Highly flexible dual and tri band impedance transformation techniques and their applications in advanced passive and active circuits

Abstract

The proliferation of multi-band and multi-standard wireless systems has led to frequent advancements in their design techniques and schemes. In essence, the multi-band devices ensure a reduction in the resources required for the development of a multifrequency transceiver, considering that individual components can be utilized to achieve the desired functionalities at multiple frequencies. Furthermore, the operation of one device at different frequencies of interest increases the re-usability of the same. For example, for a tri-band power divider, a single device can split power at three different frequencies of interest, without other additional supporting circuitry.

However, the design of multi-band circuits has been found to be extremely challenging. The key constraints are the achievable frequency ratios (or band separation) and the achievable impedance transformation ratios (or impedance gradient). The designs reported in the literature are limited in frequency ratio, which limits their usefulness in applications having widely separated frequency bands. For example, the design of a device to operate at GSM downlink (900MHz) and WiFi-LTE (5.8GHz) is not practically realizable using current techniques due to wide band separation. Another vital factor for multi-band devices is the requirement of “per-band bandwidth” according to the wireless standards. It has been accepted that for closely separated bands, the perband bandwidth is either too low or too-high (essentially making it wideband), and this is another pressing issue requiring attention. The other key challenge of impedance transformation ratio or “Impedance Gradient”, when appropriately addressed, can be potentially useful for numerous applications, including RF Energy Harvesting, Buttler Matrix in Beamforming, etc. Once again, the existing designs and schemes are pretty good for lower transformation ratios but are extremely limited for a wide range of impedance variations. The literature is replete with dual-band architectures, but the reports on tri-/quadand other higher-bands are still in infancy. There have been proposals of some generalized designs, but these have definite limitations. The major limitation of such designs is extremely tedious design procedures and very complicated mathematical formulations when extended to tri-band and above applications. Moreover, most of these generalized techniques rely on convergence technique or graphical approach requiring optimization, thereby essentially leading to a hit-and-trial approach. This issue can be attributed to the absence of closed-form design equations. In addition, most of the existing design techniques are limited in frequency and impedance transformation ratio. The doctoral research, therefore, envisages addressing some of the most important concerns mentioned above. This necessitates the determination of closed-form design equations for proposed circuits, development of simplified design procedures, and miniaturization of the proposed designs. All of these developments include various aspects such as enhancement in frequency and impedance transformation ratios, increased bandwidth per frequency band, and inherent DC blocking ability at all the selected frequencies. In the context of active circuits, the additional directions that are envisaged are the design, optimization, and linearization. For example, multi-band RF Power Amplifiers in IoT applications within an indoor environment need to operate at low power, but it may often require optimization and linearization techniques to achieve decent performance (PAE, IIP3, etc.) at all the frequencies simultaneously. The application focus of this research work is commercial communication, satellite and military, and the upcoming IoT. In all of these applications, the RF front-end modules are one of the key players, and therefore the planned objective of this research work has the potential to significantly advance the current state-of-the-art that may eventually lead to a paradigm shift in the design of such applications.