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Rigol DHO804 Test and Compare Thread

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Martin72:
Hi,
I just wanted to go through various time bases and capture the displayed values in the FFT window via screenshot, once with automemory, once with max memory.
But there are already inconsistencies at the beginning...
I started with 2µs/div., was over at 10µs, went back to 10µs/div.
And I have 2 different resolutions (RBW)
How does that work... ???
Oh yes, the unit Hz is not displayed at any time.
Instead, there are decimal numbers such as 19.99k, and a sample rate of 1.26GSa/s was also displayed...
I'll continue, but here are two pictures with the different RBW with the same time base

Performa01:

--- Quote from: dmulligan on December 01, 2023, 05:47:19 pm ---The discussion about FFT has been very interesting and made me fire up my DHO804 to see what it can do.  I was disappointed when I couldn't select any of the math operations as a source.  I checked and according to the manual FFT on the DHO800 only works with one of the 4 channels as the source.  It doesn't work from either a math or reference source.

--- End quote ---
It cannot do an FFT on reference traces, because these are heavily decimated (maybe even screen) data. You would need so called Memory Traces for this, which are similar to Reference traces, but with the the original data, hence original sample rate.

It's similar for math traces. Math channels as a source for math (aka math on math) is an advanced feature, and makes mostly sense in scopes that work on undecimated data for the math.

gf:

--- Quote from: TurboTom on December 01, 2023, 11:15:28 am ---Wouldn't worry too much about this since the most relevant window functions von Hann, Blackman-Harris and rectangular are available and appear to work correctly.

--- End quote ---

I would. A flattop window is important to get good amplitude accuracy for frequencies that are not integer multiples of the bin spacing. The maximum amplitude error of the flattop window 1) is only 0.016 dB. Hann, Blackman and various others still have significant scalopping loss for frequencies that fall in the middle between two bins. Blackman-Harris has also ~0.83 dB. And a rectangular window leaks as hell if the frequency is not an integer multiple of the bin spacing.

1) I consider the Matlab variant here, but there exist several other variants too

ebastler:

--- Quote from: Performa01 on December 01, 2023, 06:12:38 pm ---Don’t make it more complicated than it is. We can of course just select the record length so that it makes for the desired FFT length.
We can also take the freedom to select the record length using one criterion and a different criterion for the FFT length. What’s wrong with that?

--- End quote ---

Nothing wrong with that, of course. But on the other hand, I can't see where the serious omissions in adjustable parameters are in Rigol's FFT implementation. That's what I was trying to understand, after hearing that complaint many times from the Siglent crew.

It seems to me that you can adjust all relevant parameters on the Rigol: Record length and sampling interval (and hence FFT frequency step) via the scope's regular acquisition and horizontal controls; center frequency and span via the FFT dialog (and the touch screen too).

I get the impression that there is mainly a difference in approach here: Rigol sees this as a regular time series acquisition, with its standard controls, followed by a specific math operation -- while Siglent pulls all parameters together in an FFT-specific menu, to make FFT mode behave similar to a spectrum analyzer.

Having said that -- yes, Rigol's displayed "RBW" is not an RBW and lacks a unit, and Rigol really should implement Averaging and Max Hold modes! I hope they do that sooner rather than later...

Martin72:

--- Quote --- I hope they do that sooner rather than later...
--- End quote ---

Just had a look, even the MSO8000A does not offer more than what the DHO800 offers in terms of FFT.
But at least the unit is given with RBW, although I know this from the MSO5000 as well.
Apart from that, if you can't set the number of points, it would be great if the rigol could at least show how many points are currently available.
"Max. 1Mpt" (from the manual)doesn't say anything at first.

Apropos, It's been a long week, it's evening, you can have a bit of a mental block...
Like me right now, operating the rigol from the living room. ;)
Span= half of the current samplerate
RBW= 1/capturetime
If the timebase ist set to say 50µs/Div, capturetime should be 500µs, so RBW should be 1/500µs  = 2000Hz.
With the Rigol, I have an RBW of 5000(Hz) precisely under these parameters.
What else does the RBW depend on?
And:
Span is currently 625Mhz, which makes sense if the sample rate is 1.25GSa/s.
I know from siglentscopes that the FFT sample rate is equal to the sample rate of the time domain if the number of FFT points is greater than the number of points of the current memory points.
Since the FFT sample rate is the same, I assume that rigol "has" more than 625kpts for the FFT (unfortunately it is not displayed).
If I now set the memory from auto to manual and let's say 10Mpts, the FFT sample rate should go down.
But it doesn't, Instead, the RBW is now reduced to 2000Hz - i.e. to the value actually to be expected at 50µs/div.
And that's where my mental block is, I must be missing something. ;)

Edit: Forget it, the rigol drives me crazy...
While typing this post, the rigol shows 5K RBW until I´ve set the memory 10 Mpt as mentioned above.
So I make a pic from it, then I want to make a pic from the state before, switch back to Automemory and 625kpts - RBW remains 2K...
And it´s getting better:
OK, let´s change the timebase and go back to 50µs/Div.
Now I have 1K RBW.... ::)

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