The effect of varactor nonlinearity on the phase noise of completely integrated VCOs

This work discusses variations in phase noise over the tuning range of a completely integrated 1.9-GHz differential voltage-controlled oscillator (VCO) fabricated in a 0.5-mu m bipolar process with 25-GHz f(t). The design had a phase noise of -103 dBc/Hz at 100 kHz offset at the top of the tuning range, but the noise performance degraded to -96 dBc/Hz at 100 kHz at the bottom of the tuning range. It was determined that nonlinearities of the on-chip varactors, which led to excessively high VCO gain at the bottom of the tuning range, were primarily responsible for this degradation in performance. The VCO has a power output of -5 dBm per side. Calculations predict phase noise with only a small error and provide design insight for minimizing this effect. The oscillator core drew 6.4 mA and the output buffer circuitry drew 6 mA, both from a 3.3-V supply.