Pulse Shaping
Signal processing at the transmitter modifies the shape of a digital pulse to compensate for the predictable losses of a transmission line. Using feed forward equalization, the system adjusts the amplitude of the current bit based on the values of the bits that preceded it. This action boosts the high frequency components of the signal before they enter the copper trace.
Tap Weight
Pre emphasis and de emphasis are achieved by using a series of delay elements and multipliers known as taps. The weights assigned to these taps determine how much the signal is boosted at each transition. By overshooting the voltage at the start of a bit, the transmitter ensures that the signal reaches the receiver with a clean and recognizable profile.
This compensation is necessary because long PCB traces act as low pass filters that smear the edges of digital pulses.
Channel Loss
Implementation of this technique allows the use of standard laminate materials for higher data rates than would otherwise be possible. It reduces the impact of inter symbol interference by narrowing the transition times between states. The setting of the tap weights is often automated during a training sequence when the link first powers up.
