Transfer characterization of CMOS ring voltage controlled oscillators

One of the most important properties of any voltage controlled oscillator (VCO) is the transfer characteristic, the relationship between the control voltage and the frequency of the output signal. In a voltage controlled ring oscillator, a number of identical delay stages are used. The most common delay stage is a delay cell with a pair of differential inputs and diode-connected loads paralleled with adjustable current sources. In this paper, a detailed analysis of the transfer characteristics based on the actual transistor operating regions is presented. It is shown that the relationship between the control voltage and the oscillation frequency is not monotonic. Simulation results verify the theoretical development. A design strategy is introduced that assumes that the VCO operates in the monotonic transfer characteristic region