Be aware of FFT SNR gain. The longer the FFT windows, the lower noisefloor it will show, but this isnt true noisefloor of the device under test.
FFT window works as averaging and random incoherent signals like noise add up +3db per window lenght doubling while coherent signals like the test sinewave adds up +6db per window lenght doubling.
EDIT: I just noticed that the db scale starts at -60db and not 0db, that means it shows 100db SNR and not 160db as I first thought from quick glance at the picture, thats more realistic value.