An Ultra-Low Phase Noise Dual-Tank Oscillator Featuring Tank Impedance Scaling

This paper aims to achieve ultra-low phase noise for a voltage-controlled oscillator (VCO) through the adoption of a dual-tank configuration. The tank, based on a 1:1 transformer, incorporates an impedance scaling mechanism to reduce the equivalent parallel resistance of the tank without compromising its quality factor. Additionally, the tank provides a passive voltage gain from the drain to the gate of the core transistors, resulting in a reduction of the phase noise of the core transistors and aiding the start-up of the VCO. The tank also provides impedance peaks at the fundamental and second harmonic frequencies, leading to a flattened voltage waveform for half an oscillation cycle at the core transistors’ drains, further reducing their phase noise contribution. The VCO is fabricated in 180-nm CMOS technology. The VCO, with a die size of 0.14 mm2, demonstrates a state-of-the-art phase noise of -157.2 dBc/Hz at an offset frequency of 10 MHz, with the center frequency of 5.08 GHz. The oscillator’s peak Figure-of-Merit is 193.2 dBc/Hz.