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LC Oscillator - Cadence

rsrk

 Hi

This is could be quite obvious to many of you. Still ...

When I tried to simulate ideal LC oscillator with initial conditions(voltage across the capacitor) using cadence tools ... I realised that output voltage is decaying as time progresses. I mean ... without any series resistance across either capacitor and/or inductor how could I expect output voltage to decay ??

Secondly ... though output voltage is oscillating, I expect voltage across the capacitor, at t=0, to be the one I imposed ... rather in transient analysis I am observing it to be zero. I wanted to be clear as I need to work phase differences.

Could someone throw light on it.

Regards
SRK

Parents

  • This is almost certainly dependent upon what you set errpreset to in the tran analysis (unfortunately you didn't post your input.scs netlist as I asked you to, which would have enabled me to see what analysis options you had set).

    If you run the above circuit with a transient analysis for 100ns with no options set on the tran form, it will not decay. The same is true if you set errpreset to moderate. However, if you set it to conservative or liberal, the method that is used will be gear2only or  trapgear2 (it actually depends a bit on which specific version of spectre you're using - see "spectre -h tran" to see what errpreset does in the version of spectre you're using).

    When gear2 methods are used, they introduce a small amount of numerical damping - which in the case of an ideal oscillator will cause the oscillation to die out. In many simulations that small numerical damping is useful because it stops trapezoidal ringing, but is something you often want to avoid in a real oscillator.

    If you really want to use errpreset=conservative or liberal, you can do - just go to the Options form at the bottom of the transient form and change method to be traponly - that will stop the decay. 

    Regards,

    Andrew.

Reply

  • This is almost certainly dependent upon what you set errpreset to in the tran analysis (unfortunately you didn't post your input.scs netlist as I asked you to, which would have enabled me to see what analysis options you had set).

    If you run the above circuit with a transient analysis for 100ns with no options set on the tran form, it will not decay. The same is true if you set errpreset to moderate. However, if you set it to conservative or liberal, the method that is used will be gear2only or  trapgear2 (it actually depends a bit on which specific version of spectre you're using - see "spectre -h tran" to see what errpreset does in the version of spectre you're using).

    When gear2 methods are used, they introduce a small amount of numerical damping - which in the case of an ideal oscillator will cause the oscillation to die out. In many simulations that small numerical damping is useful because it stops trapezoidal ringing, but is something you often want to avoid in a real oscillator.

    If you really want to use errpreset=conservative or liberal, you can do - just go to the Options form at the bottom of the transient form and change method to be traponly - that will stop the decay. 

    Regards,

    Andrew.

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