Voltage Controlled Metasurfaces, Self-Biased Circulators and Passive Nonlinearities in RF Front-End

The talk will cover the following three topics:
A. Passive and active metasurfaces formed by periodic arrays of interwoven conductor patterns;
B. Self-biased nonreciprocal devices: hexaferrite based circulators and isolators;
C. Passive nonlinearities and intermodulation in multicarrier wave packets.
They address both the device and signal integrity aspects of RF front-end of communications systems.
A. Doubly periodic arrays of interwoven conductors represent a distinct class of passive and active metasurfaces and high impedance surfaces (HIS) with substantially subwavelength unit cells. Such arrays have a broadband response, high angular and polarisation stability and low loss at RF frequencies. The fundamental properties and the physical mechanisms underlying the main features of these structures are very well described by the developed qualitative models.
Interleaved conductor arrays with embedded pin diodes proved to be particularly apt for realisation of tunable metasurfaces with integrated voltage controlled active components. They exhibit high isolation (~15 dB) between the transparency and reflectance states and are also resilient to parasitic effects of real switches - insertion loss in the on-state only improves isolation between the on and off states. The main features of such metasurfaces with interwoven conductor patterns are illustrated by the examples of doubly periodic arrays of interleaved quadrifilar spirals and Brigid’s crosses.
B. Integration of ferrite devices in RF front-end poses a major challenge - conventional circulators are bulky and incompatible with the emerging fabrication processes. The existing alternative magnet-free circulators suffer from excessively high losses. The main causes of high losses in self-biased circulators and a means of loss reduction are outlined, and the new arrangements of miniature self-biased microstrip and CPW circulators are presented. It is shown that the insertion loss of the new hexaferrite based circulators can be reduced for more than a half without compromising the high isolation and return loss in a frequency band about 5%.
C. Distortions of multitone wave packets by passive nonlinearities in RF front-end have a major impact on integrity of information signals. Mitigation of the effects of weak distributed nonlinearities is a challenge because of multiphysics and multiscale nature of the underlying physical phenomena. A new approach is proposed for the quantitative analysis of multicarrier wave packets and the products of their nonlinear mixing. It reveals the fundamental properties and mechanisms of the wave packet evolution and shows that beat frequencies of high-power carriers can notably contribute to the wave packet spectral content, losses, and intermodulation products. An example of multicarrier wave packet interactions with an impedance surface of finite conductivity illustrates the mechanisms of the intermodulation products generation due to the electro-thermal nonlinearity.