Novel narrowband microstrip filters for wireless communications systems

Farhat, Mohamad (2010) Novel narrowband microstrip filters for wireless communications systems. Doctoral thesis, London Metropolitan University.


The novel resonant structures based on microstrip integrated circuit technology have been developed and extensively studied in this research project for filter applications in next generation wireless communication systems. Several novel microstrip bandpass filter structures have been introduced that meet the stringent requirements for high-performance (shaip-cutoff frequency response, low insertion-loss, high return-loss, and high out-of-band isolation), compact size, low cost, and ease of integration in subsystem and systems. The analysis and modelling methods for these resonators includes a transmission-line theory, ABCD matrix, and EM simulation based on method of moments. The measured results are used to validate the performance of the devices developed and show good agreement with the theory and simulation results. The various ‘C-shape’ open-loop ring resonators were developed and fabricated, namely ‘Embedded-Square’ and ‘Embedded-Triangle’ resonators, exhibited a quasielliptic function bandpass filter response free from spurii across an ultra-wide bandwidth.
The theoretical analysis on these structures revealed that the transmission zeros (or attenuation poles) disposed at either side of the passband were a function of the length of the open-ends relative to the feed-line. It was also discovered that the input/output feed-lines had to be located asymmetrically relative to each other to yield the desired bandpass response.
The ‘C-shape’ resonator is able to produce a very broad 3-dB bandwidth of 1027 MHz, which has a fractional bandwidth of 24.63%. This broad passband is achieved with the resonator with line width of 200 microns. In addition, it has a return-loss of 13.16 dB and insertion-loss of 1.22 dB at centre frequency of 4.17 GHz. The newly designed ‘Embedded-Square’ and ‘Embedded-Triangle’ resonators were fabricated and the measured results showed an excellent suppression of out-ofband spurious responses up to 13.8 GHz and 17.7 GHz, respectively, where the rejection is greater than 10 dB. These new structures not only achieve fractional bandwidth of around 18% and 22% for the ‘Embedded-Square’ resonator and ‘Embedded-Triangle’ resonator, respectively; but also maintain an insertion-loss at maximum of 0.7 dB. These proposed filters exhibit good stopband rejection and the measured results confirm the validity and the usefulness of these proposed filters in many practical applications.
Compact dual bandpass filters were also developed using a variant of the openloop resonator and interconnected using an inter-digital capacitor. These microstrip structures have the ability to tune the upper passband without affecting the characteristics of the lower passband response. The tuning range is across a frequency range of 1.51 fo, where fo is centre frequency of the primary passband. The novel filter designs presented in this thesis provide advantage of compact size, low insertion-loss, high selectivity, and high out-of-band isolation across an ultra-bandwidth frequency range.

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