Spurious Tone Mitigation in Fractional-N Phase-Locked Loops

Fractional-N phase-locked loops (PLLs) are widely used to synthesize local oscillator signals for modulation and demodulation in communication systems. Such PLLs generate and subsequently lowpass filter DC-free quantization noise as part of their normal operation. Unfortunately, the quantization noise and its running sum inevitably are subjected to nonlinear distortion from analog circuit imperfections which causes spurious tones in the PLL output signal that can degrade communication system performance. This paper presents the first general mathematical analysis of this phenomenon. It proves that if the running sum of the quantization noise, t[n], satisfies tlow < t[n] ≤ thigh for all n, where tlow and thigh are integers, then subjecting t[n] to kth-order distortion for at least one k {1, 2, 3, ..., thigh − tlow} will result in spurious tones for most fractional-N PLL output frequencies regardless of how the quantization is performed. It also shows that quantizers exist which are optimal in the sense that subjecting the running Manuscript received January 25, 2013; revised April 26, 2013; accepted May 07, 2013. Date of publication May 16, 2013; date of current version July 22, 2013. The associate editor coordinating the review of this manuscript and approving it for publication was Prof. Ljubisa Stankovic. This work was supported by the National Science Foundation under Award 0914748. The authors are with the Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TSP.2013.2263504