Products > Test Equipment
Rigol DHO804 Test and Compare Thread
gf:
--- Quote from: Performa01 on December 01, 2023, 02:37:15 pm ---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.
--- End quote ---
I'm just wondering: When does it make sense to average in the time domain instead of averaging the spectral power of the FFT bins?
I would always prefer the latter.
The former suffers from trigger jitter due to noise, which slightly misaligns the traces to be averaged.
OTOH, the sampling phase (or IOW the trigger point of the traces) does not matter at all for spectral power averaging.
--- Quote ---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
--- End quote ---
What is the blue "Ref" trace?
--- Quote from: rf-loop on December 01, 2023, 03:29:09 pm ---In DHO800 image by @Martin72 (Reply #956) it looks that it is simply sample frequency / FFT length. (62500000/625000=100)
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I agree with that. It is obviously just the FFT bin spacing, not the -3dB bandwidth of the corresponding spectrum analysis filter.
ebastler:
--- Quote from: Performa01 on December 01, 2023, 02:37:15 pm ---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.
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Yes, that's what I meant. Agree, if there is a dedicated averaging mode in the FFT, nothing else is needed. I just wanted to confirm my understanding what Averaging FFT mode does, and whether a secondary math averaging function is equivalent (if it can be applied to the FFT result, of course). Thanks!
--- Quote ---[...] 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?
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But what would the FFT operation do then? Split the long record into multiple segments, Fourier-transform them individually, and combine them via Averaging or Max Hold? If so, isn't that equivalent to setting a shorter record length in the first place and keeping the operation running?
(Honest question; I might be missing something.)
--- Quote ---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.
--- End quote ---
OK, thanks. So Max Hold would be used in scenarios where you sweep the input signal, as 2N3055 also suggested? It's a bit counter-intuitive to me that you would always do a full FFT during such a scan, rather than a Bode-plot style detection of the known fundamental frequency. If a system has strong distortion, say due to clipping, at a low input frequency, that might appear as an apparent response at a higher harmonic?
Anyway -- if it's a feature that is commonly available, it seems like an omission on Rigol's part not to implement it. Like averaging mode, it should be relatively low-hanging fruit.
Performa01:
--- Quote from: gf on December 01, 2023, 04:07:11 pm ---I'm just wondering: When does it make sense to average in the time domain instead of averaging the spectral power of the FFT bins?
I would always prefer the latter.
The former suffers from trigger jitter due to noise, which slightly misaligns the traces to be averaged.
OTOH, the sampling phase (or IOW the trigger point of the traces) does not matter at all for spectral power averaging.
--- End quote ---
Of course it makes no sense to average the acquisitions before an FFT is applied. It kills all modulation and other dynamic effects.
Yet I think I need not stress how important Average acquisition mode and/or math function can be when we do not intend to do the FFT processing. This mode even allows the scope to act as an autocorrelator to clearly display weakest signals deeply buried in noise, if only we have a synchronous reference signal for the trigger available.
And no, trigger jitter absolutely isn’t a concern on these machines, thanks to the incorporation of a fully digital trigger system with proper trigger point interpolator. It is measured in the picoseconds...
--- Quote from: gf on December 01, 2023, 04:07:11 pm ---What is the blue "Ref" trace?
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That is the frequency response in the 2nd Nyquist zone, i.e. from 1 GHz – 500 MHz 😉
dmulligan:
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.
Performa01:
--- Quote from: ebastler on December 01, 2023, 04:07:32 pm ---
--- Quote ---[...] 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?
--- End quote ---
But what would the FFT operation do then? Split the long record into multiple segments, Fourier-transform them individually, and combine them via Averaging or Max Hold? If so, isn't that equivalent to setting a shorter record length in the first place and keeping the operation running?
(Honest question; I might be missing something.)
--- End quote ---
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?
Of course the record length cannot be shorter than the FFT length. But it could be way longer.
As to the question what happens then – the same what happens whenever we capture e.g. 10 Mpts but have only 1 Mpts FFT available. The data gets decimated before FFT processing.
--- Quote from: ebastler on December 01, 2023, 04:07:32 pm ---OK, thanks. So Max Hold would be used in scenarios where you sweep the input signal, as 2N3055 also suggested? It's a bit counter-intuitive to me that you would always do a full FFT during such a scan, rather than a Bode-plot style detection of the known fundamental frequency. If a system has strong distortion, say due to clipping, at a low input frequency, that might appear as an apparent response at a higher harmonic?
--- End quote ---
Just because we can always find a scenario where something might not work well, this doesn’t make a certain feature less valuable. In my example it worked just perfect.
Of course we don’t have a frequency synchronous measurement and yes, because of the very high dynamic range of the FFT we can see the harmonics as false signals even from sources with low distortion – but these spurious signals get overwritten in most wideband measurements anyway – except for the cases where we want to measure the stop band of a LP filter of all things.
If we stick to a decent signal source (AWG or signal generator) the harmonic level shouldn’t be much of a problem. And if it still is, we can do a separate scan for the stop band alone - there are so many ways to solve a problem. Finding solutions is the main job of engineers, by the way. But for this, they need reliable tools and the knowledge how to use them properly – which includes a clear judgement what is going to work and what is not.
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