CS2000-OTP
10
DS758F3
Figure 6. 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
). 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.
Figure 7. Fractional-N Source Selection Overview
N
Digital Filter
Frequency
Comparator for
Frac-N Generation
Frequency Reference
Clock
Delta-Sigma Fractional-N Frequency Synthesizer
Digital PLL and Fractional-N Logic
Output to Input Ratio for Hybrid mode
Fractional-N
Divider
Timing Reference
Clock
PLL Output
Voltage Controlled
Oscillator
Internal
Loop Filter
Phase
Comparator
Delta-Sigma
Modulator
Frequency Reference Clock
Output to Input ratio for Hybrid Mode
Timing Reference Clock
PLL Output
Fractional-N
Frequency Synthesizer
Digital PLL & Fractional-N Logic
Output to Input Ratio for Synthesizer Mode
N
