A 6.75-8.25-GHz -250-dB FoM Rapid ON/OFF Fractional-N Injection-Locked Clock Multiplier

A rapid ON/OFF LC-based fractional-N injection-locked clock multiplier (ILCM) is presented. The proposed architecture extends the merits of ILCMs to fractional-N operation. It employs a high-resolution digital-to-time converter to align the injected pulses to the oscillator's zero crossings. An all-digital frequency-tracking loop continuously tunes the oscillator free-running frequency toward the target output frequency. The proposed clock multiplier can be powered ON from a completely OFF state almost instantaneously. Background calibration techniques ensure that robust operation across process, voltage, and temperature. Fabricated in 65-nm CMOS process with an active area of 0.27 mm², the prototype ILCM generates output clock in the range of 6.75-8.25 GHz using a 115-MHz reference clock. It achieves integrated jitter performance of 109 fs_rms (integer-N) and 177 fs_rms (fractional-N), while consuming only 2.65 (integer-N) and 3.25 mW (fractional-N). This translates to the best-reported FoM_J of -255 (integer-N) and -250 dB (fractional-N). The turn-on time is less than 4 ns in both the integer- and fractional-N modes, illustrating almost instantaneous settling.