Ultra-wideband variable signal generator using series shunt switch

Ultra-wideband (UWB) technologies are at the forefront of wireless communications, offering the possibility to provide extremely high data rate wireless solutions. This paper works on the UWB variable signal generator that is capable of fast switching between different center frequencies. The target switching time is 4ns and the three center frequencies at the lower band are selected, which are 3.35 GHz, 3.85 GHz and 4.35 GHz. The structure of our variable signal generator is based on the topology of active oscillators. By using a switching network, the different frequencies from different local oscillators are switched within 4ns. In this project, a basic NMOS switch is studied and the weaknesses are justified. A new topology of NMOS switch based on series-shunt configuration is proposed so that it is able to pass the signal from oscillators to output without transition spike. Three oscillators operating at targeted center frequencies are designed based on cross-coupled LC topology. The problems and issues after the integration of switches and oscillators are identified and solved. A modified switch is developed specially for solving the loading effect design issue. By optimizing the transistor size for switch and other design parameters in oscillator, it is possible to generate the signal at desired frequency and amplitude without transition noise. The performance of our variable signal generator is estimated through simulation with a target technology of Silterra 0.18 mum CMOS at a supply voltage of 1.8 V. The simulation results indicate that our variable signal generator is able to produce a signal with three center frequencies, i.e. 3.35 GHz, 3.85 GHz and 4.35 GHz at 400 mV peak-to-peak swing within every 4ns.