依我的了解(有錯請糾正),目前的tool都是要求先在circuit stable的情況下(frequency不會再變化)來先求出PSD,user 可以看這個PSD的分佈,或再進一步run transient noise。While running transient noise, it doesn't restrict the frequency change. However, since the PSD is from the analysis where the frequency stays stable, this will definite introduce some error(margin誤差). I don't know if there any tool so powerful that can analyze the phase noise of unstable VCO(也不是說unstable,而是說頻率還在變化).