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Fractional- frequency synthesizers have long been known to suffer from a set of spurious components, often referred to as fractional spurs, which are usually attributed to the operation of the modulator. This paper proposes a new phenomenon-based on cross-coupling and sampling of the nonharmonically related signals present in such synthesizers-which is capable of producing a family of spurious components of identical form to fractional spurs, with support from a range of analytic, simulated and measured results. A unique experimental arrangement is described, allowing controlled cross-coupling of signals within a synthesizer, which provides convincing experimental validation of this mechanism. Finally, techniques are suggested to allow at least partial mitigation of the effect.