On the Mechanisms Governing Spurious Tone Injection in Fractional PLLs

In fractional phase-locked loop driven by ΣΔ modulators there can be spurious tones in the power spectral density (PSD) of output signals even if the PSDs of the sequences used to drive the frequency divider are spur-free. This is due to undesirable nonlinear effects notably occurring in the charge pump (CP). In this brief, we focus on static and dynamic mismatch of the CP and its interaction with the digital delay usually present in the reset path of the phase-frequency detector. Despite the presence of nonlinearities, we show that spurs may not appear, and mechanisms governing their existence are unveiled. A compact and simple model is presented allowing the onset of spurs and their magnitude to be reliably estimated.