Patent Grant
6885258
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
As shown in
Referring to
In
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
Referring to
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
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:
E=12.6×10−7f·h·i/d(V/m)∝f·h·i/d(V/m) (1),
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
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
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
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.
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.
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.
Circuit Configuration
Referring to
In the first embodiment of the present invention as shown in
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
Common Mode Choke
1. Normal Mode Signal
Referring to
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 Eab between terminals a and b is represented by:
Eab=(−jωM+jωL1+ZL+jωL2−jωM)IS (2),
where j2=−1 and ω=2πf (f is a frequency).
Assuming that a coupling coefficient is 1,
M=L1=L2
Eab=ZL·IS (3).
In other words, the load impedance viewed from the terminals a and b remains ZL without any change even if the common mode choke coils are inserted.
2. Common Mode Noise
Referring to
A voltage Eac between terminals a and c is represented by:
Eac=jωL1·Ic1+jωM·Ic2 (4).
A voltage Ebd between terminals b and d is represented by:
Ebd=jωL2·Ic2+jωM·Ic1 (5).
Assuming that a coupling coefficient is 1, L1=L2=M.
When balanced, Ic1=Ic2 and
Eac=Ebd=2jωL1·Ic1 (6)
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
On the other hand, a common mode radiant noise energy EC2 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 EFG in the common mode radiant noise energy 30 of the housing 10.
The present invention focuses on the following points:
More specifically, referring to
In other words, since EC1≈EC2, 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
Ec1=Eac+jXc·Ic1 (7), and
Ec2=Ebd+EFG (8).
Also, referring to
Eac=jωL1·Ic1+jωM·Ic2 (9), and
Ebd=jωL2·Ic2+jωM·Ic1 (10),
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, Ic1=Ic2 and
Eac=Ebd=2jωL1·Ic1 (11).
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, jXc·Ic1, which is a voltage drop |XcIc1| caused by a current flowing through the distributed capacity impedance Xc equivalently connecting the signal line to the ground, and the noise energy voltage EFG 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 Eac=Ebd, that is,
|XcIc1|≈EFG≈0.
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:
EC1=Eac=EC2=Ebd (12).
From the this equation (12),
Ic1=Ic2 (13)
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
As shown in
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
As shown in
Referring to
E(V/m)∝S·i·f2/d (14),
where S is a loop area, i is a loop current, f is a frequency, and d is distance from antenna.
As shown in
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
As shown in
To make the radiant noise energy from all communication wires falling into the regulation range in the state as shown in
The first embodiment as described above is directed to the case where, as shown in
Similar to the method described in Japanese Patent Application Unexamined Publication No. 11-307986, the present invention is applicable to the case of EC1=EC2<EFG.
Third Example
Referring to
As shown in
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.
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
Assuming that a normal mode current I flows through the ferrite bead as shown in
When the common mode current I flows in a direction shown in
Therefore, the ferrite bead can suppress the common mode current without influencing the transmission signal.
First Example
As shown in
In the second embodiment of the present invention as shown in
Second Example
Referring to
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
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.
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
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 of the operational amplifier 291 are connected through a fourth resistor R.
All the first to fourth resistors of operational amplifiers 290, 291 have the same resistance R. In such a circuit arrangement, an input signal of an operational amplifier is inverted in phase only and is not amplified or attenuated. As a result, the output of the operational amplifier has the same level as the input.
In the present invention, a voltage of common mode noise 51 existing in the E (earth) pin 3E of communication LSI 3F is applied to the non-inverting input terminals of operational amplifiers 290, 291. The voltage of common mode noise 51 and the normal mode signals, which are superposed on the communication wire output pins 8H, 8J of communication LSI 3F, are applied to the inverting input terminals of operational amplifiers 290, 291, respectively.
Since an operational amplifier, as known well, operates such that a potential on the inverting input terminal is equal to that on the non-inverting input terminal, the voltage of common mode noise 51, which is included in output signals on the signal output pins 8H, 8J of communication LSI 3F, is consequently canceled. That is, it shows that even the circuit, employing operational amplifiers can realize the same EMI suppression effect as that of the above embodiments.
The + and −power supply voltages of operational amplifiers 290, 291 described in this case are obtained by using an inductance and a capacitor, which are similar to the inductance 3N and the capacitor 3P for decoupling as shown in FIG. 4A. Here, sufficient EMI suppression is subject to the power supply voltages to eliminate common mode noise 51 from the power supply voltage to the extent that the noise level of the power supply voltage is substantially equal to the level of the quasi-ground surface 20.
Although a metallic housing is taken as an example in the above embodiments, a mold or plastic housing may be used instead of the metallic housing. In this case, a portion having the smallest noise level within the EMI regulation value is determined as the quasi-ground. The present invention can be applied to such a case by replacing the quasi-ground surface 20 in the above embodiments with the determined portion.
The relationship between the present invention and the previously proposed method described in Japanese Patent Application Unexamined Publication No. 11-307986 will be briefly described.
As shown in
As shown in
As shown in
Finally, a basic operation of the present invention will be described briefly taking as an example the case using common mode chokes.
Referring to
More specifically, the common mode choke 80 has two coils having a mutual inductance, one of which connects the output pin 8H of the LSI 3F to the load ZL, and the other of which connects the E (earth) pin 3E to the quasi-ground surface 20. Similarly, the common mode choke 81 has two coils having a mutual inductance, one of which connects the output pin 8J of the LSI 3F to the load ZL, and the other of which connects the E (earth) pin 3E to the quasi-ground surface 20. The common mode chokes 80 and 81 are provided at positions near the quasi-ground of the housing 10. Here, the common mode chokes 80 and 81 have the same characteristics.
As shown in
Since |Ina|=|−Inb|, |−Ina·fg|=|−Inb·fg|, |Ina|=|−Ina·fg|, |−Inb|=|Inb·fg|, |Ina|=|−Inb|=|−Ina·fg|=|−Inb|=|Inb·fg|. Accordingly, no inductance is generated for a normal mode signal, resulting in no attenuation of the normal mode signal.
Further, the current Ina·fg and the current Inb·fg that flow between the quasi-ground 20 and the E (earth) pin 3E of the communication LSI 3F cancel each other out, so that no current flows through the common mode impedance 40. Accordingly, the common mode no se level is not affected.
As described above, according to the present invention, one coil of a common mode choke having at least two coils therein is inserted between communication wires and the other coil is inserted between the quasi-ground surface of housing and the earth pin of a communication LSI. Alternatively, an inexpensive device such as a ferrite bead may be used instead of the common mode choke, resulting in reduced cost of the device. Employing such a circuit, the common mode radiant noise which is generated between the earth pin of communication LSI and the housing can be eliminated effectively without attenuating a normal mode signal level on the communication wires.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2000-144988 | May 2000 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3697896 | Sarkozi et al. | Oct 1972 | A |
| 3832646 | Szabo et al. | Aug 1974 | A |
| 5220298 | Nagase | Jun 1993 | A |
| 5321373 | Shusterman et al. | Jun 1994 | A |
| 5969583 | Hutchison | Oct 1999 | A |
| Number | Date | Country |
|---|---|---|
| 11-307986 | Nov 1999 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20020117318 A1 | Aug 2002 | US |
