Products > Test Equipment
Functional comparison of R&S RTB2000, Siglent SDS2000X and Keysight DSOX1000
RBBVNL9:
Dear Performa01,
Many thanks, that is a very extensive analysis, and it is in line with my intuition as well. I think we nailed it! Now on a road trip, but I will check in more detail once I'm back in the lab.
--- Quote ---Meanwhile I had a totally different question: where does the claim of 128 kpts FFT-length for the RTB come from? I could not find the slightest hint in the datasheet or user manual.
--- End quote ---
It's in the R&S RTB2004 brochure (here or here).
Like with all information I use, I tried to provide the source in my comparison document. For the FFT points, I noted there (B, p.3), which means Brochure, page 3. PS like with all three scopes I look at, not all information is systematically in the data sheet / spec sheet, sometimes it's in the manual, sometimes in the brochure, or somewhere in other documentation...
Performa01:
--- Quote from: PeDre on July 23, 2022, 06:06:58 am ---Here are the read-in FFT data as a comparison to the screenshots from the RTB2000.
--- End quote ---
So what we can see is that in contrast to the “Acquired Points”, the "Data Points" are actually bins. The provided text files contain 64k data entries - and that is equivalent to 128k FFT-points.
We can also see the effect of the time gate. But the screenshots also seem to reveal a major limitation of the RTB approach:
It looks like the FFT analysis bandwidth is solely defined by the start/stop or center/span frequencies. In this particular example, the upper bandwidth limit is about 809 kHz. Now what the RTB obviously does is multiplying that by two and then rounding up to the next feasible sample rate, which happens to be 2.05 MSa/s for an FFT-bandwidth of 1.025 MHz. I get the impression that it’s not possible to select the spectrum view independently of the FFT-bandwidth. Is there a way to set the spectrum analysis application up for e.g. 100 MHz analysis bandwidth and then zoom into any narrow span like 1MHz for closer inspection?
For 131072 samples and 2.05 MSa/s we should get an acquisition length of 63.93756 ms.
It is not quite clear why the RTB reports a time gate width of 63.96 ms in case of the minimum RBW of 60 Hz, but maybe the reported sample rate is not entirely correct due to some rounding errors. In any case we can assume that all the acquired data is used for the FFT.
For a RBW of 350 Hz, the time gate is narrowed down to 10.97 ms, hence we get about 131072 * 10.97 / 63.96 = 22480 points = 11240 bins. This reflects the exact ratio of the resolution bandwidths, as was to be expected. The originally acquired bin data gets decimated to 60 / 350 * 64k by simply cropping them. This way, there will be no reduction in sample rate and no unexpected aliasing.
RBBVNL9:
>> This way, there will be no reduction in sample rate and no unexpected aliasing
For me, the ultimate test is to create a signal that results in a known aliasing tone. Then we can change (RBW) settings and confirm that FFT sample frequency remains constant. Will try when I’m back.
Performa01:
--- Quote from: RBBVNL9 on July 23, 2022, 07:06:26 am --->> This way, there will be no reduction in sample rate and no unexpected aliasing
For me, the ultimate test is to create a signal that results in a known aliasing tone. Then we can change (RBW) settings and confirm that FFT sample frequency remains constant. Will try when I’m back.
--- End quote ---
You're absolutely right - I already wanted to suggest that. You don't need a pulse generator - any generator that can provide sine waves up to a few megahertz should be fine.
If for example you use that 2.05 MSa/s setup again, then you can feed a 900 kHz and 1.15 MHz into the RTB in sequence. In both case, you should see a spectral line at 900 kHz. Then try different RBWs and see if it always behaves the same. I'm pretty positive that it will...
Performa01:
--- Quote from: PeDre on July 23, 2022, 07:35:52 am ---
--- Quote from: Performa01 on July 23, 2022, 07:17:14 am ---If for example you use that 2.05 MSa/s setup again, then you can feed a 900 kHz and 1.15 MHz into the RTB in sequence. In both case, you should see a spectral line at 900 kHz. Then try different RBWs and see if it always behaves the same. I'm pretty positive that it will...
--- End quote ---
This does not work like this. There are more FFT points than are displayed on the screen. You can zoom out in stop mode, and then the 900 kHz or 1.15 MHz signals appear.
--- End quote ---
Of course you have to perform this test without altering any parameters.
There are not more FFT points right from the start, but a good chance that the FFT will be recalculated with a different effective sample rate after altering the span even in stop mode, because the original raw acquisition was unlikely to be actually limited to 2.05 MSa/s.
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