SpectreRF gives you both SSB_NF and DSB_NF.
SSB_NF and DSB_NF are following.
vn^2/Rl + kTo(G1+G2+...+Gn)
Fssb = -------------------------------------
kToG1
vn^2/Rl + kTo(G1+G2+...+Gn)
Fdsb = -------------------------------------
kTo(G1+G2)
kTo is the noise power at the input termination.
G1 is the primary frequency conversion from RF input frequency to IF output frequency.
G2 is the other frequency conversion from RF input image to the same IF output frequency.
G3 and higher are the higher order, low efficiency conversions
that occur as other input frequencies mix with harmonics of the LO to
appear at the IF output frequency.
If mixer is noiseless, vn=0.
G3 and higher are all zero in mixer behavioral model.
So NFdsb=0dB and NFssb=3dB.
Also see followings.
http://www.designers-guide.org/Forum/YaBB.pl?num=1162399761