Wave transmission lines and networks – Attenuators
Reexamination Certificate
2001-03-09
2003-03-04
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Attenuators
C333S185000, C333S012000
Reexamination Certificate
active
06529091
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an absorptive circuit element and an absorptive low-pass filter, utilizing frequency selective absorption of a magnetic material, and to a manufacturing method thereof.
DESCRIPTION OF THE RELATED ART
In a digital equipment operating at a high clock frequency and in an equipment processing signals with a wide frequency range inside a narrow housing such as a mobile communication equipment, elimination of unnecessary frequency components contained in the signals is important for stabilizing the operation of the equipments.
According to a conventional method for eliminating an unwanted signal, a capacitor with a large capacitance or a general low pass filter was inserted in a source which might produce the unwanted signal to damp unnecessary frequency components in the signal. However, since the large-capacitance capacitor or the general low pass filter was a reactance with a small loss to limit the transmission of the unwanted signal by reflection, the signal reflected by such countermeasure element detoured in other circuits and became new interference sources.
The essential elimination of unnecessary frequency components therefore should not be performed by reflection, but should be performed by absorption.
As for elimination of the unwanted signal by absorption, ferrite beads are currently used. A signal line is covered by the ferrite beads array to configure an inductor with a certain loss so that the unwanted signal is eliminated by the increase in reactance due to frequency and by the magnetic loss. However, since the impedance of the signal line changes depending upon reactance change of the ferrite beads, within a frequency band where the impedance of the ferrite beads is not matching with the output impedance of the unnecessary frequency component source, the unwanted signal is suppressed from passing by reflection. Thus, using of the ferrite beads also cannot be an essential countermeasure. Furthermore, absorption of the ferrite beads rapidly decreases at 1 GHz or more due to performance restriction of the material. Hence, the ferrite beads are insufficient for a countermeasure against the unwanted signal in a recent mobile communication equipment and a high-speed data bus circuit.
In order to solve these problems, the present applicant has proposed an absorptive low-pass filter element exhibiting a small reflection and a large absorption in an unwanted signal processing frequency band (Japanese patent unexamined publication No.08204486A).
FIG. 1
is a partially cutaway oblique view schematically illustrating a structure of this conventional absorptive low-pass filter element.
In the figure, reference numeral
10
denotes a magnetic material core provided in a center section and formed by ferrite or fine powder of pure iron bound with a resin,
11
a conductor (inner conductor) helically wound around the magnetic core
10
,
12
a magnetic material provided outside the inner conductor
11
and formed by binding fine pure iron powder with a resin, and
13
an outer conductor formed on the surface of the magnetic material
12
to be made conductive, respectively.
By electrically dividing this outer conductor
13
as shown in
FIG. 1
into three sections, by applying a signal across the two conductor sections in both end surfaces (input and output terminals)
13
a
and
13
b
and by grounding the central conductor section (ground conductor)
13
c,
the inner conductor
11
and the outer conductor
13
configure a transmission line with a certain loss. Since this line is distributed-constant structure, a characteristic impedance of the filter element is determined by its line structure and by real parts of a permeability and a dielectric constant of the magnetic material, and loss is determined by the magnetic loss of the magnetic material. If the characteristic impedance is set at a value near to a drive impedance, it is possible to absorb the unwanted signal energy by the loss in the filter element while suppressing reflection from the filter element as much as possible.
When such low pass filter is terminated, as shown in
FIG. 2
, a reflection coefficient (reflection loss) S
11
of an input terminal is −10 dB or less over the entire frequency range, but a transmission coefficient (transmission loss) S
21
exhibits a low-pass or high-cut filtering characteristic. If this filter element is inserted into a high frequency circuit, signals at or below a cutoff frequency will pass as it is, but signals over the cutoff frequency will be absorbed inside the element and will not be transmitted resulting that it is possible to eliminate the signals over the cutoff frequency from the high frequency circuit. If an element other than a terminator is connected to the output side of this filter element however, its impedance is reflected to the input side in a low-pass frequency region of the transmission coefficient S
21
(Japanese patent unexamined publication No.08204486A).
It is difficult to precisely find a characteristic impedance Z
0
of such compact filter element. Nevertheless, by modeling the line structure on a microstrip line, it is possible to roughly calculate the characteristic impedance Z
0
from formula (1) with a width W
0
of the inner conductor, a thickness h of the magnetic material, and a relative permeability &mgr;
r
and a relative dielectric constant ∈
r
of the magnetic material.
Z
0
=
μ
r
ϵ
r
601
n
(
8
h
W
0
-
0.358
+
1
0.931
h
W
0
+
0.736
)
(
1
)
Now, let W
0
be 0.15 mm, and h be 0.2 mm. Because &mgr;
r
=9 and ∈
r
=90 in the magnetic material containing 85 wt % of fine pure iron powder, the characteristic impedance of this filter element becomes Z
0
=45.2 &OHgr; from formula (1).
However, if such filter element is connected to a drive element with a high impedance, an unwanted signal suppression effect becomes insufficient due to the reflection caused by impedance-mismatching. In addition, if contents of the fine pure iron powder in the magnetic material
12
is increased to enhance the magnetic loss (absorption amount), the input impedance of the element remarkably drops because of the increase of an effective dielectric constant resulting the unwanted signal suppression effect to become further insufficient due to the mismatching in impedance.
If the contents of iron powder is made at 90 wt % or more in the magnetic material
12
in order to increase the magnetic loss, breakdown may occur because of contact between particles of the iron powder and a short-circuit with the ground conductor may occur. This is called a leakage current failure that should be avoided in any electronic component.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an absorptive circuit element and an absorptive low-pass filter, in which input impedance can be determined independently of an absorption characteristic, and a manufacturing method thereof.
According to the present invention, an absorptive circuit element includes a core body made of non-conductive material, an inner conductor formed by winding a conductive wire around the core body with a gap provided between adjacent turns, a magnetic material surrounding outside of the inner conductor, the magnetic material being made of composite material containing ferromagnetic fine metal powder and insulating resin, a dielectric surrounding outside of the magnetic material, and an outer conductor formed on a surface of the dielectric. An absorptive low-pass filter according to the present invention is provided with this absorptive circuit element.
An absorptive circuit element according to the present invention processes unnecessary frequency components in a signal by absorption, not by reflection. Owing to this, the circuit element of the present invention is remarkably useful for elimination of an interfering wave inside a computer with a high frequency clock and a mobile communication equipment which processes a signal with a wide frequency range in a narr
Endou Kenji
Miura Taro
Armstrong Westerman & Hattori, LLP
Harris Wesley
Lee Benny
TDK Corporation
LandOfFree
Absorptive circuit element, absorptive low-pass filter and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Absorptive circuit element, absorptive low-pass filter and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Absorptive circuit element, absorptive low-pass filter and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3082551