Airomate is the best software based RDS encoder on the market which generates a RDS or RBDS signal at the highest possible quality! Read more about Airomate RDS encoder and stereo encoder and where to download.
Also check out the other products from Diffusion software like the freeware FM stereo encoder named StereoCoder and the freeware Little SineGen which generates sine waves on any sample rate and don't forget Chronos the freeware RDS messages scheduler for Airomate.