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)
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.
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.