I’m wondering how you used an oscilloscope to diagnose the ~1ghz bandwidth DOCSIS signals on broadband cable. I have a (expensive!) gigahertz bandwidth scope but I’m not sure what I’d look for on it if I connected it to my cable.

The high capacity of an internet link does not translate directly into high bandwidth signals in the analog domain. The DOCSIS standard includes modulation patterns as high as 32768-QAM which allows 15 bits to be transmitted per symbol change. For 1Gb/s, that means that you only need <70M baud.

The upstream channels are squarely in the HF to VHF range. The downstream channels (which typically require more bandwidth) start at about the same HF frequency (42MHz) but can extend above 1GHz. Each channel, however, is relatively bandwidth limited.