Because the discussion applies equally well to both types of systems with only minor obvious variations, for the sake of brevity, the systems will only be described with reference to FIG. 6 in which a VCO 60 is used.) An analog signal z(t) comes from a receiver (not shown), and is multiplied using complex multiplier 61 by cos 2 ??fcr t and sin 2 ??fcr t which are generated by the VCO 60.