Design and Development of Planar Lowpass Filters with Sharp Roll-off and Wide Stopband for RF and Wireless Communication Systems

Abstract

Filters are one of the essential components in the RF and wireless communication systems. Small sized planar lowpass filters with good electrical characteristics along with low cost, light weight and ease of fabrication are highly desirable for the front end of modern communication systems to suppress harmonics and spurious signals. Design of compact lowpass filters with improved performance and diverse specifications for numerous applications is a huge challenge. In this thesis, high performance planar compact lowpass filters using multiple patch resonators on high impedance transmission line are developed. Design techniques of different types of patch resonators and their modifications to enhance the performance of the filters are presented.Patch resonators are designed by using high impedance short circuited stubs and low impedance open circuited patches. In the first stage of filter realization, compact lowpass filter having sharp roll-off using triangular and funnel patch resonators is presented. The structure is modified further to enhance the relative stopband bandwidth of the filter. In the third stage, another resonator has been introduced near the feed line to achieve sharp roll-off for the same cutoff frequency, stopband bandwidth and suppression level. To obtain compactness, high suppression level and wide stopband in filter design, low thickness substrate is tested and proved in the fourth stage. Realizations of planar compact lowpass filter with very sharp roll-off near the cutoff frequency have been presented using stepped impedance polygonal patch resonators. By increasing the patch size and number of resonators, the stopband bandwidth and suppression level have to be enhanced to a great extent. Enhancement of performance characteristics of lowpass filter design is continually being extended. By using high value capacitance patch, the stopband suppression level of the filter with sharp roll-off rate is achieved. Elliptic function lowpass filter with ultra-sharp roll-off is also developed using elliptic shaped patch resonators. Throughout the study, low cost substrate having permittivity 4.4 is used for the filter design. All the designed filters have been fabricated and predicted results are validated by the measurements.