This paper deals with the timing jitter in high speed optical fiber digital communication systems. From the concept of time-varying spectrum

a group general expressions of the jitter spectral densities are proposed

which are applicable to conditions of both arbitrary timing extraction （BPF or PLL

etc.） and arbitrary non-linear circuit （NLC）. Their specific results in the simple timing extraction circuit（STC） condition are compared with those from other deductive methods. It proviues convenience for both comparing the effect of every jitter source in specific circuits and choosing appropriate NLC forms.