A Harmonic Class-C CMOS VCO-Based on Low Frequency Feedback Loop: Theoretical Analysis and Experimental Results

A novel harmonic Class-C CMOS VCO architecture with improved phase noise performance and power efficiency is presented in this paper. The VCO is based on the widely adopted topology consisting in a crossed pair of NMOS devices refilling a symmetric resonator with a center tapered inductor and biased by a top PMOS current generator. The Class-C operation mode is obtained through a low frequency feedback loop constituted by an operational transconductance amplifier operating the difference between the inductor center tap voltage and a reference voltage, pushing gate polarization voltage of VCO crossed pair devices well below their threshold voltage. The Class-C VCO achieves a theoretical 2.9 dB phase noise improvement compared to the standard differential-pair LC-tank oscillator for the same current consumption. A prototype of the VCO is implemented in a standard RF 55 nm CMOS technology and compared to both a standard and an optimized VCO implemented in the same technology. All these VCOs share a copy of a unique resonator. The Class-C VCO is tunable over the frequency band 6.5-7.8 GHz and displaying an average phase noise lower than -127 dBc/Hz @ 1 MHz offset with a power consumption of 18 mW, for a state-of-the-art figure-of-merit of -187 dBc/Hz @ 1 MHz and -191 dBc/Hz @ 10 MHz offsets, respectively.