This is an Agilent E4406A Vector Signal Analyzer that I bought super cheap on eBay. I used to use these at work, but I didn't know all of their capabilities. This photo shows the spectrum analyzer in its basic mode at a resolution bandwidth of 100 mHz (yes, millihertz!), measuring the instrument's internal 50 MHz calibration signal. Instruments with capabilities like this used to cost more than my house...now they cost less than a decent bicycle.