A Millimeter-Wave Frequency Doubler With Wideband Transformer Balun Output Matching and Enhanced Harmonic Suppression

This brief presents the analysis, design, and implementation of a millimeter-wave frequency doubler fabricated in a 130-nm SiGe process. Guided by the asymmetric coupled-line (ACL) model of the transformer, we derive the broadband impedance matching conditions and accordingly design the output balun. To achieve enhanced fourth-harmonic suppression over the wide operational bandwidth, the two push-push cores are configured to generate a differential second harmonic and a common-mode fourth harmonic, enabling the output balun to combine the desired second harmonic while suppressing the fourth. The measured doubler chip achieves a maximum output power of 15.9 dBm with a 3-dB bandwidth ranging from 52 to 88 GHz. The measured in-band rejection ratios for the fundamental, third, and fourth harmonics exceed 26 dBc.