An Alternative – Fractional Synthesis
Fractional N synthesis has several advantages
- Lower feedback-divider ratio results in lower phase noise with finer step size
- Lowpass-noise contributors are lowered by 20×log(L), where L is the fractional modulus
- Channel spacing is 1/L times smaller than an integer N loop
Fractional synthesis induces phase modulation, causing increased output spurious
- Spur cancellation schemes reduce output spurious, but are limited by the cancellation-circuitry linearity
- Phase detectors and DAC cancellation circuits need high linearity and fast settling time despite small cancellation currents
- Additional filtering can be achieved from the low-pass loop transfer function and from discrete filtering