one end of a decoupling inductance
3N and one end
of a capacitor
3P. The other end of capacitor
3P is connected with
E (earth)
3D of the printed board (PWB). The other end of inductance
3N
is connected with a power layer
3M of the printed board via a connector
3L and a power layer
3K of the back board.
Also, the communication wires
3H and
3J, connected with communication
output pins
8H and
8J of the communication LSI
3F through
a common mode choke
3G, are taken outwards from the housing
10 via
an hole opened on the quasi-ground surface
20 of the front side of the housing
10.
Here, the E (earth)
3D of the printed board and the power
3M
of the printed board are respectively an E (earth) layer and a power layer on the
printed board of the package having the same communication wire connected thereto.
Radiant Noises
By the way, there are two kinds of noise energies radiated from the communication
wires
3H and
3J: one is a normal mode noise current having a noise
current with a same level that goes between communication wires; and the other
is a common mode noise current flowing in the same direction through the communication
wires
3H and
3J.
The normal mode noise current is a noise energy generated by an individual communication
LSI of each communication wire package. On the other hand, the common mode noise
energy is an energy obtained by combining all of earth noises generated by the
control system packages
22 and the communication system packages
21,
which are accommodates in the housing
10 of FIG.
3. The common mode
noise energy is much larger than that of the normal one, and therefore it is the
biggest factor of radiation as if the communication wire is a monopole antenna.
As will be described later, the present invention provides a method and device
allowing a reduction of the common mode noise to mainly induce radiant noises.
The common mode noise will be described below in detail.
Common Mode Noise
FIG. 5 shows an outline of impedance factors causing a great common mode noise
between the quasi-ground
20 and the E (earth) pin
3E of the communication
LSI
3F.
Referring to FIG. 5, the presence of impedance (housing impedance) of a
metallic member composing the housing IC is a big factor that the common mode noise
is generated between the housing
10 and the quasi-ground surface
20
with the smallest noise energy.
The E (earth) current flows through the impedance (boss-E impedance) of a plurality
of metallic bosses
3A from the E (earth) layer
3B of the back board
(BWB), and thereby a common mode noise is generated at this boss-E impedance.
The E (earth) current from all packages of the control system packages
22
and the communication system packages
21 flows through the impedance (BWB-E
impedance) of the E (earth) layer
3B of the back board, and thereby a common
mode noise is caused.
The current branching out from the E (earth)
3D of the printed board of
the communication system packages flows through the impedance (connector-E impedance)
of a plurality of E (earth) pins of the connector
3C, and thereby a common
mode noise is caused.
The current from many LSI or IC components implemented to this package flows
through the impedance (PWB-E impedance) of the E (earth)
3D provided on
the printed board (PWB) of a package having a communication wire, and thereby a
common mode noise is caused.
As shown in FIG. 6A, therefore, when the E (earth) currents flow through many
kinds of above-described impedances existing between the quasi-ground surface
20
and the E (earth) pin of communication LSI
3F, which is indicated by a common
impedance
40 in FIG. 5, a large amount of common mode noise energy as denoted
by a reference numeral
51 of FIG. 6B exists in the E (earth) pin
3E
of communication LSI
3F implemented to the printed board of the package
having communication wire. Since the communication. LSI
3F functions with
respect to this E pin
3E, the large amount of common mode noise
51
(see FIG. 6) generated in the common impedance
40 is radiated as it is towards
the communication wires
3H,
3J connected with the communication output
pins
8H,
8J of the communication LSI
3F.
FIGS.
6(
a) and
6(
b) each show a relationship between
the common impedance
40 and the quasi-ground
20, wherein FIG.
6(
a)
schematically shows components forming the common impedance
40.
Since the internal E (earth) of communication LSI
3F as shown in FIG.
6(
b) works with respect to the E pin
3E, the common mode noise
on the internal E (earth) of communication LSI
3F, that is, the common mode
noise
51 on E pin
3E of communication LSI
3F is radiated as
it is to the communication wire output pins
8H,
8J of communication
LSI
3F. Therefore, the internal E (earth) of communication LSI
3F
and the E (earth) pin
3E can be recognized as a common E (earth)
50
as shown in FIG.
6(
c) and it can be also regarded that the communication
wire output pins
8H,
8J of communication LSI
3F are also connected
to the common E (earth)
50.
As shown in FIG.
6(
c), if the communication LSI
3F is omitted,
an equivalent configuration in which the common mode choke
3G inputs the
same common mode noise
51 is obtained. As the result, the communication
wires
3H,
3J play the role of a monopole antenna connected to the
quasi-ground surface
20 to radiate the noise energy exceeding the regulation value.
As described before, this radiant noise energy means the direct radiation of
common
mode noise energy
51 caused by the E (earth) current flowing through the
common mode impedance
40.
