  The article considers the problem of identifying the variable frequency of a
sinusoidal signal. To obtain a regression model of the signal, an iterative
differentiation of the original analytical expression is performed, and the
swapping lemma is applied. The estimation of the parameters of the
non-stationary frequency is implemented using the dynamic expansion of the
regressor and mixing (DREM) procedure and the Luenberger observer. As a result
of the numerical simulation, the efficiency of the proposed algorithm is
demonstrated, showing the convergence of the frequency estimate to the true
value.
