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Functional comparison of R&S RTB2000, Siglent SDS2000X and Keysight DSOX1000

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RBBVNL9:
dear pdenisowski,


--- Quote --- 1) You can change the horizontal scaling of the spectrum using the horizontal knob
2) Autoset sets both the channel parameters and the FFT parameters
--- End quote ---

Thanks!

I think I had (1) already covered in my video.

Concerning (2), yes, that is interesting, had not realised it. It is not mentioned in the FFT chapter of the RTB manual, though, and only quite limited in the manual's general section (page 51). So this feature could be highlighted a bit more in the manual!   

nctnico:

--- Quote from: RBBVNL9 on July 15, 2022, 07:15:04 am ---I also see RBW shown on the screen of some other brands of scopes with FFT, such as the GW INSTEK MDO-2000E (screenshot here).

--- End quote ---
Actually that is a spectrum analysis mode. Basically you can divide oscilloscopes in two categories:

1) ones that offer plain FFT and depending on time/div you get a certain bin width (frequency resolution)
2) ones that offer a spectrum analyser-ish interface that optimises time/div and other parameters based on a requested frequency resolution

BTW: The GW Instek 2000E series can do both; there is an FFT mode and spectrum analysis mode (the latter may need some magic using a key generator to enable it on non-MDO versions).

IMHO having a spectrum analyser style interface on top of FFT makes it easier to use. Gettting FFT to do what you want can be tedious while a spectrum analyser interface allows you to go from what you want and let the oscilloscope sort out which settings to use.

RBBVNL9:
For my RTB, I am doing the calculations as posted above by Performa01 and rf-loop, but I’m not getting what I would expect. Probably I’m doing something wrong; I'm sure someone can help me.

If I feed the RTB with an 800kHz sine, set its FFT to 8kHz centre and 200kHz span. The RTB has 128k FFT points (RTB2004 product brochure V06.00). The scope reports it runs at 2.27MSa/s. If I get Performa01’s calculations right, that means bins of 17.7Hz (*). Using a Flat Top window, I’d expect (on the basis of Performa01 and rf-loop’s posts above) an RBW that is about 3.73 times larger, which would be 66.1Hz. And, indeed, I can set my RTB to a minimal RWB of 66.6Hz, close enough, so all seems right. So far.

Now choosing a Hanning window. The scope still reports it runs at 2.27MSa/s, so the bin size should remain unchanged 17.7Hz, but the window-specific factor for Hanning is around 1.62 (source), so I’d expect an improved RWB of 17.7Hz*1.62=28.7Hz. But, to my surprise, when I try to set the RBW on the scope, the minimum I can set it to is 119Hz (so it's larger instead of smaller than in the Flat Top window scenario). I tried several other scenarios (8kHz sine, 8MHz sine, different span settings), but the results are always different by the same degree. I’m attaching the calculations.

The only possibility that comes to my mind is that the RTB FFT points is not fixed at 128k, but can go to lower values depending on the settings (the window used) and, in the above Hamming window example, would have dropped to simewhere in the range of 32k. But that is pure speculation, I guess it's more likely I am making a mistake in the above. Anyone?

(*) I did those bin size calculations as well for the SDS and they are always exactly as expected.
 

nctnico:
It is possible the FFT size varies on the RTB. In recent firmware R&S improved the performance when using low frequencies but I don't know whether 8kHz counts as 'low'. If you use a higher frequency (8MHz for example) I guess (!!!) variable FFT size won't be used. It is worth a try.

Performa01:

--- Quote from: RBBVNL9 on July 15, 2022, 04:42:12 pm ---For my RTB, I am doing the calculations as posted above by Performa01 and rf-loop, but I’m not getting what I would expect. Probably I’m doing something wrong; I'm sure someone can help me.

--- End quote ---

I have no idea what's going wrong here, but a few general remarks.

Never forget that there can be two different sample rates: the first one is the sample rate of the scope that depends on its maximum record length, the second one is the FFT sample rate, that depends on the FFT length hence might be further decimated. So it's essential to look at the FFT samplerate, like it's reported by the SDS in the FFT info block and not the sample rate in the timebase tab. I just mention this for completeness as this obviously isn't your problem right now.

The only other thing I can think of is the beauty of the sprectrum analyzer mode, which not only makes things easier for the inexperienced, but might also do some funny and unexpected things behind your back. Such as Keysight who prove unable to provide a valid averaging, but do some additional HiRes unexpectedly and claim it's because they think the users want to reduce noise anyway. (The truth is, that the tiny secondary buffer of just 64 kpts forces decimation and they need to use HiRes in order to avoid aliasing.) I cannot know, but since the FFT in the RTB appears to be somewhat limited in certain regards, you cannot rule out some unexpected tricks behind the scenes to overcome certain limitations.

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