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

Intel® Pentium® 4 Processor in the 423-pin package EMI Guideline
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1.4 EMI Regulations and Certifications
Personal Computer Original Equipment Manufacturers (PC OEMs) ensure EMC compliance by
meeting EMI regulatory requirements. PC designers must ensure that their computer systems do not
exceed the emission limit standards set by applicable regulatory agencies. Regulatory requirements
referenced in this document include:
United States Federal Communication Commission (FCC) Part 15 Class B
International Electrotechnical Commission’s International Special Committee on Radio
Interference (CISPR) Publication 22 class B limits.
The FCC rules are viewed to require any PC OEM who sells an “off-the-shelf” motherboard in the
United States to pass an open chassis test. Open chassis testing is defined as removing the chassis
cover (or top and 2 sides) and testing for EMI compliance (although permitted emission levels are
allowed to be higher). Removing the cover greatly reduces the shielding provided by the chassis and
increases the amount of EMI radiation. The purpose of this regulation is to ensure that system boards
have reasonable emission levels since they are one of the main contributors to EMI.
2.0 EMI Design Considerations
The following sections discuss design techniques that may be applied to minimize EMI emissions.
Some ideas have been incorporated into Intel enabled designs (differential clock drivers, selective
clock gating, etc) and some must be implemented by motherboard designers (trace routing, clocking
schemes, etc).
2.1 Spread Spectrum Clocking (SSC)
Spread Spectrum Clocking is defined as continuously ramping (or modulating) the processor clock
frequency over a predefined range, Figure 1. SSC reduces radiated emissions by spreading the radiated
energy over a wider frequency band, Figure 2. Thus, instead of maintaining a constant system
frequency, SSC modulates the clock frequency along a predetermined path (or modulating profile,
Figure 1) with a predetermined modulation frequency. The modulation frequency is usually selected to
be larger than 30 kHz (above the audio band) while small enough not to upset the PC system’s timings
(less than 0.8% of the clock frequency). SSC has been demonstrated to effectively reduce peak
radiation levels, making EMC compliance easier to achieve.
To conserve the minimum period requirement for bus timing, the SSC clock is modulated between f
nom
and (1-δ) f
nom
where f
nom
is the nominal frequency for a constant frequency clock. δ specifies the total
amount of spreading as a relative percentage of f
nom
. The modulation percentage is always a function
of 1-δ and not 1+δ, as increasing the clock frequency above the rated speed of the processor will cause
unpredictable operation.