DPECL White Paper
Design Subtleties
Besides clock generation and distribution component considerations, there are other design subtleties to consider.
Ground plane. The most important statement that may not be intuitively obvious to the PCB or system designer when moving up to higher frequencies is the absolute requirement for a ground plane.
The ground plane is an integral part of a transmission line and cannot be omitted. It serves as the only absolute for all levels, including threshold and noise margins (excluding the power supply). It is required for shielding between critical sub-circuits and to the outside world.
The term ground plane may well refer to an AC ground, such as that represented by the +5V supply bus. The +5V plane may be the only solid plane on the PCB. That is acceptable as long as there is one which can serve the purpose described.
Circulating currents are generated even if fully differential signals are used. The loops swept by these circulating (output) currents must be minimized and voltage drops due to such currents must be taken care of by decoupling capacitors; usually one 0.1microfarads per IC, placed in close proximity.
Balance and symmetry. Balance and symmetry should be maintained for the clocking net and other time-sensitive data nets. Differential lines must be of equal length, contain equal loads and have similar surroundings.
Emissions and crosstalk. Emissions and crosstalk can be attributed to two major causes: voltage and current transients.
Voltage transitions, which are not locally compensated by an equal transition in the reverse direction, will capacitively couple a noise current of the magnitude C x dV/dt.
Current transients, which are not locally compensated by an equal transition in the reverse direction, will inductively couple a noise voltage of the magnitude L x dI/dt.
Simultaneous switching of address buses, for instance, can generate sufficient crosstalk into ill designed clocking systems to cause runt clock pulses which are extremely difficult to analyze, locate and eliminate.
View the methodology document in PDF format