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

Question

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 1

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.

Answer 2

A sampling scope can indeed make a good spectrum analyzer. The three main limitations are in the areas of span width, dynamic range, and spur rejection.

The span width of a modern sampling instrument is usually constrained by the sample clock rate and/or various throughput limitations of its baseband FFT pipeline. Meanwhile, its dynamic range is lower than that of a classical spectrum analyzer because the front-end sampler 'sees' the whole DC-to-daylight spectrum worth of noise. The instrument folds the noise at every harmonic of the sampling clock, so noise and signals near all of the resulting aliases are visible at once.

Using a sampling front end with an FFT baseband analyzer section is not a new idea -- check out the HP 71500A, and its main module, the 70820A introduced circa 1992. (Large .PDFs but worth the wait.) This 'microwave transition analyzer' was a bit too far ahead of its time due to the wimpy processing power available, but the concept is very sound, and it can be implemented very economically compared to a traditional microwave spectrum analyzer.

Answer 3

You use a sampling scope for repetitive signals, e.g. clocks and other signal-integrity issues. You use a regular DSO (that is the correct term, not a SO) for most everything else. You use a scope at baseband and a spectrum analyzer for RF. The FFTs in scopes are for lower-frequency signals, not at RF.

Answer 4

This is a guess (I would have put this into a comment but I don't have the points for it).

I would guess that Digital Sampling O-scopes probably use heterodyning to mix the incoming high frequency with a fixed carrier frequency, to bring it down to something more manageable, i.e. an Intermediate Frequency, just like a HF radio receivers do. For instance if you mix a ~5.1Ghz incoming signal with a fixed 5Ghz carrier signal, you end up with a ~.1Ghz beat frequency signal. (I've long since forgotten all the RF stuff my dad taught me years ago so I may have that wrong)

I suspect SAs do the same thing, at least those capable of working with RF frequencies, anyway.