Variable frequency ring oscillator design

[viperpaki007]

Hi,

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.



regards

 

[FvM]

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.