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Why PSS gives OScillation to Free Running VCO without any initial condition but NOT transient ?

RFStuff

Dear All,

I simulated a LC oscillator in PSS in Oscillator mode even if I have not given any initial condition (Convergence Aid).

While if I run the same oscillator in TRAN  without any initial condition it never oscillates.

Could anybody please tell me what is the reason behind this ?

Kind Regards,

  • Almost certainly this is because the default option when enabling oscillator mode in PSS is "Calculate initial conditions (ic) automatically":

    I don't know which IC subversion you're using but this is from IC616 (recent ISR) - it's been there for a while though. Before that it use to be called something like "use linear osc ic" or something helpful like that. It's a mode that computes the initial condition for you for oscillators.

    This is also available for transient, but since transient doesn't know that it's an oscillator - and a linear oscillator at that - you have to tell it. Again, from the transient options form in IC616 ISR14:

    If you click on the Calculate ic automatically "yes", you can then specify the estimated frequency.

    If you're stuck with an older version of the IC environment (but are using an MMSIM version from MMSIM12.1 onwards), then you can add the relevant parameters in manually, via the additionalParams field on the Misc tab of the transient options. For example: linearic=yes oscfreq=2G


    Regards,

    Andrew.

  • Dear Andrew,

    Thanks a lot ! I am using IC616-ISR14.
    So, by running transient ( with the IC set as suggested by you),can I know the Initial Conditions ( i.e. the capacitor voltage and inductor current) of the oscillator which leads to oscillation ?
  • Probably you'd just need to save them at time 0 - it's not recorded anywhere else.
  • Dear RfStuff,

    If you are not doing so already, might I suggest you set the value of maxstep in your pss and transient simulations to a value of say, 1/20 or less, then the expected period of the oscillator. Often an oscillator will not start in an analog simulation if the default timestep (which depends on the value of errpreset you have chosen and the total simulation time period) is not sufficiently small. This may avoid the need to use an initial condition file.

    Shawn

  • Shawn,

    This may not be necessary if it's a linear oscillator and  you can use the parameter to tell it to use the linear oscillator ic method. So I'd try that first, and then if it fails to start, set maxstep.

    Regards,

    Andrew.

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