A wideband fractional-N ring PLL with fractional-spur suppression using spectrally shaped segmentation

Abstract

Fractional-N PLLs play an important role in both wireless and wireline circuits. Finer timing resolution is needed to reduce the quantization noise in the phase domain. In conventional designs, a phase interpolator (PI) is used to suppress the quantization noise due to its simplicity, but at a cost of gain error and non-linearity. These sub-phase non-idealities result in large fractional spurs [2-5]. Techniques for reducing these spurs include using a PI mismatch and spur-cancellation scheme [2], digital correlation and cancellation [3], use of a successive requantizer with switched loop filter and offset charge-pump [4], and foreground calibration [5]. This paper presents a ring-oscillator based 2MHz bandwidth fractional-N PLL that uses a spectrally shaped segmented-feedback approach to alleviate fractional spurs induced by the PI non-idealities. This approach results in a compact design and, in contrast to previous work, achieves a 26dB spur reduction without need of correlation, cancellation, or calibration methods.