Quasi-elliptic Microstrip Filters in K-Band

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 • ButterworthChebychevElliptical • 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

Quasi-elliptic Microstrip Filters in K-Band