A Tail-resonance Calibration Technique for Wide Tuning Range LC VCOs

Tail resonance at double the oscillation frequency is widely used in LC oscillators for reducing phase noise. The phase noise improvement is quite sensitive to shifts in the tail resonance frequency. A 6% error in the tail inductance can cause up to 4 dB increase in the close-in phase noise from the optimum value. Phase noise degradation is quite severe for widely tuned high frequency VCOs where modeling errors and changing output frequency contribute to substantial shift from the desired resonance frequency. We propose a technique to automatically calibrate the tail resonance to obtain the best phase noise. The amplitude of the oscillation across the tail resonant tank is sensed using a peak detector and the capacitor array used to tune this tank is adjusted to maximize this amplitude. This setting maximizes the phase noise improvement due to the tail resonance. A 4 stage left handed LC ring VCO with a tuning range of 8.5 GHz to 13.2 GHz maintains a fairly constant phase noise figure of merit of 190 dBc/Hz for offsets greater than 100 kHz, over the entire tuning range when the proposed technique is incorporated.