Circuit for recovering the carrier wave in digital transmission systems

Abstract

A circuit for recovering the carrier of a digitally modulated wave having a phase symmetry 2.pi./M, wherein M is the order of the symmetry, includes means for automatic and fast acquisition comprising a voltage-controlled oscillator having an output supplying said carrier and a control input to which an error signal .epsilon.(.phi.) is applied in order to change the oscillator phase and to adjust it to the phase of the digital modulated wave. This wave is introduced in two channels, one being in phase and the other being in quadrature with the carrier, each comprising the series arrangement of: a demodulator, a low-pass filter, reconstruction means for reconstructing a signal and for determining the error between the filtered and reconstructed signals, the quadrature channel comprising in addition a 90.degree. phase shifter, the two channels being joined together with the aid of a phase comparator arrangement which produces a comparison signal and a variable-rate sampling clock. With the representation of the signals from the in-phase and quadrature channels defining the states of a signal constellation of the modulated wave, the phase comparator arrangement comprises means for selecting received signal points with the aid of zones which are centered on or eccentric to certain diagonal states of the signal constellation, and means for generating the sampling clock by eliminating from a recovered symbol clock an active edge capable of validating zero crossings of the comparator signal which do not correspond to a phase difference equal to 2k.pi./M (k and M integers).