Products > Test Equipment
REVIEW - Rigol DS2072 - First Impressions of the DS2000 series from Rigol
tinhead:
--- Quote from: Teneyes on July 14, 2013, 08:39:37 pm ---
--- Quote from: tinhead on July 14, 2013, 12:24:10 pm ---wondering a bit about that picture, FFT center is 3kHz (so full span 6kHz) but FFT sample rate 10kSa/s ? That didn't make any sense. Or is that zoomed in and moved to left side?
--- End quote ---
The display was 50Hz/div , span of the display =14 x 50 = 700Hz
therefore the display was from 2.65 - 3.35 KHz
"According to the Shannon Sampling Theorem"
--- End quote ---
i haven't asked about Shannon but about settings. It is not like you think it is, you can't simply count the visible DIVs and multiply with per DIV value as it could be zoom or part of zoomed window (which is here the case). Center is set to 3kHz, so worst case full span would be 6kHz, what is too much for 10kSa/s. And this is exactly what i asked, and the proper answer was "yes, it is zoom/cut/moved so span 2.65 - 3.35 KHz".
poida_pie:
we seem to be looking at the FFT function of the DS2072 right now.
Have a look at the results I get.
Input is via 50 ohm bnc, with terminator, from Rigol DG1022, 6.32 kHz sine, 1v p/p into
my DS2072 channel 1 200mV/div.
Even though I have chosen a timebase to yield a decent FFT sample rate for the input signal
to display the FFT (the purple text in the first image says "50.000kSa/s") it seems there are lots of
rubbish peaks in the FFT. There is no way the function generator has that kind of signal from a 1 v sine
into 50 ohms.
See the second image. This is from a custom program I wrote that obtains the raw sample data from the DSO
and then performs FFT. the db scale is incorrect. forget about that for this discussion, it is immaterial.
Notice how you see what you expect to see, a clear peak at 6.32kHz and another much smaller one at 2x 6.32kHz. This is to be expected from a cheap function generator. The specs say about 1% THD anyway.
The DS2072 sample rate in this case was 50 kSa/s. I download only the first 2048 samples of the 14,000 available and process it. I also draw the waveform's first hundred or so points above the FTT.
My program can obtain more of the samples, up to 8K and it shows the same story. I use 2048 samples each transfer
to save time and increase frame rates.
You can see the sample points too, they are drawn in white. So it's a 6.32kHz sin, sampled at 50 kSa/s.
The correct information is there. No rubbish peaks. (I dare not use the word alias or I will be shouted at again)
To get a realistic FFT of a signal on the DS2072 you need to speed up the time base to much faster than
the FFT display's sample rate. At 100K there are clear problems still, I need to go to 200kSa/s to get something
approaching reality. This is what I used for the third image.
I conclude by saying that the Rigol DSOs I have owned (DS1102e and DS2072) BOTH perform the same way with respect to generating FFT displays.
They use the display data to make the FFT, not the raw sample data.
This limits very severely the quality of the FFT results.
You need to choose a timebase or sample rate to provide a DISPLAY that has not much fine detail in it
if you want the DS2072 to perform an FFT on it. Using the DISPLAY data to produce FFT power spectra is a poor choice when I have shown the information IS PRESENT AND IS ACCURATE in the raw data.
I estimate about a factor of 20 at least improvement is possible with using the raw data.
I suspect it would take a long time for the DS2072 to produce FFTs from (portions of the) raw data compared with the display data since the latter is already scaled and processed in preparation for drawing on the screen. So the choice was made on that basis by Rigol.
tinhead:
--- Quote from: Teneyes on July 15, 2013, 04:56:08 am ---
The Rigol 'Center' is Not the Center frequency of the Scanned span as in a Spectrum Analyzer.
For Rigol DSOs (D2000,DS4000,DS6000),
The 'Center Freq.' is that Frequency that is set at the Center of the display.
--- End quote ---
When the center of screen is marked as "center", then on left and right side there are exact the same amount of DIVs. As long the FFT is from one edge of the screen to the opposite edge, the center marking is center of full span, a typical center frequency. When you move the FFT to left or right direction then of course that "center" marking if showing the actual frequency on the center line location, so when you move to max. right it will be 0 (or whatever is set to start freq) and when you move to left the full span freq. So whatever you set as DIV, the center marking (again, when position not changed) is always the center frquency of the span. When you zoom in, still the same situation - as long the FFT has been not moved to right or left direction.
tinhead:
--- Quote from: Teneyes on July 15, 2013, 08:34:44 am ---Here is what My DS2072 shows at the left end of the FFT freq.spectrum , always 0 HZ
see display 2 (Rigol center =0kHz )
--- End quote ---
really? Even of your picture on left side there is no zero
but as you said
--- Quote from: Teneyes on July 14, 2013, 08:39:37 pm ---therefore the display was from 2.65 - 3.35 KHz
--- End quote ---
and this is exact what i said here (zero to full span or range x to range y, depends on settings):
--- Quote from: tinhead on July 15, 2013, 07:49:52 am ---When the center of screen is marked as "center", then on left and right side there are exact the same amount of DIVs. As long the FFT is from one edge of the screen to the opposite edge, the center marking is center of full span, a typical center frequency. When you move the FFT to left or right direction then of course that "center" marking if showing the actual frequency on the center line location, so when you move to max. right it will be 0 (or whatever is set to start freq) and when you move to left the full span freq. So whatever you set as DIV, the center marking (again, when position not changed) is always the center frquency of the span. When you zoom in, still the same situation - as long the FFT has been not moved to right or left direction.
--- End quote ---
and the center is always a center of the screen, so when zero to full span the center marker is center frequency, and when range x to range y then it it still center frequency (of the specific range). Only when you move it's not center frequency anymore, which is exactly what it should be (but maybe you expecting something unexpected from Rigol? )
Teneyes:
--- Quote from: tinhead on July 15, 2013, 09:19:36 am ---and the center is always a center of the screen, so when zero to full span the center marker is center frequency, and when range x to range y then it it still center frequency (of the specific range). Only when you move it's not center frequency anymore, which is exactly what it should be (but maybe you expecting something unexpected from Rigol? )
--- End quote ---
@Tinhead
Here are more displays to show what the Rigol DSO does ?
The input in all displays is 3KHZ, that is the Carrier signal Peak shown
The displays are shown with the Center at 1k, 2k, 3k, 3k
you sure are making this Blog bigger
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version