CA2306529A1

CA2306529A1 - Cable simulator

Info

Publication number: CA2306529A1
Application number: CA 2306529
Authority: CA
Inventors: William Louise
Original assignee: Eagle Comtronics Inc
Current assignee: Eagle Comtronics Inc
Priority date: 1999-04-26
Filing date: 2000-04-25
Publication date: 2000-10-26

Abstract

A cable simulator includes an inductor, a capacitor, and two resistors connected to provide low attenuation on low frequency (50 MHZ and below) signals and increasing attenuation with increasing frequency. The simulator provides a broadband impedance match.

Description

CABLE SIMULATOR
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from U.S. Provisional Application Serial No.
60/130,951 entitled CABLE SIMULATOR filed on April 26, 1999, incorporated S herein by reference.
FIELD OF THE INVENTION
The invention pertains to the field of broadband impedance matching, and in particular, to the field of cable simulators for simulating a specified length of cable.
BACKGROUND OF THE INVENTION
In cable television systems that are running 2-way transmissions, there is a loss differential which depends on the direction of the signal. The cable plant is designed around the forward properties (50 MHz-750 MHZ) of the distribution equipment, i.e., amplifiers, taps, and cable so that the drop level remains a constant about a given design level.
1 S Reverse path signals experience lower signal losses than forward signals due to the low frequencies used (5 MHz-40 MHZ). Since the system is designed for the forward signal, there is more amplification at the higher frequencies which is overcome by the distribution equipment losses. In those cases where distribution equipment is close to an amplifier, cable system operators have found that they need to attenuate the higher frequencies and not the lower frequencies. Only those modem connections closest to an amplifier or line extender have this need. The operators have found that using 100 feet of coaxial cable right before the modem effectively attenuates the forward path without significantly impacting the reverse path.
There are problems with simply adding 100 feet of coaxial cable to an installation, such as the time involved and the need to hide 100 feet of coiled cable at a user's location.
Accordingly, an alternate solution is required.

SUMMARY OF THE INVENTION
Briefly stated, a cable simulator includes an inductor, a capacitor, and two resistors connected to provide low attenuation on low frequency (50 MHZ and below) signals and increasing attenuation with increasing frequency. The simulator provides a broadband impedance match.
According to an embodiment of the invention, a cable simulator circuit includes an input; an output; an inductor; first and second resistors; said inductor and said first and second resistors being connected in parallel between said input and said output; and a capacitor having one end connected between said first and second resistors and another end connected to ground.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a schematic diagram of a cable simulator according to the present invention.
Fig. 2 shows a housing for the cable simulator of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Fig. 1, a cable simulator 10 includes an input 12 and an output 14.
An inductor L1 is in parallel with two equal resistors R1. One end of a capacitor C1 is connected at 16 between the two resistors Rl, and the other end is connected to ground. To simulate 100 feet of RG-6A/U cable, the following values are used:
L 1 = 22 nH
C 1 = 2.2 pF
Rl = 75 S2 These values can be varied to simulate other lengths and types of cable. To obtain the component values, a simulation program such as Genesys, Touchstone, or Microwave Office is used. The program is used to optimize the cable simulator response curve to match approximately the values for the desired cable attenuation.
Cable simulator 10 has low attenuation on low frequency signals, i.e., 50 MHz and below, with increasing attenuation with increasing frequency. The difference between simulator 10 and a low pass filter is that simulator 10 provides a broadband impedance match whereas the low pass filter is reflective in its stopband.
Inductor L1 allows low frequencies to pass through the device with little loss while impeding some of the higher frequencies. Resistors Rl match the circuit to the system impedance for good return loss. Capacitor C1 shunts off some of the high frequencies to ground. The capacitor to inductor ratio determines the slope of the cable simulator.
Referring to Fig. 2, cable simulator 10 fits inside a housing 20. Housing 20 is approximately 1.6 inches long and 0.8 inches in diameter. Cable compatible connectors 22, 24 on each end of housing 20 facilitate connecting cable simulator in line with a cable. Cable simulator 10 is preferably of discrete components on a printed circuit board. A ground cable connects the circuit to the outside of housing 20. Housing 20 is preferably a standard filter tube housing with connectors 22; 24 built into the housing.
While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.

Claims

1. A cable simulator circuit, comprising:
an input;
an output;
an inductor;
first and second resistors;
said inductor and said first and second resistors being connected in parallel between said input and said output; and a capacitor having one end connected between said first and second resistors and another end connected to ground.

2. A cable simulator according to claim 1, wherein said first and second resistors are of equal resistance.

3. A cable simulator according to claim 1, wherein a capacitance of said capacitor equals 2.2 pF, an inductance of said inductor equals 22 nH, and said first and second resistors each have a resistance equal to 75 ohms.

4. A cable simulator according to claim 2, wherein a capacitance of said capacitor equals 2.2 pF, an inductance of said inductor equals 22 nH, and said first and second resistors each have a resistance equal to 75 ohms.

5. A cable simulator according to claim 1, wherein said cable simulator circuit is encased in a housing wherein said housing includes a first cable connector connected to said input and a second cable connector connected to said output.

6. A cable simulator according to claim 2, wherein said cable simulator circuit is encased in a housing wherein said housing includes a first cable connector connected to said input and a second cable connector connected to said output.