The lowest level signal you will be able to see captured on a spectrum analyser will be limited by the DANL or displayed average noise level so the lower the DANL the better.
To obtain the lowest DANL the RSA3000/RSA5000 series analyzers will need to be operating within these parameters and settings:
Attenuation = 0 dB
Trace Average greater or equal to 50
Tracking Generator set to off
Normalized to 1Hz
Operating between 20°C to 30°C
Input Impedance set to 50 Ohms