JITTER IN THE SQUARE SYNCHRONIZER ON DIRECT-DETECTION OPTICAL COMMUNICATIONS .3. NUMERICAL RESULTS CONCERNING WHITE CIRCUIT NOISE

Theoretical assessment of the square synchroniser performance, develop ed in a previous paper, is used to present and discuss numerical resul ts in the presence of white circuit noise. This fully analytical metho d permits the optimal design (i.e. minimising timing jitter) of square synchronisers, taking into account all the relevant parameters, for d irect detection optical communication systems. A thorough physical exp lanation of the presented results and their relevance for practical re ceiver design is provided. The work is increasingly important nowadays because multigigabit per second transmission rates are being used on direct detection optical systems. It is shown that, for low signal-to- noise ratios and under quite general conditions, the performance of th e square synchroniser is better in a signal shot-noise-dominant situat ion (typical of long-distance communications with APD receivers), foll owed by the white-circuit-noise-dominant situation (typical of long-di stance communications with PIN receivers), and is worst for the dark c urrent-noise-dominant situation (typical of germanium APD receivers). The noise contribution to square synchroniser performance can be very high. The timing jitter properties respecting the circuit noise domina nt situation usually hold also on the signal shot-noise dominant situa tion, except for a very narrow (compared with the bit period) optical pulse shape incident on the photodetector.