Intel Xeon Processor Multiprocessor Platform Design Guide

57
Mechanical and EMI Design Considerations
7.2.7 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 7-6). Lower H-fields will result in reduced EMI radiation.
Figure 7-5. Impact of Spread Spectrum Clocking on Radiated Emissions
∆
SCC
ssc.vsd
non-SCC
(1- δ )f
nom
f
nom
Figure 7-6. Cancellation of H-fields through Inverse Currents
CLK - clock trace
CLK' - clock bar
trace
H-field
caused by
Iclk
Iclk
Iclk'
H-field
caused by
Iclk'