In this article a sinusoidal process of direct frequency conversion is shown in an illustrative way. In this case input phase voltages of constant value frequency are modulated (multiplied) by sinusoidal modulating functions. All value ranges of modulating frequency have been discussed. It has been shown that the sum of all modulated input voltages makes up a sinusoidal output voltage, besides its similar frequency absolute value can be obtained by high as well as low value modulating frequency. It allows one to make practical conclusions about new possibilities of direct frequency conversion, e.g., to widen the converter line commutating output frequency range up to its input frequency. Also characteristic conversion cases have been discussed, e.g., at the modulating frequency value which is similar to the input frequency when the output voltage form is a straight line at the rectifier mode but the value of the voltage changes depend on the output voltage phase. The discussed processes can be used when working out new conversion methods.