Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
1999-07-12
2001-07-03
Kee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S08100R, C333S202000, C333S246000
Reexamination Certificate
active
06255920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a low-pass filter that is used to attenuate high-frequency components in VHF, UHF, microwave and milliwave bands and, more particularly, to a low-pass filter of the type that has a ground and a signal conductor, such as a coaxial line filter or a stripline filter.
2. Description of the Prior Art
FIG. 1
is a partly exploded, perspective view depicting the structure of a conventional coaxial line filter (a low-pass filter) disclosed in G. L. Matthaei et al., “Microwave Filters, Impedance-Matching Networks, and Coupling Structures,” pp.365-374, McGrawHill, 1962. Reference numeral
1
denotes a hollow, cylindrical external ground conductor;
2
denotes a columnar or rod-like signal conductor disposed in the external ground conductor
1
along its axis but spaced apart therefrom;
3
denotes an input terminal connected to one end of the signal conductor
2
;
4
denotes an output terminal connected to the other end of the signal conductor
2
;
5
,
6
and
7
denote disc-shaped, capacitive conductors of the same size which are mounted on the signal conductor
2
concentrically therewith at predetermined intervals in such a manner that the signal conductor
2
extends through the capacitive conductors
5
,
6
and
7
at the center thereof; and
8
,
9
and
10
denote dielectric rings tightly inserted between the perimeters of the capacitive conductors
5
,
6
and
7
and the interior wall of the external ground conductor
1
.
The coaxial line filter of the above configuration serves, in its entirety, as an LC ladder circuit wherein those parts of the signal conductor
2
having mounted thereon the capacitive conductors
5
,
6
and
7
function as low-impedance lines and the other parts as high-impedance lines.
When supplied at the input terminal
3
with a signal of the VHF, UHF, microwave or milliwave band, the coaxial line filter attenuates a signal component above a cut-off frequency fc determined by the LC ladder circuit, permitting the passage therethrough of a signal component below the cut-off frequency fc for output via the output terminal
4
. Thus, the coaxial line filter operates as a low-pass filter.
Because of such a configuration as described above, however, the conventional low-pass filter has some drawbacks; for example, in the case of its multi-stage connection, high-impedance lines of a predetermined electric length produce therebetween resonance at a frequency where the phase of the input signal varies by &pgr; for the length of one of the high-impedance lines. As a result, the low-pass filter permits the passage therethrough of signal components of frequencies around resonance.
FIG. 2
is a graph showing the attenuation characteristic of the traditional coaxial line filter. The abscissa and the ordinate represent signal frequency and attenuation value, respectively. Reference character fc denotes the cut-off frequency and fs denotes the resonance frequency of the high-impedance line. As depicted in
FIG. 2
, the coaxial line filter exhibits a transmission characteristic at the frequency (the resonance frequency fs) corresponding to the electric length of the high-impedance line, resulting in a failure to provide a large attenuation value over a wide frequency band above the cut-off frequency fc.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a low-pass filter which has a plurality of stages of high-impedance lines to secure a sharp cut-off characteristic but suppresses the occurrence of resonance between the high-impedance lines, thereby providing a large attenuation value over a wide frequency band above the cut-off frequency.
According to an aspect of the present invention, there is provided a low-pass filter which comprises: a ground conductor; a signal conductor disposed in the ground conductor but spaced apart therefrom; a plurality of capacitive conductors mounted on the signal conductor at predetermined intervals lengthwise thereof to form electric fields higher in intensity than that of the signal conductor between the capacitive conductors and the ground conductor, the plurality of capacitive conductors forming low-impedance lines, respectively, and defining a high-impedance line between each pair of capacitive conductors so that the signal conductor is composed of an alternate arrangement of low- and high-impedance lines; and second capacitive conductors each carried upon the signal conductor in one of the high-impedance lines at the mid-point in its lengthwise direction to form between it and the ground conductor an electric field of an intensity lower than that by each of the capacitive conductors.
With such a low-pass filter, since the signal conductor consists of an alternate arrangement of the high-impedance lines defined by the capacitive conductors therebetween and the low-impedance lines formed by the capacitive conductors themselves, it is possible to achieve excellent attenuation of signals over a wide frequency band above the cut-off frequency that is determined by the alternate arrangement of the high- and low-impedance lines.
In addition, even in the case where the high-impedance lines symmetrically located along the signal conductor have the same electric length and hence produce therebetween resonance at a frequency where the phase of the input signal varies by &pgr; for the electric length of each high-impedance liner, the second capacitive conductor secured to each high-impedance line at the center thereof ensures effective attenuation of a signal at the resonance frequency. Furthermore, even if the signal of the resonance frequency is not sufficiently attenuated by the second capacitive conductor itself, the energy transmittance of the resonance frequency, which is dependent solely upon the capacitive conductor, can be lowered because the resonance frequency of the signal conductor practically shifts toward the higher-frequency side due to the provision of the second capacitive conductor on the signal conductor for each high-impedance line at the mid-point in its lengthwise direction. Hence, the low-pass filter exhibits a sharp cut-off characteristic at the cut-off frequency by the multistage high-impedance lines and, at the same time, suppresses the occurrence of resonance between the high-impedance lines to thereby provide a large attenuation value over a wide frequency band which is impossible to achieve with the prior art above the cut-off frequency.
According to another aspect of the present invention, the second capacitive conductors are geometrically similar to the first-mentioned capacitive conductors.
Because of their geometrical similarity, the both capacitive conductors can be fabricated by common design criteria. Hence, the additional provision of the second capacitive conductors does not ever require extra time to do so.
According to another aspect of the present invention, there is provided a low-pass filter which comprises: a ground conductor; a signal conductor disposed in the ground conductor but spaced apart therefrom; and a plurality of capacitive conductors mounted on the signal conductor at predetermined intervals lengthwise thereof to form electric fields higher in intensity than that of the signal conductor between the capacitive conductors and the ground conductor, the plurality of capacitive conductors forming low-impedance lines, respectively, and defining a high-impedance line between each pair of capacitive conductors so that the signal conductor is composed of an alternate arrangement of low- and high-impedance lines; and wherein that part of the signal conductor forming at least one of the high-impedance lines has a sectional area different from those of the other parts of the signal conductor forming the other remaining high-impedance lines; and when the sectional area of that part of the signal conductor forming said at least one high-impedance line differs from the sectional area of that part of the signal conductor forming that one of the remaining high-impedan
Miyazaki Moriyasu
Mukai Kazuhiro
Ohwada Tetsu
Glenn Kimberly E
Kee Benny
Mitsubishi Denki & Kabushiki Kaisha
Rothwell Figg Ernst & Manbeck
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