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maybe you can input some random voltage noise to some point in the ideal simulink circuit. that is to emmulate the 1/f or thermal noise in the actual circuit. the phase noise is caused by the circuit inner noise inserted when the oscillation wave is at or near its cross of zero point.
the book "The design of low noise oscillators" written by Ali Hajimiri (Caltech), Thomas H. Lee (Stanford) talks about this issue more.