Products > Test Equipment
A High-Performance Open Source Oscilloscope: development log & future ideas
Someone:
--- Quote from: tom66 on November 20, 2020, 06:38:43 pm ---Fractional plotting does not appear to be implemented by any mainstream OEM, I have tried Tek 3000 series, Siglent 5000X, Agilent/Keysight 2000X and 3000X, and various Rigol scopes.
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The reconstruction (plotting) filter in the megazoom IV is matched to the expected bandwidth of the front end so it may be difficult to see with the slower models. But on faster models the plotting is most certainly not hard aligned to the trigger, and can be seen to move with at least 1 px of precision at 2ns/div (64px, 31ps). Noting that scope uses an analog trigger so there is additional jitter from that hardware which isn't eliminated as would be with a digital trigger.
rhb:
--- Quote from: Someone on November 20, 2020, 10:09:27 am ---
--- Quote from: tom66 on November 20, 2020, 09:08:31 am ---The principle is, if the input filter has the correct response (it needs to roll off before Nyquist so that you avoid the described Nyquist headaches) you can calculate slope from the delta from the presumed trigger point (which is at t=0 - centre of the waveform) and the actual trigger point at t=? ... the actual trigger point will be offset from the point that the oscilloscope triggered at by a fraction of the sample rate (0-1ns).
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This is getting silly, you still can't provide a mathematical example of your proposed method. How can the slope be known a-priori? With an ideal AFE filter, any frequency (slope) less than the cutoff could be occurring around the trigger point.
Even with the trivial example of a perfect sine wave of constant frequency being sampled perfectly (and below Nyquist) shifting it with DC while keeping the trigger threshold static would present different slopes at the trigger point. Just the phasing of the points when the frequency isn't rational with the sampling frequency causes significant shifts and jitter as the waveform approaches the Nyquist rate.
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Old military electronics tech trick: Measure the harmonics of a square wave on the spectrum analyzer to determine the slew rate.
All of this is a basic application of the Fourier transform and causality.
We have given much thought to triggering and I think we can do better than anyone else. Not implemented yet, but nothing difficult to do. I spent quite a bit of time on the subject. The limitation is free time to devote to the project.
If anyone wants to commit their time to working on the task I shall be pleased to advise. It's actually quite easy if you know how.
Have Fun!
Reg
nctnico:
It would be better to just explain the math in detail so people know what they are getting into instead of pulling up smoke screens. Open source means full disclosure ;D
snoopy:
--- Quote from: tom66 on November 20, 2020, 06:38:43 pm ---
--- Quote from: 2N3055 on November 20, 2020, 06:07:28 pm ---Pico 3406D supports up to 20GS/s in ETS mode, so needs triggering that can cope with that.
I'm afraid I don't understand what you mean by "doesn't plot sub-pixels (same as most scopes.)"?
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Right - OK, I didn't consider ETS. I'm not planning on implementing it, I don't see a major benefit from it. It may be possible to do it at lower wfm/s but at higher rates it would require the PLL to hop frequency too often.
But, even if you have ETS, at the end of the day, when you have a sample to plot, say, at 50ps in time... it is going to land on exactly one pixel. Fractional plotting does not appear to be implemented by any mainstream OEM, I have tried Tek 3000 series, Siglent 5000X, Agilent/Keysight 2000X and 3000X, and various Rigol scopes.
If you have a 50ns/div timebase (12 divs so 600ns span), and 1920 pixels to plot your waveform points on, then each pixel would represent 31ps of time. You cannot represent finer than this: you do not have the pixels to do so. So, there is no benefit to achieving any better than pixel-perfect representation, so in this case, anything better than 31ps jitter is no better information.
This applies for sinx/x too, as a sinx/x interpolator works like a regular FIR filter with most of its inputs set to zero. (10x interpolator would have 9 samples at zero and 1 sample at your input value) so you can only shift by interpolated-sample intervals.
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Tek TDS7XX, TDS7XXX, TDS5XXX all offer ETS. The TDS7XXX, and TDS5XXX also offer real time sinx/x interpolation probably because it has much more computational power compared to the earlier TDS7XX scopes. The ETS works extremely well on these scopes. I don't think any other vendor does it as well as Tek does. The downside to ETS is that it requires a repetitive waveform :(
cheers
tom66:
The trade off with ETS is that your waveform rate has to fall because you need to hop the PLL frequency often.
The ADF4351 I'm using takes about 80us to lock in "Fast Lock Mode" which is intended for fast channel changes, not including the time required to write the registers on the device over SPI. In the most optimistic case, that sets your acquisition rate at 12,500 wfm/s. Faster devices do exist but they would still end up being the ultimate limit in the system.
ETS is making up for poor sinx/x interpolation, you can do everything ETS does, and arguably more accurately, with a good interpolator. (Assuming your input is correctly bandlimited for the normal ADC sampling rate.)
Working on the Python sampling model now.
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