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Hi Wany,
As I undersand, the ring oscillator is controlled by the Voltage (VCO) and hence there should be no problem with the startup (I don't see any problems with the VCO that I have).
There can be issues with the startup if it is self-biased or does not have any inputs.
Thank you for ravirajdv and FvM. I have read a paper there is a start-up circuit in the VCO. I have some quesiton about it. The fig is the schematic. When Vc(control voltage) is 0 . M5 is closed . And M11 and M4 is opened because start up circuit. Then there is current through M1 and M2. The VCO will has a frequency when Vc is 0. But i think it is not right output.
So i want to know if the current source in the ring VCO need a start up circuit?
1. add cap to ground on one of the output nodes of value 0Fand give it initial condition of Vdd
2. in ADE goto simulation->convergence aids->initial condition and set the initial voltage on one of the output nodes.
3. set VDD as a ramp input
4. exponentially decaying current source at one node
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