Digital phase reversal detector

Details Digital phase reversal detector Digital phase reversal detector

1998-06-25

2001-07-10

Chin, Stephen (Department: 2734)

Pulse or digital communications

Receivers

Automatic frequency control

C379S413020, C370S286000, C370S289000

Reexamination Certificate

active

06259750

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of telecommunications, and more particularly to a method of detecting a valid phase reversal.
BACKGROUND OF THE INVENTION
There are certain situations in telecommunications where a phase reversal is used to signal a particular condition. For example, in ITU (International Telecommunications Union) standard G.165 entitled “General Characteristics of International Telephone Connections and International Telephone Circuits”, a 2100 Hz tone signal that has a phase reversal every 450±25 ms is used to disable an echo canceller. A valid phase reversal is defined as a phase variation in the range of 180°±25° of a 2100 Hz (±21 Hz) tone. An invalid phase reversal is defined as a phase variation in the range of 0°±110°.
Some means must be provided for detecting valid phase reversals and rejecting invalid phase reversals. The detector employed must operate perfectly on signals having a level of −31 dBmø to 06 dBmø in conditions of white noise less than or equal to 11 dB below the level of the 2100 Hz tone signal. For white noise levels between 11 dB and 5 dB below the level of the tone signal, the percentage of correct operation should fall by no more than 1% for each dB.
The tone disabler is required to operate (disable the echo canceller) within one second of the receipt of the disabling signal.
While the G.165 standard specifies additional requirements, reliable phase reversal detection is essential for the proper functioning of the echo canceller tone disabling detector.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a reliable phase reversal detector.
According to the present invention there is provided a phase reversal detector comprising means for receiving an input signal subject to occasional phase reversals, means for generating signals representing the quadrature components thereof, and means responsive to a migration in the quadrature plane of the position of said quadrature components by an amount greater than a predetermined threshold to generate a proportional signal indicative of a valid phase reversal.
The detector preferably includes means for adjusting the frequency of a local signal used in deriving the above-mentioned quadrature components.
The detector may also include means for bandpassing the incoming signal, which may be hard limited to eliminate the need for an automatic gain control circuit while ensuring the functionality of the detector over a large dynamic range of the input signal. The hard limiting operation is equivalent to applying the sign function to the band passed signal.
The invention also provides a method of detecting valid phase reversals in an input signal subject to occasional phase reversals, comprising the steps of generating signals representing the quadrature components thereof, determining the distance between said signals at different times in the quadrature plane, and generating a signal indicative of a valid phase reversal when said distance, or a value dependent thereon, exceeds a predetermined threshold value.


REFERENCES:
patent: 5528632 (1996-06-01), Lin
patent: 5815568 (1998-09-01), Trump

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