Low phase-noise VCO utilizing NMOS symmetric load for Frequency-based Delta-Sigma Modulators

The voltage-controlled oscillator (VCO) in frequency-based delta-sigma modulator (fdsm) systems behaves as a voltage-to-phase integrator converting an analog input voltage to phase information. In this paper, a low phase noise CMOS VCO utilizing modified NMOS symmetric load and PMOS tail transistor for the differential delay cells is presented. By using modified NMOS symmetric loads and a PMOS tail for delay cells, the VCO phase noise can be reduced over 10 dB compared to that of conventional one. The circuits are designed using a 90 nm CMOS process.