CS2000-OTP
Delta-Sigma Fractional-N Frequency Synthesizer
Timing Reference
Clock
Phase
Comparator
Internal
Loop Filter
Voltage Controlled
Oscillator
PLL Output
Fractional-N
Divider
Delta-Sigma
Modulator
N
Digital PLL and Fractional-N Logic
Digital Filter
Frequency
Comparator for
Frac-N Generation
Frequency Reference
Clock
Output to Input Ratio for Hybrid mode
Figure 3. Hybrid Analog-Digital PLL
4.2.1
Fractional-N Source Selection for the Frequency Synthesizer
The fractional-N value for the frequency synthesizer can be sourced from either a static ratio or a dynamic
ratio generated from the digital PLL (see Figure 4). This allows for the selection between operating in the
static ratio based Frequency Synthesizer Mode as a simple frequency synthesizer (for frequency gener-
ation from the Timing Reference Clock) and in the dynamic ratio based Hybrid PLL Mode (for jitter reduc-
tion and clock multiplication). Selection between these two modes can either be made automatically
based on the presence of the Frequency Reference Clock or manually through the mode select pins.
Fractional-N
Frequency Synthesizer
Timing Reference Clock
PLL Output
N
Output to Input Ratio for Synthesizer Mode
Frequency Reference Clock
Digital PLL & Fractional-N Logic
Output to Input ratio for Hybrid Mode
Figure 4. Fractional-N Source Selection Overview
DS758PP1
9
CIRRUS [ CIRRUS LOGIC ]
