Altera Corporation 1–29
July 2005 Stratix Device Handbook, Volume 2
General-Purpose PLLs in Stratix & Stratix GX Devices
Figure 1–16. VCO Frequency Modulation Waveforms
Software Support
You can enter the desired down-spread percentage and modulation
frequency in the MegaWizard Plug-In Manager through the Quartus II
software. Alternatively, the MegaWizard Plug-In Manager can set the
downspread parameter in the altpll megafunction to the desired
down-spread percentage. Timing analysis ensures the design operates at
the maximum spread frequency and meets all timing requirements.
f For more information on PLL software support in the Quartus II
software, see the altpll Megafunction User Guide.
Guidelines
If the design cascades PLLs, the source, or upstream PLL should have a
low bandwidth setting, while the destination, or downstream PLL should
have a high bandwidth setting. The upstream PLL must have a low
bandwidth setting because a PLL does not generate jitter higher than its
bandwidth. The downstream PLL must have a high bandwidth setting to
track the jitter. The design must use the spread-spectrum feature in a low-
bandwidth PLL and, therefore, the Quartus II software automatically sets
the spread-spectrum PLL’s bandwidth to low.
1 Designs cannot use spread-spectrum PLLs with the
programmable bandwidth feature.
Stratix and Stratix GX devices can accept a spread-spectrum input with
typical modulation frequencies. However, the device cannot
automatically detect that the input is a spread-spectrum signal. Instead,
the input signal looks like deterministic jitter at the input of the
downstream PLL.
Spread spectrum should only have a minor effect on period jitter, but
period jitter increases. Period jitter is the deviation of a clock’s cycle time
from its previous cycle position. Period jitter measures the variation of a
clock’s output transition from its ideal position over consecutive edges.
count2 values
count1 values
VCO Frequency
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