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 frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i>O for achieving simultaneously low phase noise (PN) and high power efficiency, i.e., highest FoM. We show that the optimal <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i>O 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 log<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub>(L/(CR<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub>)) and 20 log<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub>(R<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> √C/L) derived from analytical expressions for PN and FoM are optimized over frequency for various C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub>/C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> ratios. 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 0.05mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>.