Triaxial Balun With Inherent Harmonic Reflection for Millimeter-Wave Frequency Doublers

This article presents the design and utilization of a triaxial balun as a harmonic reflector (HR) for millimeter-wave frequency doublers (FDs). The triaxial structure consists of two vertically integrated transmission lines (TLs). The impedance of the asymmetric couplers can be tuned to independently control the reflection coefficients at the fundamental and second-harmonic frequencies. The performance of this reflector is analyzed in detail, and the effects of parasitics and nonideal terminations are investigated. Two FDs utilizing this approach were implemented in a 90-nm silicon-germanium (SiGe) BiCMOS platform, both with and without an output buffer. The bufferless design achieves a record peak output power and efficiency of 9.4 dBm and 12.1% at 130 GHz, respectively, with a bandwidth of 128-140 GHz. The buffered design achieves a peak output power of 8.85 dBm and 8.8% efficiency while operating over a 118-146-GHz bandwidth.