Intermodulation-borne fractional-N frequency synthesiser spurious components

Fractional-N frequency synthesisers have long been known to suffer from a set of spurious components, often referred to as ´fractional spurs´. The authors propose an intermodulation mechanism that is responsible for the production of these components and present numerical and analytic techniques to predict their distribution. These predictions are validated against a range of simulated and measured results to determine the role of intermodulation in fractional-N frequency synthesisers. Finally, two techniques are suggested to allow at least partial mitigation of this effect.