Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1999-06-28
2001-10-02
Gaffin, Jeffrey (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S760000, C361S777000, C361S780000, C361S794000, C361S818000, C333S012000, C333S247000, C307S089000, C307S091000
Reexamination Certificate
active
06297965
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a printed circuit board constituting a part of an electronic device, more in particular, to the printed circuit board for suppressing unnecessary electromagnetic waves radiated therefrom.
(b) Description of the Related Art
With the demand of high speed operation and high integration of electronic devices, unnecessary electromagnetic wave radiation occurs from a device of which a function is not to radiate the electromagnetic waves, to raise public concern. Therefore, the unnecessary electromagnetic waves, especially those of 30 MHz to 1 GHz, must be legally controlled. Manufacturers of the electronic devices are requested to design and manufacture articles satisfying this legal standard.
In order to suppress the unnecessary electromagnetic wave radiation from the electronic devices, it is most effective that the radiation is suppressed at a printed circuit board in the device. Conventionally, a number of printed circuit boards have been devised having a means for radiation-control.
Examples of these printed circuit boards include that described in JP-A-06(1994)-244562, a circuit board having a low EMI structure disclosed in JP-A-09(1997)-205290, a low EMI multi-layered circuit board described in JP-A-09(1997)-283974, and electronic devices employing these printed circuit boards.
The characteristic features of these prior arts will be described referring to
FIGS. 1
to
3
.
In the printed circuit board shown in
FIG. 1
which is disclosed in JP-A-06(1994)-244562, a part
131
of a power source layer
133
is separated from the remaining part, and is located on a substrate
134
near a ground layer
132
. The power source layer
133
and the separated part (power source layer)
131
are connected by a coupling means
135
to increase an electrostatic capacity between the separated power source layer and the ground layer.
In the circuit board having the low EMI structure shown in
FIG. 2
which is described in JP-A-09(1997)-205290, a printed circuit board
150
having a power source layer
151
on one surface and a ground layer
152
on the other surface is illustrated. Minute conductive patterns
154
,
155
and minute conductive patterns
153
,
156
are alternately disposed on the peripheries of the one layer and of the other layer, respectively. Every other conductive pattern
154
disposed on the periphery of the power source layer
151
is connected with the ground layer
152
, and the other every other conductive pattern
155
is connected with the power source layer
151
. The every other conductive pattern
153
disposed on the periphery of the ground layer
152
opposing to the above conductive patterns
154
is connected with the power source layer
151
, and the every other conductive pattern
156
opposing to the above conductive patterns
155
is connected with the ground layer
152
.
In the low EMI multi-layered circuit board shown in
FIG. 3
which is described in JP-A-09(1997)-283974, a power source layer
162
and a first ground layer
163
form a capacity C
1
interposing a dielectric layer
166
, the power source layer
162
and a second ground layer
164
form a capacity C
2
interposing a dielectric layer
167
, and a resistor
165
is connected between the first ground layer
163
and the second ground layer
164
.
In the circuit boards shown in
FIGS. 1
to
3
, all the circuit boards have a function for suppressing the variation of the power source voltage between the power source layer
162
and the first ground layer
163
both of which are a source of radiation. However, even if the power source layer and a part of the ground planar layer are so disposed that they are close to each other in the printed circuit board shown in
FIG. 1
, the increase of the electrostatic capacity obtained thereby is extremely small and the sufficient suppression of the power source voltage variation cannot be expected. In the circuit board
150
shown in
FIG. 2
, the polarities of voltages generated between the conductive patterns
154
and
153
and between the adjacent conductive patterns
155
and
156
are reversed to each other to have a function of implementing the radiation suppression by means of compensating the electric fields at the ends of the circuit board. Since, however, the voltage variation itself between the power source layer
151
and the ground layer
152
is unchanged, the unnecessary electromagnetic wave radiation from these layers
151
and
152
cannot be suppressed. While, in the multi-layered circuit board
161
shown in
FIG. 3
, the radiation of the unnecessary electromagnetic waves due to the voltage variation between the power source and the ground, and the malfunction of the device can be suppressed, the additional ground layer
164
and the second dielectric layer
167
are required in addition to the ordinary ground layer
163
to make its structure more complicated and to increase the cost.
All the above prior arts unavoidably require large alterations to the board structures. In order to apply the structural alteration to a printed circuit board already supplied as a product, redesign of the circuit board is required from the first step.
On the other hand, in order to suppress the variation of the power source voltage of IC or LSI, a capacitor having a large capacity is conventionally located between a power supply terminal and a ground terminal of IC or LSI. This conventional method utilizes a principle that variation of a power source voltage due to a switching of IC is reduced by making an impedance between a power supply terminal
171
and a ground terminal
172
lower than that of a capacitor ZC as shown in FIG.
4
.
However, in a frequency band between 30 MHz and 1 GHz in which the radiation of the unnecessary electromagnetic waves raises the public concern (controlled frequency band), the capacitor element ZC shown in
FIG. 4
cannot be regarded as a mere capacitor because of its parasitic inductance. A source line and other capacitors connected to IC are connected between the power supply terminals and the ground terminal of IC in addition to the capacitor element ZC, and the influence of these elements cannot be negligible.
A power supply circuit having a capacitor ZC
1
, and a source line and a capacitor ZC
2
connected thereto is shown in FIG.
5
. In order to reduce variation of a power source voltage in this circuit, an input impedance |Zin | observed from IC toward the power supply circuit must be reduced as much as possible. If, for example, a serial circuit including a source line having a length of 200 mm, its characteristic impedance of 50 ohms, its wavelength reduction rate of 0.5 and the capacitors ZC
1
and ZC
2
having 0.04 ohms, 0.7 nH and 0.1 &mgr;F which are close to those of an actual capacity product is assumed to exist, its input impedance |Zin | becomes that shown as a solid line in
FIG. 6. A
broken line in
FIG. 6
is a frequency characteristic of the absolute value |ZC | of an impedance of the single capacitor ZC. The impedances |Zin | and |ZC | function as capacitive resistances of which absolute values decrease proportional to wavelengths until about 17 MHz. Since the influence of the parasitic inductance of the capacitor becomes dominant over the above wavelength, the impedances function as an induced reactance in which the absolute value of the impedance increases proportional to the wavelength. Moreover, |Zin | becomes large at specified frequencies by means of the influence of the source line and ZC
2
. The variation of the power source voltage becomes large when a higher harmonic of an operating wavelength of IC is consistent with a wavelength of the source line and ZC
2
.
The reason why |Zin | becomes larger is occurrence of parallel resonance of the power supply circuit including the capacitor ZC
1
, the source line and the capacitor ZC
2
. Therefore, a large current flows in the source line at a frequency at
Harada Takashi
Sasaki Hideki
Gaffin Jeffrey
McGuire Woods LLP
NEC Corporation
Vigushin John B.
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