Digital Signal Processing Reference
In-Depth Information
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Illustration 93:
FOURIER analysis and symmetrical spectrum
In the FFT module the "Real FFT of a real signal" was selected as before and then the setting "FOURIER-
Analysis". Our standard setting block length = sampling rate = 1024 is selected. The sawtooth frequency
is 16 Hz.
If the frequency values are not a power of two - for example 21 Hz - , additional spectral lines result which
will be dealt with in Chapter 9.
The result is an symmetrical spectrum measured by DASYLab. However, not with positive and negative
frequencies at the top as previously shown, but at all events symmetrical. In the new 6.0 version and in the
educational version there is now the possibility of selecting the lower representation with a positive and
negative frequency range by means of the setting (bottom right) "symmetrical axis".
However, the negative amplitudes are lacking and there is no information on the phase spectrum. The
FOURIER-analysis does not therefore provide all the information that we would need in order to obtain a
clear representation of the signal in the frequency domain. For this reason we shall now examine what
other variants of the FFT module there are ( Illustration 94). There are two forms of the "complex FFT"
which have two outputs. We shall now use these.
