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Check VCO oscillating in transient and PSS

lswei1984

I designed a low power LCVCO. When I tried to lower it's current as more as possible, I must check what minimum current the VCO needed for starting oscillation. 

At first, I used PSS and enable "Oscillator" item. I found the VCO very easy to oscillate.

But when I use transient, it's difficult for the VCO to start oscillating, even if I chose traponly in transient options.

My question is which one could I trust? PSS or transient?

Any help will be appreciated.

  • Dear Iswei1984,

    I would suggest you not rely on the presence or absence of an oscillation in a transient or a PSS analysis to indicate if sufficient design margin exists to assure a robust start-up process. The amount of numerical noise in the simulator and the DC operating point analysis result can impact the start-up process. Hence, it is likely that the either the DC operating point or simulator settings between the PSS and transient simulations are different and resulted in your observed behavior.

    In my opinion, I would suggest you examine the negative resistance of your sustaining amplifier as you vary its power and compare it to the real impedance of your resonator. This is a far more direct and reliable means to establish if sufficient design margin exists to provide reliable start-up performance.

    Shawn

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