Monolithic Frequency Filter Designs Bsed On A Sampled-Data Analogue Wave Filter Approach

Abstract

Frequency filters are widely used in communications systems today. Historically they have been constructed from inductors, capacitors and resistors and such circuits are well understood. However they suffer from the disadvantages of large physical size and high cost. A monolithic realisation of frequency filters would greatly reduce the cost of systems in which filters are used and would increase their reliability in hostile environments. In this thesis a novel approach to the realisation of sampled-data monolithic frequency filters is presented. The method is based on the use of sampled analogue signals and is related to the wave digital filter in its design techniques. The eventual monolithic realisation in NMOS technology is in the form of a switched-capacitor structure. While the main body of the research is concerned with the implementation of filters which are based on distributed prototype networks, some work on integratable filters which are based on lumped element prototypes is also presented. For either type the design model is exact and the resulting filter is optimally insensitive to parameter variations. The filters are implemented using a technique which ensures that performance is limited by capacitor ratios, which are moderate as compared with alternative published approaches. A prototype integrated circuit suitable for use in the audio frequency band has been designed to demonstrate the validity of the approach for the filters based on distributed prototypes and contains. filters of third, fifth and seventh order. Results are presented for these filters and also for a pseudobandpass filter which uses the periodicity of the prototype distributed network. The seventh order filter had a cut-off frequency of one eighth of the filter sample rate and achieved a stop band attenuation of well over 70dB An implemetation of a wave filter based on a lumped element prototype is also described and results are presented. This work is expected to form the basis of a novel implemetation of bandpass filters. The responses achieved for all the wave filters constructed show excellent agreement with the theory.