The 4, 5 and 6 Series MSO Oscilloscopes employ dedicated digital downconverters on all channels that operate independently of time-domain sample rate and record length. This enables their Spectrum View capability that provides improved results in frequency response analysis over traditional FFTs.  In this video see the kind of insights available for frequency domain analysis due to the independent control of time and frequency domains.

Additional videos:

Using Spectrum View Spectrum Analysis on Multiple Channels (2 of 3)

Triggering on RF Frequency Changes with Spectrum View Spectrum Analysis (3 or 3)