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NORMA 4000/5000
Operators Manual
10-6
Frequency Analysis
Basic calculation method for analysis of harmonics is done by FFT algorithm (Fast Fourier Transform):
T … Analysis interval length (2
n
data points)
k … Order of the spectral component
Magnitude of spectral component k (RMS value)
DC value
Three different calculation methods are implemented:
1. FFT mode uses fixed sampling frequency. A Hanning window function (“Cosine Bell”) is applied on the
input data to suppress spectral leakage from unsynchronized frequency components. Various frequency
ranges from 0…310 Hz up to half of the instruments sampling frequency are available. An additional
smoothing of the FFT result is applied to reduce the picket fence effect in magnitude caused by the
windowing.
Results are available as RMS values only.
2.
DFT mode. Unsmoothed results from FFT (see 1. above) are post-processed to derive fundamental
frequency and magnitude of its integer harmonics by interpolation.
Results can be viewed as absolute RMS values or as relative % of fundamental.
3.
STD mode (compliant to standard EN61000-4-7 Ed 2.1). This calculation uses a SW-based synchronization
technique to run the FFT over an integer number of fundamental cycles, therefore no windowing is needed.
Only intervals of 10 or 12 cycles (f
nom
= 50Hz or 60Hz, resp) are specified in the standard, some more are
provided by the device for lower frequencies.
Different grouping modes of the spectral components can be selected:
Basic Spectral Components Y
C,k
[rms] only
Harmonic Components
Y
H,h
[rms] or [% fundamental]
Harmonic Groups
Y
g,h
[rms] or [% fundamental]
Harmonic Subgroups
Y
sg,h
[rms] or [% fundamental]
Centered Inter-harmonic Subgroups
Y
isg,h
[rms] or [% fundamental]
Harmonic and Inter-harmonic Subgroups
Y
sg,h ,
Y
isg,h
[rms] or [% fundamental]
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