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Effect of load capacitor at the output of an oscillator

Jithin

Hi,

While designing an oscillator ,which had a ring oscillator(differential ended) architecture , the frequency sensitivity of the vco had changed, that is if initially my vco works in a frequency range of 300MHz - 500MHz aor a control voltage range (1.3V-1.85V). After adding a capacitor

at the output,say 400fF of each differential amplifier stage then the same frequency(300MHz- 500MHz) swings at a voltage range of (.9V - 1.65V).Thereby decreasing the  gain of vco. The problem is that I can't find a reason for this phenomenon and can't answer the questions like how the range of the control voltage had changed due to the addition of a load capacitor.

Please help me.

Thanking you in advance.

with regards,

Jithin 

  • Quek

    Hi Jithin

    It seems that this is a design issue and it would be a little hard for us to help. : )  Hope that you will be able to get more help in the designer's forum.

    Best regards
    Quek

  • Andrew Beckett

    Especially without seeing the design - you didn't even say how the control voltage controls the VCO frequency, for example. But Quek's right - a more design-centric forum would probably make more sense.

    Andrew.

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