18.2 Response of LTI-Syst,riiis to Randum Signals 449 wfiert’ U is very siuell Thc change of tlie iiitlex with time convenitwtly prochces a tiiffereritiator w-itli tlic tiaiisfer fun(-tion The lcsiilting dmivative y ( t ) = + ( t )has the power tlwsity spectriini