Quasi-elliptic Microstrip Filters in K-Band Allen Chang Cornell University Advisor: Dr. Pearson SURE 2003

Overview NASA sponsored project: noise measurements in a specific frequency band Front end filter needed for receiver Filter goals: Low loss High selectivity Low complexity Preliminary filter constructed by grad student Joel Simoneau

Background Three Common Types of Filters Butterworth Chebychev Elliptical None are particularly adequate Proposed alternative: Quasi-elliptical filters

Quasi-elliptic Filters Combines features of elliptical and chebychev filters Advantages in selectivity over Butterworth and Chebychev Disadvantages in loss, and attenuation in comparison to Butterworth/Chebychev Easier to synthesize than elliptic Ralph Levy proposed idea in 1976, but wasn’t fleshed out In recent years, Hong and Lancaster have explored this design at low microwave frequencies

Filter Theory Modification 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 Implementation Microstrip format Dielectric sandwiched between conducting surfaces Design etched or milled on top surface Supports quasi-TEM mode Why microstrip?  Compact, low cost, high volume  Drawbacks: lossy at high frequencies, low resonator Q factor

Physical Implementation Our specifications: Conductor: Copper – high conductivity, low loss Dielectric: RT/Duroid 5880 Note: 1 mil = 25 um

Filter Design Open loop resonator design chosen Demonstration filter (N=6) fabricated:

Demonstration Results

Filter Design Spacing between resonators dependent upon coupling configuration and open loop dimensions Three primary coupling configurations: Simulation software (Agilent-ADS) used to achieve desired coupling coefficient

Filter Design Middle and cross coupling need to have opposite signs Input/output tapping position also determined using simulation

Open Loop Layout Final open loop filter layout at 24 Ghz

Simulation Results:

Alternative Design Fabrication problems with open loop Hairpin design is a viable alternative Operates on similar principles Hairpin Layout:

Layout Comparison Standard chebychev parallel coupled filter Utilizes coupled input/output instead of tap Only 2 half-wave resonators

Performance Comparison

Future Work Ways to decrease loss? Majority of losses stem from ohmic(metal) loss, which can’t be helped Focus on decreasing dielectric loss One possibility: air dielectric filter Suspended on thin polyimide sheet Wet etch process, gold conductor

Conclusions Quasi-elliptic filters can improve selectivity with minimal increase in fabrication complexity Metallic losses may dominate at high frequencies Applications must be loss-tolerant

Acknowledgements Dr. Pearson SURE coordinators Dr. Noneaker & Dr. Xu Joel Simoneau Venkatesh Seetharam Chris Tompkins