Insights on Indirect Synthesis
PLL acts as a tracking lowpass filter to the reference
Lowpass noise sources (reference, phase detector, and loop-filter op amp) dominate phase noise below loop natural freq.
- Lower feedback-divider ratios directly decrease these contributions
- Lower feedback-divider ratios increase loop step size (channel spacing)
- Increased phase detector gain decreases op amp and phase detector noise
Above loop natural frequency, VCO noise dominates
- High-pass transfer function provides no noise suppression
Near the loop natural frequency, multiple contributors and loop peaking affect the phase noise