Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2000-09-25
2003-04-08
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S202000, C333S207000, C361S330000
Reexamination Certificate
active
06545565
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a filter, an antenna sharing device, and a communication device which are operable, e.g., in a microwave band.
2. Description of the Related Art
FIG. 9
shows an example of a conventional circuit configuration for a band elimination filter. The band elimination filter
1
comprises two resonance circuit stages coupled together, and includes resonators
2
and
3
electrically connected in series with trap capacitors C
1
and C
2
, respectively. Capacitors C
3
and C
4
electrically connected in parallel to the resonators
2
and
3
control the resonance frequencies of the resonators
2
and
3
, respectively.
A resonance circuit comprising the resonator
2
, the trap capacitor C
1
, and the frequency control capacitor C
3
is electrically connected via a coupling coil L
1
to a resonance circuit comprising the resonator
3
, the trap capacitor C
2
, and the frequency control capacitor C
4
. Furthermore, capacitors C
5
and C
6
are electrically connected in parallel to these two resonance circuits, respectively. In
FIG. 9
, reference numerals
5
and
6
designate input and output terminals
5
and
6
, respectively.
FIG. 10
shows an example in which the conventional circuit configuration of
FIG. 9
is realized on a circuit substrate
20
. The resonators
2
and
3
are &lgr;/4 coaxial dielectric resonators. The coupling coil L
1
is formed by winding as shown. The capacitors C
5
and C
6
are monolithic chip capacitors.
The trap capacitors C
1
and C
2
and the frequency control capacitors C
3
and C
4
are formed with one coupling plate
11
as shown in FIG.
11
. The coupling plate
11
comprises a dielectric substrate
12
and capacitor electrodes
13
to
18
formed on the front and back surfaces of the dielectric substrate
12
. The capacitor electrodes
13
and
16
form the trap capacitor C
1
. The capacitor electrodes
14
and
17
form the trap capacitor C
2
. The capacitor electrodes
13
and
15
form the frequency control capacitor C
3
. The capacitor electrodes
14
and
18
form the frequency control capacitor C
4
.
Individual monolithic chip capacitors can also be used, as the trap capacitors C
1
and C
2
, and the frequency control capacitors C
3
and C
4
. However, in this case, variations in capacitance between the capacitors C
1
to C
4
are large, causing the characteristics of the filter
1
to vary considerably, as compared with capacitance-variations between the capacitors formed with the coupling plate
11
.
The open end-faces of the dielectric resonators
2
and
3
are connected to the capacitor electrodes
13
and
14
of the coupling plate
11
via conductors
10
, respectively. That is, the resonators
2
and
3
are electrically connected to the capacitors C
1
and C
3
and the capacitors C
2
and C
4
.
As the design frequency of the filter
1
becomes higher, the required capacitances of the capacitors C
1
to C
4
are decreased. For example, for the filter
1
to be used in the 800 MHZ band, the capacitances of the capacitors C
1
to C
4
are set at 1.5 to 3.0 pF. On the other hand, for the filter
1
to be used in the 1.5 GHz band which is a quasi-microwave band, the capacitances of the capacitors C
1
to C
4
are set at 1.0 pF or lower.
In the case in which the capacitors C
1
to C
4
are formed with the coupling plate
11
, an unnecessary capacitance Cs is generated between the capacitor electrodes
13
and
16
, and the capacitor electrodes
14
and
17
. When the filter
1
is used in the 800 MHZ band, the unnecessary capacitance Cs is very small as compared with the capacitances of the capacitors C
1
to C
4
, and exerts no influence over filter characteristics.
However, when the employed frequency of the filter
1
becomes high, the required capacitances of the capacitors C
1
to C
4
decrease. Therefore, the unnecessary capacitance Cs considerably affects the filter characteristics. In the case of the band-block filter
1
, an unnecessary coupling is caused between the resonance circuits. Thus, there arises the problem that the attenuation characteristics of the filter deteriorate and so forth.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a filter, an antenna sharing device, and a communication device in which generation of an unnecessary capacitance between resonance circuits is suppressed.
To achieve the above object, according to the present invention, there is provided a filter which comprises plural resonance circuits each having a resonator, a trap capacitor, and a frequency control capacitor electrically connected to the resonator, respectively, the trap capacitor and the frequency control capacitor for each respective resonance circuit being made up of one corresponding capacitor plate. Preferably, a voltage-controllable reactance element such as a PIN diode, a field-effect transistor, a variable capacitance diode, or the like is electrically connected between each of the frequency control capacitors and ground.
The trap capacitor and the frequency control capacitor for each reactive resonance circuit are made up of one capacitor plate. Thereby, an unnecessary capacitance between the resonance circuits can be suppressed, which prevents the resonance circuits from being coupled.
The antenna sharing device and the communication device of the present invention include the filter having the above-described characteristics. Therefore, coupling can be suppressed between the resonance circuits, which provides excellent frequency characteristics.
Other features and advantages of the invention will be understood from the following description of embodiments thereof.
REFERENCES:
patent: 5726608 (1998-03-01), Arlin
patent: 11046102 (1999-02-01), None
Dickstein , Shapiro, Morin & Oshinsky, LLP
Jones Stephen E.
Murata Manufacturing Co. Ltd.
Pascal Robert
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