The Curious Case of Seed-Related Fractional Spurs

Fractional-N frequency synthesizers are notorious for producing spurious tones (spurs) whose frequencies depend explicitly on the fractional part of the frequency control word. This paper addresses spurs that appear in the spectrum when that fraction is zero. In principle, this setting should not produce any spurs other than the reference spur and its harmonics. In practice, if the divider controller is clocked, it introduces a set of spurs whose locations are determined explicitly by its initial state (usually called the "seed"). Experimental observations are consistent with theoretical analysis and simulations.