A 68.6fs_rms-Total-integrated-Jitter and 1.5us-Locking-Time Fractional-N Bang-Bang PLL Based on Type-II Gear Shifting and Adaptive Frequency Switching

To pursue the ever-growing trend in mobile data-rates, modern transceivers exploit carrier aggregation and high-order modulations, at the price of calling for ultra-low jitter (below 100fs) and frequency-agile local oscillators. Although the bang-bang digital-PLL (BBPLL) architecture can meet the stringent 5G jitter requirements at low power consumption and silicon area [1], its inability to quickly recover from a frequency jump, caused by the narrow linear range of a bang-bang phase detector (BBPD), has so far prevented its application when frequency agility is an important requirement. A high-resolution and wide-range time-to-digital converter (TDC), rather than a BBPD, would solve this issue at the price of larger power and area. As a matter of fact, fast-hopping ADPLLs using multibit TDCs have been demonstrated in applications with relaxed jitter specifications [2], [3]. To break the trade-off between frequency agility and jitter-power product, auxiliary frequency-acquisition loops have been recently adopted in BBPLL architectures [1], [4], [5]. Those auxiliary loops, based on extra BBPDs, control the digitally-controlled-oscillator (DCO) frequency with a gain larger than that of the main loop, thus decreasing the settling time. Unfortunately, above a certain control-gain value, the system nonlinearity introduces an unwanted dependence between the main and auxiliary loops, limiting locking time to several thousands of reference cycles.