Signal Scrambling; Spectral Comparison Of A Clock Scrambled Vs. Unscrambled - Intel Quark SoC X1000 Design Manual

The frequency modulation in the time-domain results in a frequency-domain energy
redistribution of the constant-frequency clock harmonics. The shape of the spectral
energy distribution of the SSC is determined by the time-domain modulation profile,
while the energy distribution width is determined be the modulation amount (
two combined determine the amount of EMI reduction ( ).
The clock frequency deviation (
the corresponding nominal frequency (+0% / -0.5%). The width of its spectral
distribution is between the -3 dB roll-off points. The ratio of this width to the
fundamental frequency cannot exceed 0.5%. The total power can be estimated by
measuring the power density and integrating over the frequency of interest to obtain
total power. Normally, clock signals are "down-spread", that is, modulated so that all
the sideband power is lower than the original center frequency. This prevents "over-
clocking", or running clocks higher than specified, a condition that might cause a
system to hang.
17.4.8

Signal Scrambling

Most EMI problems in electronic systems can be attributed to repetitive signals. In
high-speed communications these repetitive signals are not limited to the clocks. For
instance, data that is periodic itself can lead to an EMI problem. To minimize EMI risk
due to such events, every character, except for those encoded into special symbols (K
codes), and the characters that are within the training sequences are scrambled. In this
way the repetitive nature of data such as 'H' patterns, as dictated by the FCC for testing
of video interfaces, do not exhibit an increase in EMI over random data. Additional
information on the specific scrambling algorithm used can be found in technology
specifications, e.g, PCI Express base specification.
Figure 71
after getting scrambled. The first harmonics show benefit from scrambling, but higher
order harmonics are equivalent to the scrambled amplitudes. In fact, a scrambled clock
can be worse in terms of impact due to the broadband nature of the signal versus the
discrete nature of the unscrambled clock. This is probably more of a concern for RF
interference in radio channels than for EMI compliance.
Figure 71.

Spectral Comparison of a Clock Scrambled vs. Unscrambled

®
Intel Quark™ SoC X1000
PDG
122

shows a comparison of a 100 MHz 50% duty cycle clock unscrambled and
dbm dbm
Frequency in MHz Frequency in MHz
20 20
0 0
-20 -20
-40 -40
500 500
100 MHz clock, 1Vpp, 100 pS rise&fall-times 100 MHz clock, 1Vpp, 100 pS rise&fall-times
®
Intel Quark™ SoC X1000—Electromagnetic Interference

) is required to be at least 0.5% down-spread from
Note that in the limit ~ 20 harmonics Note that in the limit ~ 20 harmonics
The clock amplitude is = to scrambled The clock amplitude is = to scrambled
3000 3000 5500 5500
 ). The
10 GHz 10 GHz
8000 8000 10000 10000
June 2014
Order Number: 330258-002US