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| FFT - Rohde & Schwarz RTB2002 vs. Siglent SDS2104X Plus |
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| jjoonathan:
Is that a gate cutting down the RTB to a fraction of the acquired length? |
| maginnovision:
Here is what I got with a 100kHz square wave and averaging(like the siglent seemed to be doing). Flat top window, dBm. The waveform fills the capture screen as well. I'm attaching another image with the window cut down from 1.13ms to just 500us and switched off high res mode. |
| 2N3055:
--- Quote from: maginnovision on September 19, 2020, 02:03:52 am ---Here is what I got with a 100kHz square wave and averaging(like the siglent seemed to be doing). Flat top window, dBm. The waveform fills the capture screen as well. I'm attaching another image with the window cut down from 1.13ms to just 500us and switched off high res mode. --- End quote --- Yes, thank you, that's better.. FFT is tricky, settings are critical. Here it is how it looks like, on 8bit scope with good FFT implementation. Averaging used. If you sample with very low sample rate, and feed FFT with very wideband signal (like square wave with many harmonics ), those frequencies that are too high for sample rate will get folded down (basically downsampled) and falsely shown as low frequency components. On a perfect square wave there should not be any even harmonics. Any even harmonics are either distortion (scope, siggen or interconnect) or aliasing from too low sample rate. |
| tautech:
--- Quote from: maginnovision on September 19, 2020, 02:03:52 am ---Here is what I got with a 100kHz square wave and averaging(like the siglent seemed to be doing). Flat top window, dBm. --- End quote --- Here zero averaging, instead Max Hold which does indeed do a type of average in each sweep. Signal source SDG6022X 0dBm Tee'd to supply SDS2104X Plus and SVA1032X so unmatched load for the AWG but serves well enough for this demonstration. It need be noted the SVA in FFT mode cannot see the even harmonics only the fundamental and its odd harmonics. This is something about square wave harmonics I have not understood until recently where it was discussed in this post: https://www.eevblog.com/forum/beginners/what-an-oscilloscope-recommended-for-a-woman-passionate-about-electronics/msg3216564/#msg3216564 Why the scope can see the even harmonics IDK. :-// |
| 2N3055:
--- Quote from: tautech on September 19, 2020, 10:13:19 am --- --- Quote from: maginnovision on September 19, 2020, 02:03:52 am ---Here is what I got with a 100kHz square wave and averaging(like the siglent seemed to be doing). Flat top window, dBm. --- End quote --- Here zero averaging, instead Max Hold which does indeed do a type of average in each sweep. Signal source SDG6022X 0dBm Tee'd to supply SDS2104X Plus and SVA1032X so unmatched load for the AWG but serves well enough for this demonstration. It need be noted the SVA in FFT mode cannot see the even harmonics only the fundamental and its odd harmonics. This is something about square wave harmonics I have not understood until recently where it was discussed in this post: https://www.eevblog.com/forum/beginners/what-an-oscilloscope-recommended-for-a-woman-passionate-about-electronics/msg3216564/#msg3216564 Why the scope can see the even harmonics IDK. :-// --- End quote --- Try enabling 20MHz limiting and try again with same settings.. We are looking at 1 MHz bandwidth anyways. Also enable averaging, not peak hold. Try with different V/div settings. With spectrum, we want least distortion with best noise floor. Try capturing at 200mv/div. That will decrease dynamic range but lower distortion in scope frontend. What you see now?. My point is that FFT must be understood first, than you must get used to implementation on your scope. After that you need play with it for a while to characterize it's strengths, weaknesses, where the spurs are etc. It is same as spectrum analyser. It is practically another whole instrument inside scope that you have to learn in detail to be able to use well.. EDIT: posted too soon,sorry.. in bold what was added later.. |
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