Lossy resonator with high Q for switchable absorptive bandstop to bandpass filter

• Main Author: Zahari, Mohd Khairy
• Format: Thesis
• Language: English; English
• Published: 2018
• Subjects: T Technology (General); TK Electrical engineering. Electronics Nuclear engineering
• Online Access: http://eprints.utem.edu.my/id/eprint/23307/1/Lossy%20Resonator%20With%20High%20Q%20For%20Switchable%20Absorptive%20Bandstop%20To%20Bandpass%20Filter.pdf; http://eprints.utem.edu.my/id/eprint/23307/2/Lossy%20resonator%20with%20high%20Q%20for%20switchable%20absorptive%20bandstop%20to%20bandpass%20filter.pdf; http://eprints.utem.edu.my/id/eprint/23307/; https://plh.utem.edu.my/cgi-bin/koha/opac-detail.pl?biblionumber=112323
• Institution: Universiti Teknikal Malaysia Melaka

New developments in the design of the switchable microwave filters in some cognitive radio system are essential to meet the ever increasing demands to discriminate between wanted and unwanted signals. There also has a demand for miniaturization of microwave communications systems. A compact design can be achieved through the implementation of planar microstrip technology. However, conventional electronically tunable bandstop filters suffer performance degradation due to the finite unloaded Q of the resonators and also the loss associated with the switching elements. Therefore, two low Q lossy resonator filter topology has been implemented where the topology can be used to partially compensate for the loss where a high Q absorptive bandstop filter can be achieved. The filter consists of λ/2 resonator with K-inverter, parallel with an Allpass nominally-90°-phase-shift element. A frequency agile bandstop filter based on this topology has been developed, but such filters as well as conventional switchable bandstop filters encounter performance degradation in terms of tuning bandwidth and stopband bandwidth due to the frequency dependant losses and couplings. Through this thesis a new switchable microwave filter is investigated and developed, where this filter is able to switch from high Q absorptive bandstop response (ON state) to bandpass response (OFF state). This switchable filter is designed using four different types of resonator which are parallel coupled, dual mode ring, stepped impedance dual mode and T-shape. The parallel coupled resonator consisted of two low-Q lossy resonator connected with 90° wavelength and with correct k-inverter to produce high Q absorptive bandstop response. T-shape resonator consisted of T resonator coupled with 90° wavelength. While for the dual mode ring resonator structure is composed by two degenerate modes or splitting resonant frequencies, where the ring can be excited by perturbing stub. For stepped impedance resonator, the structure is consisted of the stepped impedance resonator with mid-plane of via hole and connected with 90° wavelength to achieve the high Q absorptive bandstop response. The filters are integrated with switching element, such as PIN and a varactor diode to switch the filter response and biasing circuit is needed to make the PIN or the varactor diode working properly. The absorptive bandstop filter operates at 2.4 GHz where S11 is below than 15 dB and S21 has high selectivity with the narrow bandstop response with high Q factor. The unloaded Q factor of the absorptive bandstop filter is more than 60 for measuring and 150 for simulation. For a bandpass response, the response depends on the filter structure. Where, each resonator produced different character of a bandpass filter. The dual mode bandpass response for stepped impedance, was achieved by switched ‘OFF’ the PIN diodes, where the insertion loss, S21 4.9 dB, return loss, S11 is below 15 dB, and passband bandwidth is 200 MHz at centre frequency of 2.35 GHz. A good agreement is observed between simulated and measured results. The benefits of this filter is not only can produce a bandpass response, but also high quality factor in bandstop response which offer a better performance and high selectivity. The outcomes of the proposed switchable filters may facilitate improvements and the solution in cognitive radio.