Using a suitable modem with inbuilt statistics is the most affordable method.
Agreed, but I need the raw data out of the scope for other purposes as well.
More on 'suitable modem': the spectrum analysis isn't an advertised feature. It's pretty hard to figure out which ones have it and which ones have not.
Moreover, my connection provider doesn't support external VOIP adapters, forcing me to their router; there are community efforts to sniff credentials and make a fake handshake using some python glue, but it isn't very reliable atm. And If you put together a price of a decent router and a decent VOIP ATA, it's about the same of the cheapest vdsl line testers. Consider also that I don't really rely on my telco's router for any feature more than NAT&VOIP, as I use my custom router right after it, before my machines.
That said, I'd like also to learn something about my scope and get a way, in case of need, to turn it into a primitive spectrum analyzer. The scope can do FFT; it's ridiculous, but it does it. I mean: it's kind of 'half away' between the time domain and the frequency domain. Can't be that hard to make a step forward.