I. Introduction
Been widely used for decades, phase-locked loops (PLLs) have become one of the basic devices in modern electronic systems [1]–[3]. They can be used as modulators, demodulators, oscillator stabilizers, data synchronizers, frequency multipliers, etc [4]. A classic PLL consists of a phase detector (PD), a loop filter, a voltage-controlled oscillator, and sometimes a frequency divider for frequency multiplier applications.