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Trying to display bandwith via Math on Siglent SDS2k+/2kHD/800X HD
gf:
--- Quote from: Performa01 on March 25, 2024, 06:19:17 pm ---
--- Quote from: gf on March 25, 2024, 01:34:49 pm ---We could go up to say 1200MHz with a steep transition band to 1250MHz. Then you could see the generator's intrinsic roll-off up to 1200 MHz.
--- End quote ---
Maybe you feel like providing such a waveform, so we could have a go at it?
--- End quote ---
Sure. See attachment.
--- Quote ---So we actually have deviations of ~0.4 dB up to 500 MHz and <0.85 dB up to 1 GHz with the AFG version, yet ~0.35 dB up to 500 MHz and <0.65 dB up to 1 GHz with the AWG version. I’m pleased with both versions! 😉
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So the AFG works nicely too 8)
Performa01:
Many thanks to member gf, who provides the arbitrary waveforms that give me the opportunity to get the most out of my Siglent gear. I was really curious what we’d get from the 1200 MHz version of the Sinc pulse. And yes, this is going to be spectacular, fasten your seat belts!
As always, I’ve analyzed it in the time domain at a fast time base first, to obtain the precise pulse parameters.
SDS6204_Pro_H12_PR-Sinc_Gf-AWG1200
The pulse width is narrower again and has now finally landed at just 500 ps, while the rise time is less than 200 ps. My calculation says ~188 ps, but this is rather uncertain territory, as the numbers are now even below the rise time of the DSO. Yes, we can still calculate the signal rise time from the measured value and the DSO’s own rise time – as I did, but we cannot absolutely rely on the specification and I suspect that the rise time of the SDS6204 might actually be a little faster than 230 ps. If that’s the case, the signal rise-time is correspondingly higher. For instance, if the DSO rise time actually is just 200 ps, then the generated Sinc pulse would have a rise time of 220 ps. Either way, we’re dealing with pretty fast edges here.
Now we expect a rather spectacular result for the frequency domain analysis, and I can state in advance that the result managed to surprise and impress me at the same time…
SDS6204_Pro_H12_FFT-Sinc_Gf-AWG1200
Above is the frequency response graph with measured values at strategic frequency values from 10 MHz to 1.2 GHz. And this is the summary of it:
Up to 200 MHz, the deviations are <0.1 dB.
Up to 1.12 GHz, the deviations are <0.3 dB.
At 1.2 GHz the deviation is about -3 dB, so that appears to be the actual bandwidth of the generator.
I can only repeat over and over again that we see the combined tolerances of AWG, cabling and DSO here.
gf:
So it is approximately flat up to 1.1 GHz :-+
Another aspect which limits the maximum "usable" frequency is the image rejection in the 2nd Nyquist band. We do not want frequencies beyond 1.25 GHz in the generated signal. Still they are present. The image of 1200MHz (at 1300MHz) is attenuated by ~15dB, and the image of 1100MHz (at 1400 MHz) is attenuated by ~25dB. But that's already the noise floor of your FFT, and that's also the point where your frequency scale ends. Could you connect it also to your SA and swep from say 1GHz to 2.5GHz, hoping that you can get a better dynamic range if you use a small enough RBW? At which frequencies beyond 1.3GHz do we get say -30dB, -40dB, -50dB, -60dB?
EDIT: And what's the attenuation at 1500MHz, since this is the image frequency of the nominal 1GHz bandwidth.
EDIT: And where does the spur at 1.25GHz come from? Is it from the siggen or from the scope? Iguess the latter. The SA will reveal.
Performa01:
--- Quote from: gf on March 26, 2024, 09:23:56 am ---Could you connect it also to your SA and swep from say 1GHz to 2.5GHz, hoping that you can get a better dynamic range if you use a small enough RBW? At which frequencies beyond 1.3GHz do we get say -30dB, -40dB, -50dB, -60dB?
--- End quote ---
I don’t have a high-end spectrum analyzer and while my SA44 is clearly better than average at low frequencies and narrow spans, it doesn’t provide narrow RBW-settings at wider spans, and above 1.5 GHz its noise floor starts rising considerably. Consequently, I can only provide a scan over 1-2.5 GHz with 6.5 kHz RBW:
SA44_Sinc_Gf-AWG1200_2-1-2.5GHz
This first scan however already shows pretty much everything relevant. The signal doesn’t seem to get any lower than -45 dBc. I still feel more comfortable with the DSO, even though it has lots of spurs at these low levels. For a more detailed view, I divided the spectrum in several 500 MHz wide sections. First is the critical part around the Nyquist frequency of the AWG at 1.25 GHz:
SDS6204_Pro_H12_FFT-Sinc_Gf-AWG1200_1-1.5GHz
We see the first minimum at 1.25 GHz – unfortunately the DSO generates a spurious signal there, so we have to measure the level at a 1 MHz higher frequency and get -36.46 dBc. At 1.29 GHz, the maximum level beyond Nyquist is measured as -13.25 dBV. At a random frequency of 1.462 GHz, the level is -40.7 dBc and we won’t see much less as the initial spectrum has already hinted on.
Here is the frequency range around 1.5 GHz:
SDS6204_Pro_H12_FFT-Sinc_Gf-AWG1200_1.25-1.75GHz
There is absolutely nothing special to see, except for the pulse spectrum at levels between approximately -42 and -47 dBc extending to 2 GHz.
In the range from 1.75 – 2.25 GHz the spectrum decreases in level very slowly, yet not falling below -48 dBc.
SDS6204_Pro_H12_FFT-Sinc_Gf-AWG1200_1.75-2.25GHz
--- Quote from: gf on March 26, 2024, 09:23:56 am ---EDIT: And where does the spur at 1.25GHz come from? Is it from the siggen or from the scope? Iguess the latter. The SA will reveal.
--- End quote ---
Yes, even without SA I could verify that this stronger spur comes from the SDS6204. But here is the proof from the SA:
SA44_Sinc_Gf-AWG1200_1.5GHz
gf:
Thanks for trying. My conslusions are:
1) We cannot go higher than 1000-1050 MHz if we want to avoid the "hill" between 1250 and 1000 1400 MHz (i.e. we should not see this hill with the previous 950 MHz impulse).
2) The stopband attenuation of the digital upsampling/reconstruction filter seems to be not more than about 40 dB.
The next point of interest would be the region around 3900 MHz (say +-300 MHz). That's where the analog reconstruction filter is mostly challenged.
I think you'll understand what I mean if you look at the attached figure, which shows what the (continuous) staircase output of an ideal DAC is expected to look like after 2x upsampling to 5 GSa/s with a digital reconstruction filter. This staircase is the signal going into the analog reconstruction filter.
And the second figure shows the spectrum of this staircase (and yes, it is not bandwidth-limited, but repeats periodically, decaying proportionally to 1/f). The "valley" up to 3600 MHz was already suppressed by the digital reconstruction filter (which is very helpful), but anything we see in this spectrum beyond 3600 MHz must still be eliminated (or sufficiently attenuated) by the analog filter. I have also drawn in orange what the frequency response of an example analog filter might look like.
EDIT:
--- Quote from: Performa01 on March 26, 2024, 03:53:20 pm ---Yes, even without SA I could verify that this stronger spur comes from the SDS6204. But here is the proof from the SA:
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Looks like typical interleaving spurs (offset spurs). Calibration may help, but I guess that the resolution of the calibration is limited to integral ADC codes, while fractional resolution might be required to get rid of the spurs.
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