Intel Xeon Processor and Intel E7500/E7501Chipset Compatible Platform Design Guide

High-Speed Design Concerns

210 Intel

Xeon™ Processor and Intel

E7500/E7501 Chipset Compatible Platform Design Guide

12.4.4 Differential Clocking

Differential clocking requires that the clock generator supply both clock and clock-bar traces.

Clock-bar has equal and opposite current as the primary clock, and is also 180 degrees out of

phase. To maximize the benefit of differential clocking, both clock lines must be routed parallel to

each other for their entire length. Devices connected to the clock must also be designed to accept

both the clock and clock-bar signals.

EMI reduction due to differential clocking is caused by H-field cancellation. Since H-field

orientation is generated by and is dependent upon current flow, two equal currents flowing in

opposite directions and 180 degrees out of phase will have their H-fields cancelled

(see Figure 12-6). Lower H-fields will result in reduced EMI radiation.

Figure 12-5. Impact of Spread Spectrum Clocking on Radiated Emissions

∆

SCC

non-SSC

(1-δ)f

nom

Figure 12-6. Cancellation of H-fields through Inverse Currents

CLK - clock trace

CLK' - clock bar

trace

H-field

caused by

Iclk

Iclk'

H-field

caused by

Iclk'