Today's radio frequency front-ends (RFFEs) employed in cellular devices achieve duplex operation by utilizing discrete, fixed-frequency surface acoustic wave (SAW) or bulk acoustic wave (BAW) filters. The state-of-the-art tunable filters cannot meet the demanding requirements for steep roll-off and low insertion loss, supporting multi-band operation. As a result, they require multiple fixed-frequency duplexers and switches for band selection. However, supporting all the frequency bands increases circuit design complexity, area consumption, and manufacturing cost. It is a crucial challenge and research interest to develop a tunable RFFE.
This thesis starts with an introduction to this work. The first chapter presents the motivation and objective, then in the following chapter, fundamental and background knowledge of duplexers is reviewed. From the works of literature and state-of-the-art, various architectures in hardware domains such as SAW-, circulator- and hybrid transformer-based or software domains like digital cancellation have been proposed to achieve full-duplexing (FD). Motivated by the concept of electrical balance, two alternative duplexer design approaches from literature are presented for LTE low band application. One of them is the Wheatstone bridge balanced duplexer (WBD). This architecture combines the electrical balanced duplexer (EBD) and the Wheatstone bridge circuit. It uses the SAW technology and digitally tunable capacitors (DTCs) to realize the circuit blocks in the system, as shown in Chapter 3. Another proposed architecture is a phase gradient supported balanced duplexer (PBD) which has been investigated at the Institute of Electronics Engineering in FAU Erlangen-Nürnberg to overcome the insertion loss limitation that features at least 3 dB in the EBD concept. Through the equation-defined design parameters of circuit blocks and detailed system analysis, the PBD concept is described theoretically in Chapter 4. With these two presented architectures, the key performance indicator of duplexer insertion loss at Tx and Rx band can achieve 2.9 and 2.8 dB, respectively, and the transmit-to-receive isolation can reach over 50 dB. Last, this thesis summarizes and reviews the WBD and PBD performance in Chapter 5. It is feasible to have a small form-factor and tunable duplexer realization targeted at LTE low band application by implementing complementary metal-oxide-semiconductor (CMOS) and metal-based micro-electro-mechanical systems (MEMS) technology in the proposed architectures
