CMOS Class-C VCO Design Using Open Loop Analysis

This paper presents a design method of LC cross-coupled oscillators using a large signal S-parameter open loop approach instead of typical negative resistance methodology presented in the literature. The open loop technique allows extraction of loaded quality factor of the complete oscillator circuit and observe how oscillation conditions change with increasing oscillator signal amplitude. As a result, highly non-linear modes of oscillator operation (class-C in this case), can be analysed without necessity of conducting time consuming transient simulations. The presented method is not technology specific and allows fast calculations under changing bias conditions. The simulated class-C 130 nm CMOS oscillator operates at 5 GHz from a reduced power supply of 350 mV, achieving average SSB phase noise better than -115 dBc/Hz at 1 MHz offset from the carrier, using a relatively low loaded quality factor (QL ≈ 10) LC resonator. The presented VCO has tuning range of 280 MHz to compensate for process and temperature variations. In steady state, MOSFET devices in the oscillator operate in class-C i.e. for VGS <Vth, resulting in low power consumption of less than 0.5 mW RMS.