In this paper describes a voltage inverter with galvanic isolation. It is shown that for efficient use of a high-frequency transformer, it is advisable to perform voltage modulation according to the law of bipolar frequency-pulse modulation with a constant pulse duration. In this case, the transformer is magnetized by the hysteresis limit loop, and therefore the maximum amount of energy is transmitted in one commutation of the inverter switch elements, which reduces the dynamic losses in the converter. At the output of the inverter, the voltage, modulated by the proposed law, is rectified and smoothed by the filter, after which the output of the converter generates a voltage close to sinusoidal. The main objective of the study is to research the voltage spectrum of a high-frequency inverter modulated by the law of bipolar frequency-pulse modulation and rectified voltage, modulated by the law of unipolar frequency-pulse modulation. For the analysis of the voltage spectrum it is proposed to use a Double Fourier series, which allows to divide the influence of the carrier and the modulating function on the spectrum of the modulated signal. Since the frequency-modulated signal is formed on the basis of a variable-frequency carrier function, its spectrum is calculated using a variable-time conversion function based on a pulse-width modulated signal with a constant-frequency carrier function. Bipolar modulation is obtained by describing the bipolar signal as the sum of two unipolar signals - one with a positive and the other with a negative polarity. The calculated spectrum of the modulated signal at the output of the inverter indicates that energy is transmitted mainly through the high-frequency transformer at an intermediate frequency, which indicates the efficient use of the transformer. The rectilinear unipolar signal spectrum is transmitted with first harmonic and a low higher harmonic composition, which simplifies their filtration and reduces filter dimensions. Therefore, the application of the proposed modulation law in practice will reduce the size of the high-frequency transformer and the level of dynamic losses by reducing the commutations of switch elements of the inverter.