Two compact power-combining CMOS VCOs with excellent 2^nd-harmonic rejection using three-conductor coupled-line

This paper presents two novel differential power-combining VCOs using a modified Marchand balun. The balun incorporating three-conductor coupled-lines achieves a high coupling coefficient to obtain a low-loss power combination of fundamental signals. And the two compact designs are realized by meandering the edge-coupled lines. Moreover, the balun not only acts as a part of LC tank but also suppresses the 2^nd-harmonic signals significantly. An NMOS-based VCO and a PMOS-based VCO are both designed at X-band and are fabricated in a standard 0.18-μm CMOS process to demonstrate the design methodology. The NMOS-/PMOS-based VCOs feature -1.8/-3.3-dBm output power and 33.3/43.6-dB 2^nd-harmonic rejection with power consumption of 4.8/8 mW.