The description of electric field intensity, which is selected because of easy
theoretical analysis although electromagnetic wave is composed of electric field
and magnetic field, is shown in FIG.
7.
An electric field strength E (V/m) can be obtained by the following formula:
where i: a current value (A), h: antenna height (m), and f: frequency (Hz),
and d: distance (m) away from the monopole antenna.
In order to make this electric field intensity E smaller, current i, antenna
height
h, and frequency f are made smaller, and/or distance d greater.
In order to equivalently set the current i longitudinally flowing through he
antenna
to zero, a pair of quasi-grounding capacitors are employed in a conventional communication
system package as shown in FIG.
8.
Conventional Circuit Configuration
There has been proposed a quasi-grounding method and device in Japanese Patent
Application Unexamined Publication No. 11-30798, which was filed by Suzuki (the
present inventor) et al. The details thereof will be described hereafter.
As shown in FIG. 8, a capacitor
60 having a property of high frequency
is connected between the quasi-ground surface
20 and a point
6H connecting
one coil of common mode choke
3G with the communication wire
3H,
and a capacitor
61 having a property of high frequency is connected between
the quasi-ground surface
20 and a point
6J connecting the other coil
of common mode choke
3G with the communication wire
3J. The capacitors
60,
61 are hereinafter called quasi-grounding capacitors because
the respective capacitors
60 an
61 make short circuits in a high
frequency range between the communication wires
3H,
3J and the housing.
As the result, the radiant noise energy level of the communication wires
3H
and
3J are substantially equal to that of the quasi-ground (housing)
20.
In other words, the quasi-grounding capacitors
60,
61 cause the
radiant noise energy to flow into the quasi-ground surface
20 to short-circuit
the common impedance in high frequencies, preventing common mode noise energy
51
from being generated. The example shown in FIG. 8 is composed to combine the common
mode choke
3G with the quasi-grounding capacitors
60,
61 so
that a large amount of common mode noise energy is eliminated effectively.
FIG. 9A shows a sectional view of a conventional package having the circuit
configuration as shown in FIG.
8. Compared with the circuit configuration
of FIG. 4A, the quasi-grounding capacitors
60,
61 are additionally
connected between the communication wires
3H,
3J and the quasi-ground
surface
20.
Since the radiant noise energy of communication wires
3H,
3J
flows into the quasi-ground surface
20 through the quasi-grounding capacitors
60,
61, with the increase of number of communication system packages
21 accommodated in the housing
10, the radiant noise energy level
30 of quasi-ground surface
20 is also increased (see FIG.
9B).
In this case, even if all of the communication system packages
21 are accommodated
in the housing, it is necessary to keep the radiated noise energy level
30
of the quasi-ground surface
20 within the regulation value
70. For
this purpose, various countermeasures against noise energy (EMI) have been employed
for each internal circuit accommodated in he housing, such as back board, control
system packages, and communication system packages.
According to the conventional method described referring to FIGS. 8 and
9A,
9B, the quasi-grounding can be effectively realized in the case where
a common mode radiant noise energy of 30 MHz or more is reduced from analog communication
wires for a voice frequency band of 4 KHz or less. The reason is that a frequency
difference between the voice signal on the communication wires and the noise energy
to be eliminated is large. Therefore, after the common mode radiant noise energy
is removed by the common mode choke
3G and the quasi-grounding capacitors
60,
61, the normal mode signal level is not substantially reduced.
Here, the frequency of 30 MHz is adopted because the radiant noise of 30 MHz
or more is regulated by the International Special Committee on Radio Interference
(CISPR) international standards, the Voluntary Control Council for Interference
by Information Technology Equipment (VCCI) domestic standards, the FCC federal
standards, and the EN55022 European standards.
However, with the increase of signal speed on digital communication wires,
for example, a gigabit Ethernet with a transmission rate of Giga bps order in recent
years, the frequency difference between the signal and the common mode noise is
getting smaller. Accordingly, the quasi-grounding capacitors employed in the prior
art cause the normal mode signal level on the communication wires to be also attenuated
when the frequency of a signal on communication wires is increased to near that
of the common mode noise.
Now it is required to develop such a technology that can achieve an economical
countermeasure against EMI under high-speed transmission on communication wires
with reliability and stability.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for taking
countermeasures against EMI, that allows a common mode radiant noise energy to
be reduced without attenuating a normal mode signal level of communication wire
and reducing the high-speed transmission.
Other objects, characteristics, advantages and the like of the present invention
may be apparently understood from the following embodiments described in the disclosure.
