Robust Frequency-Adaptive PLL with Lyapunov Stability Guarantees

The Global Quadrature Phase-Locked-Loop (GQPLL) is a recently-devised PLL-type architecture (based on Quadrature Signal Generation) able to track a biased sinusoidal signal with unknown frequency and amplitude. The original GQPLL formulation resorted to a non-standard non-normalized adaptive law, able to guarantee the global convergence of the estimates for arbitrarily large adaptation gains, thus enabling arbitrarily fast adaptation transients. On the other side, the non-conventional adaptation law used in the original formulation makes it difficult to apply robustifying modifications of adaptive control. In this connection, the present work presents a new formulation for the PLL internal dynamics in order to obtain a convenient 1st order linear-in-the parameters error model, which can be dealt with by a conventional non-normalized adaptive law, to which a robustifying modification such as projection can be applied. The large-gain global stability of the adaptive system is proven by Lyapunov arguments. The overall adaptive PLL is named Robustified GQPLL (RGQPLL).