This paper explores the attainable cancellation bandwidth for practical TX-RX self-interference (SI) in simultaneous transmit and receive (STAR) RF front-ends. We present a passive, lumped feed-forward self-interference cancellation (SIC) network to neutralize a measured SI of a commercial single antenna interface. The proposed SIC transfer function is a reflection coefficient linearly related to the SI, enabling SIC filter design and analysis following impedance matching theory and techniques. Accordingly, the SIC circuit adheres to the Fano-Bode limit, where the TX-RX SIC is inversely proportional to the achieved bandwidth (BW). Lumped Chebyshev SIC equalizers enable efficient extraction of the Fano-Bode limit, ensuring low insertion loss, linear response, and on-chip integration. Furthermore, we establish that the rational function approximation of the SI sets a numerical SIC-BW bound corresponding to the SIC Chebyshev equalizer transfer function of the same order. We present a practical application of the proposed SIC design approach in a commercial single antenna circulator RF front-end around 5.3 GHz, which comprises a Wi-Fi antenna. The cancellation path includes commercial Wilkinson and quadrature hybrid SMA components, and two simulated$4^{th}$order LC Chebyshev equalizer networks. The realizable SIC filter of this work achieves an insertion loss of 6 dB and enables 40 dB of TX-RX isolation over 300 MHz. The 40 dB numerical TX-RX isolation BW bound for the same order of the Chebyshev equalizer is 450 MHz, further validating the SIC-BW conclusions of this work.

We present a fully integrated and tunable transceiver for frequency-division duplex (FDD) and half duplex (HD) operation based on a quadrature balanced N-path mixer-first receiver (MFRX) architecture. The quadrature balanced N-path transceiver (QBNT) comprises a quadrature hybrid (QH) and two identical MFRXs, presenting a short circuit and a matched impedance at the transmitter (TX) and receiver (RX) bands, respectively. The proposed transceiver achieves low TX to antenna loss while maintaining high RX linearity, and is capable of cancelling both TX noise and reciprocal mixing (RM) at the RX under antenna voltage standing wave ratio (VSWR) variations. Analysis and design equations of the QBNT are shown, and the design considerations of each block are presented. A channel estimation algorithm is proposed to cope with the frequency-dependant antenna reflection QH response. An integrated QBNT prototype was fabricated in TSMC 65nm CMOS process as a proof of concept, occupying an active area of 2.96$mm^2$. The QBNT operates at the frequency range between 0.75-2 GHz with a TX-RX offsets above 200 MHz. It achieves RX noise figure (NF) of 2.8-5.8 dB, RXB1dB of 18 dBm, TX-ANT OIP3 of 27.3 dBm and 29.5 dBm in FDD and HD modes, respectively. The demonstrated FDD operation of the QBNT shows that in our implementation we achieve a simultaneous 6.5 dBm TX output power and an RX EVM of$-$40.8 dB after digital cancellation. The RX and TX (at OP1dB) consume DC power of 82-130 mW and 254 mW, respectively.

A mixer-first receiver (MFRX) that breaks the baseband (BB) input linearity, matching, and noise trade-off is proposed. The MFRX is composed of a quadrature hybrid and two identical transimpedance (TIA) based MFRXs, with a noiseless antenna matching achieved by placing a termination at the hybrid's isolated port. The receiver (RX) performance is analyzed utilizing a linear time-invariant (LTI) model and a comparison to previous approaches is presented. A TSMC 65nm CMOS design is presented as a proof of concept. Simulation results show superior performance over voltage-mode topologies of MFRXs in merits of in-band (IB) and out-of-band (OOB) linearity at the cost of power and area, with almost no noise penalty.

This article presents a new electronic device – the four-port quasi-circulating quadrature hybrid (QCQH), which combines desirable features of electronic circulators and quadrature hybrids. The QCQH comprises three quarter-wavelength transmission lines and a 90° non-reciprocal phase shifter (NRPS) and is suitable for inband full duplex applications. We derive the four-port S-parameter matrix of the QCQH and the transfer functions under termination scenarios of interest. The resulting closed-form expressions are compared with the prior art electronic circulator. A 90° transmission line and lumped transformer alternatives are considered for designing a QCQH based on a two-port N-path circuit. TX isolation and transmission expressions are derived and verified against simulations, and a new wideband leakage cancellation approach is proposed. A TSMC 65 nm CMOS N-path chip is integrated on-board with a discrete, lumped, LCL transformer implementation. TX-to-antenna insertion loss of 1.4 dB and an RX NF of 4.7 dB at the frequency of 1 GHz were measured with a primary passive TX-RX isolation of 21 dB. Total isolation of more than 50 dB for an 80 MHz OFDM WiFi TX signal employing digitally equalized active leakage cancellation was achieved along with better than −40 dB TX EVM with SIC ON.