Apparatus for matching unbalanced R. F. baseband signals to balanced signals on a twisted two-wire line

• US 4,766,402 A
• Filed: 08/06/1987
• Issued: 08/23/1988
• Est. Priority Date: 08/06/1987
• Status: Expired due to Term

• Current Assignee: 3Com Corporation (HP Inc.)
• Original Assignee: 3Com Corporation (HP Inc.)
• Inventors: Crane, Ronald C.
• Primary Examiner(s): Gensler, Paul
• Application Number:
  US07/083,499
• Time in Patent Office: 383 Days
• Field of Search: 333/25, 333/26, 379/93, 379/442, 375/36
• US Class Current: 333/25
• CPC Class Codes: H03H 7/42 Networks for transforming balanced signals into unbalanced signals and vice versa, e.g. baluns

Abstract

Impedance of each termination in a wiring environment, wherein unshielded twisted pair wires with a characteristic impedance at 10 MHz of about 85 to 130 ohms are used as a communication medium for a local area network where run lengths do not exceed about 250 feet between terminations, is matched at each termination by an impedance matching termination and line conditioning apparatus, hereinafter a matching device. The matching device matches impedance of twisted pair wiring to a nominal 50 termination over a bandwidth of d.c. to above about 10 MHz. The matching device comprises a low-pass filter coupled at a first port to a fifty-ohm standard BNC connector and at a second port to a tapped autotransformer means with a d.c. blocking capacitor for impedance matching and balanced to unbalanced conversion, which in turn is coupled to a common mode inductor, which in turn is coupled to terminals for each side of a twisted pair cable. The common mode inductor is for passing differential signals over the entire frequency spectrum while rejecting all common mode signals over substantially all of the frequency spectrum.

Claims

1. An apparatus for coupling an unbalanced termination of a first characteristic impedance to a substantially balanced termination of a second characteristic impedance with matching from d.c. to above about 10 MHz comprising:

a low-pass filter means for limiting bandwidth to a system passband, said low-pass filter means coupled at a first filter port to a first connector suited for termination in an unbalanced line of said first characteristic impedance and at a second port coupled to an autotransformer means, said unbalanced line having a center conductor coupled to a center conductor side and a signal common coupled to a common node;

autotransformer means with a first d.c. blocking capacitor coupled to said common node for impedance matching and for balanced to unbalanced signal conversion; and

a common mode inductor means for providing common mode rejection over a bandwidth up to at least said system passband, said common mode inductor means to be coupled to a second connector suited for termination in a substantially balanced line of said second characteristic impedance.

2. The apparatus according to claim 1 wherein said common mode inductor means comprises:

a first transformer, said first transformer having a first winding and a second winding, said first winding being coupled on said center conductor side to pass differential signals to a first terminal of said second connector and said second winding being coupled on a signal common side to pass differential signals to a second terminal of said second connector, said first terminal and said second terminal for direct connection to a twisted pair cable, for providing a large common mode impedance at high-range frequencies above about 1 MHz;

a second transformer comprising a third winding and a fourth winding, said third winding being coupled on said center conductor side to pass differential signals to said first winding, and said fourth winding being coupled on said signal common side to pass differential signals to said second winding, for providing a large common mode impedance at midrange frequencies between about 10 kHz and about 1 MHz; and

4. The apparatus according to claim 2 wherein said common mode inductor means further comprises:

a first resistor;

a second resistor; and

a first coupling capacitor;

said first resistor being coupled between said signal common side of said second winding and said common node;

said second resistor being coupled between said center conductor side of said first winding and said signal common through said first coupling capacitor for reducing effects of capacitively-coupled noise from external sources through said second connector.

5. The apparatus according to claim 2 wherein said first transformer and said second transformer each have a differential characteristic impedance matched to the impedance at its insertion point in its signal path.

6. The apparatus according to claim 1 wherein said autotransformer means includes means for selecting a tap to match the characteristic impedance at said first connector with the characteristic impedance at said second connector when said second connector is connected to said substantially balanced termination.

3. a first inductance means coupled in series with a second d.c. blocking capacitor between said center conductor side of said first winding and a signal common of said unbalanced conductor, said inductance means having an inductance value matched to the inductance value of said third winding, for diverting current from said third winding to cancel common mode offset current in said third winding.

7. The apparatus according claim to 2 wherein said autotransformer means includes means for selecting a tap to match the characteristic impedance at said first connector with the characteristic impedance at said second connector when said second connector is connected to said substantially balanced termination.

8. The apparatus according claim to 5 wherein said autotransformer means comprises an autotransformer, coil and frequency compensation means, said frequency compensation means to minimize high frequency attenuation and to establish a minimum frequency for impedance matching.

9. The apparatus according claim to 7 wherein said frequency compensation means comprises a second inductance means coupled in parallel with magnetizing inductance of said autotransformer coil and a parallel resistor-capacitor combination coupled between said autotransformer coil and said first d.c. blocking capacitor, wherein the values of said d.c. blocking capacitor and said second inductance means establishes said minimum frequency for impedance matching, and wherein the values of said parallel resistor-capacitor combination control input to output frequency response.

10. The apparatus according to claim to 1 wherein said low-pass filter comprises diode means coupled between said center conductor side and said common node, said diode means for providing voltage clamping between said center conductor side and said common node, said diode means having a parasitic capacitance included as a component of said low-pass filter.

11. The apparatus according claim to 2 wherein said third winding is coupled between said first winding and a first tap of said autotransformer means and said fourth winding is coupled between said second winding and a second tap of said autotransformer means, said second tap being of equal voltage and opposite polarity to said first tap.

12. The apparatus according claim to 2 wherein said third winding is coupled between said second port and a primary tap of said autotransformer means and wherein said fourth winding is coupled between said second port and a common reference of said autotransformer means.

13. The apparatus according to claim to 1 wherein said common mode inductor means further provides common mode rejection from a low frequency limit determined by noise.

14. The apparatus according claim to 2 further including means coupled between a primary tap of said autotransformer means and a common reference of said autotransformer means for compensating for parasitic leakage inductance of said autotransformer means.