The Agilent E4406A is a Vector Signal Analyzer that can be used as a Spectrum Analyzer with some constraints. It's a real bargain for RF hobbyist, it can be found around 1000$ everywhere on eBay. It was mainly a GSM transmitter tester for production line (this explains the low cost and the huge availability on the used market).
Be aware that it exists at least 2 differents versions of this VSA. A 12 bits one and a 14 bits one, usually seller are not aware of the differences. If possible try to have a 14 bits version (1).
An other option is often missing on this model, the Digital IQ board with input on the Left side of the front panel. If someone has pictures (or even has one to sell) please let a message.
UPDATE: Internal reference of this board is E440660158
Assembly Instructions printed on the Rack. The two cards on the left are mandatory. Analog Front End seems to be the same for each version of E4406A. The digital IF card is either 12 or 14 bits version.
Detailed view of the Analog Front End Board.
Detailed view of the 14 bits version Digital Board.
We are missing the IQ dedicated board. If you have it or any pictures of it, please leave a message.