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Rigol DHO804 Test and Compare Thread

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ebastler:

--- 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?

2N3055:

--- 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 ---

Basic problem is that this is a measurement instrument but nobody knows what it shows.

If it measures and shows kilobananas then say that so user knows.

If RBW is RBW in a SA sense then it should be configurable and documented how it does rescaling.
If they use RBW for frequency bin width, that is simply wrong.

Also since real FFT parameters are not shown, and RBW is aparently dimensionless number, how you know it is correct in what it shows. And since you can see how the rest of the scope is tested, at this point I would not trust FFT for any measurements.
Until this is untangled, FFT is simply a graphical visualization, like Fungus nicely shows in his multi window image. You have 3 spectrum windows without basic parameters shown. You know the window function used and resampled sample rate. nothing else.

2N3055:

--- Quote from: ebastler on December 01, 2023, 01:47:35 pm ---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 ---

1. Max hold is as much used as averaging. For instance frequency sweep..
2. Number of bins can be separate from sample rate and acquistion length.
3. I apologize for confusion. I mentioned RBW as it is now only thing they have. If they have RBW then it should be configurable like in a propper SA. In which case by setting RBW, scope changes number of bins to get close and then recalculates FFT bins to RBW bins taking into consideration used window.

ebastler:

--- Quote from: 2N3055 on December 01, 2023, 01:51:57 pm ---Basic problem is that this is a measurement instrument but nobody knows what it shows.

If it measures and shows kilobananas then say that so user knows.

If RBW is RBW in a SA sense then it should be configurable and documented how it does rescaling.
If they use RBW for frequency bin width, that is simply wrong.

Also since real FFT parameters are not shown, and RBW is aparently dimensionless number, how you know it is correct in what it shows.

--- End quote ---

Sorry, but I feel this did not add anything to the prior discussion besides negative emotions. Can we please keep separate matters separate, and be specific about them?

(a) What Rigol displays is apparently not RBW but frequency step, so they should fix either the label or the way the calculate it. Already said more than once.
(b) Of course it should always be displayed with a unit. Already said more than once.
(c) Which "real FFT parameters" are you missing? The number of FFT points, as already said more than once; what else?

ebastler:

--- Quote from: 2N3055 on December 01, 2023, 01:57:14 pm ---2. Number of bins can be separate from sample rate and acquistion length.

--- End quote ---

How would that be done? Doesn't the time resolution and length of the transformed time series define the spectral resolution and range of the FFT? Unless you do a piecewise FFT of partial segments of the time series, or preprocess the time series by averaging etc., of course.

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