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Why the overall RMS level of my Spectrum mismatches with the sum of spectrum lines ?

ENBW is the key... :)


On NVGate you can find the RMS overall level on several windows. For exemple, on FFT spectrum it can be found here:





If you want to recompute this value using the FFT spectrum line you need to do the following computation:



First you have to sum the amplitude of the frequency line between the freq min and the freq max A(f). (be careful if you have RMS value you have to put each amplitude to square before sum it. A(f)² ). Then you have to devide that by the ENBW of the ponderation windows (for information it’s 1.5 for haning windows.) then do the square root of this sum.


That is to say:


Ap (f) = A(f)²/ENBW


Then you sum S (f1, f2) = sum from f1 to f2 of Ap(f).


Then you have to put the square root of this sum to have the overall power : P(f1,f2) ) = SQRT (S (f1, f2)) 



Question :

How to find ENBW coefficent ? 


You will find it on NVGate folder manuals:  appendix 2 weighting windows . To sum up:


ENBW rectangular windows = 1


ENBW hanning windows = 1.5


ENBW hamming windows = 1.36


ENBW kaiser bessel windows = 1.8


ENBW Flat Top windows = 3.77




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