The mean value of phase noise as a stochastic process

  1. What is the mean value of phase noise as a stochastic process?
  2. Where can I get a theoretical analysis of this topic?

PS: PLL produces cos(2*πfct+φ(t)). The phase noise refers to φ(t). The mean value of the phase noise what I say refers to the mathematic expectation of phase noise stochastic process φ(t), namely E{ φ(t) }.