I am designing a ring oscillator whose output frequency is not sensitive to process and supply voltage variations. In order to do this i made the following current controlled ring oscillator. However, changing the bias current does not change the oscillation frequency of ring oscillator due to some reason. Can somebody tell how can i control the oscillation frequency from outside the chip so that i can adjust the frequency to compensate the supply and process variations.
Did you ever inspect the ring oscillator waveforms to understand how the current control works? There might be an unwanted voltage swing at the inner supply nodes. A standard VCO technology uses differential stages, and if I remember right, voltage rather than current control.