| Products > Test Equipment |
| Siglent - 11/20 - New SDS1104X-U, 4 channel 100MHz, 1Gsa/s economy oscilloscope |
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| tautech:
--- Quote from: tttonyyy on June 19, 2021, 10:08:56 pm ---I'd not really thought about this before. I would guess that the scope firmware just sees that two successive samples cross the trigger threshold, and linearly (or sin(X)/X) interpolates where that crossing point would be in time and shifts the display of all sample positions appropriately to make the zero point line up like we see above. --- End quote --- Good, so some more for you to ponder on. These scopes (all DSO's) covert the input analog signal to an entirely digital stream (data points) where in modern Siglents an entirely digital trigger awaits a data point that meets its threshold before interpolation is applied to reconstruct the waveform into something we all understand and expect to see. For the most part you can use these entirely in Dot mode although for screenshot captures and Single shot it's better to be in Vector mode so to have interpolation applied. |
| QuitButton:
--- Quote from: tttonyyy on June 19, 2021, 06:27:29 pm ---This only works with repetitive signals. If you single trigger the scope you'll see the true reflection of number of sample points on screen. --- End quote --- That's just it, this IS (supposed to be) a single shot capture. Which doesn't make sense why Hantek shows the result it does. |
| StillTrying:
--- Quote from: tttonyyy on June 19, 2021, 10:08:56 pm ---I'd not really thought about this before. I would guess that the scope firmware just sees that two successive samples cross the trigger threshold, and linearly (or sin(X)/X) interpolates where that crossing point would be in time and shifts the display of all sample positions appropriately to make the zero point line up like we see above. --- End quote --- I've looked at loads of scope shots where there's only 2 or 3 full ADC speed samples per division trying to determine if the trigger interpolation is linear or sinx, most of the time the X timing looks too good for the trigger interpolation to be just a linear line between the 2 samples. It's difficult to tell much when the trigger is in the middle of a fast edge where any interpolation is going to be very nearly a straight line between the 2 nearest samples anyway. We need someone to experiment with dots + persistence ~2 samples division and the trigger nearer the false pre and over shoot that sinx creates when there's not enough samples on the edge. If the middle of the fast edge now wobbles we'll know the trigger interpolation is exactly the same as used for the displayed trace, - I think. :) |
| rf-loop:
Here is 100MHz sine in CH4. Also other channels are on for reduce samplerate to 250MSa/s. (Ch 1-3 trace display hidden) Attached image 1. First one shot with Sinc interpolation. Persistence infinite. Attached image 2. Then same acquistion but turned display for dots only. (note that Siglent do not produce fake dots as many other scopes do in this case) Now it still keep this persistence but its intensity is now reduced and real ADC sample dots are visible. If trig is based to linear interpolation there can see where trigger position is in this case. And if this interpolated trig position is now placed to trigger position we can see really high horizontal positioning jitter because samples are randomly in different positions in every sequential acquisitions. But as next image show trigger is rock solid. Attached image 3. All same but now just running with infinite persistence and trigger position is rock solid and as can see in image 2 samples can be where ever. -------- This last example (SARI) is old, made with SDS1104X-E , (it works also in X-U but with bit reduced performance due to different trigger system, due to single ADC system) Also when I read this thread... one tiny tip. In Siglent there is two different acquisition modes. Fast and slow. Slow mode works like conventional DSO. one acquisition and display... one acquisition and display... Fast mode... working as DPO (Siglent name SPO) depending timebase and signal, trigger etc... it acquire as many acquistion as it can before need display. Display interval is normally 40ms. In some cases it can acquire up to thousands acquisition in one display interval and they are displayed overlaid in this one display frame. Note in image what is sampling speed and what is signal. This is NOT Equal Time. This is Siglent SARI (Sequential Acquistion Random Inteleaving) what is also not LeCroy RIS mode but somehow tiny tiny bit its cousin. Siglent have not advertised it in any place, and also no need advertise. But experienced user of course can use it because he also then understand its all limits without ranting this and that. But this is useful in some special cases when Sinc may produce Gibbs ears when working with samplerates. |
| bdunham7:
--- Quote from: rf-loop on June 21, 2021, 12:47:05 pm ---Note in image what is sampling speed and what is signal. This is NOT Equal Time. This is Siglent SARI (Sequential Acquistion Random Inteleaving) what is also not LeCroy RIS mode but somehow tiny tiny bit its cousin. Siglent have not advertised it in any place, and also no need advertise. But experienced user of course can use it because he also then understand its all limits without ranting this and that. But this is useful in some special cases when Sinc may produce Gibbs ears when working with samplerates. --- End quote --- You can get the same result with slow mode and persistence, but either case the usefulness seems quite limited. Your sample rate still has to be pretty close to the desired bandwidth otherwise this too falls apart. I take it that this example demonstrates that when reducing the sample rate to fit in a smaller memory space, the scope does the sinc trigger interpolation before decimation? Your statement that Siglent doesn't intentionally lie about dots seems true as far as I can tell--but the reason the scope can't do something like this example on aliased signals is because it can't properly place the dots in the correct horizontal position because it is relying on a sinc interpolation of an aliased signal. So it is precise but incorrect about the horizontal position, resulting in a very clean but wrong aliased signal, dots or vectors don't matter. In this example, it can place the dots correctly even though they are over Nyquist for the decimated sample rate--and when overlaid, the represent the signal correctly. |
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