PLL Design Tradeoffs

• Small N-divide ratios reduce in-band phase noise

  • Small N Þ large channel spacing (poor frequency resolution)

• Fractional synthesis allows fine tuning steps with small N

  • Channel spacing is 1/L times smaller than an integer-N loop (Lºfractional modulus)
  • Fractional synthesis Þ loop phase modulation

• Loop phase modulation causes output spurs

  • Cancellation and filtering can reduce spurs

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