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Hmmm... the age old debate... hsim or powermill. Interestingly enough I was a big hsim lover for no other aparent reason than it was something new. Painful experience has shown me that although both have annoying shortcomings, Powermill's I can overcome, Hsim's I cannot.
Case in point: pll's. I simulated the exact same pll on both simulators. Hsim was slower, but you are always left with the nagging thought, "maybe hsim was more accurate, thats why it was slower". Sadly this was not the case, as Hsim was acting in a non-physical manner. Externally it looked fine, but inside the vco it was oscillating common-mode. Which it did not do in spectre, hspice, or powermill.
And on the topic of Hsim being developed by the same people that did powermill... they use fundamentally different approaches. Hsim uses the heirarchical approach, which means that if you have identical cells (i.e. segmented DAC, memories, digital parts) you save much overhead, and gain speed. But for analog circuits that have non-repetitive parts hsim seems much slower. And both can be used on a linux system (nanosim being basically powermill). I am still not completely sure that powermill is better for everything, but even NASDA was unable to help me with correct settings. The long and short of it is this... I would be very careful when someone tells you that hsim is the cureall simulator. But let the debate continue!
There is no such a cure all simulator. VCO circuit is a very analog nature and Hsim is not a good candidate in this job. In your case I think Hspice is the best simulator. There are alway a trade off between speed, capacity and accuracy. AS I mention before HSIM IS THE BEST SIMULATOR IN TERM OF SPEED, CAPACITY AND ACCURACY COMBINE. You can
't use a single circuit to compare. In order to fairly compare we have to use a wide range of circuit for benchmark. Of course in some case Hsim is not the best one, but overall it beats other simulators