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Doing an AC analysis in a time-varying circuit.

Alli

Hi every one :-)

I wanted to do a simulation on an oscillator but I couldn't find any tools in cadence for that. Basically what I want to do is :

Considering an oscillator which is oscillating. During the oscillation I want to do a frequency domain transfer function (AC analysis). I need to linearize the circuit at several point in an oscillation period and find an AC transfer function at a single frequency from an input AC source to the output, and finally plot the magnitude of the transfer function in a period. Basically I want to simulate how the magnitude of a transfer function at a specific frequency is changed during the time (Oscillation period.) 
The frequency which I need the transfer the function is not important it could be exactly at the oscillation frequency or even a little off.
I read about PAC & PXF and if I'm right none of them gives me what I need.
I would appreciate it, if you have any idea.
Parents

  • Dear Ali,

    During the oscillation I want to do a frequency domain transfer function (AC analysis).

    I have been thinking about your question and request for suggestions to simulate the transfer function over the oscillation period of your VCO. I cannot help but wonder what the motivation for your analysis might be. For any transfer function that I am familiar with for an oscillator, the transfer functions of interest are not meaningful over a portion of the oscillation period. If you are concerned about the sustaining amplifier's open loop or closed loop gain transfer functions, these are only of interest over the entire oscillation period I believe. Why do I suggest this? Basically, one is interested not in whether the sustaining amplifier can support oscillation over just a portion of the period but whether when considering the entire period the sustaining amplifier has sufficient gain to support steady-state oscillation.

    If you are considering the initial gain of the sustaining amplifier prior to the establishment of steady-state oscillation (for example in a ring based VCO consisting of several delay stages whose initial states are not known, then a series of actran simulations will serve you well. I have done this before to verify that the combined delay stages provide sufficient gain to establish oscillation. (An example for the motivation for this type of simulation is if one or more stage may be saturated at start-up. In this case, the near zero gain of the saturated delay stage may prevent the buildup of steady-state oscillation.)

    Transfer functions of control voltage bandwidth will not be sensitive to any variation in their value over a single VCO period as the VCO modulation bandwidth is typically far less than the frequency corresponding to the oscillation period. Therefore, the transfer function consists of an average over the oscillation period and therefore does not appear to me, anyway, a reason to explore its behavior over a period of oscillation.

    Perhaps if you might provide some added insight into the goal of your effort (i.e., its motivation), I can provide some added thoughts. Others, I am sure, will have good suggestions.

    I hope I did not misunderstand your request - and apologize if I did!

    Shawn

Reply

  • Dear Ali,

    During the oscillation I want to do a frequency domain transfer function (AC analysis).

    I have been thinking about your question and request for suggestions to simulate the transfer function over the oscillation period of your VCO. I cannot help but wonder what the motivation for your analysis might be. For any transfer function that I am familiar with for an oscillator, the transfer functions of interest are not meaningful over a portion of the oscillation period. If you are concerned about the sustaining amplifier's open loop or closed loop gain transfer functions, these are only of interest over the entire oscillation period I believe. Why do I suggest this? Basically, one is interested not in whether the sustaining amplifier can support oscillation over just a portion of the period but whether when considering the entire period the sustaining amplifier has sufficient gain to support steady-state oscillation.

    If you are considering the initial gain of the sustaining amplifier prior to the establishment of steady-state oscillation (for example in a ring based VCO consisting of several delay stages whose initial states are not known, then a series of actran simulations will serve you well. I have done this before to verify that the combined delay stages provide sufficient gain to establish oscillation. (An example for the motivation for this type of simulation is if one or more stage may be saturated at start-up. In this case, the near zero gain of the saturated delay stage may prevent the buildup of steady-state oscillation.)

    Transfer functions of control voltage bandwidth will not be sensitive to any variation in their value over a single VCO period as the VCO modulation bandwidth is typically far less than the frequency corresponding to the oscillation period. Therefore, the transfer function consists of an average over the oscillation period and therefore does not appear to me, anyway, a reason to explore its behavior over a period of oscillation.

    Perhaps if you might provide some added insight into the goal of your effort (i.e., its motivation), I can provide some added thoughts. Others, I am sure, will have good suggestions.

    I hope I did not misunderstand your request - and apologize if I did!

    Shawn

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