  In this paper we discuss the application of the harmonic balance method for
the global analysis of the classical phase-locked loop (PLL) circuit. The
harmonic balance is non rigorous method, which is widely used %,often without
rigorous justification, for the computation of periodic solutions and the
checking of global stability. The proof of the absence of periodic solutions is
a key step to establish the global stability of PLL and estimate the pull-in
range (which is an interval of the frequency deviations such that any solution
tends to one of the equilibria). The advantages and limitations of the study of
the classical PLL with lead-lag filter using the harmonic balance method is
discussed.
