Following a pilot study initiated by the CCTF WG on TWSTFT and coordinated by the BIPM, since the summer of 2016 several laboratories have been testing a new type of TWSTFT receiving equipment based on a Software Defined Radio. This type of receiver has been developed with the aim of mitigate some of the time transfer instabilities observed in the TWSTFT links based on SATRE modem. One of the major feature of the SDR receiver is the low AM-PM conversion, thank to the particular design of the software correlator. At present the SDR system is only used as a receiver in parallel with the regular TWSTFT SATRE system and the investigation is mainly focused on the instability improvement. Nevertheless, considering that in the frame of the TAI network the TWSTFT links are mostly used for their accuracy, it is useful to develop a calibration method for such devices to allow a direct comparison with the traditional links, and to investigate the long term instability.