Tunable narrow-band X-band bandpass filters

Meyer, Elmine (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: This dissertation proposes two narrow-band tunable X-band filters from different classes, one planar and one waveguide. The first is a suspended substrate varactor-tuned doubly end-loaded staircase filter that incorporates a geometrically decoupled biasing structure. The second design consists of a varactor-tuned evanescent mode waveguide filter that incorporates tuning screws and a symmetrical varactor mounting structure inside the guide. The filters are designed for miniaturization with competitive filter performance in terms of centre frequency tunability, insertion loss and bandwidth performance. A planar staircase filter with electrically decoupled biasing and centre frequency tunability is realised using a suspended substrate and surface mount varactor diodes. Each half-wave resonator is terminated at both ends with identical varactor diodes, controlled by a single biasing line running through the roof of the enclosure. For symmetrically loaded half-wave resonators such as used in the staircase filter, each resonant line is DC decoupled from the next and a zero-voltage point exists in the centre of each line. This point is stationary under changes in centre frequency, which is a significant advantage for tunable filters. The biasing structure is also non-resonant, which allows for good out-ofband performance. The proposed planar structure is illustrated by a fifth order staircase X-band filter. Using varactors at each open end of the resonators together with spatially decoupled bias feeds, a measured centre frequency tunability of 400 MHz is achieved around a nominal frequency of 11.9 GHz. The tunable evanescent mode waveguide filter is realised with in-waveguide mounted varactors and tuning screws. The mechanical tuning screws enable rough-tuning, while the varactors provide fine-tuning capabilities. The varactors are connected back-to-back on central metal plates which are grounded to the enclosure. The other end of each varactor is connected to a biasing wire which exits the enclosure through a small hole in the side of the waveguide. A centre frequency tunability of 1:3 GHz around a nominal frequency of 10:45 GHz is achieved by the manufactured filter prototype. Both filters are evaluated through theory and simulation in terms of the thermal resistance of the mounting structure and input power limitations due to the non-linearity and power dissipation of the varactor diodes. Across the full tuning range of the planar filter, the maximum input power limit is 8 dBm. For the waveguide filter, the simulation yielded a maximum input power limit for full range tunability of 2 dBm due to the nonlinearity of the hyperabrupt varactor diode, the applied voltage, and the power dissipation in the varactors. The proposed biasing structure is a novel method for biasing planar filters, and to the knowledge of the author, this is the first application of the specified in-waveguide mounted diode structure in tunable evanescent mode waveguide filters.

AFRIKAANSE OPSOMMING: Twee nouband verstelbare X-band filters vanaf verskillende klasse, een planêr en een golfleier, word aangebied in hierdie proefskrif. Die eerste filter is 'n gesuspendeerde substraat varaktor-verstelde dubbelsydig-belaaide trapfilter wat 'n geometries-ontkoppelde voorspanningsnetwerk insluit. Die tweede ontwerp behels 'n varaktor-verstelde onder afknip golfleier filter wat verstelbare skroewe en simmetriese varaktor monteringstrukture behels. 'n Planêre trapfilter met elektries-ontkoppelde voorspanning is gerealiseer met die gebruik van gesuspendeerde substraat en oppervlakgemonteerde varaktor diodes. Elke half-golflengte resoneerder word aan beide kante getermineer met identiese varaktor diodes wat beheer word deur 'n vertikale voorspanningslyn. 'n Punt van nul RF spanning bestaan op simmetries belaaide half-golflengte resoneerders, soos in die geval van 'n trapfilter waar elke resoneerder ook GS ontkoppel is. Hierdie punt is stasionêr met verandering in senter frekwensie, wat 'n groot voordeel vir verstelbare filters bied. Die voorspanningstruktuur is nie resonant nie, wat bydra tot goeie buiteband werkverrigting. Die voorgestelde filter word geïllustreer deur 'n vyfde orde X-band trapfilter. 'n Verstelbare senter frekwensie van 400 MHz is gemeet rondom 'n nominale frekwensie van 11:9 GHz. Die verstelbare onder afknip golfleier filter is gerealiseer deur van geïntegreerde varaktormonteringstrukture en verstelbare skroewe gebruik te maak. Die meganiese verstelbare skroewe stel die verbruiker in staat om growwe verstelling te doen, terwyl die varaktors fyn-verstelling moontlik maak. Die varaktors word rug-aan-rug konnekteer op 'n sentrale metaalplaat wat gegrond is teen die behuising. Die ander kant van elke varaktor is gekonnekteer aan 'n voorspanningsdraad wat deur 'n klein gaping in die behuising gevoer word. 'n Verstelbare senter frekwensie van 1:3 GHz rondom 'n nominale frekwensie van 10:5 GHz is gemeet. Beide filters is geëvalueer met teorie en simulasies in terme van die termiese weerstand van die monteringstrukture en die intreedrywingsbeperkings as gevolg van die nielineariteit en drywingsverkwisting van die varaktors. Oor die volle bereik van die frekwensie verstelling is die maksimum intreedrywingsbeperking vir die planêre filter 8 dBm terwyl dit 2 dBm vir die golfleier filter is. Die laer beperking van die golfleier filter is as gevolg van die nie-lineariteit van die varaktor tipe, die voorspanningsvlakke, en die drywingverkwissing in die varaktors. Die voorgestelde voorspanningsstruktuur is 'n nuwe metode vir die voorspanning van planêre filters, en so ver die kennis van die skrywer strek, is hierdie die eerste toepassing van die golfleier monteerde diode struktuur vir onder afknip golfleier filters.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103568
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