Using the stationary phase concept, find the instantaneous frequency for the waveform whose envelope and complex spectrum are respectively given by \[
r(t)=\frac{1}{\sqrt{\tau_{0}}} \operatorname{Rect}\left(\frac{t}{\tau_{0}}ight) ; \quad 0\]
and \[
|X(\omega)|=\frac{2}{\sqrt{B}} \frac{1}{\sqrt{1+(2 \omega / B)^{2}}}
\]