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Aspects of two-photon Ramsey fringes in SF6 at 30 THz obtained with a 1-m separation between the two absorption zones are presented. The experiment is referenced to a primary standard, the Hydrogen maser/Cs fountain located at BNM-SYRTE, via a femtosecond laser frequency comb generator. This results in a stability below 1 Hz for 1000 s of averaging, and absolute frequency measurements to ±2 Hz which corresponds to 6×10-14 in relative frequency. Resolution of the hyperfine structure now approaches 10 Hz, while the fringe period of 200 Hz provides a frequency discriminator 100 times narrower than any other currently in use in the spectral region.