ABSTRACT
The incoming signal is filtered in such a way that the overall impulse response—concerning the signal path from the laser/modulator-transmitter filter up to the matching filter output—produces a pulse that satisfies the second Nyquist criterion. A comparator set to the appropriate threshold produces a signal that is then both differentiated and rectified. The output signal contains a bit frequency component with data-independent phase. Clock and amplitude regeneration can be carried out independently: whereas an amplitude decision has to be made within each symbol period, which is subjected to extremely slow fluctuations, and can be regarded as constant throughout thousands of symbol periods. Clamping circuits are only of limited use for baseline regeneration. In an amplitude regeneration circuit a decision is made as to whether the momentary value of the amplitude of the received signal is above or below the reference level, that is the threshold.
