The trend is toward wider bandwidths in modern communications work -- up to hundreds of MHz, in fact. The analysis bandwidth of some of the newer signal analyzers is in the 1 GHz range. 3 MHz or even 10 MHz isn't enough to take in a whole WiFi signal, and it only gets worse from there. But if you don't
know that you need wideband and/or real-time analysis capabilities, the answer is that you don't.
Meanwhile, resolution bandwidths down to 1 Hz or even less can be very useful in low-level RF work with stable sources. This is one thing that the E4406A really excels at. Second place in this category goes to the 856xE series portable analyzers -- which are also the quietest in terms of their own LO noise floor. But if you don't
know that you need extremely narrow resolution bandwidth or low PN, the answer is that you don't.
What that means is that you should take the same advice that would/should be given to someone who's just learning to play guitar for the first time. Buy something dirt cheap and use it to learn on. Then, when (
if) you're ready to move on from that first guitar, you'll know what to look for in your next one.
Possible mistakes:
- Buying an 8568 or 8566 if you don't have plenty of room on your bench and actively enjoy playing with test equipment.
- Paying too much for an 8590 series model. These are warmed-over incarnations of much older instruments, released by HP when Tektronix caught them flat-footed with no portable instruments.
- Turning down a great deal on an 856xE-series model. These are among the best SAs ever built in the analog era, even though they were sold as portables and not "lab grade" benchtop models. I prefer the display and UI on the 8566/8568 but performance-wise, the 856xE series is superior.
Also keep an eye out for an LCD-based R&S FSP in good shape. I can't stand their user interfaces myself, but you may not agree. Their performance is in the 8560E class in most respects, and they are a more modern design. They do take forever to boot, but their fan isn't stupidly loud like the 8566/8568, so you won't mind leaving them on. The biggest drawback is that if anything goes wrong with an off-brand spectrum analyzer (read: anything other than HP or Tek, and yes, that includes R&S and Advantest and other well-respected names) you are hosed.
I don't know anything about Rigols, but I will say that some of the low-cost Asian offerings are starting to look pretty competent. They will not perform up to HP standards in pretty much any respect, but they have other advantages that you shouldn't ignore off because a bunch of spectrum analyzer snobs on EEVBlog told you to.
To go back to the earlier metaphor, you will not find it useful as a beginning musician to listen to Eric Clapton and David Gilmour argue about whose guitar is better.