Pulse generator in Simulink Matlab
There is no alternative idea when it comes to fix step. everything must be an integer number of the step. 

Your solver is set at 1us yet you want a pulse that is 20ns. You need to reduce you step to at least 20ns to realise such a pulse,

There are three options 


Reduce the fix-step to at least 20ns to realise such a pulse
Change to a variable step to realise such a pulse
Re-evaluate whether you need a pulse this narrow being simulated, is the edge good enough?