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Rigol DHO804 Test and Compare Thread

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2N3055:

--- Quote from: ebastler on December 01, 2023, 02:09:39 pm ---
Unless you do a piecewise FFT of partial segments of the time series, or preprocess the time series by averaging etc., of course.


--- End quote ---

Exactly like that....

rf-loop:
Just for satisfy some peoples curiosity, not compete with any,. Here one poor sample using SDS1204XHD (at this time it is China only model)
Around same kind of signals but need run low level LSB peak instead of USB  and 10MHz for keep generator high peak level around 20Vpp (because >10MHz can not this level) (Looks like this is bit too challenging for generator what I use  and it somehow produce also USB peak etc...  )

FFT F1  normal, and Hanning
FFT F2  Max Hold, Hanning
FFT F3  Average 16, Hanning
Eta: do not care Trigger setting in image. it is Auto and other things there do not matter anything here.

Performa01:

--- Quote from: ebastler on December 01, 2023, 01:34:06 pm ---
--- Quote from: Performa01 on December 01, 2023, 01:22:51 pm ---Average acquisition mode and/or math function are something completely different than averaging of an FFT trace.

Full featured DSOs have Average as
1.   Acquisition mode, yet with reduced max. memory depth
2.   Math function
3.   Substitute for VBW setting of an SA for the FFT traces.

--- End quote ---

But if a Math function 2. were available which averages over successive sweeps, couldn't it be be applied to the FFT to obtain 3.?

--- End quote ---
I’m not sure if I understand correctly what you mean. Of course there is a difference whether we average the records, i.e. the waveform in the time domain, which are the basis for the FFT, or the converted math result (FFT) which is now in the frequency domain.

If you mean an advanced scope with math on math, where you want to apply averaging on the FFT result, either by two math traces where one acts as input for the other, or defined in a formula in a single channel. I’ve never tried it, but would not be surprised if FFT (which delivers a frequency domain result after all) would be about the only math function that cannot act as a source for other (time domain) functions.

Since Averaging is included in the FFT package anyway, why bother.



--- Quote from: ebastler on December 01, 2023, 01:34:06 pm ---As stated in my questions to TurboTom, a few posts above, I have been wondering that too. If the only flaw in the displayed "RBW" is that it is not actually an RBW but a frequency step, I can live with that (although Rigol should fix it, of course, and add a unit while they are at it). But I did see some screenshots or videos earlier where unexpected "RBW" values were displayed, which did not seem to make sense even as a frequency step and assuming a Hz unit. Maybe Tom or Martin can shed some light onto whether and when this happens?

--- End quote ---
Yes, you can live with it – once you know what it actually means. It’s annoying nevertheless – and up to now, you only know what I have found out by accident. Rght now you’re expressing doubts what this dimensionless number actually is. I can only tell what I see in a few screenshots, so others will have to verify it.



--- Quote from: ebastler on December 01, 2023, 01:42:30 pm ---
--- Quote from: Martin72 on December 01, 2023, 12:51:46 pm ---
--- Quote --- Is there something in the FFT which you would like to adjust but can't?
--- End quote ---

Number of points for example, I'm used to being able to manually set the number of memory points (which are also not displayed on the rigol).

--- End quote ---

I thought that's set via the acquisition memory setting (up to 1 MPts)? Is the issue that it's not in the FFT dialog, or that the available steps are too coarse? Or does that setting not actually set the number of FFT points at all?

--- End quote ---
Yes, this is one possibility, but can’t you imagine situations, where you have to capture long records, exceeding 1 Mpts, yet want a shorter FFT, maybe to get a wider RBW?



--- Quote from: ebastler on December 01, 2023, 01:47:35 pm ---
--- Quote from: 2N3055 on December 01, 2023, 01:41:05 pm ---1. FFT mode : Normal, Average, Max Hold (at least)
2. Number of points/bins
3. RBW cannot be set. It is displayed in window title (which by the way is not visible if you do multiple windows, negating the benefit) and is some kind of  function of timease. You twiddle timebase left and right until you like how it looks.

--- End quote ---

1. I think we all agree that lacking Average mode is an annoying omission, as discussed separately above. I don't know about the relevance of Max Hold mode, but that's probably due to my lack of knowledge about some applications (regulatory/EMC maybe?).

2. As discussed, it's set via the acquisition memory size in my understanding. That way of doing it may be lacking something; see my questions to Martin just above.

3. Now I am confused. If you choose the window function, record length and sampling rate -- how can you set the RBW independently?

--- End quote ---
Ad 2: see above.

Ad 3: Max Hold is at least as important as Averaging. It allows the scope to do frequency response plots. Other then Bode Plot, this can use the full bandwidth up to half the sample rate and it can plot the frequency response of the scope itself. The attached screenshot shows an example of a measurement graph demonstrating the frequency response of an SDS2000X Plus in 10 bit mode, that could not be produced otherwise.


SDS2354X Plus_FR_BFull_1GSa_500MHz_10bit

TurboTom:
RBW value that gets displayed on DHO1000 FFT screen is 3dB reolution bandwidth in units kHz. To me it appears that on the DHO800 screen, the RBW value is in units Hz but I may wrong.

rf-loop:

--- Quote from: TurboTom on December 01, 2023, 03:14:53 pm ---RBW value that gets displayed on DHO1000 FFT screen is 3dB reolution bandwidth in units kHz. To me it appears that on the DHO800 screen, the RBW value is in units Hz but I may wrong.

--- End quote ---

In DHO800 image by @Martin72 (Reply #956)  it looks that it is simply sample frequency / FFT length.  (62500000/625000=100)
And then there read just plain RBW:100   and also then same RBW even for Hanning and Flattop..  They did not use  window factor but print only Bin interval as RBW. RBW we normally think as Resolution Band Width.
Of course it can say it is RBWBins  but more commonly with spectrum analyzers etc we use RBW3dB  and for some purposes RBW6dB (example EMI measurements). But tightly speaking it is not absolute error to tell RBWBin but why then not tell it using example  Δf as example Siglent use what do not leave anything unclear. Also then can tell "normal" RBW also and its width depending used window. As can see example in my last image.

Well, if someone names it RBW, shame on him (Rigol) and even without a unit... well, what about the units of measurement... this is just a measuring device where guessing is part of the game. (sarc.)

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