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Comment by thatcherc | Hacker News Reader

The scope's sample rate really jumped out to me:

> I configured the oscilloscope to collect 100M samples at 1 TSPS (tera-sample per second, 10^12) ...

Wow! Sampling at 1000 GHz? I wouldn't have thought that was possible! Does anyone know how the scope can even do that?

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.

> 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".

Huh, analogous to RAW photography and postprocessing rather than on-camera jpg creation, I guess.

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.)

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.

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.

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

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)

The Signal Path has a teardown of the Keysight UXR oscilloscope, which has a BW of 110GHz and samples 4 channels at 256Gbps/channel, with 10bit resolution: https://www.youtube.com/watch?v=DXYje2B04xE.

In general the slow part is getting the data out of the ADCs, so by interleaving multiple you can increase the speed. To reduce costs, if you have a repetitive signal, you can trigger the ADCs at different points in the waveform to build up a better idea of what is going on.

For scopes that can do these high sample rates they frequently use two lasers and have the scope capture the beat between them as a demo of how fast they are.

That's a trick called equivalent input sampling. Basically shifts the sample clock a little bit around while looking at the same waveform. The catch is it only works on repetitive waves, and you are still limited by the analog frontend's rise time.

The fastest scope I know of is a 256 GSPS done by interleaved ADC's. Analog bandwidth is 100 GHz Here's a recent submission on the bad boy:

https://news.ycombinator.com/item?id=31774734

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