CS2000-OTP

DS758PP1

9

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.

Figure 4. 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