Circuit Design: Frequency Generation

Our ISSCC 2025 LO generator combines a dual-core VCO with magnetic mode switching and a compact, tuning-free, octave-bandwidth common-mode resonator. Magnetic coupling suppresses undesired oscillation modes without directly loading the VCO cores. The common-mode resonator controls and extracts the second harmonic without manual tuning. Combining the fundamental and second-harmonic outputs provides 9.05–37.0 GHz coverage, a 4.09:1 tuning range, and a worst-case FoM of 190.7 dBc/Hz at a 10-MHz offset.

Our TMTT 2025 and CICC 2023 paper reports a W-band LO generator with a world-record FoM of >190 dBc/Hz. We start with a high-FoM fundamental VCO around 7 GHz and employ cascaded implicit frequency quintupling and tripling to generate the output at 15× the fundamental frequency. The TMTT journal paper further delves into the fundamental trade-off in using a Class-F₂₃ VCO as an implicit frequency tripler. Contrary to conventional wisdom, we prove that having strong 2^nd and 3^rd harmonics simultaneously is actually detrimental for the fundamental VCO’s FoM. To address this, we propose a new VCO topology that enables implicit harmonic extraction without FoM degradation, along with a new third harmonic enhancement technique through 2^nd harmonic control in injection-locked oscillators. A must-read if you work on mmWave frequency generation circuits or VCOs.