Heaviside problem

Hello for everybody ,

I am trying to make a square pulse train, and was read a lot of forums for this..but unfortunately I didn't get pulse train...just one 5us pulse with DV amplitude..

My rectangular pulse train function is:

DV*(flc2hs(mod(t[1/s],0.1)-1e-9,1e-9)-flc2hs(mod(t[1/s],0.1)-tp,1e-9))

there is :
DV - pulse amplitude (30000 V),
0.1 s - pulse period
tp - pulse width (10us)

So where is a problem and why I don't get a correct pulse train.. ???

Thanks a lot,

Mindaugas