how to generate a flat power density from gaussian signal?

I want to generate a normal random signal as an input(displacement) to my manipulator. I have these feature about it: flat power density which is in m^2/〖Hz〗^0.5 . so I cannot find the concept of 〖Hz〗^0.5 in power density. so my question is about how I can generate this signal ? I finally want to scale them between [-3cm,3cm]