Investigations into Impedance Tuners for the Next Generation of Communication Systems

Abstract

Novel communication systems, including 5G and the upcoming 6G cellular networks and software defined/cognitive radio technologies, call for compact, multi-band, and reconfigurable circuit components. Specifically, stringent requirements of 5G networks in terms of high data rates and spectrum efficiency have paved the way for the widespread incorporation of full-duplex radios, multi-band Power Amplifiers (PAs), and multiband antennas in communication systems. Simultaneously, the transceivers need to be tunable so that they can cover the numerous bands of the wireless networks. The implication in research is to push the existing fabrication technologies to their performance extremes, high precision tuning techniques, and to develop and devise reconfigurable subcomponents. Impedance tuners are integral parts of any communication systems. Their ability to regulate the power transfer between different components has led to novel applications for these components in full-duplex front-ends. This includes supressing the self-interference problem by improving the isolation between the transmitter (Tx) and receiver (Rx). Impedance tuners are also required to address the antenna impedance variation over the frequency and due to the variable environmental conditions. Moreover, an increasing number of multiband components such as antennas, PAs, power dividers, and baluns, are gaining momentum in communication systems. This highlights the need for multiband matching networks, which should simultaneously address the variable frequency bands (shift in frequency) and variable impedance of the different components. The requirements for such multiband impedance tuners include ability to handle variable frequency bands, wide impedance coverage at each frequency band, independent operation at each band, low loss, compact, and high power performance. The presented thesis is focused on novel applications for the impedance tuners. It presents a detailed analysis of doubly terminated impedance tuners and a novel approach for designing fixed and tunable dual-band matching networks using RF filters and phase-shifters. The intended frequency of operation is Sub-6 GHz frequency region, yet the discussions are kept applicable to other frequency ranges. The thesis first presents a detailed analysis of doubly terminated impedance tuners in the form of tunable loaded-lines and π- networks. The intention here is to address the stringent requirements for the full-duplex transceiver. This includes improving the matching between the impedance-varying antenna and the circulator, as well as the isolation between circulator ports. Accordingly, Radio-Frequency Micro-Electro-Mechanical System (RF MEMS) packaged switches are used in a distributed π-network matching network. Accurate modeling of the RF switch parasitic elements, as well as inclusion of the electrical length of the switches in the circuit design stage allows for low-loss and high precision switched matching networks. A doubly terminated prototype for achieving a 30 dB of matching at both ends is designed and tested at 2 GHz. Moreover, Barium Strontium Titanate (BST) variable capacitors are employed in a CPW loaded line configuration to achieve at least 40 dB of matching between a variable antenna and a circulator with return losses of 15 dB and 18 dB, respectively. A prototype is designed and tested for operation between 3.4 GHz and 3.7 GHz frequencies. Accordingly, a detailed model for connecting Transmission Line (TL) segments is developed and optimized. The measurement results demonstrate that the designed matching network can achieve 40 dB matching over at least 20 MHz bandwidths over the band of 3.4-3.7 GHz with better than 0.6 dB of insertion loss. In addition, a Reconfigurable Impedance Tuner (RIT) for full-duplex transceiver front-ends is presented. The RIT employs multi-port Phase Change Material (PCM) switches to improve the isolation between the circulator ports. These switches are extremely and have extremely low insertion loss over Sub-6 GHz frequencies. This allows for achieving compact, fully integrated, passive, and low loss RITs, where the main line is loaded by switched high quality factor capacitors. Employing 6 PCM switches allows for more than 2.6×105 switching states and a high precision uniform coverage. This allows for achieving circulator isolation of more than 40 dB over the 3.4-3.7 GHz frequency band, while negligibly affecting the matching. In the second part of the thesis, a novel approach for designing multi-band matching networks is investigated. The novel concept is based on cascade configurations of filters and phase shifters to develop fixed and tunable dual-band matching networks. Two types of the fixed Dual-band Cascade Matching Network (DBCMN) are designed and developed. The first type of DBCMN uses individual filters and matching networks for each frequency band. This offers a simple and straightforward design procedure. The first type is demonstrated through a design example, which employs a Low Pass Filter (LPF), a High Pass Filter (HPF), and two microstrip-line phase-shifters. The second type combines the two filters and uses a dual-band phase-shifter. For this purpose, a Band Pass Filter (BPF) and dual-band Reflection Type Phase Shifter (RTPS) are used. The two types are compared and a few guidelines for selecting the appropriate type of the DBCMN according to the application are presented. The advantages of the proposed DBCMN includes miniaturization, low loss performance, and independent matching of the two bands. The most significant contribution of this thesis is the development of a design and implementation methodology for Dual-band Reconfigurable Impedance Matching Networks (DB-RIMNs). To our knowledge, this is the first time a tunable solution is offered for the cases where the impedance of the dual-band load and/or the intended matching frequency of the two bands are varied. First, the requirements for the proposed method are presented. This is followed by developing single band and dual-band tunable phase shifters. Then, circuit typologies for the proposed DB-RIMN are presented. Finally, the proposed DB-RIMN is implemented using tunable band pass and band stop filters, based on the frequency ratio of the two bands.