In this work, we propose a new method for online calibration of recently proposed Modulated Wideband Converter (MWC), which digitizes wideband sparse signals below the Nyquist limit without loss of information by using compressive sensing techniques. Our method requires a single frequency synthesizer card, which can generate clean tones along the operation band of the system, rather than much expensive measurement instruments such as network analyser or vector spectrum analyser, which are not appropriate for online calibration. Moreover, low computational complexity of the proposed method enables its implementation on FPGA so that it can be embedded into the system. Hence, on each power on, the system can utilize self calibration without requiring any additional measurement instruments.