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.