I am having trouble believing or understanding what I am seeing on this scope with regards to the FFT feature. I started off trying to measure the noise on my home built amplifier and I have been playing with it for several days now but I'm new to FFT and while I may not fully understand what I am seeing I surely don't believe it. Long story short, I'm now looking at the noise level of the scope itself meaning I'm connecting the probes input to its own ground and looking at the FFT trace on the scope. Right now it is showing a noise floor (average) of -12 dbVrms. Does that sound accurate for a probe shorted to ground? Seems it should be much lower than that. I measured the noise on my amplifier output and the average there is -8 dbVrms. This does not seem nearly low enough either. I was expecting something around -45 dbVrms. The amp may be a little noisier that normal because it has a high gain of x1000 but the dbVrms readings don't match up with my peak to peak signal on the scope channel which is about 4mv peak to peak. I'm using AC coupling on the scope which is supposed to provide better FFT performance. I also noticed that every time I adjust the scope channels volts/division the average dbVrms value of the FFT plot changes drastically. Basically if I increase the volts/division for CH1 from 10mv to 1v the dbVrms reading goes up from -8 dbVrms to .6 dbVrms. Should this be happening?
thanks
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thanks
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