Pulse Width Distortion

Electrical Source

Unbalanced drive currents in the pull-up and pull-down networks of a digital driver cause unequal transition times. When the rising edge of a pulse is faster than the falling edge, the pulse duration increases as it passes through each successive gate. This effect is compounded by threshold voltage mismatches in the receiver, which shifts the decision level relative to the switching point of the signal.

In optical systems, chromatic dispersion causes different wavelengths to travel at different speeds, which further distorts the pulse shape. These electrical and optical non-idealities accumulate over long transmission paths, which reduces the timing budget of the link.

Calibration Test

Measurement of this temporal error requires high-bandwidth oscilloscopes to compare the incoming pulse width with the known reference. Test patterns containing alternating sequences of ones and zeros are used to isolate the duty cycle variations. This analysis allows engineers to quantify the pulse width distortion under different operating conditions.

Temperature and supply voltage fluctuations can worsen the transition asymmetry, which necessitates active compensation in the transmitter.

Timing Constraint

High levels of distortion close the timing window of the receiver, which increases the likelihood of sampling errors. This limitation forces system designers to reduce the clock frequency or to implement active pulse width correction circuits. These corrections restore the symmetrical duty cycle and ensure the reliability of the high-speed data link.