Compact Ultra-Wideband Bandpass Filters Using Composite Microstrip–Coplanar-Waveguide Structure

Compact ultra-wideband bandpass filters are proposed
based on the composite microstrip–coplanar-waveguide
(CPW) structure. In this study, the microstrip–CPW transitions
and the CPW shorted stubs are adopted as quasi-lumped-circuit
elements for realizing a three-pole high-pass filter prototype. By
introducing a cross-coupled capacitance between input and output
ports of this high-pass filter and suitably designing the transition
stretch stubs, a compact three-pole ultra-wideband bandpass filter
is implemented with two transmission zeros located close to the
passband edges. To further improve the selectivity, two microstrip
shorted stubs are added to implement a five-pole ultra-wideband
bandpass filter with good out-of-band response. Being developed
from the quasi-lumped elements, and not from the transmission
lines, the proposed ultra-wideband filters have sizes more compact
than those of the published wideband filters. The proposed ultrawideband
filters have the merits of compact size, flat group delay,
good insertion/return loss, and good selectivity. Agreement between
simulated and measured responses of these filters is demonstrated.