How suitable for use as a general purpose spectrum analyzer are these? They're quite cheap on eBay.
They look as if they'd be very good for general RF work, but I'm nervous that there's a gotcha I don't know about.
There's a reason they're so cheap. There are a lot of drawbacks to the E4406A when used as a general-purpose spectrum analyzer. Limited span width, no front-end RF filtering, lots of controls and features are disabled, and deliberately-crippled usability for the ones that are provided -- e.g., the knob tunes at the same rate regardless of span.
But they're still very nice instruments. If your application involves characterizing a known signal source (as opposed to hunting spurious signals, doing EMC precompliance, or otherwise looking at sources with unknown characteristics), they're very capable. They're especially well-suited for use in automated test applications.
Thanks. I felt sure there was a reason they were so cheap. It looks as it it would make a hell of an SDR with the I/Q output option. Rather larger than my SDRplay RSP2 though. I'll have to search around to see if they can be hacked into something more capable.