Fractional-N PLLs, particularly delta-sigma fractional-N PLLs, are some of the most popular integrated frequency synthesizers. They offer very fine grain frequency synthesis and find use in a wide range of applications such as LO synthesis, phase/frequency modulation, spread-spectrum clocking, frequency hopping, etc. While very useful, fractional-N PLLs present interesting technical challenges in the form of excess phase noise and fractional spurs. However, with the help of clever quantization noise shaping techniques and careful circuit design, phase noise and spurious tone performance close to integer -N PLLs can be achieved without sacrificing their attendant benefits.

Chapter Contents:

• 2.1 Basics
• 2.1.1 Fractional-N operation
• 2.1.2 Excess phase noise
• 2.1.3 Delta-sigma fractional-N PLLs
• 2.1.3.1 Model of excess phase in a delta-sigma fractional-N PLL
• 2.1.4 Applications
• 2.1.5 Challenges
• 2.2 Digital delta-sigma modulator design
• 2.2.1 Modulator order
• 2.2.2 DDSM architecture
• 2.2.3 Dithering
• 2.3 Circuit design issues
• 2.3.1 PFD and CP
• 2.3.1.1 Increased charge pump noise contribution
• 2.3.1.2 PFD/CP non-linearity
• 2.3.2 Multi-modulus frequency dividers
• 2.4 Wideband fractional-PLLs
• 2.4.1 Phase noise cancellation
• 2.4.1.1 Handling high dynamic range of cancellation path
• 2.4.1.2 Matching the main and cancellation paths
• 2.4.2 Multi-phase fractional-N PLLs
• 2.5 Conclusion
• References

Inspec keywords: quantisation (signal); delta-sigma modulation; network synthesis; phase locked loops; phase noise; frequency synthesizers

Other keywords: phase-frequency modulation; fractional-N frequency synthesis; delta-sigma fractional-N PLLs; spread-spectrum clocking; fractional-N PLLs; frequency hopping; fractional spurs; spurious tone performance; integrated frequency synthesizers; clever quantization noise shaping techniques; circuit design; grain frequency synthesis; LO synthesis; phase noise

Subjects: General circuit analysis and synthesis methods; A/D and D/A convertors; Signal generators; Modulators, demodulators, discriminators and mixers