Transversal filter MMIC design for multi-Gbit/s optical CDMA systems
In this thesis, the approach of the distributed-amplifier based transversal filter for multi-Gbit/s Optical CDMA systems is addressed. Of particular interest is the research into circuits that enable handling high rate sequences for high-speed system applications. Different distributed transversal filter structures were considered, in particular those that allow extending the range of filtering functions by including positive and negative tap gain weight control. A novel transversal filter topology that enables operations at multi-Gbit/s application is designed and its behaviour is studied. The work demonstrates that pulse generation and correlation functions in the electrical domain can be accomplished using the designed versatile and reconfigurable transversal filters. The newly developed filter is dubbed the triple-line transversal filter in reference to the filter cell design that distributes the parasitic capacitances of the devices along three artificial transmission lines; a common-input line and two output drain lines. It is demonstrated that this topology can be designed for chip rates exceeding 40 Gbit/s. The practicalities of the triple-line transversal filter were assessed using a 0.2 mum-gate length HEMTs and following the design rules of a commercially available MMIC foundry process. A new tap gain weight control technique was specifically designed for the triple-line filter topology. A special bias level calculation technique was developed and used so that the filter can be modelled and operated with constant distributed characteristics. Wideband delay transmission lines were modelled and designed for filter implementations. An assessment of the MMIC transversal filter is provided via computer simulations in the time and frequency domains. A novel framework based on differential-mode scattering parameters was derived to investigate the various frequency responses of the triple-line structure. The filter satisfies the first Nyquist criterion and is suitable for decoders and encoders of CDMA systems. Overall, this thesis addresses design issues that provide an insight into the practicality of receivers and transmitters for multi-Gbit/s CDMA systems.