Intel Pentium 4 Processor in the 423-pin package EMI Guideline

Intel® Pentium® 4 Processor in the 423-pin package EMI Guideline
5
®
Figure 1. Spread Spectrum Modulation Profile
∆
SCC
ssc.vsd
non-SCC
(1-
δ
)f
nom
f
nom
Figure 2
.
Impact of Spread Spectrum Clocking on Radiated Emissions
Radiated emissions are typically confined in a narrow band centered around clock frequency harmonics. By uniformly
distributing the radiation over a band of a few MHz, regulatory measurement levels will be reduced.
2.2 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, Figure 3. Lower H-fields will result in
reduced EMI radiation.
(
1-
δ
)
f
nom
f
nom
1/f
m
time
System clock
as a result of
SSC