4 ms·

Good question – it's actually sampling at 50 GS/s (the scope's maximum sample rate), then upsampling by interpolation. The higher sample rate is useful, becaus

Good question – it's actually sampling at 50 GS/s (the scope's maximum sample rate), then upsampling by interpolation. The higher sample rate is useful, because it effectively gives higher precision when finding the zero crossings; however, you could reduce the .wfm size by sampling at 50 GS/s on the scope then upsampling on the computer.

patrick451 4y ago
How does this get around the aliasing problems that Nyquist says should exist?
- mkeeter 4y ago
  It doesn't – you'd still theoretically see aliasing of frequencies above the Nyquist limit, but the 5 GSPS QSGMII signal is well below that. (Also, the probe itself has a max bandwidth of 10 GHz, so it will filter those frequencies before they're sampled by the oscilloscope)
guenthert 4y ago
I'm not sure, I follow. I believe you describe what is often called equivalent-time sampling. That however obviously works only on strictly periodic signals. UDP packets would be a fairly large waveform, requiring a corresponding large buffer and they would have to be repeated with very precise timing. Further, I don't see the need for such an absurd high sampling rate for signals transmitted at a mere 5Gbps (50GSps - itself a pretty high sampling rate - should be plenty). I sense some confusion.
exmadscientist 4y ago
> upsampling on the computer That would be better. Upsampling on a scope is dangerous, it's just too easy to fool yourself about what you're really seeing. Much better to use a slightly more clever algorithm on the PC than just "nearest crossing".
- matthberg 4y ago
  Huh, analogous to RAW photography and postprocessing rather than on-camera jpg creation, I guess.
  - exmadscientist 4y ago
    Also just that the interpolation and curve fitting algorithms on scopes are often utter garbage. We tested the 5 Series predecessor to your 6 Series and I was amazed to see 10 or so volts on a 3.3V logic signal... until I remembered to check the interpolation setting. There was, in fact, no measurement above 3.3V (ok, within the usual tolerances) but the dumb shit machine had made things up and displayed them as if they were real. Even though they would have meant there was a serious hardware fault (not impossible, it was a new design going through bringup and there were in fact higher voltages on the board!), nope, it was the fault finding tool that was faulty. I've had zero interest in the 5 Series since that demo week. Terrifying machine. (And it was supposed to have been "debugged" by that time.)
jeffwheeler 4y ago
You can use the 8b/10b decoding directly on your Tek MSO scope to do most of the preprocessing here. You can even trigger on errors. It would be easier than analyzing the analog stream directly.

patrick451 4y ago

How does this get around the aliasing problems that Nyquist says should exist?

mkeeter 4y ago
It doesn't – you'd still theoretically see aliasing of frequencies above the Nyquist limit, but the 5 GSPS QSGMII signal is well below that. (Also, the probe itself has a max bandwidth of 10 GHz, so it will filter those frequencies before they're sampled by the oscilloscope)

guenthert 4y ago

I'm not sure, I follow. I believe you describe what is often called equivalent-time sampling. That however obviously works only on strictly periodic signals. UDP packets would be a fairly large waveform, requiring a corresponding large buffer and they would have to be repeated with very precise timing. Further, I don't see the need for such an absurd high sampling rate for signals transmitted at a mere 5Gbps (50GSps - itself a pretty high sampling rate - should be plenty). I sense some confusion.

exmadscientist 4y ago

> upsampling on the computer That would be better. Upsampling on a scope is dangerous, it's just too easy to fool yourself about what you're really seeing. Much better to use a slightly more clever algorithm on the PC than just "nearest crossing".

matthberg 4y ago
Huh, analogous to RAW photography and postprocessing rather than on-camera jpg creation, I guess.
- exmadscientist 4y ago
  Also just that the interpolation and curve fitting algorithms on scopes are often utter garbage. We tested the 5 Series predecessor to your 6 Series and I was amazed to see 10 or so volts on a 3.3V logic signal... until I remembered to check the interpolation setting. There was, in fact, no measurement above 3.3V (ok, within the usual tolerances) but the dumb shit machine had made things up and displayed them as if they were real. Even though they would have meant there was a serious hardware fault (not impossible, it was a new design going through bringup and there were in fact higher voltages on the board!), nope, it was the fault finding tool that was faulty. I've had zero interest in the 5 Series since that demo week. Terrifying machine. (And it was supposed to have been "debugged" by that time.)

jeffwheeler 4y ago

You can use the 8b/10b decoding directly on your Tek MSO scope to do most of the preprocessing here. You can even trigger on errors. It would be easier than analyzing the analog stream directly.