
Altera Corporation
1–27
July 2005
Stratix Device Handbook, Volume 2
General-Purpose PLLs in Stratix & Stratix 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.
Description
Stratix and Stratix GX device enhanced PLLs feature spread-spectrum
technology to reduce the EMI emitted from the device. The enhanced PLL
provides up to a 0.5% down spread (–0.5%) using a triangular, also
known as linear, modulation profile. The modulation frequency is
programmable and ranges from approximately 30 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 2 to 5 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 1–15) sets the
modulation frequency.
Figure 1–15 shows how spread-spectrum
technology is implemented in the Stratix device enhanced PLL.