J-Test actually remains quite relevant in our tests of USB DACs. It is dither free, high-frequency and toggles all the data bits. All of this has a way of causing jitter components to become visible far, far more than they would with music. Imagine the last time you saw near 0 dBFS music content at 12 Khz! You would go deaf listening to that to say nothing of your tweeter cooking good.
I plan to write a more detailed article on this in a future as there is a lot of confusion in this regard.
As to noise, we actually pay attention to noise floor rising and as a result, the amount of random jitter there. So by your own notion, it works there.
On packetization of USB, that actually shows up nice too at its packet timing if it bleeds into either Vref or Clock signal.
Isn't that what the question was? That there is more than "1s and 0s" here?
There is no difficulty here: we can't hear the ultrasonics so easy to interpret.
I have done wide-bandwidth measurements at times. See example of my review of Uptone LPS-1 supercap power source:
https://audiosciencereview.com/foru...ar-power-supply-review-and-measurements.1849/
As you see here, there was no reduction of noise as claimed by the manufacturer at higher frequencies higher.
Be careful of frequencies well above this as in Mhz you ask about. High-speed ADCs running at these speeds have much lower dynamic range than the 24-bit converters in my analyzer. And at any rate, things like probing matters as otherwise you pick up a bunch of common mode noise you can confuse as DUT output.
Anyway, the key here that J-Test signal was plenty to show differences here. Indeed this has caused the designers of a number of products to go back to the drawing board and rethink what they had assumed. In the case of UPtone ISO Regen for example, it showed mains AC leakage due to switchmode supply which has nothing to do with jitter:
https://audiosciencereview.com/foru...so-regen-review-and-measurements.1829/page-18