I did some searching on google, having never come across uramp before. As far as I can see from
http://www.beigebag.com/case_logical_3.htm it is simply:
uramp(f(a)) == f(a) if f(a)>0, else 0.
http://www.anasoft.co.uk/Spice3F5Manual.html says the same thing.
So in spectre, that would be:
Code:// model of what I think uramp does
//
v1 (1 0) vsource type=sine freq=1M ampl=1
v2 (2 0) bsource v=v(1)>0?v(1):0
tran tran stop=5u maxstep=10n
Regards,
Andrew.