###### Abstract  
In this article, we present an ultra-wideband, fully-differential quadrature signal generation network for 5G applications. The ultra-wideband network is composed of a passive balun and cascaded transformer-based quadrature hybrid. Passive balun converts a single-ended signal to differential with minimum insertion loss, and transformer-based quadrature hybrids are cascaded to suppress I/Q imbalance over an ultra-wide bandwidth. The coupling coefficient of the transformer-based quadrature hybrid is enhanced by adopting vertically stacked multiturn transformer topology to extend operation bandwidth and reduce passive loss and chip area. A novel layout and signal routings are proposed to reduce passive loss, undesired magnetic coupling and I/Q imbalance, making meander lines for phase matching unnecessary. The proposed ultra-wideband quadrature signal generation network is designed in GlobalFoundries 45 nm CMOS SOI process with a core area of 845 \(\upmu\)m \(\times\) 495 \(\upmu\)m. The output I/Q magnitude mismatch is less than 0.5 dB from 16 to 60 GHz, and phase mismatch is less than 2\({}^{\circ}\) from 16.5 to 54.7 GHz. The input return loss is lower than \(-\)10 dB from 22 to 45 GHz, and signal loss varies from 5.74 to 7.4 dB (including 1:2 power splitting and loss from passive balun). The effective image rejection ratio (IRR) is calculated based on I/Q mismatch and is higher than 40 dB from 21.5 to 53.5 GHz. 5G communication; CMOS SOI; image rejection (IR); quadrature generation; transformer; quadrature hybrid 20221112508. [[https://doi.org/10.3390/electronics11162508](https://doi.org/10.3390/electronics11162508)]([https://doi.org/10.3390/electronics11162508](https://doi.org/10.3390/electronics11162508))  
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decent performance and great success [12; 13; 14; 15; 16], making it possible to support 23-44 GHz operation using a single system. In 5G wideband receivers, quadrature signals have been widely used to perform beamforming (together with vector modulator) and avoid image jamming. Figure 1 demonstrates one such application in image rejection. A single-ended LO signal is first converted to a differential signal by using a passive balun and fed to an I/Q signal generation network. Then, a differential quadrature LO signal down-converts the RF signal in mixers. After the down-conversion operation, the differential quadrature signal is amplified in an IF variable-gain amplifier (VGA) and then combined. IRR highly depends on the quality of the quadrature signal, i.e., phase mismatch and magnitude mismatch. However, it is a great challenge to generate a perfectly matched quadrature signal that supports wideband 5G applications. RC-CR polyphase filters (PPFs) [17] are popular for low-RF frequency bands due to their relatively compact size and simplicity. The quadrature signal generation bandwidth could be extended by cascading multiple stages of RC-CR PPFs. However, in the 5G frequency band, the resistive components in the signal path attenuate RF signal substantially. Moreover, RC-CR PPFs are sensitive to process mismatch [16] and vulnerable to load impedance variation. A quadrature all-pass filter (QAF) network is another possible solution to generate an I/Q signal. In QAF networks, I/Q errors due to load capacitance could be suppressed by adding series-resistive components at the cost of higher signal loss. A inductive compensation network can also be added at the output to reduce its load sensitivity and achieve voltage peaking [16] but with a substantial increase in chip area.