  The dynamics of an oscillator driven by both low- and high- frequency
external signals is studied. It is shown that both two- and three-frequency
resonances arise due to a nonlinear interaction of these harmonic forces.
Conditions which must be met for oscillator synchronization under these
resonances are estimated analytically by considering the Van der Pol oscillator
with modulated natural frequency as mathematical model. It is demonstrated that
due to the low-frequency modulation, additional synchronization regions arise
in the control parameter space. Feasibility of the theoretical findings is
confirmed in experiments with a Hartley-type oscillator.
