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Understanding FFT

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Understanding FFT

Postby Ax2_ » Mon Jul 27, 2015 1:49 pm

Hello. Could you help me to understand FFT and Spectrum analyzer. See below some issues I can't understand.
Environment: [See attached image Environment]

Setup: Generator set to white noise which is about 800mV RMS. So far so good. [Image Setup]
A jumper from signal generator output to Channel 2 input. FFT Display mode: Average.

Changed to Spectrum mode and wave looks ok [Image Wave1]... however: I have set dBu to Y axis and 800mV RMS should be around 0dBu. Here it is somewhere -43dBu... why?

Lets change Spectrum bins few times between 512, 1024 and 2048. After 3-5 change the result looks like Wave2. This must be a bug. It is repeatable using low bin values.

Changed Y-axis to linear. Level of white noise is below 5mV. Hmm... I think I should see 800mV Linear level using 800mV RMS signal (of white noise in this case).

Ok. Back to logarithm Y axis. Lets change bin values 2048, 4096, 8192. dBu levels also changes (-57dBu, -60dBu, -63dBu). How come Bin width changes the resulting dB level? (wave3 = 8192 bins)

Axel.
Attachments
wave3.PNG
wave2.PNG
Wave1.PNG
Setup.PNG
Environment.PNG
Environment.PNG (3.02 KiB) Viewed 603 times
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Re: Understanding FFT

Postby Ax2_ » Mon Jul 27, 2015 7:41 pm

Spectrum range also affects to signal level. All kinds of weird issues when using White noise. Cannot get much out of White noise -option... but...

...I switched to Sine wave+Sweep mode and everything started to work properly. Levels are shown correctly and waveforms do not get crazy etc etc.

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Re: Understanding FFT

Postby Martyn » Mon Jul 27, 2015 8:22 pm

I am out of the office and on a mobile device so can't see the pictures clearly but I think what you are seeing is:-

With a sine wave the time domain amplitude will map to a frequency domain level at a specific frequency.

White noise is constant power across the spectrum so the time domain amplitude is spread across all the frequency components hence the lower levels.
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Re: Understanding FFT

Postby Ax2_ » Fri Jul 31, 2015 7:23 pm

Hello,

Thanks for the reply. This sounds like an explanation. However I need to know an exact amplitude so white noise is not an option in this case then. It is ok since preferable methods is available.

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Re: Understanding FFT

Postby Martyn » Fri Jul 31, 2015 8:38 pm

If you can explain exactly what you are trying to achieve then hopefully we can help.
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Re: Understanding FFT

Postby Ax2_ » Sun Aug 02, 2015 4:46 pm

Hello,

Basically it was all in the initial message, but in general I'm doing an output analysis of a filter. I thought that I can do it easily with White noise.
One of the critical parameter is an exact input level and that requirement is met when sweep-method is being used (rather than White noise-method). At least it worked for me for now.

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Re: Understanding FFT

Postby Martyn » Mon Aug 03, 2015 6:46 am

Using a swept sine is the correct approach for this.
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