Spread-Spectrum Clocking
enough to meet EMI compliance. Spread-spectrum technology provides
you with a simple and effective technique for reducing EMI without
additional cost and the trouble of re-designing a board.
Spread-spectrum technology modulates the target frequency over a small
range. For example, if a 100-MHz signal has a 0.5% down-spread
modulation, then the frequency is swept from 99.5 to 100 MHz.
Figure 1–32 gives a graphical representation of the energy present in a
spread-spectrum signal vs. a non-spread spectrum-signal. It is apparent
that instead of concentrating the energy at the target frequency, the
energy is re-distributed across a wider band of frequencies, which
reduces peak energy. Not only is there a reduction in the fundamental
peak EMI components, but there is also a reduction in EMI of the higher
order harmonics. Since some regulations focus on peak EMI emissions,
rather than average EMI emissions, spread-spectrum technology is a
valuable method of EMI reduction.
Figure 1–32. Spread-Spectrum Signal Energy Versus Non-Spread-Spectrum Signal Energy
Spread-Spectrum Signal
Non-Spread-Spectrum Signal
Δ = ~5 dB
Amplitude
(dB)
δ = 0.5%
Frequency
(MHz)
Spread-spectrum technology would benefit a design with high EMI
emissions and/or strict EMI requirements. Device-generated EMI is
dependent on frequency and output voltage swing amplitude and edge
rate. For example, a design using LVDS already has low EMI emissions
1–52
Altera Corporation
July 2009
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
