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The impedance frequency response of Large signal and periodic circuit

Tom Brown

Hi ,i'm designing LC VCO and i want to simulate the frequency response of the tank impddance.At first, i use the sp simulation.

But i realize that the small signal sp simulation is not suitable for the large signal circuit like VCO,because the operation point of mos tansistor is time-varying.

So,is there any solution.thanks.

  • Dear Tom,

    Tom Brown said:
    i want to simulate the frequency response of the tank impddance.

    If you are interested in the tank impedance, I might suggest that in lieu of simulated the S parameters, you simulate the impedance (i.e., real and imaginary parts). This will also provide a much more intuitive view of the required negative resistance required from the sustaining amplifier to assure steady-state oscillation. Basically, separate the sustaining amplifier from the tank impedance (the latter can include any varactors if desired). You can then directly simulate the small-signal impedance of the tank impedance over frequency. If you are including the varactors in the tank impedance, you will need to simulate the impedance over the full range of the control voltage used to vary the varactor impedance.

    If you can set the operating point of the sustaining amplifier to its large signal operating point at start-up, you may then also determine its small-signal impedance (at start-up). As you correctly noted, if the sustaining amplifier's negative real impedance exceeds in magnitude the real impedance of the tank at a given frequency, then as oscillation builds, the negative impedance of the sustaining amplifier will decrease towards 0 until it is equal and opposite in sign to the tank's real impedance.

    If you are having trouble devising a test bench to determine the impedance of the tank/varactor network, I might suggest you examine an example at URL:

    https://community.cadence.com/cadence_technology_forums/f/rf-design/38996/capacitance-vs-bias-voltage-curve-for-ferroelectric-varactor

    I hope I understood your question correctly and this is somewhat useful Tom,

    Shawn

  • in reply to ShawnLogan

    Hi Shawn,

    Thanks for your answer.And i learned a lot.

  • in reply to Tom Brown

    Hi Tom,

    You are most welcome - please don't be discouraged! Oscillators remain one of the most challenging circuits to both design and simulate. They appear to be such simple circuits, yet have the most subtle behaviors that never ceases to interest, educate, and "entertain". I continue to be humbled by them.

    Shawn

  • in reply to ShawnLogan

    Thank you for cheering me up

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