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Issue in Transition Filter transient slope settings in Verilog-A model

RFStuff

Dear All,

I have set my transition filter for a signal in my model as below:-

 V(out) <+ transition( quantized, td, trise,tfall,ttol );

I have set 

td=0 , trise=1p tfall=1p and ttol=1p.

But, after my transient simulation, I get something below

I am expecting trise=1ps but, here I am getting trise=63 ps also the rise is not smooth and it has a break point inbetween. 

Can anybody please tell why it is happening and how it can be actually set to have trise=1ps ?

  • You're running out of numerical resolution on the. The time here is at 7 seconds, and so a rise time of a picosecond is nearly 1e-13 of the current time. Time is a double precision quantity (as is pretty much everything else in the simulator), and double precision numbers have a 53 bit mantissa - which means you have log10(2)*53 decimal digits of resolution (approximately 15-16). This means you are close to the absolute resolution of the time. The simulator needs some margin around that, so the default minimum transition time is 1e-9*tstop - see the transres option of the transient analysis. That doesn't mean that the transition can't be faster, but that it will combine events within this time to prevent the timestep collapsing too much.

    You can reduce transres a bit, but don't go too far because you'll then get convergence problems.

    Andrew

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