Ndujiuba, Charles Uzoanya; (1999) Predistortion technique and its application for loss compensation in high frequency filter design. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This thesis outlines a novel and systematic method for high frequency active filter design based on predistortion of the filter transfer function by appropriate loss/or gain factor. The approach enables the designer to realize all his filter specifications without resorting to the traditional circuit optimization and tuning (tweaking). The strength of this technique is demonstrated in terms of the savings in computation time and memory requirement, when compared with the commercial EM-Simulators. As a proof of practical usefulness of this method, a couple of 3rd order coupled-line microstrip bandpass filters are described, designed and fabricated using this approach. The results show a remarkable compliance between theory and practice without resorting to optimization. Using the material properties of the microstrip line, the filter loss is calculated to provide an estimate of the amount of compensation (predistortion) required. As an extension of this work, a negative resistance circuit is designed and fabricated using the GEC-Marconi GaAs F20 process, which is coupled to the passive filter to compensate for the resulting passband insertion loss. Both simulated and measured results of the predistorted active microstrip bandpass filters are presented to demonstrate the effectiveness of this approach. The results suggest that the usual phenomena of shift in centre frequency, loss of poles and zeros, and rounding of passband edges are all corrected. Complicated tuning design iterations are therefore avoided. The application of predistortion technique was then examined in the context of Monolithic Microwave Integrated Circuit (MMIC) filters, where full integration of chip sets has been hindered by the innate resonator low Q-factors. Using active inductor circuits, two predistorted 2nd order MMIC filters, one operating at 5GHz centre frequency and bandwidth of 150 MHz, and the other operating at DCS 1800 base-station receive band (1.71GHz - 1.785GHz), are designed and fabricated using GEC-Marconi GaAs F20 process. Simulation and measured results also confirm that the filter specifications are realized, hence validating our proposed method.

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