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| vk6zgo:
--- Quote from: BillyO on June 08, 2024, 11:27:35 pm ---Well, it is "ringing" before the edge because of the propagation skew between the different inverters? Edit: But, wait, you don't think it's because of that, do you? That's good because like others that have suggested that, you'd be wrong. No, you think it's being caused by the scope. Aliasing because, in your opinion, it's a "shitty Chinese scope", right? Well, if that's what you think then you are still wrong. If that's not what you think, then I'd love to hear what you think it is. --- End quote --- A square wave can be regarded as a fundamental & all the odd harmonics of that fundamental. Real world networks suffer from the phenomenon of "Group Delay", where the fundamental & harmonics are displaced slightly in time. |
| BillyO:
--- Quote from: vk6zgo on June 09, 2024, 01:24:04 am ---A square wave can be regarded as a fundamental & all the odd harmonics of that fundamental. Real world networks suffer from the phenomenon of "Group Delay", where the fundamental & harmonics are displaced slightly in time. --- End quote --- I'm not sure group delay would produce what we are seeing in the pink trace. If somehow it can, then it's all happening within the 74AC14 used to create the waveform. |
| T3sl4co1l:
Looks like sinc interpolation to me -- zoom in to better show the edge (and hopefully get a faster [equivalent, if applicable] sampling rate), and try with and without interpolation, or show vectors/dots. Ed: note that the sample rate is only 2G/s: for an edge rate ~1ns, what does this suggest about the number of points in the rise itself? Tim |
| BillyO:
--- Quote from: T3sl4co1l on June 09, 2024, 04:44:53 am ---Looks like sinc interpolation to me.. --- End quote --- Hmm, okay but I'd like to draw you attention to the traces I posted in reply #486. Same sampling rate and a pulser with an risetime of < 40ps. No "ring" before the edge. |
| David Hess:
--- Quote from: vk6zgo on June 09, 2024, 01:24:04 am ---A square wave can be regarded as a fundamental & all the odd harmonics of that fundamental. Real world networks suffer from the phenomenon of "Group Delay", where the fundamental & harmonics are displaced slightly in time. --- End quote --- Some analog oscilloscopes suffer from that because of their delay line. The delay line produces group delay which would limit performance except that a phase compensation network is included to remove it. The faster Tektronix 24xx series of analog oscilloscopes will show a small amount of preshoot when a fast enough edge is applied. --- Quote from: BillyO on June 09, 2024, 03:59:09 am ---I'm not sure group delay would produce what we are seeing in the pink trace. If somehow it can, then it's all happening within the 74AC14 used to create the waveform. --- End quote --- Group delay could do it, but I do not know how a simple 74AC14 oscillator could produce group delay. If a bunch of sections are in parallel for higher speed, could some sections be switching before the others? Maybe the paths connecting the sections are too unequal? I have used parallel AC gates before and never had that happen, but I do not remember specifically using AC Schmitt triggers. I have used HC and other Schmitt triggers that way, but they must be too slow to create this problem. --- Quote from: T3sl4co1l on June 09, 2024, 04:44:53 am ---Looks like sinc interpolation to me -- zoom in to better show the edge (and hopefully get a faster [equivalent, if applicable] sampling rate), and try with and without interpolation, or show vectors/dots. --- End quote --- It does to me also, although I am fuzzy as to how this would differ from the Gibbs phenomena when the number of samples is limited. They both seems to describe the same problem. When I was evaluating the now old Tektronix MSO5000 series with a 350 picosecond edge source, which was significantly slower than the oscilloscope bandwidth, I could produce exactly this type of display by enabling the software bandwidth limiting, which lead me directly to the Gibbs phenomena. The Tektronix application engineers supervising us could not explain it either. The hardware bandwidth limiting with the same cutoff frequency produced an analog type display without preshoot. If my understanding is correct, then Sinc interpolation should do exactly the same thing when the Nyquist frequency is not far above the edge frequency. |
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