QAM phase jitter and frequency offset correction system
AssigneeRixon, Inc.
InventorSteven Alan Tretter
Life of this patent
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Abstract
A system is provided for correcting phase jitter and frequency offsets in QAM modems. The system comprises (i) a narrow bandwidth, decision directed, phase-locked loop for tracking the average phase and frequency offsets followed by (ii) a phase jitter corrector located outside of the loop. The phase error signal for the jitter corrector is produced by passing the phase error signal of the phase-locked loop through a low pass filter designed to pass the expected jitter frequencies. Inphase and quadrature signals from the loop are delayed to compensate for the delay of the low pass filter, and the delayed signals, together with the filter output, are applied to a phase shifter which provides phase jitter correction using a rotational transformation.
Claims4
A QAM receiver system for correcting phase jitter and frequency offsets in a transmitted QAM signal, said system comprising a decision directed phase-locked loop for receiving the transmitted signal and for generating a phase error signal based on estimates of the transmitted symbols, and phase jitter correction means, located outside of said phase-locked loop and including a low pass filter for passing the expected jitter frequencies, for receiving the inphase and quadrature components of the transmitted signal and said phase error signal, and for correcting said inphase and quadrature components for phase jitter.
Dependencies are inferred from the claim text.