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Problem with understanding the noise integration

Greetings,

I want to understand the total integrated output noise of my circuit. Using "Results > Direct Plot > Main Form" I can get the expression for plotting the squared of my output noise (which I believe to be the Power spectral density):

(getData("out" ?result "noise")**2)

This will give me the following graph:

I think by integrating this waveform and then take the square root should give me the rms value of my integrated output noise. (Assuming similar frequency range setup)

sqrt(integ((getData("out" ?result "noise")**2) 0.1 1k " " ))

(Which is indeed agrees with the value given in my Noise Summary.)

To my understanding, integrating the graph above from 1mHz to 1Hz should give a much larger result compared to the range from 1Hz to 1kHz (here there's almost nothing). However the result is very contradictory to my understanding:

sqrt(integ((getData("out" ?result "noise")**2) 1m 1 " " )) = 6.205E-6

sqrt(integ((getData("out" ?result "noise")**2) 1 1k " " )) = 10.4E-6

Here's the attached .vcsv file exported from my noise curve.

I'm still learning myself so any feedback would be extremely appreciated.

Thanks a lot!

Andrew Beckett over 4 years ago

You're being confused by the log x-axis. Try plotting on a linear x-axis (right mouse button over the x-axis allows you to change it). The bandwidth of the second integration is 1000 times higher than the first integration, and so whilst the magnitude is smaller, you have a much larger x-range to integrate over. A narrow peak on a log x-axis looks big, but it's fairly insignificant over the wider range.

Andrew
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Phong Phan over 4 years ago in reply to Andrew Beckett

Thank you Andrew! That's very silly of me for not realizing.
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