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QVCO designing

VIVEK1729

hi,

I am trying to make QVCO @5GHz in 180nm but i am  not getting the required sine wave, could you please suggest me the model no. of capacitor and Inductor which can be used in RF region. I also tried this in 130nm but my result are not sine wave due to which oscillation are not getting sustained i am sending you one image where transient response of my design can be seen, as you will see lower part of the wave is getting small 'kink'. Please help me out. 

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  • ShawnLogan

     Dear Vivek1729,

     

    > i also attached a transient response in the end where you can see the negative part of the wave is

    > not proper due to which oscillation are not getting sustained. I am also thinking that may be Inductor

    > and Capacitor are not functioning well in the RF region of operation,

     I believe your oscillator is functioning as expected. There is no guarantee in your circuit that the oscillation waveform will have low distortion (which I believe is what you are looking for). The amplitude of the oscillation - and the waveform itself - will be determined by the magnitude and phase of the waveform that provides a steady-state impedance that exactly balances the steady-state impedance of your resonator (inductor in this case). There is some filtering by the LC tank, but that is not narrow band enough to eliminate distortion products. If you are looking for a very low distortion waveform, there are other techniques that are used to limit the amplitude of the waveforms or to use a very narrrow band resonator. Limiting the amplitude basically causes the negative resistance of the sustaining amplifier to drop in value as the amplitude grows and hence reduces distortion.

     However, if I have understood your question (I hope I have!), the behavior does not reflect any simulator issue nor an issue with your capacitor or inductor.

    Shawn

     

Reply
  • ShawnLogan

     Dear Vivek1729,

     

    > i also attached a transient response in the end where you can see the negative part of the wave is

    > not proper due to which oscillation are not getting sustained. I am also thinking that may be Inductor

    > and Capacitor are not functioning well in the RF region of operation,

     I believe your oscillator is functioning as expected. There is no guarantee in your circuit that the oscillation waveform will have low distortion (which I believe is what you are looking for). The amplitude of the oscillation - and the waveform itself - will be determined by the magnitude and phase of the waveform that provides a steady-state impedance that exactly balances the steady-state impedance of your resonator (inductor in this case). There is some filtering by the LC tank, but that is not narrow band enough to eliminate distortion products. If you are looking for a very low distortion waveform, there are other techniques that are used to limit the amplitude of the waveforms or to use a very narrrow band resonator. Limiting the amplitude basically causes the negative resistance of the sustaining amplifier to drop in value as the amplitude grows and hence reduces distortion.

     However, if I have understood your question (I hope I have!), the behavior does not reflect any simulator issue nor an issue with your capacitor or inductor.

    Shawn

     

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