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Rigol DHO804 Test and Compare Thread

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ebastler:

--- Quote from: Martin72 on December 02, 2023, 10:20:23 pm ---But how could you find out how many points are currently being used if this value is not displayed?

--- End quote ---

Seems like the number of data points listed in your "memory" column is also used for the FFT. With that assumption, the listed "RBW" (which is really delta_f) is consistent with the sample rate and number of data points given, delta_f = samplerate / memory.


--- Quote ---I don't understand the values at 500µs, these "crooked" values in the FFT, nor the memory, which is 625Kpt before and after, but 781.25kpt in this time base.

--- End quote ---

Bit strange indeed. It looks like they wanted to maintain the systematic 500/200/100 cadence for delta_f. If delta_f = 200 Hz and sampling rate = 156.25 MHz are given, you land at that odd number of data points.

gf:

--- Quote from: Martin72 on December 02, 2023, 10:20:23 pm ---FFT again, going through several times/div.

--- End quote ---

Have you ever seen a FFT sample rate different from the acquisition sample rate?
What happens if you increase the memory depth beyond 1M points (i.e. beyond the maximum FFT size)?

Martin72:
Right here and now, just shooting from the hip:
500µs Auto Memory, 500µs 10Mpts.
Note the FFT sampling rate.

ebastler:

--- Quote from: Martin72 on December 02, 2023, 11:45:34 pm ---Right here and now, just shooting from the hip:
500µs Auto Memory, 500µs 10Mpts.
Note the FFT sampling rate.

--- End quote ---

The 156.4 can't be right; as gf said, how could it be different from the acquisition rate? Must be a rounding error, which also results in the awkward 199.9 bandwidth figure. We have seen in other places (axis scales and table entries) that Rigol struggles with proper rounding...

gf:

--- Quote from: Martin72 on December 02, 2023, 11:45:34 pm ---10Mpts.png

--- End quote ---

Seems that it down-samples*) if the record length is larger than the maximum FFT size.
And "RBW: 200" suggests that we got 625000 FFT points @125MSa/s.

So eventually it seems to do both here:
1) 10x down-sampling, which reduces 10M points to 1M points
2) Truncate the 1M points to 625k FFT points

*) And since the noise fllor does not roll off sharply a little bit below Nyquist (62.5 MHz), it's likely down-sampling w/o decimation filtering.

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