Products > Test Equipment
Rigol DHO804 Test and Compare Thread
Martin72:
OK, it´s a bug... ;)
When changing the timebase, the scope "forget" (often but not everytime) to refresh the RBW value.
If you then move the position (vertical offset) of the FFT, the RBW value is updated. :phew:
Mechatrommer:
--- Quote from: Martin72 on December 01, 2023, 09:04:36 pm ---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.
--- End quote ---
istr when i ordered my dho804 few weeks ago, someone keep complaining about dho800/900, how 900 (same HW as 800) can easily aliased (by struggling hard enough to show gibbs effect phenomenon as aliasing etc) how bad things including its fft, so i let him find the formula by himself. usually if people think he is clever i'll let him find the formula himself... not sure if he already figure that out... the formula is very simple..
number of FFT point (bins you saw plotted on screen) = FFT frequency range / rbw
FFT frequency range = DC - sampling rate / 2.
both frequency range and rbw can be seen on screen..
working example: sampling rate: 1.25GSps, hence FFT range = DC - 625MHz (sampling rate / 2), RBW (shown) = 2K (Hz).... so number of FFT bins = 625MHz / 2KHz = 312500 points (bins).
since FFT only use/show half of calculated points (another half is only mirror image), number of sampled points used to generate FFT is twice bins number. you are right what you have figured, dho800 has some bug on showing RBW, i reported in bug thread but not sure if people understood when i reported it.. i figure when you increase time/div and then back down/ reduce it again, it will show correct rbw, but not when you increase time/div.
so in conclusion, we can indirectly change how many FFT or sampled points used by dho800 by changing time/div and use formula above to figure how many points currently used for FFT, ymmv.
and btw, if you have traditional real/sweeping SA, you'll know you only have freq range and RBW figure, you wont have sampled/fft points count anywhere there is no such thing, so rigol is being traditious ymmv.
Performa01:
When trying to sound smart, people should at least post correct information.
That includes not using points and bins as synonym first, and making contradicting statements later by (accidentally?) stating something that sounds right, namely that FFT bins are half the FFT points.
One should also not confuse frequency step (df) with resolution bandwidth (RBW).
One should also understand the vast difference between a traditional swept SA and the FFT on a DSO – or even a combination of both to form a powerful realtime SA…
When in doubt, one can always look up the fundamentals in this old post (reply #23):
https://www.eevblog.com/forum/testgear/rohde-schwarz-rtb2002-vs-siglent-sds2104x-plus/msg3239832/#msg3239832
gf:
--- Quote ---number of FFT point (bins you saw plotted on screen)
--- End quote ---
In common terminology, the "number of FFT points" usually refers to the number of points in the time domain from which the FFT is computed, not to the subset of frequency bins which happen to be displayed.
gf:
When I see screenshots showing 62.5MSa/s, 625000 points, and RBW 100 (assuming that 100Hz is not really the -3dB RBW, but actually the frequency bin spacing), then I get the impression that the FFT size is the full record length of 625000 points, which would also imply that this FFT implementation can handle sizes that are not a power of two. Of course, there has to be a limit to the maximum FFT size, so the question remains what happens if the acquired records are longer than the limit. What subset of samples is selected? Start of the record? Center? Or are long records even split into multiple chunks of the maximum FFT size?
If we distrust the numbers displayed on the screen, ist it possible to export the FFT math trace to a text file? If yes, how many points does it export if full span (0...fs/2) is selected? The number of saved points and the difference between adjacent frequencies should give an indication for the actual FFT size and frequency bin spacing (unless only decimated screen data are saved :( - that's useless, of course).
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