Hi, how could i implement a voltage source with this behavior?: at 0 s -> 0 V at 1 s -> 2 V at 3 s -> 3V ... The graphic should be like a stairs

If you're using ngspice, you can use the piece-wise linear command PWL, has the form PWL(T1 V1 T2 V2 T3 V3...), so your voltage source might look like:

VCLOCK 0 1 PWL(0 0 1 2 3 3)