Question related zu FFT in RTB2000 and Siglent SDS2000X Plus  - Page 1
Forum section: electronics / testgear

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 23, 2020, 04:03:05 pm
content: Hello, I have an FFT question for the experts here in the forum. The RTB2000 oscilloscope is based on the 64kpts-FFT and the siglent on the 2Mpts-FFT.I haven't done FFT in detail for a long while, and need a little bit support for that ;-)but are the 2Mpts at Siglent really better now or is that just marketing ?At the moment I would be interested in FFT in the audio (up to 20Khz) and I know that a soundcard is actually better suited for it ;-)Thanks,Frank

author: CH1
author_rank: Newbie
timestamp: October 23, 2020, 07:15:02 pm
content: I just want to point out that the RTB2004 has 128K points FFT, plus a 10-bit ADC for a lower noise floor.However, one thing that has annoyed me is that it doesn't let you choose the number of FFT points. If you are trying to look at low frequency signals the RTB insists on filling the entire 128K FFT memory before computing it, which takes an awful long time.

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 24, 2020, 05:23:00 pm
content: Quote from: CH1 on October 23, 2020, 07:15:02 pmThanks for you correction related to 128K Points FFT.How long does the calculation take with the RTB2000 ?And does anyone know how long the FFT calculation within the SDS2000x Plus will take ?

author: Performa01
author_rank: Super Contributor
timestamp: October 25, 2020, 09:11:37 am
content: Quote from: Frank_MV on October 23, 2020, 04:03:05 pmOf course it's better, because it means much more versatility. The number of FFT points (aka FFT length) determines the frequency resolution for a given FFT bandwidth.Look here:https://www.eevblog.com/forum/testgear/rohde-schwarz-rtb2002-vs-siglent-sds2104x-plus/msg3239832/#msg3239832It means that the Siglent can have 16 times finer resolution in the frequency domain.It also means that you can have 0.1 Hz frequency steps, hence about 0.4 Hz RBW (resolution bandwidth) for 100 kHz FFT bandwidth. The max. FFT length is selectable on the Siglent, so you can go down even to 64 points if you don't need any higher resolution - in short, you have the full versatility.Quote from: Frank_MV on October 23, 2020, 04:03:05 pmBoth the Siglent and the R&S scopes have excellent low noise frontends, but as with all this modern split path input buffer designs, this is only true for frequencies above some 100 kHz. Below that, noise rises significantly. This has to do with the input protection of the LF/DC path, which has to withstand e.g. 400 volts. So this is certainly not an microphone amplifier design...

author: tautech
author_rank: Super Contributor
timestamp: October 25, 2020, 09:30:46 am
content: Sold out of SDS2000X Plus however SDS5000X has almost identical FFT specs and performance therefore a few screenshots as examples.1 KHz sine 0dB from the 14 bit SDG1032X.SDS5000X DSO is 8 bit.FFT sweeps are ~2s in 2 Mpts mode, roughly halving for 1 Mpts and so on down the FFT Mpts selection menu.Max mem depth selected can also impact FFT speed as can FFT Averaging or Max Hold selections however FFT settling time is longer for these but the sweep time remains the same.Just like a SA the Span setting dictates the time required to get a result.Nevertheless with the OSD info in the following screenshots you can nearly all the settings used.

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 26, 2020, 02:21:19 pm
content: Many thanks for your answers and the helpfull images.br,Frank