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LC VCO startup issue for different transient algorithms

Cherag

My LCVCO is converging .i.e. starting when I am not checking any transient algorithm. In this case, I am getting the required tuning frequency.

But when I turned on gear2 or gear2only algorithm, then my LC is not starting for any vcontrol signal. The oscillations are getting damped. 

The same setup when I am doing transient simulation without checking algorithm in transient options, then my LCVCO is working perfectly.

Is it necessary that it should work with gear2 or gear2only algorithm?
What might be the reason for this ?
Can anyone please help me out with this?

-Cherag

  • Hi Cherag,

    The gear methods (gear2 and gear2only) both end up applying a small amount of numerical damping, which can be deleterious to oscillation in oscillators. It's useful in other situations because it damps out numerical damping (such as you see with the trapezoidal integration method). It's not as severe as the euler method, but it still can cause a problem particularly with high Q oscillators such as LC tank oscillators. 

    So with analyses such as periodic steady state and harmonic balance (when simulating oscillators), traponly is used for the transient assist (aka tstab) phase and then gear2only used for the shooting interval (obviously not for harmonic balance as that's in the frequency domain after the transient assist phase). gear2only tends to give the best accuracy for the cost, but you don't want to run it for a long time in transient with gear2only.

    Depending on which errpreset setting with transient, gear2 might be applied. With the default (moderate) it uses traponly, so that's OK. If you need to switch to conservative (which you may not need to), then you might in that case need to explicitly set method=traponly yourself.

    Hope that helps,

    Regards,

    Andrew.

  • in reply to Andrew Beckett

    Thank you so much Andrew. It helped me a lot.

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