Combined adaptive equalization and demodulation circuit

Claim 1

An adaptive equalizer arrangement for a digital transmission system, characterized in that it comprises: (1) a first in-phase path provided at the output of the transmission channel of the system and whose structure is that of a transversal filter having n branches and (n-1) delay circuits between the n inputs of these branches, each of these n branches comprising, arranged in series: (a) a mixer; (b) a lowpass filter; (c) a multiplier; the outputs of these n branches being connected to means for deciding the symbols a k to be transmitted from the output of this in-phase path and of the arrangement; (2) arranged in parallel with this first path, a second quadrature path which also has the structure of a transversal filter having n branches and (n-1) delay circuits between the n inputs of these branches, each of these n branches comprising, arranged in series: (d) a mixer; (e) a lowpass filter; (f) a multiplier; the outputs of these n branches being connected to means for deciding the symbols b k to be transmitted from the output of this quadrature path and of the arrangement; the multipliers of these first and second paths being placed in each branch either at the head, before the corresponding mixers, or at the output of the lowpass filters; (3) a third control path comprising: (g) two subtracting circuits for determining the differences e' k and e" k between the signals before decision, X k and Y k , and after decision, a k and b k , expressed for the in-phase path and for the quadrature path by the respective expressions: e'.sub.k =X.sub.k -a.sub.k (where X.sub.k =X(t).sub.t=kt+t.sbsb.o) and e".sub.k =Y.sub.i -b.sub.k (where Y.sub.k =Y(t).sub.t=kT+t.sbsb.o) where X(t) and Y(t) are the output signals of the transversal filter of the in-phase path and the quadrature path, respectively, and t o is the sampling instant; (h) a voltage-controlled oscillator producing a signal of the type sin (ω o t+φ) where ω o corresponds to the frequency of the carrier, which oscillator is controlled by the signal ε.sub.k =e'.sub.k Y.sub.k -e".sub.k X.sub.k (i) at the output of this oscillator, n first parallel-ranged phase shifters whose outputs apply to the respective n second inputs of the mixers in the in-phase path a demodulation signal of the type sin (ω o t+φθ m ) where θ m is the phase shift for the (m+1) st branch and m varies from o to (n-1); (j) also at the output of the voltage-controlled oscillator, a π/2 phase shifter followed by n second parallel-arranged phase shifters whose outputs apply to the n second inputs of the mixers in the quadrature path a demodulation signal of the type cos (ω o t+φ+θ m ); (k) a control circuit for the voltage-controlled oscillator, the 2n phase shifters and the 2n multipliers.