According to the present invention, a method for suppressing a radiant
noise of an electronic circuit having at least one signal transmission wire connected
thereto, wherein the electronic circuit includes an electronic device having an
earth terminal and at least one signal terminal, the earth terminal being connected
to a housing of the electronic circuit via a common mode impedance, includes the
steps of: forming a plurality of current paths such that a first current path affects
any other current path corresponding to one of said at least one signal transmission
were; transmitting an output signal of an electronic device to a corresponding
signal transmission wire through a corresponding one of he current paths; flowing
a current between an earth terminal of the electronic device and a quasi-ground
through the first current path; and suppressing a common mode radiant noise included
in the output signal by flowing the current through the first current path.
The plurality of current paths may be formed by a common mode choke having a
plurality of coils corresponding to respective ones of the current paths.
The plurality of current paths may be formed by a ferrite bead having a plurality
of through holes corresponding to respective ones of the current paths, wherein
each of the current paths is passed through a corresponding one of the through holes.
Alternatively, the plurality of current paths may be formed by a resistor
and at least one operational amplifier, which correspond to respective ones of
the current paths, wherein the resistor is inserted in the first current path to
generate a reference voltage from the current flowing the first current path, and
said at least one operational amplifier inputs the reference voltage at a non-inverting
input terminal thereof.
According to another aspect of the present invention, an electronic apparatus
includes: a housing having at least one signal transmission wire extending to outside
the housing; an electronic device having an earth terminal and at least one signal
terminal corresponding to the at least one signal transmission wire, wherein the
earth terminal is connected to the housing via a common mode impedance; at least
one common mode choke having a plurality of coils having a mutual inductance, wherein
one of the plurality of coils connects a signal terminal of the electronic device
to a corresponding signal transmission wire; and another one of the plurality of
coils connects the earth terminal of the electronic device to a quasi-ground surface
of the housing.
According to an embodiment, the electronic device has the earth terminal
and first and second signal terminals corresponding respectively to first and second
signal transmission wires, wherein a first common mode choke having first and second
coils having a mutual inductance, wherein the first coil connects the first signal
terminal of the electronic device to the first signal transmission wire; and the
second coil connects the earth terminal of the electronic device to a quasi-ground
surface of the housing, and a second common mode choke having third and fourth
coils having a mutual inductance, wherein the third coil connects the second signal
terminal of the electronic device to the second signal transmission wire; and the
fourth coil connects the earth terminal of the electronic device to the quasi-ground
surface of the housing, wherein the first and second common mode chokes cause a
common mode radiant noise generated by the common mode impedance to be removed
from the first and second signal transmission wires without affecting a normal
mode signal flowing through the first and second signal transmission wires in opposite
directions to each other.
According to further another aspect of the present invention, an electronic
apparatus includes: a housing having at least one signal transmission wire extending
to outside the housing; an electronic device having an earth terminal and at least
one signal terminal corresponding to the at least one signal transmission wire,
wherein the earth terminal is connected to the housing via a common mode impedance;
at least one ferrite bead having a plurality of holes therein, wherein a signal
wire connecting a signal terminal of the electronic device to a corresponding signal
transmission wire is passed through one of the plurality of holes; and another
signal wire connecting the earth terminal of the electronic device to a quasi-ground
surface of the housing is passed through another one of the plurality of holes.
According to an embodiment, the electronic device may have the earth terminal
and first and second signal terminals corresponding respectively to first and second
signal transmission wires, wherein a first ferrite bead having first and second
holes, wherein a first signal wire connecting the first signal terminal of the
electronic device to the first signal transmission wire is passed through the first
hole; and a second signal wire connecting the earth terminal of the electronic
device to a quasi-ground surface of the housing is passed through the second hole,
and a second ferrite bead having third and fourth holes, wherein a third signal
wire connecting the second signal terminal of the electronic device to the second
signal transmission wire is passed through the third hole; and a fourth signal
wire connecting the earth terminal of the electronic device to the quasi-ground
surface of the housing is passed through the fourth hole, wherein the first and
second ferrite beads cause a common mode radiant noise generated by the common
mode impedance to be removed from the first and second signal transmission wires
without affecting a normal mode signal flowing through the first and second signal
transmission wires in opposite directions to each other.
According to another embodiment, the electronic device may have the earth
terminal and a plurality of signal terminals corresponding respectively to a plurality
of signal transmission wires, wherein the ferrite bead having a plurality of first
holes and a second hole, wherein a plurality of signal wires each connecting the
signal terminals of the electronic device to the signal transmission wires are
passed through respective ones of the first holes; and a signal wire connecting
the earth terminal of the electronic device to the quasi-ground surface of the
housing is passed through the second hole.
According to still another aspect of the present invention, an electronic
apparatus includes: a housing having at least one signal transmission wire extending
to outside the housing; an electronic device having an earth terminal and at least
one signal terminal corresponding to the at least one signal transmission wire,
wherein the earth terminal is connected to the housing via a common mode impedance;
a resistor for generating a reference voltage from a current flowing between the
housing and the earth terminal of the electronic device; and at least one operational
amplifier having an inverting input terminal connected to a corresponding signal
terminal of the electronic device, wherein the reference voltage is inputted at
a non-inverting input terminal thereof.
