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time varying spectre alter statement

markbeck

I have no idea whether this is possible or not but what I'd like to do is create a variable that changes with time.

e.g.

  alter1 alter dev=Q2 param=mult value=5*sin(2*pi*1G*$time)"

But when I try simulating this, it complains about "unknown variable $time".  However, I can create a bsource which is dependent on the variable $time

e.g.

    vb1 (x1 vee) bsource v=sin(2*pi*1G*$time)

So the time variable does appear to be available to some instances, but not necessarily to all?

Is there another spectre variable name for time that is generically available?

Thanks,
Mark
 

  • This is not possible. bsource is a special case (you can use the special variable on resistor, capacitor, inductor, vsource and isource which morph into bsource when such expressions are used) - effectively it becomes a VerilogA model.

    Changing a parameter on a device at every timestep could cause significant changes in the operating point - and so would not be desirable as it could cause convergence problems (plus a lot of work having to re-build the model, possibly with topology changes). You do have the "dynamic parameter" capability in a tran analysis to allow you to set up a piece-wise time-value pairs, which provided you change smoothly enough shouldn't cause convergence difficulty - but it's not quite what you want as it's not a function of time.

    So it's not supported. If you want this, you'll need to create a service request and ask for an enhancement (ideally with good justification as to why it would be useful).

    Thanks,

    Andrew.

  •  Thanks for the feedback. At least I can stop trying to go down that road. 

    As for a service request, I doubt that I could make a strong enough case that would be considered worthwhile from a financial standpoint.  My desire for a time-varying alter statement is to allow me to simulate with a smaller / faster circuit than I'll have to simulate with now.  From Cadence's point of view, I should just purchase some aps licences...

    Thanks,
    Mark

  • Mark,

    Actually Cadence are interested in improving customer's productivity - it's far better for you to be able to simulate things efficiently (that's why there has been so much effort put into APS, and various other techniques to simulate circuits more smartly rather than just throwing licenses at it). So I'd still say it's worth it if you can suggest technical and productivity benefits.

    Regards,

    Andrew.

  • Fair enough... I created the SR 42617518.

     Mark

  • You can do this in PSpice. PSpice support expression based on TIME (simulation time point)

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