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The design of low-jitter VCO-based PLLs is quite challenging as high VCO control gain, KVCO, increases the phase noise contribution arising from the charge pump and loop filter. To resolve this problem, dual-tuning PLLs (DT-PLLs) have been studied [1-4]. The DT-PLL structure adds a narrow-bandwidth coarse (high-KVCO) path to the fine (low-KVCO) path consisting of a type-II PLL. The narrow-bandwidth analog filter in the coarse path plays an important role in preventing the charge pump and the loop filter from increasing the output jitter, while a wide-tuning range is maintained. Moreover, the coarse path adds another pole at origin to the fine path and transforms it from a type-II to type-III PLL [3-4]. Compared to a type-II PLL, owing to its boosted low-frequency loop gain, a type-III PLL can better suppress a low-frequency disturbance to the ring VCO, such as temperature drift. However, a type-III PLL has stability problems. To ensure sufficient phase margin (PM), a type-III PLL requires an extremely narrow-bandwidth (e.g. ~10-100Hz) analog filter in the coarse path or must make the KVCO of the fine path larger. The former requires a nano-Farad capacitor or a fairly complex design for shrinking the capacitance, while the latter way increases total jitter. This paper presents a digitally stabilized type-Ill PLL with a ring VCO. It employs a DT-PLL structure and improves its stability by composing the coarse path with a digital integrator and a digital-to-analog converter (DAC). It can set the Kvco of the fine path to 10MHz/V, which is 1000χ lower than that of the coarse path with a sufficient PM. For further in-band phase-noise reduction, the proposed type-Ill PLL adopts a sub-sampling PLL (SS-PLL) in its fine path , and achieves 1.01 psrms integrated jitter.
Date of Conference: 19-23 Feb. 2012