Claims
- 1. A branching filter comprising:
- an input/output (I/O) port;
- a transmitter filter and a receiver filter, each said filter having a respective port, each said filter being a dielectric filter; and
- a branching filter circuit coupling said filters together, the branching filter circuit including:
- a first conductive strip line having a first line length l.sub.12 coupling said I/O port with said receiver filter port;
- a second conductive strip line having a second line length l.sub.23 coupled between said receiver filter port and said transmitter filter port; and
- a third conductive strip line having a third line length l.sub.20 coupled between said receiver filter port and ground,
- said branching filter being configured so that for Z.sub.L12 =.infin., Z.sub.L12 being the impedance corresponding to l.sub.12, an input admittance Y.sub.in looking toward said transmitter filter and said receiver filter from the I/O port is expressed by the following equation:
- Y.sub.in =Y.sub.T +Y.sub.RF +Y.sub.20
- wherein
- Y.sub.T is a synthetic admittance of the input admittance of the second conductive strip line having the line length l.sub.23,
- Y.sub.RF is the input admittance of said receiver filter, and
- Y.sub.20 is the input admittance of the third line having line length l.sub.20,
- Y.sub.T being expressed by the following equation, ##EQU14## wherein Y.sub.TF is the input admittance of said transmitter filter,
- .theta..sub.23 is the phase constant of the second line having line length l.sub.23, and
- Y.sub.20 being expressed by the following equation, ##EQU15## where .theta..sub.20 is a phase constant of the third line having line length l.sub.20 ;
- wherein said input admittance Y.sub.RF of said receiver filter at a pass band of said transmitter filter is expressed by Y.sub.RF .apprxeq.Y.sub.20, and wherein the synthetic admittance Y.sub.T at a pass band of said receiver filter is expressed by Y.sub.T .apprxeq.Y.sub.20, the third line length l.sub.20 of the third line being such that a selected one of Y.sub.RF .apprxeq.Y.sub.20 and Y.sub.T .apprxeq.Y.sub.20 is satisfied; and
- wherein the input impedance Z.sub.in looking toward said transmitter filter and said receiver filter from the I/O port is expressed by the following equation, ##EQU16## wherein .theta..sub.12 is a phase constant of the first line having line length l.sub.12,
- the input admittance Y.sub.TF contained in the admittance Y.sub.in being such that the input impedance Z.sub.in is increased at a receiving frequency band, and the input admittance Y.sub.RF contained in the admittance Y.sub.in being such that the input impedance Z.sub.in is increased at a transmitting frequency band.
- 2. A branching filter according to claim 1 wherein said input impedance Z.sub.in is such that a return loss from the input/output port is larger than 10 db.
- 3. A branching filter according to claim 1 further comprising an insulative substrate, said input/output port, transmitter filter, receiver filter and branching filter circuit being mounted thereon.
- 4. A branching filter comprising:
- an input/output port coupled through a branching filter circuit to a transmitter filter and a receiver filter, respectively for use in transmitting and receiving, each said filter having an input port and an output port; said branching filter circuit comprising:
- a first conductive strip having a first line length l.sub.12 and connecting said input/output port and said input port of said receiver filter;
- a second conductive strip having a second line length l.sub.23 and connecting said input port of said receiver filter and said output port of said transmitter filter; and
- a third conductive strip having a third line length l.sub.20 provided between said input port of said receiver filter and a first potential,
- wherein for Z.sub.L12 =.infin., Z.sub.L12 being the impedance corresponding to l.sub.12, an input admittance Y.sub.in at the input port is expressed by the following equation:
- Y.sub.in =Y.sub.T +Y.sub.RD +Y.sub.20
- where Y.sub.RF is an input admittance of said receiver filter, Y.sub.20 is an input admittance of the third line having line length l.sub.20, and expressed by the equation: ##EQU17## where .theta..sub.20 is a phase constant of said third conductive strip having line length l.sub.20, and Y.sub.T is a synthetic admittance, including an input admittance of the second conductive strip having line length l.sub.23 and an input admittance of said transmitter filter, and is expressed by the equation: ##EQU18## where Y.sub.TF is an input admittance of said transmitter filter and .theta..sub.23 is a phase constant of the second conductive strip having line length l.sub.23,
- said input admittance Y.sub.RF of said receiver filter at a pass band of said transmitter filter and said synthetic admittance Y.sub.T at a pass band of said receiver filter are expressed by Y.sub.RF .apprxeq.Y.sub.20 and Y.sub.T .apprxeq.Y.sub.20, respectively, said third line length l.sub.20 of the third conductive strip satisfying a selected one of equation Y.sub.RF .apprxeq.Y.sub.20 and equation Y.sub.T .apprxeq.Y.sub.20, and
- wherein an input impedance Z.sub.in at the input/output port is expressed by the equation ##EQU19## where .theta..sub.12 is a phase constant of the first conductive strip having line length l.sub.12,
- said input impedance Z.sub.in having at least a selected one of the components of said input admittances Y.sub.TF and Y.sub.RF contained in said admittance Y.sub.in to achieve an increased return loss at said input/output port.
