Decision feedback loop for tracking a polyphase modulated carrier

Claim 1

A tracking loop for reconstructing a carrier reference signal, ru(t), from an N-phase modulated carrier, x(t), where N is an integer that is a power of 2 greater than 1, comprised of a voltage controlled oscillator for generating said reference signal, said oscillator having a control input terminal, a 90* phase-shift network connected to receive said reference signal and provide a quadrature phase reference signal, rl(t), means responsive to said modulated carrier and said reference signal for producing an inphase demodulated carrier signal, Epsilon u(t), equal to the product x(t) ru(t), means responsive to said modulated carrier and said quadrature phase reference signal for producing a quadrature demodulated carrier signal, Epsilon l(t), equal to the product x(t) rl(t), means responsive to said inphase and quadrature demodulated carrier signals for producing a phase-estimate signal, theta k, proportional to an estimate of the phase of a transmitted symbol during each symbol period of said N-phase modulated carrier, cosine and sine function generating means responsive to said phase-estimate signal, theta k, for generating cosine and sine signals equal to the functions cos theta k and sin theta k, respectively, first and second means for delaying respective signals Epsilon u(t) and Epsilon l(t) a period equal to the signal transfer delay through said phase estimating means and said cosine function generating means, said period also being equal to the signal transfer delay through said phase estimating means and said sine function generating means, first and second means responsive to said inphase and quadrature demodulated carrier signals Epsilon u(t) and Epsilon l(t) and to said cosine and sine signals for producing first and second feedback signals zu(t) and zl(t), respectively, equal to the products thereof, namely Epsilon u(t) cos theta k and Epsilon l(t) sin theta k, summing means for adding said first and second feedback signal into a phase error signal, Epsilon (t), and a low-pass filter coupling said phase error signal Epsilon (t) to said control input terminal of said voltage-controlled oscillator.