Hi, I have no Soft66DB2 SDR, but I tested the DDS daughter board DDS-34B offered by JA7TDO for this receiver. It's a DDS followed by a frequency multiplier ICS512M to increase the frequeny range. A number OM wanted to operate this DDS together with the SoftRock RXTX. I will upload output spectra of the board to the photos area. I think it's obvious why to avoid using this board especially in a RXTX.
Bodo, DJ9CS