According to an embodiment, the electronic device may have the earth terminal
and first and second signal terminals corresponding respectively to first and second
signal transmission wires, wherein a first resistor for generating a first reference
voltage from a first current flowing between the housing and the earth terminal
of the electronic device; and a first operational amplifier having a first inverting
input terminal connected to the first signal terminal of the electronic device,
wherein the first reference voltage is applied to a first non-inverting input terminal
thereof, and a second resistor for generating a second reference voltage from a
second current flowing between the housing and the earth terminal of the electronic
device; and a second operational amplifier having a second inverting input terminal
connected to the second signal terminal of the electronic device, wherein the second
reference voltage is applied to a second non-inverting input terminal thereof,
wherein the first and second operational amplifiers cause a common mode radiant
noise generated by the common mode impedance to be removed from the first and second
signal transmission wires without affecting a normal mode signal flowing through
the first and second signal transmission wires in opposite directions to each other.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram for explaining the background of the invention;
FIG. 2 is a schematic diagram for explaining a quasi-ground;
FIG. 3 is a perspective view of an electronic device having a quasi-ground surface
at the front thereof;
FIG. 4A is a sectional view of the electronic device taken along the X—X
line of FIG. 3;
FIG. 4B is a diagram showing a change of radiant noise energy outside the housing;
FIG. 5 is a block diagram showing impedances within the housing for explaining
a common mode impedance (common mode noise);
FIGS. 6A, 6B and 6C show circuit diagrams for explaining the
common impedance;
FIG. 7 is a diagram explaining the electric field strength of a monopole antenna;
FIG. 8 is a diagram showing a circuit diagram of a conventional device using
quasi-grounding capacitors;
FIG. 9A is a sectional view of the conventional device;
FIG. 9B is a diagram showing a change of radiant noise energy outside the housing
in the conventional device;
FIG. 10A is a sectional view of a device according to a first example of a first
embodiment of the present invention;
FIG. 10B is a diagram showing a change of radiant noise energy outside the housing
in the first embodiment of the present invention;
FIG. 11 is a diagram explaining the first embodiment of the present invention;
FIG. 12 is a diagram explaining the first embodiment of the present invention;
FIG. 13 is a diagram explaining the first embodiment of the present invention;
FIG. 14 is a diagram explaining the first embodiment of the present invention;
FIGS. 15A, 15B and 15C are diagrams explaining the first embodiment
of the present invention;
FIG. 16 is a sectional view of a device according to a second example of the
first embodiment;
FIGS. 17A and 17B are diagrams explaining the second example of the first embodiment;
FIG. 18 is a diagram explaining the electric field strength of a loop antenna
radiation in the second example of the first embodiment;
FIG. 19 is a diagram explaining the second example of the first embodiment;
FIGS. 20A and 20B are diagrams explaining the second example of the first embodiment;
FIGS. 21A and 21B are diagrams explaining the second example of the first embodiment;
FIG. 22 is a perspective view of a device according to a third example of the
first embodiment;
FIG. 23 is a sectional view of a device according to the third example of the
first embodiment;
FIG. 24A is a diagram showing a ferrite bead when a normal mode signal flows;
FIG. 24B is a diagram showing a ferrite bead when a common mode noise flows;
FIG. 25 is a sectional view of a device according to a first example of a second
embodiment of the present invention;
FIG. 26 is a sectional view of a device according to a second example of a second
embodiment of the present invention;
FIG. 27 is a sectional view of a device according to a third example of a second
embodiment of the present invention;
FIG. 28 is a sectional view of a device according to a third embodiment of the
present invention;
FIGS. 29A and 29B are diagrams explaining the prior art method disclosed in
Japanese Patent Application Unexamined Publication No. 11-307986;
FIGS. 30A and 30B are diagrams explaining a prior art method different from
the method disclosed in Japanese Patent Application Unexamined Publication No.
11-307986; and
FIG. 31 is a diagram explaining the principle of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The conventional device disclosed in Japanese Patent Application Unexamined Publication
No. 11-307986 (see FIGS. 8 and 9) uses quasi-grounding capacitors to reduce a common
mode radiant noise level within regulation.
In contrast, an electronic device according to the present invention uses a common
mode choke to reduce a common mode radiant noise level within regulation without
reducing a normal mode signal level even in the case of a higher frequency of a
normal mode signal.
Further, according to the present invention, the communication system packages
21 are separated from the control system packages
22 by a shielding
plate
23 (see FIG.
