Speaker: Shihan Qin
Affiliation: Ph.D. Candidate - UCLA

Abstract: Nonreciprocal components like circulators are often used to separate transmitting and receiving signals in a single-antenna, full-duplex wireless system that performs simultaneous transmit and receive (STAR) over the same frequency band. The isolation of the circulator is critical in preventing the receiver from being interfered or jammed by the transmitted signal. Classical ferrite circulators are intrinsically narrowband, bulky, and hard to be integrated on modern integrated circuits (ICs). In comparison, active circulators using transistors offer small physical dimensions and compatibility with IC technology. However, active circulators have generally limited noise and power performances.

Time-varying transmission lines (TVTL) such as Distributedly Modulated Capacitors (DMC) can 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. 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 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. 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.

Biography:  Shihan Qin received the B.S.E. degree in electrical engineering from the University of Michigan, Ann Arbor, MI, USA, in 2011, the B.S. degree in computer engineering from Shanghai Jiao Tong University, Shanghai, China, in 2011, the M.S. degree in electrical engineering from the University of California, Los Angeles (UCLA), CA, USA, in 2013, and is currently working toward the Ph.D. degree in electrical engineering at UCLA. His research focuses on time-varying circuit design for nonmagnetically nonreciprocal, low-noise microwave components in high-performance full-duplex wireless systems. He is the recipient of Outstanding Master’s Thesis Award from UCLA EE Department in 2013 and IEEE MTT-S Graduate Fellowship in 2015.

For more information, contact Prof. Ethan Wang ()

Date/Time:
Date(s) - Sep 13, 2016
2:00 pm - 4:00 pm

Location:
E-IV Tesla Room #53-125
420 Westwood Plaza - 5th Flr., Los Angeles CA 90095