The paper considers primary frequency properties of parametric LPTV (linear periodically time-varying) circuits in steady state under the action of a harmonic input signal and periodic changes of parametric elements. Based on the representation of the transfer function by a finite Fourier series, an analytical expression for the output signal of a parametric circuit has been obtained. The conditions for the coincidence of pair harmonic components forming characteristic frequencies with amplification or attenuation effects have been investigated. A method for determining such frequencies has been proposed, and examples of its application in a synchronous detector, frequency converter, modulator, and in single-loop and dual-loop parametric amplifiers have been presented. The results obtained can be used to design radioelectronic devices with due regard for the parametric effects.