22). Since the communication system packages
21
work at lower speeds compared with the control system packages
22, it is
easy to make the noise energy on the communication wires
3H and
3J
smaller than the quasi-ground
20 (see FIG.
22), although the quasi-ground
surface
20 provides a large amount of radiant noise energy from the control
system packages
22 and communication system packages.
Hereafter, it is assumed for simplicity that the radiant noise energy
flows from the communication wire side
3H,
3J into the quasi-ground
surface
20. This assumption provides the case being reversed to the conventional
case as disclosed in the Japanese Patent Application Unexamined Publication No.
11-307986 where the radiant noise energy from the housing flows into the communication
wires through the quasi-grounding capacitors.
As a matter of course, the arrangement similar to the conventional case may be
formed, where the wires have a smaller radiant noise energy than that of the quasi-ground
housing and a plurality of wires having smaller radiated noise energies than that
of the quasi-ground housing are connected to the quasi-ground housing through quasi-grounding
capacitors at high-frequency range. Since the current flows from the housing with
a high noise energy level to the wire with a low high-frequency noise energy level,
the noise energy level of the quasi-ground point of housing is reduced and the
high-frequency noise energy is not increased regardless of the increase of the
number of communication wires.
In the embodiments as described hereinafter, in the connected state of all communication
wires provided in the device, the radiant noise energy level of housing is assumed
to be equal to or smaller than the radiant noise regulation value, although the
increase of number of communication wires causes the radiant noise energy of the
housing (quasi-ground) to be increased.
First Embodiment
Circuit Configuration
Referring to FIG. 10A, common mode chokes are connected between the quasi-ground
surface 20 and the E pin 3E of the communication LSI 3F. In
FIG. 10A, components similar to those previously described with reference to FIG.
9A are denoted by the same reference numerals or symbols and the details will be
omitted. In FIG. 10B, a radiant noise energy level 30 outside the housing
10 (see FIG. 10A) is shown, wherein the reference numeral 70 indicates
the regulation value.
In the first embodiment of the present invention as shown in FIG. 10A, one coil
of common mode choke 80 is inserted between the communication output pin
8H of communication LSI 3F and the communication wire 3H,
and one end of the other coil of the common mode choke 80 is connected to
the quasi-ground surface 20 and the other end of the other coil is connected
to an E (earth) pin 3E of the communication LSI 3F.
Similarly, one coil of common mode choke 81 is inserted between
the communication output pin 8J of the communication LSI 3F and the
communication wire 3J, and one end of the other coil of the common mode
choke 81 is connected to the quasi-ground surface 20 and the other
end of the other coil as connected to the E (earth) pin 3E of the communication
LSI 3F.
In order to understand easily the circuit configuration as shown in FIG. 11A,
the property of the common mode chokes 80 and 81 is theoretically
analyzed referring to FIGS. 11 and 12.
Common Mode Choke
1. Normal Mode Signal
Referring to FIG. 11, in the case where common mode choke coils L1
and L2 receive a normal mode signal from a normal mode signal source Es,
the load of the normal mode signal source Es is only a load impedance Z
L,
which means the absence of the common mode choke, resulting in no reduction in
the normal mode signal level. When a current I
S flows from the normal
mode signal source E
S to the load Z
L, the direction of the
current flowing through the common mode choke coil L1 is opposite to that
of the current flowing through the common mode choke coil L2.
Assuming that the respective common mode choke coils L1 and L2
have self-inductances of L1 and L2, a mutual inductance of M, a voltage
E
ab between terminals a and b is represented by:
where j
2=-1 and ω=2πf (f is a frequency).
Assuming that a coupling coefficient is 1,
In other words, the load impedance viewed from the terminals a and b remains
Z
L
without any change even if the common mode choke coils are inserted.
2. Common Mode Noise
Referring to FIG. 12, the case where a common mode noise current is supplied
from a common mode noise source E
C will be described.
A voltage E
ac between terminals a and c is represented by:
A voltage E
bd between terminals b and d is represented by:
Assuming that a coupling coefficient is 1, L1=L2=M.
When balanced, I
c1=I
c2 and
In this manner, an impedance of 2jωL1 is added to each wire with
respect to the common mode noise, and thereby the common mode noise flowing into
the common mode choke coils L1 and L2 in the same direction can be
blocked effectively.
Referring to FIG. 13, the communication wire pin 8H or 8J
of the communication LSI 3F is connected to the communication wire 3H
or 3J through the coil L1 of the common mode choke 80 or 81.
Further, there is a distributed capacitance Xc between the communication wire 3H
or 3J and the ground of the earth. In other words, the communication wire
3H or 3J is connected to the ground by the distributed capacitance
Xc at high frequencies. Accordingly, the common mode radiant noise energy E
c1
at the communication wire pin 8H or 8J causes a current I
c1
to flow from the communication wire pin 8H or 8J to the ground
through the common mode choke coil L1 and the distributed capacitance Xc.
