PLLs in Stratix II and Stratix II GX Devices
because of the low-voltage swing. The differential LVDS signal also
allows for EMI rejection within the signal. Therefore, this situation may
not require spread-spectrum technology.
1
Spread-spectrum clocking is only supported in Stratix II
enhanced PLLs, not fast PLLs.
Implementation
Stratix II and Stratix II GX device enhanced PLLs feature
spread-spectrum technology to reduce the EMIs emitted from the device.
The enhanced PLL provides approximately 0.5% down spread using a
triangular, also known as linear, modulation profile. The modulation
frequency is programmable and ranges from approximately 30 kHz to
150 kHz. The spread percentage is based on the clock input to the PLL
and the m and n settings. Spread-spectrum technology reduces the peak
energy by four to six dB at the target frequency. However, this number is
dependent on bandwidth and the m and n counter values and can vary
from design to design.
Spread percentage, also known as modulation width, is defined as the
percentage that the design modulates the target frequency. A negative (–)
percentage indicates a down spread, a positive (+) percentage indicates
an up spread, and a ( ) indicates a center spread. Modulation frequency
is the frequency of the spreading signal, or how fast the signal sweeps
from the minimum to the maximum frequency. Down-spread
modulation shifts the target frequency down by half the spread
percentage, centering the modulated waveforms on a new target
frequency.
The m and n counter values are toggled at the same time between two
fixed values. The loop filter then slowly changes the VCO frequency to
provide the spreading effect, which results in a triangular modulation. An
additional spread-spectrum counter (shown in Figure 1–33) sets the
modulation frequency. Figure 1–33 shows how spread-spectrum
technology is implemented in the Stratix II and Stratix II GX device
enhanced PLL.
Altera Corporation
July 2009
1–53
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
