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njit-etd2003-081 - New Jersey Institute of Technology

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called the cross Wigner distributions. Therefore the Wigner distribution <strong>of</strong> the sum <strong>of</strong><br />

the two signals is not the sum <strong>of</strong> their respective Wigner distributions. In general the<br />

Wigner distribution puts cross terms in between any two frequencies and any two times<br />

[40].<br />

Figure 3.2 The Wigner distribution <strong>of</strong> the sum <strong>of</strong> two finite duration sine waves.<br />

The figure above presents the Wigner distribution <strong>of</strong> a signal, which is the sum<br />

<strong>of</strong> two sine waves <strong>of</strong> one-second duration with frequencies <strong>of</strong> 100 and 400 hertz. Note<br />

the cross term in the middle <strong>of</strong> the two frequencies at 250 hertz. This is the most<br />

important draw back <strong>of</strong> the Wigner distribution (WD).<br />

Another draw back <strong>of</strong> the Wigner distribution is that it always has negative<br />

regions throughout the time-frequency plane, except in the case <strong>of</strong> the Gaussian signal<br />

where the amplitude is modulated [40].

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