On the other hand, a common mode radiant noise energy E
C2 at the E
(earth) pin 3E of communication LSI 3F flows through the common mode
choke coil L2 into the quasi-ground surface 20 having an energy E
FG
in the common mode radiant noise energy 30 of the housing 10.
The present invention focuses on the following points:
1) LSI functions with respect to the E (earth), and the noise energy levels
included in the E (earth) appear as they are on all I/O pins thereof; and
2) The common mode noise 51 on the quasi-ground surface 20 mainly occupies
most of the radiant noise energy as described before referring to FIG. 5 and FIG. 6.
More specifically, referring to FIG. 13, the common mode radiant noise energy
E
C2 at the E (earth) pin 3E of communication LSI 3F is
included in the common mode radiant noise energy E
C1 from the communication
wire output pins 8H and 8J of communication LSI 3F. Further,
the common mode radiant noise energy E
C2 at the E (earth) pin 3E
of communication LSI 3F occupies most of the common mode radiated noise
energy from communication wire output pins 8H and 8J.
In other words, since E
C1≈E
C2, the circuit according
to the embodiment of the present invention is provided with a common mode choke
coil that is inserted between the quasi-ground surface 20 and the E (earth)
pin 3E of communication LSI 3F so as to reduce the radiant noise
energy level to the noise energy level of the quasi-ground 20 which is equal
to or smaller than the regulation value. The common mode choke coil inserted between
the quasi-ground surface 20 and the E (earth) pin 3E cancels or subtracts
an excess amount of radiant noise energy with respect to the noise level of the
quasi-ground 20 from the radiant noise energy of the pins 8H, 6J
of the communication LSI 3F. Accordingly, the radiated noise level from
the communication wires is reduced to just the amount corresponding to the energy
of quasi-ground surface 20 that is the regulation value or less.
In FIG. 13, assuming that current of the output pins 8H, 8J are
referred to as I
C1, I
C2, terminal voltage between
both ends a, c of common mode choke L1 is as E
ac, terminal voltage
between both ends is as b, d of common mode choke L2 is as E
bd,
the common mode noise energy E
c1 from the output pins 8H, 8J
of communication LSI 3F and the common mode noise energy E
c2 of
E pin 3E of communication LSI 3F are represented by the following equations:
Also, referring to FIG. 12, the terminal voltage E
ac, E
bd
are represented by the following equations:
where M is the mutual inductance of coils L1 and L2.
In the case of a coupling coefficient of common mode choke being 1, L1=L2=M,
and when the balance is kept, I
c1=I
c2 and
Accordingly, an impedance of 2jωL1 is applied to each line
(signal line) for the common mode noise and thereby the noise flowing into the
common mode choke coils L1, L2 in the same direction such as the
common mode noise can be effectively blocked.
On the other hand, jX
c·I
c1, which is a voltage drop
|X
cI
c1| caused by a current flowing through the distributed
capacity impedance X
c equivalently connecting the signal line to the
ground, and the noise energy voltage E
FG on the quasi-ground surface
20 in the common mode radiant noise energy 30 of the housing 10,
are negligible because they are much smaller than E
ac=E
bd,
that is,
By substituting this condition and the above equations (9), (10) and (11) into
the above equations (7) and (8), the following equation can be obtained:
From the this equation (12),
is obtained. Since an impedance of 2jωL1 is applied to each line
for the common mode noise, the noise flowing into the common mode choke coils L1
and L2 in the same direction such as the common mode noise can be effectively blocked.
Equivalent Circuit
Referring to FIG. 14, it is possible to regard the communication wires
3H, 3J as a monopole antenna. The common mode chokes 80 and
81 are inserted to cancel the common mode noise 51 from the monopole
radiant noise energy of the communication wires 3H, 3J to reduce
the radiant noise energy from communication wires 3H, 3J to the noise
energy 20 of the quasi-ground surface. This is entirely equivalent to the
conventional case using the quasi-grounding capacitor 60, 61 as shown
in FIG. 8.
As shown in FIG. 15B, the quasi-grounding common mode chokes 80 and 81
can remove the common mode noise mainly composed of monopole radiant noise energy
from the communication wires 3H, 3J, resulting in the following equation:
communication wire energy=housing energy E
FG=quasi-ground surface energy.
Accordingly, it is entirely equivalent to the case where the wires (communication
wires) 3H, 3J are fixed by a screw to the housing which is the quasi-ground
as shown in FIG. 15(
c).
Second Example
Although only one communication line is focused on in the above embodiment,
there are some cases where a plurality of communication lines is provided in a
single housing.
