rf - What are the functional differences between a digital sampling 'scope and a digital spectrum analyzer?

I'm interested in measuring the spectral amplitudes / frequency content of RF frequencies up to 30 GHz. This can be done using a digital sampling oscilloscope (I'll call it a SO) with FFT or a digital spectrum analyzer (SA) [or a fantastic digital storage 'scope, noted below]. As I understand it, a SO samples the signals directly with known sample jitter, then recreates the signal using regression. A SA, on the other hand, first downconverts the high-frequency signal with a mixer, then samples. It would seem that a SA should deliver greater frequency resolution given comparable ADC sampling rates to the sampling oscilloscope.

What are the limits of functionality of each type? How is one better than the other at spectral analysis? (They both rely on the FFT, right?) What makes either expensive?

Unrelated POIs: 32 GHz Agilent, 120 GS/s Lecroy, 100 GS/s Tek, Gameboy SA.
edit: There seems to be some confusion between digital storage 'scopes (DSOs) and digital sampling oscilloscopes (what I called SOs) -- they are not the same, although they both sample digitally. I've also updated the question.

Answer

Well, you're not going to be making RF measurements up to 30 GHz without spending a bunch of money, so either path is big bucks.

Typically, Spectrum analyzers are used to do frequency domain measurements. You'll get a display of power vs frequency on the display. The controls in the SA are setup for relevant things, Center frequency, bandwidth, resolution bandwith, signal powers in dBm/dBc etc.

Digital oscilloscopes don't directly have sampling rates to directly sample a 30 Ghz signal, so they'll undersample and assume that the signal repeats. probably a safe assumption, although with no front end filters built into them, you've got dynamic range issues, as well as aliasing concerns that aren't present in a Spectrum Analyzer. You won't directly get spectral plots out of a Digital oscilloscope, you'll need to do an FFT on that. Now, that opens up a can of worms. FFT bin width/windowing function selection, etc. All stuff that can be worked through, but another question to deal with.

You won't get eye diagrams out of a spectrum analyzer, it's a useless measurement @ RF. That's a demodulated signal measurement.

Ultimately, if you want time domain data, then use an oscilloscope. If you want Spectral information, use a spectrum analyzer.