Tunable Phase Gradient Supported Electrical Balance Duplexer-System based on Digital Tunable Capacitors in RF SOI Technology

In modern mobile radio systems surface accoustic wave (SAW) filters are generally used for the frequency duplex. The receive (Rx) path is separated from the transmit (Tx) path by means of a so-called duplexer filter, which ensures a high level of transmit-to-receive (TxRx) isolation. To cover numerous mobile radio frequencies, a significant number of SAW-based duplex filters is required that take up a considerable amount of space in a mobile radio device. For this reason, research into tunable duplexer systems that do not require SAW filters, such as the electrical balance duplexer (EBD) and phase gradient supported electrical balance duplexer (PBD), has been pursued in the past. Both approaches reveal a notable interdependence between TxRx isolation and the variation in antenna impedance. In contrary to the ideal EBD system, the ideal PBD system is able to realize losses of less than 3 dB in the Rx and Tx path simultaneously. In literature, a varactor-based PBD system using a search algorithm-based phase shifter (PS) has been introduced. This work focuses on the applicability for practical use by replacing varactors with digital tunable capacitors (DTCs) based on radio frequency (RF) silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology and introducing an alternative equation-based PS (PS on the basis of analytic equations). In practical applications, maintaining a linear capacitive tuning range at high power levels is crucial. In SOI-based DTCs the power handling capability is primarily determined by its number of transistor stacks. Moreover, an equation-based PS approach can be utilized to circumvent the necessity of a search algorithm in the design phase, thereby establishing the optimal states for a minimum insertion loss in multi-band applications. An equation-based PS enhances the centering of the antenna tracker goal reflection coefficient on the Smith chart, thus enabling passive antenna trackers to cover a broad VSWR range. In the context of this work, two distinct types of DTCs are introduced. The first DTC exhibits a quality factor of greater than 68, a tuning ratio of 3.2, a resolution of less than 11 fF, a minimum capacitance of 1.417 pF, and a RF voltage handling capability of 20 V, which is equal to 36 dBm at 50 Ω. The second DTC is designed to achieve a large tuning range of 17.5, a medium quality factor with a value of 6.5, a resolution of less than 19 fF, a minimum capacitance of 1.414 pF, and a RF voltage handling capability of less than 20 V, which is equal to 36 dBm at 50 Ω. Additionally, a PBD prototype with a search algorithm-based PS targeting the long term evolution (LTE) low-band bands B71, B17, B26, and B8 is presented. The tunability of the PBD system is established by these two distinct types of DTCs. In the simulations a TxRx isolation of greater than 55 dB and an insertion loss of less than 3 dB in the Tx and Rx path is achieved. However, the determined results cannot be transferred to the examined prototype, since the wiring of the isolators differs from that in the simulation environment. A voltage standing wave ratio (VSWR) value of 1:1 at the antenna was taken into account. Furthermore, a study of equation-based PS architectures is carried out, discussing viable alternatives to search algorithm-based PS. A simulation-based evaluation of the influence on the PBD system parameters is conducted. Losses in the Tx and Rx paths are well below 3 dB. These architectures allow VSWR ranges greater than 1:1 and a centered antenna reflection goal coefficient on the Smith chart in the frequency range of 0.6 GHz to 1 GHz.