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#### A Compact 70–86GHz Bandwidth Frequency Quadrupler with Transformer-Based Harmonic Reflectors in 28nm CMOS

A frequency quadrupler based on cascaded push-push frequency doublers is presented in this work. Push-push frequency doublers suffer from limited power efficiency and conversion gain, mainly due to second-harmonic feedback. Conventional harmonic reflectors minimize this undesired feedback introducing a common-mode second-harmonic resonance, at the price of increased area and reduced bandwidth. In this design the harmonic reflector is embedded into the input matching network, resulting in a more compact design. The coupling coefficient between the multiple windings of the transformer secondary is used to decouple the differential-mode inductance from the common-mode inductance, that acts as a wideband harmonic reflector. A common-gate transistor is stacked with the push-push pair to further boost the output power while reusing the same current. Two push-push frequency doublers are cascaded without additional power amplification stages. The quadrupler, implemented in 28nm CMOS, achieves a peak output power of 0 dBm and peak power efficiency of 5% at 77 GHz and the 3-dB bandwidth is from 70 to 86 GHz.