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In this paper, we demonstrate, for the first time, an isolating bandpass filter with low-loss forward transmission and high reverse isolation by modulating its constituent resonators. To understand the operating principle behind the device, we develop a spectral domain analysis method and show that the same-frequency nonreciprocity is a result of the nonreciprocal frequency conversion to the intermodulation (IM) frequencies by the time-varying resonators. With appropriate modulation frequency, modulation depth, and phase delay, the signal power at the IM frequencies is converted back to the RF frequency and adds up constructively to form a low-loss forward passband, whereas they add up destructively in the reverse direction to create the isolation. To validate the theory, a lumped-element three-pole 0.04-dB ripple isolating filter with a center frequency of 200 MHz and a ripple bandwidth of 30 MHz is designed, simulated, and measured. When modulated with a sinusoidal frequency of 30 MHz, a modulation index of 0.25, and an incremental phase difference of 45°, the filter achieves a forward insertion loss of 1.5 dB and a reverse isolation of 20 dB. The measured nonmodulated and modulated results agree very well with the simulations. Such nonreciprocal filters may find applications in wideband simultaneous transmit and receive radio front ends.

The propagation of surface waves along spatially dispersive graphene based 2 D waveguides is investigated in detail. Graphene is characterized using a full kp conductivity model under the relaxation time approximation which allows to obtain analytical and closed formed expressions for the wavenumber of plasmons supported by sheets and parallel plate waveguides respectively. Per unit length equivalent circuits are introduced to accurately characterize the propagation in different waveguides and analytical relations between the effective TM mode circuit lumped elements and graphene conductivity are derived. The proposed circuits allow identifying the different mechanisms involved in spatially dispersive plasmon propagation explaining their connection with the intrinsic properties of graphene. Results demonstrate that spatial dispersion which significantly decreases the confinement and the losses of slow surface plasmons must be accurately assessed in the design of graphene based plasmonic components at millimeter waves and low terahertz frequencies. © 2013 IEEE.

In this paper, a practical implementation of the spatial images technique for the analysis of shielded multilayered printed circuits inside convex cavities is proposed. A new method is introduced in order to automatically locate the images surrounding the structure in order to impose the appropriate boundary conditions for the potentials. The boundary conditions are imposed at discrete points along the cavity wall and, therefore, the technique proposed is an approximation to the exact cavity modeling. Furthermore, for the analysis of electrically long cavities, the use of several rings of images surrounding the entire cavity at different heights is employed. Using the special features of the formulation, a new method of moments implementation combined with the spatial images technique is proposed in order to efficiently analyze practical multilayered printed filters, considerably reducing the computational cost. Several examples with CPU time comparisons are provided, demonstrating the accuracy and efficiency of the new technique. A novel transversal filter in a trapezium-shaped cavity is designed, manufactured, and tested for the first time using the spatial images technique. This work was supported by the Centro de Investigación Científica y Tecnólogica (CICYT) under Spanish National Project TEC2004-04313-C02-02/TCM and under Regional Seneca Project 02972/PI/05.