A Full Spectrum Synthesizer For UWB Direct-conversion Transceiver

Due to its high channel capacity, an ultra-wideband (UWB) system is an attractive solution for the implementation of very high data rate (>100Mbps) short-range wireless communication systems. Among the different options of efficient use of the UWB spectrum, the MB-OFDM approach has received strong support from several industrial organizations. UWB frequency synthesizer is a key module and a design challenge in a MB-OFDM UWB RF transceiver. At least four reasons make it difficult to design. First, the range of UWB spectrum is very broad (3.1-10.6GHz). So the output frequencies of a UWB frequency synthesizer span several gigahertzes. Second, the time to switch between different band frequencies should be limited in 9.47ns, which eliminates the possibility of a conventional PLL based frequency synthesizer. Moreover, the UWB technology is defined as a kind of low power technology. Taking the fact that frequency synthesizers are always the second biggest power consumer in various multi-band RF transceiver front-ends into consideration, a new structure for UWB frequency synthesizer should be as simple as possible to reduce the power consumption. The more simple the architecture is, the more power and circuit area it saves. Small circuit also means low cost. Last, operating as a low power device, the power emitted/received by the UWB RF transceiver is relatively weak. So, to make sure that the UWB device would not be interfered by other wireless communication systems whose power emission is high and other UWB device is very important. In this work, to deal with the problems mentioned above, a new architecture and modification to the key block are proposed on the basis of current achievements at home and abroad. The main contribution of this work is shown as follow:1) The main technical indexes of the UWB system placed on the frequency synthesizer are fixed.2) A new UWB frequency synthesizer, which can generate all the 14 band frequencies, is proposed. This new structure has both the advantages of simplicity and the bonus of its simplicity, for example, low power and less building blocks. The stages of mixing, which deteriorate the purity of the output signal is minimized.3) Structures of various circuits needed in a UWB synthesizer are mentioned and the final circuits are chosen. The stability of PLL is made sure by designing its parameters using maximum phase margin method.4) A UWB bandpass network whose centre frequency can be switched between the 14 band frequencies is proposed and it can achieve a good selectivity.5) The proposed architecture is designed and simulated in ADS using TSMC 0.18 CMOS technology. Results show that the spurs rejection ratio is over 30.094 dBc.