Spread-Spectrum Clocking
Figure 1–33. Stratix II and Stratix II GX Spread-Spectrum Circuit Block Diagram
÷ n
refclk
Up
PFD
Down
Spread-
Spectrum
Counter
÷ m
n count1
n count2
m count1
m count2
Figure 1–34 shows a VCO frequency waveform when toggling between
different counter values. Since the enhanced PLL switches between two
different m and n values, the result is a straight line between two
frequencies, which gives a linear modulation. The magnitude of
modulation is determined by the ratio of two m/n sets. The percent
spread is determined by:
percent spread = (fVCOmax - fVCOmin)/fVCOmax = 1 [(m2 × n1)/(m1 × n2)].
The maximum and minimum VCO frequency is defined as:
fVCOmax = (m1/n1) × fREF
fVCOmin = (m2/n2) × fREF
Figure 1–34. VCO Frequency Modulation Waveform
count2 values
count1 values
VCO Frequency
1–54
Altera Corporation
July 2009
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
