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Broadband Nonreciprocal RF Front-Ends Based on Time-Varying Transmission Lines


Nonreciprocal components like circulators are often used to separate transmitting and receiving signals sharing a common antenna such as those in the front-end of a monostatic continuous-wave radar or a full-duplex radio that performs simultaneous transmit and receive (STAR) over the same frequency band. In those systems, the isolation of the circulator is critical in preventing the receiver from being interfered or jammed by the transmitted signal. Classical ferrite circulators are essentially cavities supporting non-reciprocal resonant modes. They are intrinsically narrowband, bulky, and hard to be integrated on modern integrated circuits (ICs). In comparison, active circulators have been developed based on the nonreciprocal transfer characteristics of transistors. They offer small physical dimensions, versatile functionalities, and compatibility with IC technology. However, active circulators have generally limited noise and power performances which prevent them from being widely deployed in systems requiring wide dynamic ranges.

Time-varying transmission lines (TVTL) such as Distributedly Modulated Capacitors (DMC) are transmission lines loaded with varactors whose capacitances are modulated by a unidirectionally propagating wave. They translate the direction difference of waves propagating on it into the difference in frequency based on its unidirectional frequency conversion property. In such way, TVTL can behave as a circulator and exhibit broadband isolation due to the non-resonant nature of the traveling wave structure. Positive-dB parametric gain may be realized during the frequency conversion process which helps to compensate for the circuit loss.

In this work, a complete theory, based on (1) the distributed parametric effect on a TVTL and (2) the distributed capacitive mixers, is presented with emphasis on the theoretical bounds of the isolation and gain performances of the DMC. The DMCs implemented on Rogers PCB and on GaN MMIC are both presented with the measured results agreeing well with the theoretical derivations and the simulation results. Especially, the MMIC DMC has demonstrated experimentally verified positive-dB receiving (RX) gain and > 10 dB transmitting-versus-receiving (TX/RX) isolation over 0.7 - 2.5 GHz. Furthermore, a circulator prototype combining a pair of MMIC DMC units in a balanced architecture is assembled and tested. The balanced architecture offers a new level of isolation in addition to what is offered by a single DMC. More than 25 dB TX/RX isolation and less than 2 dB RX loss have been experimentally observed for almost one octave at 0.95 - 1.8 GHz.

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