[time-nuts] Very stable synthesizer, alternative to PTS(Programmed Test Sources) x10 or 040?

[Anders Time]

I have been looking around for a very stable synthesizer(<E-12 at 1s adev).
The only really good information that I have found is febo.com´s
measurements on the PTS synthesizers(http://www.febo.com/pages/pts_synth/).
Is there any other really good alternatives to the PTS synthesizers? DDS?
HP?
Best Regards
Anders