Nonreciprocal Components With Distributedly Modulated Capacitors

The concept of distributedly modulated capacitors (DMC) is proposed as a form of time-varying transmission-line structure offering nonreciprocal propagation and coupling characteristics. The nonreciprocity is achieved by taking advantage of the additional dimension of time-variance in the transmission-line property. The complete theory, based on: 1) the distributed parametric effect on a time-varying transmission line and 2) the distributed capacitive mixers, is presented with emphasis on the theoretical bounds of the isolation and insertion performances of DMC. Simulations are carried out and a prototype consisting of double-balanced varactor diodes on microstrip lines is implemented on a Rogers board. The measured results agree well with the theoretical derivations and the simulation results. It is thus confirmed that the DMC has great potential of being a circulator device with broadband, minimum insertion loss, and synthesizable isolation characteristics. It can be integrated into an RF system front-end that allows transmission and reception of signals at the same time and the same frequency.