Is this a step response curve?

[xiao]

Is the plot in the picture a step response curve？
This is how it's measured? Could you please explain the detailed steps, thank you.

 

[John Mulcahy]

Yes, it is a step response, hence the title of that graph trace. It is derived from the impulse response (the step response is the integral of the impulse response). REW determines the impulse response from sweep measurements.