Best way to switch on fields in time-dependent simulations

Hi,

Is there a kind of recommendable way of switching on fields in transient emw-simulations. Right now, I'm using the step-function, included in Comsol, and I have some problems with the converge of the solver. So is it worthy do modify the the time-profile of the turn on process? And if so, what are more suitable choices than Comsol step-function?

Thanks a lot in advance,
Felix