A frequency digitizer based on the continuous time phase domain noise shaping

A PLL-based frequency digitization scheme for frequency modulated signals is presented. It is shown that making use of a continuous time second order loop-filter provides a signal to noise ratio close to what is expected from a second-order delta-sigma modulator. The continuous time phase-domain loop transfer function is translated to the discrete time domain using impulse invariant transformation. Simulation results confirm the analytical predictions of the transformed loop-filter. The properties of this type of digitization are discussed and the benefits and limitations are compared to previous schemes.