EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: argg on April 29, 2016, 09:42:39 am
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Being a long time user of analog scopes I am not familiar with the FFT function of DSOs. I understand the FFT in this case is rather rudimentary and not of much use but still have some newbie questions
I connected my function gen, fed 1 KHz to the scope and got these screenshots. Please help me understand what I am seeing :)
- two strong harmonics can be seen, pointed by arrows, the 3rd and 5th. Is this the inherent distortion of the generator or some other nonlinearity?
- as the scale is set at 10 dBV does it mean the 3rd is only about 35 dBV below the fundamental
- is there a way to have the fundamental moved to far left so that the full window can be used?
(http://i.imgur.com/767Vnzk.png)
(http://i.imgur.com/t5ezqUO.png)
thanks in advance
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I haven't ordered my Rigol yet, but the fundamental looks lined up with the trigger (the orange T arrow at the top), so try putting the trigger display earlier (using the horizontal position control), which should move the whole graph to the left.
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Select mode = memory, and use the center attribute button to move
the display. Will occupy ~ 3/4 of screen.
As far as utility of FFT ve traditional swept spectrum analyzer. The latter generally has
a much greater dynamic range, > 100 db, and a 50 or 75 ohm environment. That being
said a great deal of work goes on where 60 db or even less all thats needed, and very handy
mixed mode display. I find both approaches of equal value depending on task at hand.
Regards, Dana.
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I connected my function gen, fed 1 KHz to the scope and got these screenshots. Please help me understand what I am seeing :)
- two strong harmonics can be seen, pointed by arrows, the 3rd and 5th. Is this the inherent distortion of the generator or some other nonlinearity?
Yes, your FG isn't putting out a very clean sine wave. It's hard to tell at this scale but there seems to be a small glitch at the positive peaks of the wave, and the overall shape seems to be distorted towards being a triangle rather than a nice sinusoid. What kind of FG are you using?
- as the scale is set at 10 dBV does it mean the 3rd is only about 35 dBV below the fundamental
Yes, that's right.
- is there a way to have the fundamental moved to far left so that the full window can be used?
Yes, as danadak says, use the "Center" frequency setting to move the trace leftwards.
For different looking displays, you might also try different "Window" modes, and also the "Unit" can be changed. Cursors can also be used effectively in the FFT window.
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I haven't ordered my Rigol yet, but the fundamental looks lined up with the trigger (the orange T arrow at the top), so try putting the trigger display earlier (using the horizontal position control), which should move the whole graph to the left.
thanks, but moving the horizontal trigger point to the left does not make a difference to the FFT graph
Select mode = memory, and use the center attribute button to move
the display. Will occupy ~ 3/4 of screen.
Yes, as danadak says, use the "Center" frequency setting to move the trace leftwards.
thank you both, that did the trick :)
Yes, your FG isn't putting out a very clean sine wave. It's hard to tell at this scale but there seems to be a small glitch at the positive peaks of the wave, and the overall shape seems to be distorted towards being a triangle rather than a nice sinusoid. What kind of FG are you using?
it' s an old BK Precision 3011B that must have gone way out of spec to show so high distortion. I will try to find a decent one to confirm
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Distortion specs for 3011B are 1% for < 20 Khz, 2% above, thats
~ -40 db and -34 db respectively, depending on the way they com-
puted the distortion.
https://en.wikipedia.org/wiki/Total_harmonic_distortion
Regards, Dana.
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truth is I did not bother before to check the nominal distortion of the FG; then the FFT measurement does make sense. Given the opportunity I downloaded the 3011B pdf and I will at last rtfm :D
thanks :-+