The trapezoidal clocking

Usually, transmission lines such as coaxial cables, twist pair cables, printed circuit board traces etc. are carefully terminated to eliminate reflections when transmitting important signals such as system clocks. However, if appropriately designed, the reflection signal from the un-terminated cable end carries useful information which can be utilized to improve system performance. In this paper, multi-tap transmission lines with one open end are studied for clock distribution. When trapezoidal clock pulses with sufficiently long rising and falling ramps are fed into such a transmission line, the timing skews at different taps due to propagation delays can be compensated naturally as a result of the superposition of the transmitting and reflecting signals. Higher order effect due to resistive loss of the transmission lines is also discussed and the curve shapes of the raising/falling ramps for reducing higher order skews are derived and tested. Measurements are made on actual cables to validate this clock distribution scheme.