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Difference in transient simulation result between HSPICE and SPECTRE for RC oscillator

FormerMember
FormerMember

dear forum members,

I would appreciate it if anyone could help me with the following problem.

When oscillator transient simulation is performed with HSPICE and SPECTER, oscillator settling time differs as shown below.

     

The left figure is the result of HSPICE simulation and the right figure is the result of SPECTRE simulation.
However, even if I perform SPECTRE simulation longer than HSPICE simulation, SPECTRE does not stabilize.

I wonder why the two simulation results are different.
SPECTRE simulation setup is defaults and HSPICE simulation options are as follows.
.options post=3 nomod
+ method=gear nowarn
+ accurate=1
+ measdgt=4
+ probe
Thanks
  • Andrew Beckett
    I’m not as familiar with the settings in HSPICE nowadays, but the “gear2” method typically has some numerical damping, so that can certainly affect the behaviour of circuit simulators when simulating oscillators. I suspect you should set it to “trap” in HSPICE (that’s the default in spectre). You could of course set spectre to use method=gear2only (pick gear2only rather than gear2), but this may give you misleading results.
    It’s quite hard to accurately simulate the settling of an oscillator - you typically want to suppress this numerical damping, and you also may need to force a reasonable number of time steps (say at least 20 points per period of the expected frequency) by using maxstep. That said, I’m not sure that would be necessary here because I’m assuming the oscillator output already has a reasonable number of points.
    It’s difficult to know precisely the issue here without seeing it (you’re only showing the result of a measurement, and not anything else).
    Regards,
    Andrew.

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