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{T}} \exp \left\{-\left(\frac{2 t}{T}ight)^{2}ight\} ; \quad 0and \[
|X(f)|=\frac{1}{\sqrt{B}} \exp \left\{-\left(\frac{2 f}{B}ight)^{2}ight\}
\]