Nonlinearity-Induced Spurs in Fractional-N Frequency Synthesizers

Fractional- $N$ frequency synthesizers are characterized by unwanted periodic components in their frequency spectra called spurs. In communications applications, spurs reduce the signal to noise ratio of the system; in clocking they add jitter; in radar and imaging, they can present ghost targets. Some spurs are due to parasitic electromagnetic coupling between components and can be mitigated by careful circuit layout. Others are inherent in the mathematics that describe the system and are best tackled using mathematical approaches. This brief considers spurs that are introduced directly by the divider controller and those that result from distortion of the control signal by nonlinearity in the synthesizer. It summarizes the state of the art in terms of divider controllers that are themselves spur-free and divider controllers that do not produce spurs when used in synthesizers with memoryless polynomial nonlinearities.