Guido PE1NNZ has been doing some very impressive work on direct SSB generation using a PLL and implemented it on a Raspberry Pi mini computer. I don't claim to understand a word of it (!) but this really is clever stuff. He tells me he has made several contacts on the 40m and 20m bands with his Raspberry Pi!
For your interest, here is more information about the SSB experiment: http://pe1nnz.nl.eu.org/2013/