This paper presents the spectral analysis of OOK modulation with rectangular carrier and the expression that shows the behavior of this signal in the frequency domain is obtained. A graphical analysis is performed from the representation of the harmonics of the OOK wave with rectangular carrier, it permit make the conversion in OOK modulated signal with sinusoidal carrier. It checked how this approximation permit attenuate the problems in low frequency passive RFID system if is using the rectangular carrier in wireless power and communication. Further, is showed a converter circuit, this circuit was simulated and checked in practice.