A first device transmits data over a first branch of a communications link toward a second device. That second device loops the received data pattern back over a second branch of the communications link. A bit error rate of the looped back data pattern is determined and a pre-emphasis applied to the transmitted data pattern is adjusted in response thereto. The first device further perturbs the data pattern communications signal so as to increase the bit error rate. The pre-emphasis is adjusted so as to reduce the determined bit error rate in the looped back data pattern in the presence of the perturbation. The steps for perturbing the signal and adjusting the pre-emphasis are iteratively performed, with the perturbation of the signal increasing with each iteration and adjustment of the pre-emphasis being refined with each iteration. The signal is perturbing by injecting modulation jitter into the signal (increasing each iteration) and adjusting amplitude of the signal (decreasing each iteration).