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