Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2001-12-31
2003-07-08
Ham, Seungsook (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S172000
Reexamination Certificate
active
06590476
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a lowpass filter; and, more particularly, to a lowpass filter for effectively suppressing a spurious in a stop-band, which is generated from a filter operating in a high frequency band including a Radio Frequency (RF) band or a microwave band.
DESCRIPTION OF THE RELATED ARTS
Generally, a conventional lowpass filter is composed of a transmission line in the Radio Frequency RF or the microwave band and has a spurious characteristic due to a periodic characteristic of the transmission line in a stop-band.
Referring to
FIGS. 1 and 2
, in a conventional lowpass filter implemented by using microstrips, an input port
210
and an output port
270
are microstrips with a characteristic impedance as an impedance of an input/output ports.
Capacitors
120
,
140
and
160
in
FIG. 1
are implemented by using Open-ended microstrips
220
,
240
and
260
in FIG.
2
. Capacitances corresponding to the open-ended microstrips
220
,
240
and
260
are determined by a line-width, a line-length and an operation frequency of the microstrips.
Microstrip lines
230
and
250
connect the open-ended microstrips
220
to the open-ended microstrips
240
and the open-ended microstrips
240
to the open-ended microstrips
260
, respectively. The microstrip lines
230
and
250
at
FIG. 2
are operated as inductors
130
and
150
of
FIG. 1
at especially cutoff frequency.
A circuitry employing with microstrips in
FIG. 2
has the similar result with a circuit in
FIG. 1
in a low frequency band. However, in a high frequency band, the results of the circuitry in
FIG. 2
show a difference results with a circuit in FIG.
1
. Such a difference in results has been a problem of the conventional lowpass filter shown in FIG.
2
.
Referring to
FIG. 3
, “A” and “B” denote an insertion loss and a reflection loss of the conventional lowpass filter shown in FIG.
2
.
A dotted line in a graph of
FIG. 3
represents a spurious response of the conventional lowpass filter. As shown, un-wanted spurious characteristics appear in the stop-band. The spurious characteristics in the stop-band are due to a periodical impedance change characteristic of microstrip structure of the conventional lowpass filter.
For suppressing such a problem, conventional techniques have been introduced and described in an article by T. Garvens, “Microwave LPF design for RF Application”, 1995,
RF Expo,
pp. 147-159, (1995); by J. W. Sheen, “A compact semi-lumped lowpass filter for harmonics and spurious suppression”,
IEEE Microwave and Guided Wave Letters, vol,
10, pp.392-93, (March 2000); by Y. Qian and et al., “Microwave applications of photonic band-gap(PBG) structures”, 1999
IEEE International Microwave Symposium,
(June, 1999).
Garvens introduces a method for improving a spurious response by avoiding a periodical structure according to a specified frequency band. By changing a shape of microstrips in a microstrips type lowpass filter, the periodical characteristic with respect to frequency can be avoided. However, the method only improves a spurious response characteristic in a specified frequency band but not in a broad frequency band. The method cannot be implemented at the broad frequency band.
Sheen proposes a semi-lumped type lowpass filter for improving a spurious response by altering a harmonic frequency according to periodical characteristic of microstrip with respect to a frequency. For avoiding the spurious response, the semi-lumped type lowpass filter is proposed, which is composed of lumped elements and microstrips in parallel connection. The semi-lumped lowpass filter improves the spurious response characteristic of the conventional lowpass filter in a narrow frequency band; however, in broad frequency band, the semi-lumped type lowpass filter cannot improves spurious response characteristic. Qian et al. introduces a method for improving a spurious response by setting the stop-band as a specified frequency band by applying a constant defect to a groundside of microstrips. The method improves the spurious response at the specific frequency band. However, beyond the aimed the specific frequency band, its effectiveness is disappeared or another spurious characteristic may appear in another frequency band.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a lowpass filter for effectively suppressing a spurious characteristic due to a periodic characteristic of a transmission line with respect to frequency using an open-ended microstrips employing thick/thin resistors.
In accordance with an aspect of the present invention, there is provided a lowpass filter, including: an input port microstrip element; an output port microstrip element; a first open-ended microstrip element connected to the input port microstrip element, having resistance elements both located in predetermined portion of the first open-ended microstrip line and being operated as a capacitor; a second open-ended microstrip element connected to the output port microstrip element, having resistance elements both located in predetermined portion of the microstrip line and being operated as a capacitor; a third open-ended microstrip element connected between the first open-ended microstrip element and the second open-ended microstrip element, having resistance both located in predetermined portion of the microstrip line, and being operated as a capacitor; a first connector microstrip element for connecting the first open-ended microstrip element and the second open-ended microstrip element and being operated as an inductor; and a second connector microstrip element for connecting the second open-ended microstrip element and the third open-ended microstrip element and being operated as an inductor.
The lowpass filter including microstrip have at least one open-ended microstrip element of which a predetermined portion is removed and distributed resistances inserted both located in predetermined portions of the open-ended microstrip.
The open-ended micro strip element of the lowpass filter includes distributed elements and distributed resistances connected to each other in series and operated as a capacitor, wherein distributed resistance increases impedance of the microstrips in case of a low frequency and decreases the impedance of the microstrips in case of a high frequency.
REFERENCES:
patent: 5237296 (1993-08-01), Mandai et al.
patent: 5491367 (1996-02-01), Schinzel
patent: 6140892 (2000-10-01), Uda et al.
patent: 2002/0047757 (2002-04-01), Manku et al.
patent: 2002/0070814 (2002-06-01), Dixon et al.
patent: 04186910 (1992-07-01), None
Thomas Garvens; Microstrip LPF Design RF Applications; Motorola, Inc.; p. 147-159.
Yongxi Qian, et al; Microwave Applications of Photonic Band-Gap (PBG) Structures; IEEE 1999; p. 315-318.
Jyh-Wen Sheen; A Compact Semi-Lumped Low-Pass Filter for Harmonics and Spurious Suppression; IEEE Microwave and Guided Wave Letters, vol. 10, No. 3; Mar. 2000; p. 92-93.
Lee Moon Que
Lee Seong-Pal
Ryu Keun Kwan
Yom In Bok
Electronics and Telecommunications Research Institute
Ham Seungsook
Jacobson & Holman PLLC
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