A Frequency Synthesizer With Automatic Frequency Calibration Robust to Initial Phase Error and Phase-Noise Enhanced Ring Oscillator

An automatic frequency calibration robust to initial phase error and a phase-noise-enhanced ring oscillator are proposed for frequency synthesizers. The proposed automatic frequency calibration method can quickly determine the desired sub-band tuning curve for a voltage-controlled oscillator by detecting the overshoot voltage of the oscillator control voltage of the frequency synthesizer when an initial phase error occurs between the reference signal and the divided output signal from the oscillator, thereby reducing the locking time of the frequency synthesizer. The proposed ring oscillator uses a differential inverter type delay cell with an input AC-coupled current source for phase noise improvement and low power consumption. For verification, an integer-N frequency synthesizer with the proposed automatic frequency calibration and ring oscillator was fabricated using 65-nm CMOS technology. The implemented frequency synthesizer draws 2.5 mA from a 1-V supply voltage at a 1 GHz carrier frequency. It provides frequency calibration time of less than 3.6~\mu s, phase noise of -105.5 dBc/Hz at 1 MHz offset frequency of 1 GHz carrier frequency, and figure-of-merit for integrated jitter from 1 kHz to 1 MHz of -231 dBc/Hz.