Hi Susan , thank for that, the whole idea of step input is to include "all" frequencies and be hence by applying this to the system we can able to determine the system's responce, the step function used in the function generator and spectrum analyser ( looks like DC On and Off to me ), its like switching a power supply On and OFF , which means the pulse it self is a pure DC on and off, how can the step in this case be useful if its a DC on and off , or am I missing something here? thanks.