Branching filter having specific impedance and admittance characteristics

• US 5,323,127 A
• Filed: 01/11/1993
• Issued: 06/21/1994
• Est. Priority Date: 07/27/1990
• Status: Expired due to Term

• Current Assignee: Intellectual Ventures Funding LLC (Intellectual Ventures LLC)
• Original Assignee: OKI Electric Industry Company Limited
• Inventors: Ichikawa, Kouichi, Mashimo, Akira, Komazaki, Tomokazu, Onishi, Norio, Gunji, Katsuhiko
• Primary Examiner(s): LEE, BENNY T
• Application Number:
  US08/003,837
• Time in Patent Office: 526 Days
• Field of Search: 333/134, 333/126, 333/129, 333/202, 333/132, 455/78, 455/82, 455/83, 370/36, 370/38
• US Class Current: 333/126
• CPC Class Codes: H01P 1/2136 using comb or interdigital filters; using cascaded coaxial cavities H01P1/2131, H01P1/2135 take precedence
• CPC Class Codes: H03H 7/46 Networks for connecting several sources or loads, working on different frequencies or frequency bands, to a common load or source for use in multiplex transmission systems H04J1/00

Abstract

A branching filter in which an input/output port, a first dielectric filter for use in transmission and a second dielectric filter for use in reception are coupled with each other through a branching filter circuit. The branching filter circuit includes a first line provided between the input/output port and an input port of the second dielectric filter, a second line provided between the input/output port and an output port of the first dielectric filter, and a third line provided between the input/output port and the ground. In the filter, conductor lines for use in connection between ports are used for the first and second lines.

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 l₁₂ coupling said I/O port with said receiver filter port;

a second conductive strip line having a second line length l₂₃ coupled between said receiver filter port and said transmitter filter port; and

a third conductive strip line having a third line length l₂₀ coupled between said receiver filter port and ground,

said branching filter being configured so that for Z_L12 =∞

, Z_L12 being the impedance corresponding to l₁₂, an input admittance Y_in looking toward said transmitter filter and said receiver filter from the I/O port is expressed by the following equation;

space="preserve" listing-type="equation">Y.sub.in =Y.sub.T +Y.sub.RF +Y.sub.20

wherein

Y_T is a synthetic admittance of the input admittance of the second conductive strip line having the line length l₂₃,

Y_RF is the input admittance of said receiver filter, and

Y₂₀ is the input admittance of the third line having line length l₂₀,

Y_T being expressed by the following equation, ##EQU14## wherein Y_TF is the input admittance of said transmitter filter,

θ

₂₃ is the phase constant of the second line having line length l₂₃, and

Y₂₀ being expressed by the following equation, ##EQU15## where θ

₂₀ is a phase constant of the third line having line length l₂₀ ;

wherein said input admittance Y_RF of said receiver filter at a pass band of said transmitter filter is expressed by Y_RF ≈

Y₂₀, and wherein the synthetic admittance Y_T at a pass band of said receiver filter is expressed by Y_T ≈

Y₂₀, the third line length l₂₀ of the third line being such that a selected one of Y_RF ≈

Y₂₀ and Y_T ≈

Y₂₀ is satisfied; and

wherein the input impedance Z_in looking toward said transmitter filter and said receiver filter from the I/O port is expressed by the following equation, ##EQU16## wherein θ

₁₂ is a phase constant of the first line having line length l₁₂,

the input admittance Y_TF contained in the admittance Y_in being such that the input impedance Z_in is increased at a receiving frequency band, and the input admittance Y_RF contained in the admittance Y_in being such that the input impedance Z_in is increased at a transmitting frequency band.

2. A branching filter according to claim 1 wherein said input impedance Z_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 l₁₂ and connecting said input/output port and said input port of said receiver filter;

a second conductive strip having a second line length l₂₃ 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 l₂₀ provided between said input port of said receiver filter and a first potential,

wherein for Z_L12 =∞

, Z_L12 being the impedance corresponding to l₁₂, an input admittance Y_in at the input port is expressed by the following equation;

space="preserve" listing-type="equation">Y.sub.in =Y.sub.T +Y.sub.RD +Y.sub.20

where Y_RF is an input admittance of said receiver filter, Y₂₀ is an input admittance of the third line having line length l₂₀, and expressed by the equation;

##EQU17## where θ

₂₀ is a phase constant of said third conductive strip having line length l₂₀, and Y_T is a synthetic admittance, including an input admittance of the second conductive strip having line length l₂₃ and an input admittance of said transmitter filter, and is expressed by the equation;

##EQU18## where Y_TF is an input admittance of said transmitter filter and θ

₂₃ is a phase constant of the second conductive strip having line length l₂₃,

said input admittance Y_RF of said receiver filter at a pass band of said transmitter filter and said synthetic admittance Y_T at a pass band of said receiver filter are expressed by Y_RF ≈

Y₂₀ and Y_T ≈

Y₂₀, respectively, said third line length l₂₀ of the third conductive strip satisfying a selected one of equation Y_RF ≈

Y₂₀ and equation Y_T ≈

Y₂₀, and

wherein an input impedance Z_in at the input/output port is expressed by the equation ##EQU19## where θ

₁₂ is a phase constant of the first conductive strip having line length l₁₂,

said input impedance Z_in having at least a selected one of the components of said input admittances Y_TF and Y_RF contained in said admittance Y_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_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 l₁₂ between said input/output port and said input port of said receiver filter;

coupling a second conductive strip line having a second line length l₂₃ 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 l₂₀ between said I/O port and said ground;

configuring the filters and first, second, and third conductive strip lines so that for Z_L12 =∞

, Z_L12 being the impedance corresponding to l₁₂, an input admittance Y_in looking toward said transmitter filter and said receiver filter from the input port is expressed by the following equation;

space="preserve" listing-type="equation">Y.sub.in =Y.sub.T +Y.sub.RF +Y.sub.20

where Y_T is a synthetic admittance of an input admittance of the second line having line length l₂₃ and an input admittance of said transmitter filter, and is expressed by the following equation;

##EQU20## wherein;

Y_TF is the input admittance of said transmitter filter,

θ

₂₃ is the phase constant of the second line having line length l₂₃,

Y_RF is the input admittance of said receiver filter, and

Y₂₀ is the input admittance of the third conductive strip line having line length l₂₀, and is expressed by the following equation;

##EQU21## wherein θ

₂₀ is a phase constant of the third conductive strip line having line length l₂₀,

wherein, assuming that the synthetic admittance Y_T at a pass band of said receiver filter is Y_T1, the third line length l₂₀ of the third conductive strip line is such that the equation Y_T1 ≈

Y₂₀ is satisfied; and

wherein an input impedance Z_in looking toward said transmitter filter and said receiver filter from the input/output port is expressed by the following equation;

##EQU22## wherein θ

₁₂ is phase constant of the first line having line length l₁₂ ;

adjusting the input admittance Y_TF contained in the admittance Y_in in such a manner that the input impedance Z_in is increased at a receiving frequency band; and

adjusting the input admittance Y_RF contained in the admittance Y_in in such a manner that the input impedance Z_in is increased at a transmitting frequency band.