While being frequency compact and easy to implement, Non-Return to Zero (NRZ) encoded

data does not contain any energy at its clock frequency which makes the clock extraction

impossible using any kind of Linear Time Invariant (LTI) operations. Therefore, Clock Data

Recovery circuits (CDRs) have an inherent non linear recovery process. In this work we

present a frequency domain analysis of the mechanisms leading to the energy generation at clock frequency for NRZ clock data recovery systems. We also propose a frequency domain analysis which is applicable to both Bang-Bang and linear loops. We show the theory results match the measurements very well.