TIME to FREQUENCY FFT

Dear all,

I am simulating pulse echo technique using ultrasound. I have my results in time domain. Now I need frequency spectrum of my time dependent study result. In study 1 "time dependent " time range is = range( 0, 5.0e-8, 9.0e-5 )s and generated ultrasound frequency is 1 MHz.

So I add study 2 "Time to frequency FFT" in my COMSOL model. Now I dont know what to do. I tried once and get 0 Hz frequency result.

Thank you