Something cheap and nasty from fleabay.
The knee frequency and risetime are related as indicated. The noise source needs to have power above the knee frequency.
It is an approximate technique, and any vaguely competent noise source ought to be flat enough.
Yes I understand that this technique is approximate, that's why I used the word "estimate" rather than "measure".
Was wondering if my JDS2800 signal generator was suitable (it has noise as one of possible output waveforms), but apparently it won't do: its noise power nicely rolls off after 60 MHz, which, not so surprisingly, matches its max output frequency.
I'll try to find that Johnson's book for a more detailed explanation.
This is a rabbit hole indeed (but a welcome one). Since the most difficult part of creating a white noise generator seems to be finding a suitable high-frequency op amp, and it looks like I can obtain some of those locally, now I, naturally, want to try building one myself, especially now that I have an SA with which I can test its performance, rather than just buy a ready made one from Ali (even if they are sold for just $20-$25). That will likely turn out not as easy as it seems, but that's okay.