A low-jitter 125-1250-MHz process-independent and ripple-poleless 0.18-μm CMOS PLL based on a sample-reset loop filter

Abstract

This paper describes a low-jitter phase-locked loop (PLL) implemented in a 0.18-/spl mu/m CMOS process. A sample-reset loop filter architecture is used that averages the oscillator proportional control current which provides the feedforward zero over an entire update period and hence leads to a ripple-free control signal. The ripple-free control current eliminates the need for an additional filtering pole, leading to a nearly 90/spl deg/ phase margin which minimizes input jitter peaking and transient locking overshoot. The PLL damping factor is made insensitive to process variations by making it dependent only upon a bandgap voltage and ratios of circuit elements. This ensures tracking between the natural frequency and the stabilizing zero. The PLL has a frequency range of 125-1250 MHz, frequency resolution better than 500 kHz, and rms jitter less than 0.9% of the oscillator period.