Referring to FIG. 16, according to another example of the first embodiment
of the present invention, a plurality of communication LSIs (here, two LSIs) are
provided in the housing 10, and each communication LSI is connected to communication
wires through common mode chokes, which connects the E (earth) pin of the corresponding
communication LSI to the quasi-ground surface (housing) 20, and further
connects the communication output pin of the corresponding communication LSI to
the corresponding communication wire.
As shown in FIGS. 17A and 17B, there is a difference of noise level (peak value)
between the E (earth) layers of the printed board (PWD) and backboard (BWB) depending
on installation positions thereof. Such a noise level difference causes a difference
of noise energy among plural communication wires, which forms a loop current area
as a loop antenna to radiate noises. In other words, when the various communication
wires having different noise levels are accommodated in a single cable, it is shown
that the loop current flows to form a loop antenna through the distributed capacity
between the different communication wires closely arranged in the single cable.
Referring to FIG. 18, electric field strength is obtained by the following equation:
where S is a loop area, i is a loop current, f is a frequency, and d is distance
from antenna.
As shown in FIG. 19, even if the loop current i is small, a large loop current
area would increase radiant noise energy to the regulation value or more.
In order to decrease the loop antenna electric field strength E (V/m) to a sufficient
small value, it is required to decrease S (loop area), i (loop current), and/or
f (frequency), and/or to increase d (distance from antenna). According to the present
invention, the loop current i flowing the loop area (S) is decreased in an equivalent manner.
As shown in FIG. 20A, in order to prevent loop currents from flowing through
the
distributed capacitors formed between all communication wires accommodated in a
single cable, it is necessary to make the common mode noise energies on all of
the communication wires equal to each other, that is, to generate no difference
in level and phase between them. By making the distributed capacitors formed between
all communication wires accommodated in a single cable equal to the noise energy
of the quasi-around 20. This state is equivalent to all of the communication
wires being connected to one point of the housing 20, as seen in FIG. 20B,
that is the quasi-ground by means of a screw, achieving an appropriate countermeasure
against EMI.
As shown in FIG. 21A, since there is no difference in energy between all cables
#1-#n extending from the device, it is possible to integrate all the cables
#1-#n into the quasi-ground surface 20. As if equivalent to the arrangement
as shown in FIG. 2, the noise energy of all communication wires in all cables of
the device is equivalent to that of the quasi-ground and it can be regarded as
to be fixed on the quasi-ground surface 20 by a screw as shown in FIG. 21B,
a stable countermeasure against EMI within the regulation value can be economically
realized without forming any loop antenna between communication wires.
To make the radiant noise energy from all communication wires falling into the
regulation range in the state as shown in FIG. 21A, it is necessary to take such
an appropriate EMI countermeasure that the energy of quasi-ground surface 20
is within the regulation value for all of the control system packages 22
and the communication system packages 21 in the housing 10.
The first embodiment as described above is directed to the case where, as shown
in FIG. 13, the energy of quasi-ground surface 20 increases as the number
of connected communication wires increases. That is, it is the case where the common
mode radiation energy E
C1 from communication wire output pins 8H,
8J of communication LSI 3F and the common mode radiation energy E
C2
of the E (earth) pin 3E of communication LSI 3F are larger
than the energy E
FG on the quasi-ground surface 20 of common
mode radiation energy 30 of housing 10. This case is represented
by the relationship of E
C1=E
C2>E
FG.
Similar to the method described in Japanese Patent Application Unexamined
Publication No. 11-307986, the present invention is applicable to the case of E
C1=E
C2<E
FG.
Third Example
Referring to FIG. 22, an electronic device has the housing 10 accommodating
a plurality of communication system packages 21 and control system packages
22. The control system packages 22 include high-speed CPU package,
memory package, high-speed clock package and so on, which have a large amount of
radiant noise energy. A total of the radiant noise energy from these packages 21
an 22 appear in the front plate (quasi-ground 20) of the housing
10 via only the backboard thereof. The communication system packages 21
are separated from the control system packages 22 by the shielding plate
23 in order to reduce interference from the noise energy of the control
system packages 22.
As shown in FIG. 23, the E (earth) on the printed board in a package (communication
system package 21 or control system package 22) is provided at substantial
distance from the inner wall of the housing 10 so as to prevent coupling
caused by distributed capacitance between them. Only the common mode choke can
be provided nearby the front plate. A small coupling effect caused by a floating
capacitance existing between the common mode choke and the front plate is negligible.
In such an arrangement, the radiant noise energy from the E (earth) of printed
board for all packages flows from only the backboard (BWB) side to the housing 10.
Since the communication system packages operate at low-medium speeds with respect
to the control system packages, a radiant noise energy level is small and therefore
it is easy to make the energy level of E (earth) pin 3E of communication
LSI 3F and of communication output pins 8H, 8J smaller than
the energy level of the housing 10.