- 5. A branching filter according to claim 4 wherein said input impedance Z.sub.in is such that a return loss from the input/output port is larger than 10 db.
- 6. A branching filter according to claim 4, further comprising an insulative substrate, wherein said input/output port, said transmitter filter, said receiver filter and said branching filter circuit are mounted thereon.
- 7. A method of constructing a branching filter comprising a transmitter filter having input and output ports, a receiver filter having input and output ports, an input/output (I/O) port, and a ground, said method comprising the steps of:
- coupling a first conductive strip line having a first line length l.sub.12 between said input/output port and said input port of said receiver filter;
- coupling a second conductive strip line having a second line length l.sub.23 between said input port of said receiver filter and said output port of said transmitter filter;
- coupling a third conductive strip line having a third line length l.sub.20 between said I/O port and said ground;
- configuring the filters and first, second, and third conductive strip lines so that for Z.sub.L12 =.infin., Z.sub.L12 being the impedance corresponding to l.sub.12, an input admittance Y.sub.in looking toward said transmitter filter and said receiver filter from the input port is expressed by the following equation:
- Y.sub.in =Y.sub.T +Y.sub.RF +Y.sub.20
- where Y.sub.T is a synthetic admittance of an input admittance of the second line having line length l.sub.23 and an input admittance of said transmitter filter, and is expressed by the following equation: ##EQU20## wherein: Y.sub.TF is the input admittance of said transmitter filter,
- .theta..sub.23 is the phase constant of the second line having line length l.sub.23,
- Y.sub.RF is the input admittance of said receiver filter, and
- Y.sub.20 is the input admittance of the third conductive strip line having line length l.sub.20, and is expressed by the following equation: ##EQU21## wherein .theta..sub.20 is a phase constant of the third conductive strip line having line length l.sub.20,
- wherein, assuming that the synthetic admittance Y.sub.T at a pass band of said receiver filter is Y.sub.T1, the third line length l.sub.20 of the third conductive strip line is such that the equation Y.sub.T1 .apprxeq.Y.sub.20 is satisfied; and
- wherein an input impedance Z.sub.in looking toward said transmitter filter and said receiver filter from the input/output port is expressed by the following equation: ##EQU22## wherein .theta..sub.12 is phase constant of the first line having line length l.sub.12 ;
- adjusting the input admittance Y.sub.TF contained in the admittance Y.sub.in in such a manner that the input impedance Z.sub.in is increased at a receiving frequency band; and
- adjusting the input admittance Y.sub.RF contained in the admittance Y.sub.in in such a manner that the input impedance Z.sub.in is increased at a transmitting frequency band.
Priority Claims (1)
Number |
Date |
Country |
Kind |
3-197709 |
Jul 1990 |
JPX |
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Parent Case Info
This is a continuation of application Ser. No. 07/736,701, filed Jul. 26, 1991 now abandoned.
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
5015973 |
Kawakami et al. |
May 1991 |
|
5023866 |
DeMuro |
Jun 1991 |
|
Foreign Referenced Citations (1)
Number |
Date |
Country |
8203730 |
Oct 1982 |
EPX |
Continuations (1)
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Number |
Date |
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Parent |
736701 |
Jul 1991 |
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