Design of a Tank-Optimized 20 GHz VCO With Fourth Harmonic Extraction Achieving 193.8 dBc/Hz Peak FoM in 22 nm FDSOI

In this brief, we present design considerations on achieving highest figure of merit (FoM) for a class-B LC VCO in CMOS. Following aspects are addressed: a) systematic analysis of choosing the optimal fundamental oscillation frequencyfO for achieving simultaneously low phase noise (PN) and high power efficiency, i.e., highest FoM. We show that the optimalfO is directly related to the frequency tuning range (FTR). E.g., in the given technology, an FTR of 3% results in an optimal operating range of 10 GHz to 20 GHz. A wider FTR narrows down the operation range for optimal PN and FOM; and b) we present an LC-tank step-by-step dimensioning procedure. The proposed approach is based on iterative electromagnetic (EM)-simulations of the resonant circuit components and optimization with respect to technology-independent design degrees of freedom. The terms 10 log10(L/(CRT)) and 20 log10(RT√C/L) derived from analytical expressions for PN and FoM are optimized over frequency for various Cmax/Cminratios. We use these considerations to design a multi-harmonic class-B LC-VCO in 22nm FDSOI CMOS, which offers two outputs - one at the fundamental (H1) and another at the fourth harmonic (H4), extracted directly from the core. The VCO achieves a highly competitive FoM at 1MHz offset with 0.4V supply of 193.8 dBc/Hz and 188.2 dBc/Hz at 20 GHz and 80 GHz, respectively. We demonstrate that class-B VCO can achieve PN and FoM outperforming class-C or class-F. It is achieved only by means of the proposed systematic LC-tank design, without any additional circuitry, such as waveform shaping, core coupling etc. Thus, core area is only 0.05mm2.