The development of next-generation 5G networks is ongoing. The large available bandwidth at mm-waves allows increasing channel capacity well beyond the levels offered by LTE. Wide ranges of spectra, with sub-bands centered at 28GHz, 37GHz, and 39GHz, have been appointed for 5G development to facilitate international roaming and intra-networks connections [1]. In this scenario, generation of ultra-low phase-noise quadrature (IQ) signals with >40dB image rejection ratio (IRR) over >40% fractional bandwidth is key to efficiently deliver extreme data-rates through high-order spectrally efficient modulations. Quadrature voltage-controlled oscillators are disregarded because of their limited tuning range and also due to a severe trade-off between phase noise and phase accuracy. Solutions leveraging single-phase VCOs followed by quadrature generators is seen as a better strategy. Still, the challenging phase noise, required to support higher-order modulations trading-off with tuning range, mandates at least two VCOs covering half bandwidth each. For quadrature generation, distributed couplers, e.g., Lange couplers, are bulky and not amenable to integration. Hybrid couplers based on coupled inductors offer a compact footprint with low loss, but they are disregarded, because a few percent variation in the coupling coefficient, k, leads to unacceptable phase deviations. Polyphase filters (PPFs) and their improvements are widely adopted at RF [2]. In [3], the PPF operation at mm-waves is proven through careful layout techniques. Still, wideband operation can be achieved only by cascading several stages, severely increasing signal loss and power consumption.
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