Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2001-04-19
2003-07-08
Ham, Seungsook (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S134000, C333S207000
Reexamination Certificate
active
06590475
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to filters for use in a frequency band such as a microwave band, antenna duplexers, and communication apparatuses incorporating the same.
2. Description of the Related Art
FIG. 13
shows a conventional trap filter formed by using dielectric resonators (cf. Japanese Unexamined Patent Application Publication No. 63-30003). In a trap filter
201
disclosed in the publication, between an input terminal P
1
and an output terminal P
2
, there are arranged a serial resonance section
203
composed of a capacitor C
61
and a dielectric resonator
202
, a reactance element
204
electrically connected in parallel to the serial resonance section
203
. In
FIG. 13
, a capacitive reactance element is provided by a capacitor C
62
connected to the serial resonance section
203
.
In the conventional trap filter
201
, however, with the reactance element
204
connected in parallel to the serial resonance section
203
, the position of an anti-resonance frequency fa must be on either the frequency-side higher than an attenuation-pole frequency, or the frequency-side lower than the attenuation-pole frequency, but not both. That is, when a capacitor (capacitive reactance element) is used as the reactance element
204
, the position of the anti-resonance frequency fa is always on the frequency side higher than the attenuation-pole frequency (see FIG.
14
A). In contrast, when an inductor (inductive reactance element) is used as the reactance element
204
, the position of the anti-resonance frequency fa is always on the frequency-side lower than the attenuation-pole frequency (see FIG.
14
B). Thus, in the conventional trap filter
201
, the position of the anti-resonance frequency fa cannot be freely switched between the frequency side higher than the attenuation-pole frequency and the frequency-side lower than that the attenuation-pole frequency.
In addition, as shown in
FIG. 15
, there is known a conventional band elimination filter
211
with variable attenuation-pole frequency. In
FIG. 15
, the reference numerals
212
and
213
denote distributed-constant lines having characteristic impedances. The reference numerals C
71
and C
72
denote capacitors for determining the magnitude of elimination-band attenuation. The reference numerals C
73
and C
74
denote frequency shifting capacitors for varying two attenuation-pole frequencies. These capacitors C
73
and C
74
serve to change inductive reactance generated by the distributed-constant lines
212
and
213
, respectively. The reference numerals D
71
and D
72
denote PIN diodes, the reference numerals L
71
and L
72
denote choke coils. The reference numerals C
75
and C
76
denote bypass capacitors, the reference numerals r
31
and r
32
denote voltage-control supplying resistors, the reference numeral L
74
denotes a coupling coil, and the reference numerals L
73
and L
75
denote coils.
When a positive voltage is applied to a voltage control terminal Vc
1
, the PIN diodes D
71
and D
72
are switched on. As a result, since the frequency shifting capacitors C
73
and C
74
are grounded via the PIN diodes D
71
and D
72
, the two attenuation-pole frequencies both become lower (a solid line
215
shown in FIG.
16
). When a negative voltage or 0 v is applied to the voltage control terminal Vc
1
, the PIN diodes D
71
and D
72
are switched off. As a result, since the capacitors C
73
and C
74
are open-circuited, the two attenuation-pole frequencies both become higher (a broken line
216
shown in FIG.
16
).
In the band elimination filter
211
, although the attenuation-pole frequencies can be varied by performing voltage control, it is impossible to significantly change the relationship between the pass band and the attenuation band. That is, in the conventional band elimination filter
211
, the position of the attenuation band cannot be freely switched from the frequency-side higher than the pass band to the frequency-side lower than the pass band.
SUMMARY OF THE INVENTION
The present invention can provide a filter in which the relationship between a pass band and an attenuation band can be controlled. In addition, the invention can provide an antenna duplexer and a communication apparatus incorporating the filter.
In order to accomplish the above, according to a first aspect of the present invention, there is provided a filter including a trap circuit having a serial resonance section, a capacitive reactance element and an inductive reactance element for making an admittance of the trap circuit substantially zero, both reactance elements being electrically connected in parallel to the serial resonance section, and a switching element connected to one of the capacitive reactance element and the inductive reactance element and being switchable on/off. As the switching element, for example, a PIN diode or a field effect transistor may be used. As the voltage-controllable reactance element, for example, a variable capacitance diode may be used. In addition, the serial resonance section may include a dielectric resonator or a distributed-constant line.
When the switching element is switched on/off, a parallel reactance-element circuit composed of the capacitive reactance element and the inductive reactance element becomes either capacitive or inductive. That is, by the on/off control of the switching element, the position of an anti-resonance frequency fa can be freely switched between the frequency-side higher than an attenuation-pole frequency and the frequency-side lower than the attenuation-pole frequency.
In addition, the voltage-controllable reactance element may be electrically connected to each serial resonance section via a frequency shifting capacitor. With this arrangement, the reactance element is switched on/off by applying a control voltage, by which the frequency shifting capacitor is either grounded or open-circuited. As a result, the attenuation-pole frequency of the trap circuit can be varied.
Furthermore, a plurality of trap circuits having serial resonance sections may be electrically connected to each other by one of an inductive element and a capacitive element. With this arrangement, for example, a band elimination filter is formed. In the band elimination filter, by on/off switching of the switching element, the position of the attenuation band can be freely switched between the frequency-side higher than the pass band and the frequency-side lower than the pass band.
In addition, according to a second aspect of the invention, there is provided a filter including the filter of the first aspect and a band pass filter electrically connected thereto. With this arrangement, by switching the switching element on/off, the position of the anti-resonance frequency of the trap circuit can be freely switched between the frequency-side higher than the pass-band of the band pass filter and the frequency-side lower than that the pass-band of the band pass filter.
According to a third aspect of the invention, there is provided an antenna duplexer including one of the above filters. According to a fourth aspect of the invention, there is provided a communication apparatus incorporating at least one of the filters and/or the antenna duplexer described above. In both the duplexer and the communication apparatus, the freedom of design can be increased and the size reduction can be achieved.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.
REFERENCES:
patent: 4571560 (1986-02-01), Dobrovolny
patent: 4910481 (1990-03-01), Sasaki et al.
patent: 6307448 (2001-10-01), Atokawa et al.
patent: 6351195 (2002-02-01), Atokawa et al.
patent: 63030003 (1988-02-01), None
patent: 07321509 (1995-12-01), None
Atokawa Masayuki
Miyamoto Hirofumi
Suemasa Hajime
Tsunoda Kikuo
Yamada Yasuo
Dickstein, Shaprio, Morin & Oshinsky, LLP.
Ham Seungsook
Murata Manufacturing Co. Ltd.
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