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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.