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Siglent SDS800X HD Review & Demonstration Thread

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gf:

--- Quote from: markone on May 11, 2024, 01:13:38 am ---1MHz SPAN :



...
I would say that SDS800x FFT is not usable at low RBW (in relation to the central frequency), if you guys can say with certainty that there are no bugs in the FW FFT implementation my interest in this functionality, apart from a few cases at quite low frequencies, ends here.

--- End quote ---

* Could you capture 100M samples of this signal at full 2GSa/s sample rate and post the .bin file?
   My feeling is that what you see in the FFT graph is already contained in the samples. I tend not to blame the FFT for that.

* On a larger 0...1GHz span, how do the levels of the harmonics compare between FFT and SA?

* Does a larger V/div (200mV/, 500mV/,...) reduce the amplitude of the close-in spurs (and harmonics)?


--- Quote from: ebastler on May 11, 2024, 06:27:20 am ---So, potentially an error in the down-sampling to the FFT's 2 MPt maximum?

--- End quote ---

Not necessarily an error. If there exist harmonics beyond 75 MHz then it is expected that they will fold into the first Nyquist zone, and if the carrier deviates only slightly from 50 MHz, the folded harmonics can indeed form a comb in the neighborhood the fundamental frequency.

ebastler:

--- Quote from: gf on May 11, 2024, 08:47:21 am ---Not necessarily an error. If there exist harmonics beyond 75 MHz then it is expected that they will fold into the first Nyquist zone, and if the carrier deviates only slightly from 50 MHz, the folded harmonics can indeed form a comb in the neighborhood the fundamental frequency.

--- End quote ---

Good point. I just tried to reproduce the effect, and indeed it disappears when the incoming signal's frequency is adjusted such that the SDS800X HD sees it as exactly 50 MHz. The scope's time base clock is off by approx. 1..10 ppm -- well within spec of course.

So do the other instruments, which markone and others compared with, have a better clock accuracy (either due to tighter specs or luck), and hence did not see the folded-back harmonics? And/or do they see less harmonics due to better linearity of the input stage?

(Edit: Changed the stated time base tolerance -- it seems that I got lucky initially, with a 1 ppm deviation shortly after powering up the signal generator and scope. The deviation settled at about 10 ppm after a while, still well within the +- 25 ppm spec.)

gf:

--- Quote from: ebastler on May 11, 2024, 09:39:29 am --- I just tried to reproduce the effect, and indeed it disappears when the incoming signal's frequency is adjusted such that the SDS800X HD sees it as exactly 50 MHz. The scope's time base clock is off by approx. 1 ppm -- well within spec of course.

So do the other instruments, which markone and others compared with, have a better clock accuracy (either due to tighter specs or luck), and hence did not see the folded-back harmonics? And/or do they see less harmonics due to better linearity of the input stage?

--- End quote ---

Should be easy to verify this on the other scope models, by adjusting the frequency.

Could you also do a FFT with 0...1GHz span at 2GSa/s, in order to see the levels of the first 19 harmonics (w/o folding)?
And if you have a SA, then for comparison the same also on the SA.

The -30dB reading at marker 5 in markone's screenshot is quite a lot. What max. spur level do you see?

ebastler:
Screenshots of the full 1 GHz spectrum and of my recreation of markone's settings are attached. The input signal was nominal 50 MHz from an (upgraded) SDG2042X.

The highest spurious peak in the "folded back" setting is -30 dBm as well, as in markone's measurement. But in the full 1 GHz spectrum, all harmonics are below -50 dBm. What's happening there?

Martin72:

--- Quote from: markone ---could you try again with also 50KHz and 1MHz SPAN ?
--- End quote ---

Same results, but won´t post it until the "new" thread exist..

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