Looks like an 53310A Modulation domain Analyzer is on its way here. Someone has called it a frequency microscope, and it is. I remember watching it being demonstrated in 1992 and was amazed at its analysis of frequency hopping systems and its ability to measure jitter and phase noise to less than 1Hz resolution. Good for working with VFOs and evaluating GPS disciplined oscillators and other precision applications. Not sure what if anything has replaced it, but the price was right. The picture shows one displaying a 1.1Hz wide histogram of a 10MHz signal. Sure the signal is good but compating several signal sources could show which is the most stable and noise free. It's also a scope. Pretty cool!
there's a video on the modulation domain
https://www.youtube.com/watch?v=lBLEfVUVGyU