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Quasi-elliptic Microstrip Filters in K-BandAllen ChangCornell UniversityAdvisor: Dr. PearsonSURE 2003
OverviewNASA sponsored project: noise measurements in a specific frequency bandFront end filter needed for receiverFilter goals:
Low loss
High selectivity
Low complexity
Preliminary filter constructed by grad student Joel Simoneau
BackgroundThree Common Types of Filters
Butterworth Chebychev Elliptical
None are particularly adequate
Proposed alternative: Quasi-elliptical filters
Quasi-elliptic FiltersCombines features of elliptical and chebychev filtersAdvantages in selectivity over Butterworth and ChebychevDisadvantages in loss, and attenuation in comparison to Butterworth/ChebychevEasier to synthesize than elliptic
Ralph Levy proposed idea in 1976 , but wasnt fleshed out In recent years, Hong and Lancaster have explored this design at low microwave frequencies
Filter TheoryModification of standard filter design
Transfer function realized through cross coupling
Middle and cross J-inverters interdependent
Generalized filter parameters Qe and Mxy can then be found
Physical ImplementationMicrostrip formatDielectric sandwiched between conducting surfacesDesign etched or milled on top surfaceSupports quasi-TEM modeWhy microstrip?Compact, low cost, high volumeDrawbacks: lossy at high frequencies, low resonator Q factor
Physical Implementation Our specifications:Conductor: Copper high conductivity, low lossDielectric: RT/Duroid 5880Note: 1 mil = 25 um
Filter DesignOpen loop resonator design chosenDemonstration filter (N=6) fabricated:
Demonstration Results
Filter DesignSpacing between resonators dependent upon coupling configuration and open loop dimensionsThree primary coupling configurations:
Simulation software (Agilent-ADS) used to achieve desired coupling coefficient
Filter DesignMiddle and cross coupling need to have opposite signs
Input/output tapping position also determined using simulation
Open Loop LayoutFinal open loop filter layout at 24 Ghz
Simulation Results:
Alternative DesignFabrication problems with open loopHairpin design is a viable alternativeOperates on similar principlesHairpin Layout:
Layout Comparison Standard chebychev parallel coupled filterUtilizes coupled input/output instead of tapOnly 2 half-wave resonators
Performance Comparison
Future WorkWays to decrease loss?Majority of losses stem from ohmic(metal) loss, which cant be helpedFocus on decreasing dielectric lossOne possibility: air dielectric filterSuspended on thin polyimide sheetWet etch process, gold conductor
ConclusionsQuasi-elliptic filters can improve selectivity with minimal increase in fabrication complexityMetallic losses may dominate at high frequenciesApplications must be loss-tolerant
AcknowledgementsDr. PearsonSURE coordinators Dr. Noneaker & Dr. XuJoel SimoneauVenkatesh SeetharamChris Tompkins