BRUEL & KJAER 2006 - Page 28

Nonlinear Distortion Conversion Scales
N I
.
d' . d ya22 + a32 + a42 + .... an2 X 100 o/ ya22 + a32 + a42 + .... an2 X 100 oJ
on mear 1stort1on: =
1
o =
1
o
va1
2
+ a2
2
+ a3
2
+ .... an2 RMS value of complete signal
The harmonic distortion d in percent for sine-waves can be calculated from the number of dB the individual harmonics
lie below the r.m.s. value of the complete signal containing the fundamental and all harmonics.
Example: The harmonics of a signal are found to lie the following number of
dB below the r.m.s. value of the complete signal.
From scale A:
the value X for the individual harmonics is found
to, C:
A
A
8
8
2. harmonic a
2
= - 68 dB
3. harmonic a
3
= - 62 dB
4. harmonic a
4
= - 76 dB
2.
harmonic xl = 0.0016
3. harmonic x2 = 0.0064
4. harmonic x3 = 0.00025
.I;x = o.00825
From scale B it is now found that for .};X = o.00825 the distortion d in % will be,
d = o. 09%
-80d8 -70 -60 -50 -.40dB
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0. 0001
0.0002 0.0005
0.
001 0. 002 0.005 0.01
0.02
0.
05 0.1 0.2
0.5
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-.40d8 -30 -20 -10 0 dB
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2
5
10 20 50 100
200
500
1000 2000
5000 10000
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0.01%
-Distortion
-r:x
0. 02 0. 05
0.1
0.2
0.5
1%
1 2 5 10 20 50 100 200 500 1 000 2 000 5 000 10 000
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1%
5 10
20
50
f6239S 100%
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