Author Topic: Question related zu FFT in RTB2000 and Siglent SDS2000X Plus  (Read 317 times)

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Offline Frank_MV

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Question related zu FFT in RTB2000 and Siglent SDS2000X Plus
« on: October 23, 2020, 04:03:05 pm »
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
 

Offline CH1

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Re: Question related zu FFT in RTB2000 and Siglent SDS2000X Plus
« Reply #1 on: October 23, 2020, 07:15:02 pm »
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.
 

Offline Frank_MV

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Re: Question related zu FFT in RTB2000 and Siglent SDS2000X Plus
« Reply #2 on: October 24, 2020, 05:23:00 pm »
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.

Thanks 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 ?

 

Online Performa01

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Re: Question related zu FFT in RTB2000 and Siglent SDS2000X Plus
« Reply #3 on: October 25, 2020, 09:11:37 am »
but are the 2Mpts at Siglent really better now or is that just marketing ?
Of 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/#msg3239832

It 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.

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 ;-)
Both 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...
 
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Offline tautech

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Re: Question related zu FFT in RTB2000 and Siglent SDS2000X Plus
« Reply #4 on: October 25, 2020, 09:30:46 am »
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.
Avid Rabid Hobbyist
 
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Offline Frank_MV

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Re: Question related zu FFT in RTB2000 and Siglent SDS2000X Plus
« Reply #5 on: October 26, 2020, 02:21:19 pm »
Many thanks for your answers and the helpfull images.

br,
Frank
 


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