As described above, according to the method described in Japanese Patent Application
Unexamined Publication No. 11-307986, the noise energy of communication wires is
made smaller than that of the quasi-ground surface 20 and thereby the energy
of the quasi-ground 20 is caused to flow into the communication wires, resulting
in the energy of all the communication wires being equal to that of the quasi-ground
20. This method allows all communication wires to have no difference in
noise energy with more reliability.
Second Embodiment
Next, another embodiment of the present invention will be described.
In general, a common mode choke is produced by winding a wire around a ferrite
core. Such a choke has a withstand voltage of about 50V. If a higher withstand
voltage is required, a ferrite bead is usually adopted.
As shown in FIGS. 24A and 24B, a two-hole ferrite bead is adopted to pass two
signal lines (communication wires 3H, 3J) through respective ones
of the two holes thereof. Such an arrangement provides the suppression effect of
common mode noises equivalent to that of the case as shown in FIGS. 11 and 12.
Assuming that a normal mode current I flows through the ferrite bead as
shown in FIG. 24A, the load impedance viewed from the driver is not changed. In
this case, the ferrite bead is operated as a common mode choke and is not magnetized
by the load current.
When the common mode current I flows in a direction shown in FIG. 24B, the ferrite
bead is operated as an induction device and the load impedance from the driver
side increases by 2jωL.
Therefore, the ferrite bead can suppress the common mode current without
influencing the transmission signal.
First Example
As shown in FIG. 25, ferrite beads 260 and 261 are connected between
the quasi-ground surface 20 and the E pin 3E of the communication
LSI 3F. It is apparent from FIG. 10A that this example is formed by replacing
the common mode chokes 80 and 81 of FIG. 10A with the ferrite beads
260 and 261.
In the second embodiment of the present invention as shown in FIG. 25, a signal
transmitting wire connecting the communication output pin 8H of communication
LSI 3F to the communication wire 3H is passed through one hole of
the ferrite bead 260, and an E wire connecting the E (earth) pin 3E
of the communication LSI 3F to the quasi-ground 20 is passed through
the other hole of the ferrite bead 260. It is the same with the other ferrite
bead 261.
Second Example
Referring to FIG. 26, the E (earth) pin 3E and the communication
wire pins 8H, 8J of a first communication LSI 3F are connected
with a three-hole ferrite bead 270 to combine respective currents passing
through the three-hole ferrite bead 260. In this manner, a single three-hole
ferrite bead can be used in place of two two-hole ferrite beads as shown in FIG.
25 to suppress common mode currents without influencing the transmission signal.
In this example, the number of ferrite beads can be decreased.
In the case where a second communication LSI 3F has a plurality of signal
pins corresponding to respective ones of communication wires #0-#n and a
single common E (earth) pin 3E, the E (earth) pin 3E and the signal
pins are connected to a ferrite bead 271 having a necessary number (here,
2(n+1)+1) of holes. In this arrangement, the effect to suppress common mode currents
without influencing transmission signals can be also obtained.
Third Example
Common mode chokes, each of which has a plurality of coils, can be employed
in place of the ferrite beads 270, 271 as shown in FIG. 26.
Referring to FIG. 27, in a common mode choke 28C, a coil L1
is inserted to a wire for connecting the E pin 3E of communication LSI 3F
with the quasi-ground surface 20 of housing, coil L2 is inserted
between the communication wire 3J and the signal Fin 8J, and coil
L3 is inserted between the communication wire 3H to the signal pin
8H. The respective coils L1, L2, and L3 have self-inductances
L1, L2, and L3, and have a mutual inductance M.
Such an arrangement provides a more effective EMI suppression effect because
the inductance of a coil decreases the common mode radiant noise more effectively
compared with that of FIG. 26.
Third Embodiment
The above embodiments employ passive devices (common mode chokes and ferrite
beads) as a means for implementing the present invention. Instead of passive devices,
active devices may be used.
Referring to FIG. 28, two operational amplifiers are employed as active
devices to cancel the common mode noise 51 as shown in FIG. 6 from the communication
wires 3H and 3J.
More specifically, the common mode noise 51 between the E (earth) pin
3E of communication LSI and the quasi-ground surface 20 is input
commonly to non-inverting input terminals of the operational amplifiers 290
and 291.
The communication wire output pin 8H of communication LSI 3F is
connected to the inverting input terminal of the operational amplifier 290
through a first resistor R. Further, the inverting input terminal and the output
terminal of the operational amplifier 290 are connected through a second
resistor R.
Similarly, the communication wire output pin 8J of communication
LSI 3F is connected to he inverting input terminal of operational amplifier
291 through a third resistor R. Further, the inverting input terminal and
the output terminal
