QAM Demodulator automatic quadrature loop using n-m LSB's

Claim 1

A system for the demodulation of a Quadrature Amplitude Modulation (QAM) signal, of the type having in-phase and quadrature channels, each having an m bit demodulated output, thereby defining a 2 m ×2 m QAM vector space, said system comprising: detector means in each of said channels, each detector means responsive to said QAM signal and a signal at a carrier frequency input, and each for generating an analog output; an analog-to-digital converter, responsive to said analog signal in a first one of said channels, for providing said m bits and a bit group including at least the most significant one of the bits less significant than said m bits in the digital representation of the analog signal; a control loop including said detector means in said first channel and said analog-to-digital converter, and further including means for adjusting, in response to a control signal, the phase of a local oscillator signal which is nominally phase aligned with said QAM vector space, and for applying the phase adjusted signal to the carrier frequency input of said first channel detector means, and means for deriving from said bit group a measure of the phase difference between the phase adjusted oscillator output applied to said first channel and said demodulated output vector space, and for providing a control signal representative of said measure to the means for phase adjusting, whereby the means for phase adjusting is controlled to provide the local oscillator output to the detector in said first channel in controlled phase alignment with respect to said QAM vector space, for properly demodulating said QAM signal.

Claim 2

A system for the demodulation of a Quadrature Amplitude Modulation (QAM) signal, of the type having in-phase and quadrature channels, each having an m bit demodulated output, thereby defining a 2 m ×2 m QAM vector space, said system comprising: detector means in each of said channels, each detector means responsive to said QAM signal and a signal at a carrier frequency input, and each for generating an analog output; an analog-to-digital converter, responsive to said analog signal in a first one of said channels, for providing said m bits and a bit group including at least the most significant one of the bits less significant than said m bits in the digital representation of the analog signal; a control loop including said detector means in said first channel and said analog-to-digital converter, and further including means for adjusting, in response to a control signal, the phase of a local oscillator signal which is nominally phase aligned with said QAM vector space, and for applying the phase adjusted signal to the carrier frequency input of said first channel detector means, and means for deriving from said bit group a measure of the phase difference between the phase adjusted oscillator output applied to said first channel and said demodulated output vector space, and for providing a control signal representative of said measure to the means for phase adjusting, whereby the means for phase adjusting is controlled to provide the local oscillator output to the detector in said first channel in controlled phase alignment with respect to said QAM vector space, for properly demodulating said QAM signal.

Claim 12

A system for the demodulation of a Quadrature Amplitude Modulation (QAM) signal, of the type having in-phase and quadrature channels, each having an m bit demodulated output, thereby defining a 2 m ×2 m QAM vector space, said system comprising: detector means in each of said channels, each detector means responsive to said QAM signal and a signal at a carrier frequency input, and each for generating an analog output; an analog-to-digital converter, responsive to said analog signal in a first one of said channels, for providing said m bits and a bit group including at least the most significant one of the bits less significant than said m bits in the digital representation of the analog signal; a control loop including said detector means in said first channel and said analog-to-digital converter, and further including means for adjusting, in response to a control signal, the phase of a local oscillator signal which is approximately phase aligned with said QAM vector space, and for applying the phase adjusted signal to the carrier frequency input of said first channel detector means, and means for deriving from said bit group a measure of the phase difference between the phase adjusted oscillator output applied to said first channel and said demodulated output vector space, and for providing a control signal representative of said measure to the means for phase adjusting, including means, responsive to the value of said bit group, for providing said bit group value with the sign thereof controlled by the most significant bit of said m bits in the second one of said channels, a digital-to-analog converter responsive to the bit group value with the sign thereof controlled, means for filtering the output of the digital-to-analog converter to provide said control signal, and means for referencing the control signal for said oscillator with respect to a midpoint value of said bit group with the sign thereof controlled, whereby the means for phase adjusting is controlled to provide the local oscillator output to the detector in said first channel in controlled phase alignment with respect to said QAM vector space, for properly demodulating said QAM signal.

Claim 13

A system for the demodulation of a Quadrature Amplitude Modulation (QAM) signal, of the type having in-phase and quadrature channels, each having an m bit demodulated output, thereby defining a 2 m ×2 m QAM vector space, said system comprising: detector means in each of said channels, each detector means responsive to said QAM signal and a signal at a carrier frequency input, and each for generating an analog output; an analog-to-digital converter, responsive to said analog signal in a first one of said channels, for providing said m bits and a bit group including at least the most significant one of the bits less significant than said m bits in the digital representation of the analog signal; a control loop including said detector means in said first channel and said analog-to-digital converter, and further including means for adjusting, in response to a control signal, the phase of a local oscillator signal which is approximately phase aligned with said QAM vector space, and for applying the phase adjusted signal to the carrier frequency input of said first channel detector means, and means for deriving from said bit group a measure of the phase difference between the phase adjusted oscillator output applied to said first channel and said demodulated output vector space, and for providing a control signal representative of said measure to the means for phase adjusting, including means, responsive to the value of said bit group, for providing said bit group value with the sign thereof controlled by the most significant bit of said m bits in the second one of said channels, a digital-to-analog converter responsive to the bit group value with the sign thereof controlled, means for filtering the output of the digital-to-analog converter to provide said control signal, and means for referencing the control signal for said oscillator with respect to a midpoint value of said bit group with the sign thereof controlled, whereby the means for phase adjusting is controlled to provide the local oscillator output to the detector in said first channel in controlled phase alignment with respect to said QAM vector space, for properly demodulating said QAM signal.