Synthesis of Filters for Digital Wireless Communications

Firstly, a new synthesis method for the generation of the generalized Chebyshev characteristic polynomials has been presented. The general characteristic function is generated by a linear combination of Chebyshev basis characteristic functions. The basis functions for different filtering functions may easily be determined based on the number and position of reflection and transmission zeros. These basis functions enable direct synthesis of both lumped and distributed filter networks. Different filter functions including but not limited to low-pass, bandpass and dual bandpass filters, have been synthesised to demonstrate the general application of the synthesis method. Secondly, a new method for the design of a new class of distributed low-pass filter has been presented that enables exact realisation of the series short circuited transmission lines which are normally approximated via unit elements in other filter realisations. The filters are based on parallel coupled high impedance transmission lines which are terminated at one end in open-circuited stubs. The approach enables realisation of both finite and quarter-wave frequency transmission zeros hence giving improved stopband performance. A complete design is presented and the fabricated low-pass filter demonstrates excellent performance in good agreement with theory. Finally, design techniques for microwave bandpass filters using re-entrant resonators are presented. The key feature is that each re-entrant resonator in the filter generates a passband resonance and a finite frequency transmission zero, above the passband. Thus an Nth degree filter can have N finite frequency transmission zeros with a simple physical realization. A new synthesis technique for pseudo-elliptic low-pass filters suitable for designing re-entrant bandpass filter has also been show-cased. A physically symmetrical 5 pole re-entrant bandpass prototype filter with 5 transmission zeros above the passband was designed and fabricated. Measured results showed good correspondence with theories.