Here is a link I found to use ADS to simulate ring oscillators. The is a lot more stuff if you goggle modeling ring oscillators in ADS. I have had reasonable luck modeling phase noise using BJT’s in ADS. There is a company called Modelithics that can supply models for various transistors that are better than the manufactures data sheet. If you are designing an IC, the foundry needs, and should, supply the model for the device. When I was at Agilent (Keysight), the people in the IC design group did model the noise of diff pairs and sine to square converted, but I personally haven’t done it. I just measured it and used the data in my overall PLL model. Eric PS There was a fair bit of damage to one of the Agilent buildings in Santa Rosa due to the fires. https://www.eleceng.adelaide.edu.au/personal/dabbott/wiki/images/b/b4/OscillatorsADS.pdf