Method and apparatus for filtering asymmetric digital subscriber line (ADSL) signals

• US 6,813,343 B1
• Filed: 05/12/1999
• Issued: 11/02/2004
• Est. Priority Date: 05/12/1999
• Status: Expired due to Term

• Current Assignee: Orckit IP LLC (Orckit-Corrigent )
• Original Assignee: Orckit Communications Ltd. (Orckit-Corrigent )
• Inventors: Vitenberg, Roman
• Primary Examiner(s): Harvey, Minsun Oh
• Assistant Examiner(s): Swerdlow, Daniel
• Application Number:
  US09/310,518
• Time in Patent Office: 2,001 Days
• Field of Search: 379/93.05, 379/93.06, 379/93.07, 379/177
• US Class Current: 379/93.05
• CPC Class Codes: H04L 27/2601 Multicarrier modulation systems
• CPC Class Codes: H04M 11/062 using different frequency bands for speech and other data

Abstract

A method and apparatus for selectively filtering signals on a digital subscriber line (DSL) so as to prevent interference resulting from a telephone instrument switching between its on-hook and off-hook states. One embodiment of the invention utilizes an off-hook detector controlling a variable attenuator element. Another embodiment of the invention uses a switch in parallel with a resistive element to implement the variable attenuator element. Yet another embodiment has an inductive element with a saturable core for providing attenuation to compensate for inconsistencies that result from the changing hookswitch status of a telephone instrument.

Claims

1. A circuit for use with telephone equipment comprising:

an attenuator coupled to a communication line;

a lowpass filter coupled to said attenuator;

an off-hook detector coupled to said attenuator;

said attenuator comprising an impedance element, wherein said impedance element includes an inductive element;

a switch coupled to said impedance element;

said switch controlled by said off-hook detector; and

said switch comprising an optically-coupled MOSFET device.

2. The circuit of claim 1 wherein said inductive element is a saturable inductive element.

3. A circuit for use with telephone equipment comprising;

an attenuator coupled to a communication line;

a lowpass filter coupled to said attenuator;

an off-hook detector coupled to said attenuator;

said attenuator comprising a saturable inductive impedance element;

a switch coupled to said impedance element; and

said switch controlled by said off-hook detector.

4. The circuit of claim 3 wherein said switch is coupled in parallel with said impedance element.

5. The circuit of claim 3 wherein said off-hook detector comprises a diode bridge.

6. A method of filtering low-frequency inter-modulation distortion telephone signals for use in a splitterless ADSL system comprising a plurality of telephone instruments which are each coupled via respective lowpass filters to a central office by a digital subscriber line (DSL), wherein each lowpass filter assembly includes an off-hook detector, a lowpass filter coupled in series to said off-hook detector, and a variable attenuator coupled in series to said lowpass filter, said method comprising the following steps:

determining a hookswitch condition of at least one telephone instrument; and

altering an attenuation characteristic of the variable attenuator of the respective lowpass filter of said at least one telephone instrument; and

said altering step being performed based on said hookswitch condition and comprises the step of switching a switch which is in parallel with an impedance element of the variable attenuator;

wherein the impedance element is connected in series with the telephone instrument, and said impedance element is a saturable inductive element.