The requirement of using ferrite magnetic material for non-reciprocal microwave components like circulators and isolators prevents the designs from being small, lossless and broadband especially at the lower end of the microwave frequency spectrum. Active non-reciprocal components may have good isolation and can be integrated, but they are very poor in noise performance. A circuit with distributedly modulated capacitors (DMC) - time-varying capacitors distributed along a transmission line and modulated by a carrier wave - provides a novel realization of lossless and broadband non-reciprocity. The circuit is compatible with modern integrated circuit technology. Decent gain and low noise for receiving path are also possible as bonus advantages. Studies on the estimations of the isolation and the receiving insertion performance of the DMC circuit are focused in this thesis to show its ability to serve for a non-reciprocal component. A prototype made on Rogers circuit board has verified the theory's prediction on the performance, which is capable of achieving >13 dB isolation between the transmitting and the receiving paths and <5 dB receiving insertion loss over the frequency range of 0.5 -- 1.8 GHz.

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.